Atlas of Thymic Pathology [1st ed.] 9789811531637, 9789811531644

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Deepali Jain Justin A. Bishop Mark R. Wick Editors

Atlas of Thymic Pathology

123

Atlas of Thymic Pathology

Deepali Jain  •  Justin A. Bishop  •  Mark R. Wick Editors

Atlas of Thymic Pathology

Editors Deepali Jain Department of Pathology All India Institute of Medical Sciences New Delhi India Mark R. Wick University of Virginia Medical Center Charlottesville, VA USA

Justin A. Bishop Department of Pathology The University of Texas Southwestern Medical Center Dallas, TX USA

ISBN 978-981-15-3163-7    ISBN 978-981-15-3164-4 (eBook) https://doi.org/10.1007/978-981-15-3164-4 © Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

This book is a comprehensive, highly illustrated guide of thymic pathology for postgraduate residents, students, clinicians, and practicing pathologists. The book begins with reviewing the normal thymic structure and its immunohistochemical profile. The next section deals with the gross images of thymic lesions/tumors and histomorphology of nonneoplastic diseases, benign and malignant thymic tumors, with detailed descriptions of over 300 full-color photomicrographs accompanied by relevant clinical and radiology pictures and anatomical drawings. All photographs are complemented by short legends describing the illustration and providing relevant information. In addition to surgical pathology, a chapter is dedicated to the cytology of thymic lesions which is described rarely in available textbooks and literature. All types and stages of the most common thymic lesion that is thymoma is discussed in detail. A brief overview of the molecular pathology of thymic tumors is provided at the last along with the pathology of rare thymic lesions and lymphomas. The book is written by leading experts in thymic pathology, radiology, and surgical disciplines from the USA, Europe, Japan, and India. The book addresses the expanded role of pathologists in patient care of thymic lesions. We hope this book will serve as a practical reference handbook of thymic pathology and help readers to understand and diagnose thymic lesions/tumors for correct patient management. New Delhi, India Dallas, TX Charlottesville, VA

Deepali Jain Justin A. Bishop Mark R. Wick

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Acknowledgments

“I am thankful to the almighty God and my family including my husband Vijay and loving son Vivaan.”  —Deepali Jain “I thank my surgical pathology mentors and my loving, supportive family.”  —Justin A. Bishop “Many thanks are due to my wife, Jane, for her support during completion of this project.”  —Mark R. Wick We would like to sincerely thank all coauthors for contributing their time and expertise for the completion of this book. We also want to thank the publishing team of Springer for their untiring efforts and assistance with this book.

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Contents

1 The Normal Thymus���������������������������������������������������������������������������������������������������   1 Alexander Marx 2 Immunohistochemistry of Normal Thymus�������������������������������������������������������������  11 Maria Teresa Ramieri, Enzo Gallo, and Mirella Marino 3 Radiology of Normal Thymus, Thymic Lesions, and Tumors���������������������������������  23 Manisha Jana and Ashu Seith Bhalla 4 Surgical Approach to Thymic Lesions ���������������������������������������������������������������������  31 Manjunath Bale and Rajinder Parshad 5 Pathology of Nonneoplastic Thymic Lesions �����������������������������������������������������������  41 Alexander Marx 6 Gross Pathology of Lesions in the Thymic Region���������������������������������������������������  63 Mark R. Wick and Justin A. Bishop 7 Histomorphology of Thymomas��������������������������������������������������������������������������������  85 Prerna Guleria and Deepali Jain 8 Cytology of Thymic Lesions��������������������������������������������������������������������������������������� 113 Minhua Wang and Sinchita Roy-Chowdhuri 9 Pathology of Thymic Carcinoma������������������������������������������������������������������������������� 123 Anja C. Roden 10 Pathology of Thymic Neuroendocrine Tumors��������������������������������������������������������� 141 Deepali Jain and Prerna Guleria 11 Pathology of Ectopic Thymic Tumors����������������������������������������������������������������������� 151 Andrey Bychkov, Mitsuyoshi Hirokawa, and Kennichi Kakudo 12 Molecular Pathology of Thymic Epithelial Tumors������������������������������������������������� 169 Aruna Nambirajan, Varsha Singh, and Deepali Jain 13 Lymphomas and Other Rare Tumors of the Thymus��������������������������������������������� 173 Mirella Marino, Malgorzata Szolkowska, and Stefano Ascani

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About the Editors

Deepali  Jain  (MD, FIAC) is an Additional Professor at the Department of Pathology, All India Institute of Medical Sciences, New Delhi, India. A former postdoctoral fellow (2008–2009) at Johns Hopkins Hospital, Baltimore, MD, USA. She is trained in Thoracic and Pulmonary Pathology from Memorial Sloan Kettering Cancer Center and Mayo Clinic, USA. Her areas of interest include Cytopathology, Thoracic pathology, and Sinonasal cancers. Dr Jain has more than 260 peer-reviewed national and international publications to her credit. She authored a chapter in the 2015 WHO classification and is an editorial board member and author for the upcoming 2020 WHO classification of the lung, pleura, thymus, and heart. A member of the International Association for Study of Lung Cancer (IASLC) pathology committee and a section editor (Molecular Cytopathology) for the journal Archives of Pathology and Laboratory Medicine, Dr Jain has received many awards in recognition of her contributions. Justin A. Bishop  is the Jane B and Edwin P Jenevein MD Chair of Pathology and Director of Anatomic Pathology at UT Southwestern Medical Center, USA. Dr Bishop received his undergraduate and medical degrees from Texas Tech University in Lubbock, Texas, and completed his pathology residency at the Johns Hopkins Hospital in Baltimore. Dr Bishop is an expert in the surgical pathology diagnosis of head and neck, endocrine, and thoracic diseases. He has published more than 200 journal articles, 19 book chapters, and 7 books. Further, he contributed 15 chapters to the most recent WHO Classification of Head and Neck Tumors and is the lead author of the upcoming AFIP Fascicle of Salivary Gland Tumors. He is the editor-in-chief of Seminars in Diagnostic Pathology, an associate editor for Modern Pathology and JAMA Otolaryngology—Head and Neck Surgery and a member of the several additional editorial boards. Mark R. Wick  MD is a Professor of Pathology and Associate Director of Surgical Pathology at the University of Virginia at Charlottesville, USA. He completed his anatomic and clinical pathology residency training at the Mayo Clinic and Mayo Foundation (Rochester, MN). He has served on the pathology faculties of the Mayo Medical School (Rochester, MN), the University of Minnesota (Minneapolis, MN), and Washington University (St Louis, MO) and has made contributions to the specialty of pathology. Dr Wick has published numerous books, book chapters, and peer-reviewed journal articles. His research interests include immunohistochemistry, dermatopathology, thoracic pathology, and soft tissue pathology.

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1

The Normal Thymus Alexander Marx

The thymus is a primary lympoid organ. It consists of epithelial cells, hematopoietic cells and mesenchymal cells and generates T cells from immature, bone marrow-derived precursors. Through selection processes, the T cells become functional and largely tolerant toward self-antigens and are of key importance for adaptive immune responses. Thymic failure, particularly if congenital, predisposes to life-­ threatening infections, neoplasia, and autoimmune diseases [1].

eration of three-dimensional thymic lobes depend on interactions between NOTCH1 and DLL4 on T cells and TECs, respectively. Early Hassall corpuscles can be recognized by week 12 [5]. Mature T cells leave the thymus between week 14 and 16. The transcription factor FOXN1 is indispensable for the development of the thymus throughout embryonal and adult life [2]. Its defective expression elicits the nude phenotype and immunodeficiency in mice and humans [6, 7].

1.1

1.2

Embryology

The endoderm of the third pharyngeal pouches on both sides of the neck gives rise to “thymic epithelial cells” (TECs). From week 6 of gestation onward, the solid epithelial thymus anlage is present. By week 7, the common thymic/parathyroid primordia are established. The thymic components of the primordia descent along the carotid artery and behind the lower pole of the thyroid to the pre-cardiac region where they fuse [2]. From week 8 onward, the differentiation of cortical and medullary TECs (mTECs, cTECs) begins. By week 16, cortical and medullary compartments are established. The developmentally indispensable thymic capsule and septae originate from neural crest-derived mesenchymal cells from week 7 onward [3, 4]. The earliest T cell precursors are present at week 8 [2]. T cell maturation and the gen-

 ormal and Ectopic Location N of the Thymus

The normal position of the thymus is the anterosuperior mediastinum between the upper end of the sternum, the level of the fourth costal cartilage, the upper part of the pericardium and the pre-tracheal fascia, and the dorsal plain of the upper part of the sternum, costal cartilages, and intercostal muscles [8]. The lateral boundaries may extend beyond the phrenic nerves. Ectopic extensions comprise the cervical region up to the base of the skull, the mandibles and salivary glands, the middle and posterior mediastinum, and the intrapericardial and pleural spaces. The frequency of ectopic thoracic and cervical thymic tissue depends on the type of workup. On the microscopic level, frequencies amount to 20–50% [9, 10] but only to 1% in routine autopsies [11].

A. Marx (*) Institute of Pathology, University Medical Centre Mannheim, Heidelberg University, Mannheim, Germany e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_1

1

2

A. Marx

1.4

Histology

Thymic lobes are composed of many lobules. During childhood, each lobule shows a central medullary compartment that is completely surrounded by an outer cortical layer. From adolescence onward, the architecture gets progressively disturbed, and medullary areas more and more abut on mediastinal fat (Fig.  1.2). In a strict sense, the third thymic compartment, the perivascular space (PVS), is an extrathymic space between the continuous basal membrane of the outermost TECs of thymic lobes and the basal membrane of the vessels that enter and leave the thymus along the septae (Fig.  1.3). Hematopoietic cells that enter or leave the thymus must cross the PVS to egress from or enter into blood vessels, respectively [13]. In the thymic capsule, interlobular septae, and medulla, efferent lymphatic vessels can be found [14].

Fig. 1.1  Juvenile thymus following the removal of mediastinal fat with conspicuous upper and lower horns

1.3

Macroscopy

The thymus is composed of two lobes. Their fibrous capsules stick together in the midline. Two upper and two lower “horns” can usually be recognized (Fig. 1.1). In children and adolescents, the cut section of the thymus resembles the cut surface of a lymph node. During the course of involution (see below) it becomes more and more yellow and is barely detectable in the elderly. The average thymus weight is about 15 g at birth (range 5–25 g), reaches a maximum of around 40 g (20–50 g) at 10–15 years of age, and declines thereafter, reaching 10–15 g (range 5–30 g) by age 60 [12].

Epithelial Cells   Cortical and medullary TECs (cTECs and mTECs) show different histological features: The stellate-­s haped cTECs are quite easily detectable due to their large, round nuclei with conspicuous nucleoli, while mTECs are hard to identify among the lymphocytes due to their small, oval nuclei with inconspicuous nucleoli (Fig.  1.4a–d). The distinction of cTECs from mTECs can be achieved by immunohistochemistry, using antibodies, e.g., to the cortex-­s pecific proteasome subunit, Beta5t, and mTEC-restricted proteins such as CD40, Claudin-4, and the tolerance-­inducing autoimmune regulator, AIRE [15–17] (Table  1.1; Fig.  1.5). AIRE-positive mTECs can develop toward Hassall corpuscles (HC) on downregulation of AIRE.  HC are onion-­s haped accumulations of concentrically arranged squamoid epithelial cells that can show keratohyalin granules, lose their nuclei toward their cornified centers, and may become calcified or cystic (Fig.  1.6). In contrast to other TECs, HC express cytokeratin 10 and involucrin and fail to express HLA-DR and DP [18]. Since T cell maturation is necessary for HC development, they are lacking in thymuses from patients with T cell developmental defects (historically called “thymic dysplasia”). During aging the number of HC declines [19]. HC have immune tolerogenic functions through shedding of autoantigens and their impact on the development of regulatory T cells [20].

1  The Normal Thymus

3

a

b C

HC M

c

Fig. 1.2  Histology of thymuses in relation to age. (a) Thymus of a 1-year-old child: distinct lobular architecture with well-developed cortical areas (C) that completely envelope a medullary region (M); many small Hassall corpuscles (HC) and absence of interlobular fat are typical. (b) Thymus of a 30-year-old adult with increase of interlobular fat

d

and medullary areas directly abutting on adipocytes (arrows). (c) Thymus of a 50-year-old adult with further loss of lymphoepithelial parenchyma and more severe distortion of the cortico-medullary architecture. (d) Thymus of a 70-year-old adult with near defect of cortical areas and paucity of lymphocytes within epithelial strands (HE, a–d)

4

A. Marx

a

b M

C

HC

C

HC M

C

c

Fig. 1.3  Perivascular spaces (PVS) as highlighted by keratin 19 immunohistochemistry in a normal thymus. (a) Light-staining PVS are epithelial-­free spaces reaching from the perithymic fat and along the septae to the cortico-medullary junction (white arrows); the PVS are filled with lymphocytes, the majority of which are mature T cells (C, cortex; M, medulla; HC, Hassall corpuscle). (b) Sharp delineation

a

between an epithelial-free PVS (arrow) and cortex (C) and medulla (M) through a continuous layer of thymic epithelial cells; the disruption of the layer around PVSs is a typical sequela of lymphofollicular hyperplasia in myasthenia gravis. (c) Small capillary vessel (red arrows) within a PVS (immunoperoxidase, Keratin 19, a–c)

b

HC

Fig. 1.4  Cytological features of normal cortical and medullary thymic epithelial cells (cTEC, mTECs). (a) cTECs with medium-sized to large vesicular nuclei with conspicuous nucleoli (arrows). (b) Barely visible mTECs (arrows) outside a Hassall corpuscle (HC) with small- to

medium-sized nuclei and inconspicuous nucleoli. (c) Nuclei of cTEC highlighted by P40 staining. (d) Significantly smaller nuclei of mTEC outside HC highlighted by P40 immunohistochemistry (HE, a, b; immunoperoxidase, c, d)

1  The Normal Thymus

5

c

d

HC

Fig. 1.4 (continued) Table 1.1  Markers differentially expressed in the thymic cortex and medulla [31] Cortical markers

Medullary markers

Beta5t PRSS16 Cathepsin V TdT∗ CK10 CD40 Claudin-4 AIRE Involucrin Desmin, titin, myogenin∗∗ CD20, CD23∗∗∗

∗on immature T cells, ∗∗in myoid cells, ∗∗∗on thymic B cells

a

Fig. 1.5  Compartment-specific and largely unspecific epithelial markers in the normal thymus. (a) Expression of the thymus-specific proteasome subunit Beta5t exclusively in thymic epithelial cells of the cortex (C). HC, Hassall corpuscle. (b) Nuclear expression of the autoimmune regulator (AIRE) exclusively in a subset of medullary thymic epithelial

b

cells. (c) Expression of keratin 19 in virtually all cortical and medullary epithelial cells and epithelial cells surrounding perivascular spaces (∗). (d) P40 expression in the nuclei of almost all cortical and medullary epithelial cells (immunoperoxidase, a–d)

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A. Marx

c

d

* HC HC C

C

* *

*

Fig. 1.5 (continued)

T Cells (Thymocytes)  On immunohistochemistry, the cortex appears completely occupied by immature TdT+ thymocytes almost all of which co-express CD1a, CD99, CD3, CD4, CD5, and CD8 and are negative for CD10 and CD34 (so-called CD4/CD8 double-positive (DP) thymocytes) with a Ki67 index >90% (Fig.  1.7a, b). By contrast, the minor, CD34+ subset, CD10+ CD1a− subcapsular subset, and the CD4+CD8−CD3− immature single-positive (iSP) thymocyte subset can only be detected by flow cytometry [21]. In the medulla, almost all cells show a TdT-negative phenotype and a low Ki67 index and belong either to the CD3+CD4+CD8− or CD3+CD4−CD8+ so-called “single-­ positive” (SP) T cell subsets. Cortical and medullary thymocytes share expression of CD3 and CD5, with particularly strong CD5 expression in the medulla (Fig. 1.7c, d).

a

HC

b HC

HC

Fig. 1.6  Hassall corpuscle (HC) (a) HC with regressive changes and apoptotic cells in the medulla; inset, HC composed of vital, epidermoid cells with blue keratohyalin bodies; (b) HC with cystic enlargement containing debris; inset, typical expression of keratin 10  in the outer epithelial layers of a HC (HE, a, b; immunoperoxidase, keratin 10, inset)

B cells normally occur only in the medulla. The majority is round, while a minority is “asteroid shaped” (i.e., dendritic) if stained for CD20 [22]. The B cell content of medullary areas is highly variable, but B cells are always present. The “asteroid subset” shows a characteristic CD20+CD23+ CD21− profile (Fig.  1.8). Thymic B cells can originate through immigration from extrathymic mature B cell sources (e.g., lymph nodes) or through intrathymic development from immature precursors [23, 24]. Thymic B cells are HLA-DR+ and involved in T cell tolerance through negative T cell selection and the induction of regulatory T cells [24, 25]. Macrophages and Dendritic Cells  Macrophages occur in the cortex and medulla, while dendritic cells (DCs) are largely restricted to the medulla with a major focus on the cortico-medullary junction. Macrophages are important for the removal of dying thymocytes during T cell selection [18, 26] and are CD68+ and/or CD163+ (Fig.  1.9a). The small and round subset is strongly HLA-DR+, while the large,

1  The Normal Thymus

7

a

b

M

M

C C

c

d M M

M C

C

Fig. 1.7  T cells in the normal thymus. (a) Restriction of immature, TdT-positive T cells to the cortex (C) with labelling of virtually all lymphoid cells. (b) High Ki67 index (>90%) of cortical thymocytes as compared to few Ki67-positive cells in the medulla (M). (c) CD3

expression on immature and mature T cells in both compartments. (d) Particularly strong expression of CD5 on T cells in the medulla (M) (immunoperoxidase, a–d)

a

HC

b

M C

HC HC

HC

Fig. 1.8  B cells in the normal thymus. (a) Mainly round CD20+ B cells in the thymic medulla (M) with apparent “spillover” of single B cells into the cortex (arrows). (b) The “asteroid” B cell subset that is generally blurred by the overwhelming majority of round B cells on

CD20 immunohistochemistry can be highlighted by CD23 stains; CD23+ B cells must be distinguished from CD23+ follicular dendritic cells that form networks in thymic follicular hyperplasia (immunoperoxidase, a, b)

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A. Marx

a

C

C HC

HC

HC C

b

Fig. 1.10  Thymic myoid cells (TMCs) in the normal thymus: occurrence of desmin-positive TMCs exclusively in the medulla (here abutting on fat cells, right upper part); round, rhabdomyoblast-like TMCs (white arrows) and more elongated, myotube-like TMCs (black arrows) with vague cross-striations are present in the vicinity of a Hassall corpuscle (HC) (immunoperoxidase, desmin)

C

C

M M

C

Fig. 1.9  Macrophages and dendritic cells in the normal thymus. (a) Occurrence of CD68-positive macrophages throughout the thymus, including a Hassall corpuscle (HC); greatest frequency in the cortex (C); CD163-positive macrophages show a similar distribution and abundance (not shown). (b) CD11c-positive dendritic cells are largely restricted to the medulla, including the cortico-medullary junction; minor “spillover” CD11c-positive cells to the cortex (C)

stellate-shaped “starry sky macrophages” mainly of the cortex may contain apoptotic thymocytes and are HLA-DRlow [27]. DCs can arise from intrathymic precursors or enter the thymus as mature DCs from outside [28]. They are strongly HLA-DR+ and promote negative T cell selection and the induction of regulatory T cells [20]. Conventional DCs express CD11c (Fig.  1.9b) and may be AIRE+ [29], while the rare plasmacytoid DCs express CD123. Myoid Cells  Thymic myoid cells (TMCs) are fetal-type striated muscle cells of unknown origin in the medulla [30]. They express contractile proteins, including titin [31]. When stained for desmin, they resemble round, immature myoblasts or elongated myotubes (Fig. 1.10). Because they are non-innervated cells, TMCs express fetal and adult skeletal

muscle-type nicotinic acetylcholine receptors that likely play a role in the pathogenesis of myasthenia gravis [32, 33]. The normal function of TMCs is unclear, but it has been speculated that they release autoantigens and, thereby, endow DCs with the potential to induce muscle-specific T cell tolerance through negative selection [34].

1.5

Thymic Function

The thymus has three key functions: i) to recruit hematopoietic precursor cells from the blood into the thymus and drive their multistep maturation and expansion (Fig. 1.11) [21, 35], leading to a diverse repertoire of α/βT cells that can recognize millions of antigenic peptides if they are presented by antigen-presenting cells (APCs) on class I and II major histocompatibility (MHC) molecules (“positive selection”); ii) to eliminate from the functional T cells the subset of autoreactive T cells (“negative selection”) through the action of mTECs, DCs, and thymic B cells mainly in the medulla [29, 36, 37]; and iii) to generate immunosuppressive CD4+CD25+ FOXP3+ regulatory T cells (Tregs) that are indispensable for keeping autoreactive T cells in check that inevitably escape from negative selection and reach the peripheral immune system [38]. An essential factor for negative selection is the transcriptional “autoimmune regulator,” AIRE, that drives the expression of thousands of self-antigens in a subset of mTECs and endows them with the capacity to kill T cells if they show high affinity for MHC-presented self-peptides [39]. The thy-

1  The Normal Thymus

9 Pre-emigrants Negative selection

CD4+ SP Effector T cell

CD4+ SP Positive selection TSP

DN

DP

iSP

Negative selection CD34+ CD3CD4-CD8-

CD34+ cCD3+/CD4-CD8-

CD34-/+ cCD3+ CD4+CD8-

CD34sCD3+ CD4+CD8+

CD4+ FOXP3+ Regulatory T cell

CD8+ SP

CD8+ SP Effector T cell CMJ (TdT+)

Cortex (TdT+)

Medulla (TdT-/sCD3+)

Fig. 1.11  Maturation of alpha/beta T cells and main levels of operation of positive and negative T cell selection in the human thymus: TSP thymic-seeding precursors (the most immature T cell precursors entering the thymus), DN double-negative cells (in terms of CD4 and CD8 expression, cCD3 denotes CD3 expression in the cytoplasm), iSP immature single-positive cells, DP double-positive cells (constitute

more than 90% of the cortical thymocytes; sCD3 denotes CD3 expression on the cell surface), SP single-positive T cells, and pre-emigrants the most mature T cells generated in the thymus that are ready to egress from the thymus at the cortico-medullary junction (CMJ) into perivascular spaces (PVS) and from there into PVS-borne blood vessels and the circulation

mus is also important for the generation of γ/δ T cells [40] and NKT cells [41].

amounts of IFN-γ increase the risk for inflammatory tissue reactions and autoimmunity [46].

1.6

References

Thymic Involution

Thymic involution denotes the physiological, age-related, and gradual replacement of functional thymic tissue by fat (see above Figs. 1.1–1.3). Morphometry showed that involution starts in the first year of life and continues thereafter, leaving about 5% of thymic parenchyma by the age of 60 [19] . In a broader sense, involution includes thymic atrophy (“accidental involution”) that happens through various “stressors” such as pregnancy, infection/inflammation, malnutrition, and cancer. Factors involved in thymic atrophy are corticosteroids, sex hormones, IFN-α, adipocyte-derived factors (e.g., LIF), TNF-α, IL6, and growth factors [42, 43]. Mechanisms that are operative in relation to age are declining levels of FOXN1, decreasing proliferative activity of TECs with age, exhaustion of TEC progenitor cells, and the declining capacity of cTECs to induce T lineage commitment through NOTCH1 signaling and of mTECs to induce tolerance through expression of self-antigen [44]. These age-related changes lead to a gradual accumulation of senescent T cells and—most likely—to an increased risk of infections and cancer with increasing age [45]. On the other hand, the relative resistance of senescent T cells to regulatory signals and propensity to generate increased

1. Gupta S, Louis AG. Tolerance and autoimmunity in primary immunodeficiency disease: a comprehensive review. Clin Rev Allergy Immunol. 2013;45(2):162–9. 2. Farley AM, et  al. Dynamics of thymus organogenesis and colonization in early human development. Development. 2013;140(9):2015–26. 3. Anderson G, et  al. MHC class II-positive epithelium and mesenchyme cells are both required for T-cell development in the thymus. Nature. 1993;362(6415):70–3. 4. Patenaude J, Perreault C. Thymic mesenchymal cells have a distinct transcriptomic profile. J Immunol. 2016;196(11):4760–70. 5. von Gaudecker B, Muller-Hermelink HK. Ontogeny and organization of the stationary non-lymphoid cells in the human thymus. Cell Tissue Res. 1980;207(2):287–306. 6. Nehls M, et  al. New member of the winged-helix protein family disrupted in mouse and rat nude mutations. Nature. 1994;372(6501):103–7. 7. Frank J, et  al. Exposing the human nude phenotype. Nature. 1999;398(6727):473–4. 8. Carter BW, et  al. ITMIG classification of mediastinal compartments and multidisciplinary approach to mediastinal masses. Radiographics. 2017;37(2):413–36. 9. Kotani H, et  al. Ectopic cervical thymus: a clinicopathological study of consecutive, unselected infant autopsies. Int J Pediatr Otorhinolaryngol. 2014;78(11):1917–22. 10. Jaretzki A, Steinglass KM, Sonett JR. Thymectomy in the management of myasthenia gravis. Semin Neurol. 2004;24(1):49–62.

10 11. Bale PM, Sotelo-Avila C. Maldescent of the thymus: 34 necropsy and 10 surgical cases, including 7 thymuses medial to the mandible. Pediatr Pathol. 1993;13(2):181–90. 12. Hammar JA. Die Menschenthymus in Gesundheit und Krankheit. Ergebnisse der numerischen Analyse von mehr als tausend menschlichen Thymusdrüsen. Teil I: Das normale Organ. Zugleich eine kritische Beleuchtung der Lehre des “Status thymicus”. Zeitschrift für Mikroskopische Anatomie und Forschung. 1926;6(Suppl):1–570. 13. Maeda Y, et al. S1P lyase in thymic perivascular spaces promotes egress of mature thymocytes via up-regulation of S1P receptor 1. Int Immunol. 2014;26(5):245–55. 14. Kato S.  Thymic microvascular system. Microsc Res Tech. 1997;38(3):287–99. 15. Heino M, et  al. Autoimmune regulator is expressed in the cells regulating immune tolerance in thymus medulla. Biochem Biophys Res Commun. 1999;257(3):821–5. 16. Kyewski B, Peterson P.  Aire, master of many trades. Cell. 2010;140(1):24–6. 17. Herzig Y, et  al. Transcriptional programs that control expres sion of the autoimmune regulator gene Aire. Nat Immunol. 2017;18(2):161–72. 18. Douek DC, Altmann DM.  T-cell apoptosis and differential human leucocyte antigen class II expression in human thymus. Immunology. 2000;99(2):249–56. 19. Strobel P, et al. The ageing and myasthenic thymus: a morphometric study validating a standard procedure in the histological workup of thymic specimens. J Neuroimmunol. 2008;201-202:64–73. 20. Watanabe N, et  al. Hassall corpuscle instruct dendritic cells to induce CD4+CD25+ regulatory T cells in human thymus. Nature. 2005;436(7054):1181–5. 21. Blom B, Spits H. Development of human lymphoid cells. Annu Rev Immunol. 2006;24:287–320. 22. Isaacson PG, Norton AJ, Addis BJ. The human thymus contains a novel population of B lymphocytes. Lancet. 1987;2(8574):1488–91. 23. Akashi K, et  al. B lymphopoiesis in the thymus. J Immunol. 2000;164(10):5221–6. 24. Perera J, et  al. Self-antigen-driven thymic B cell class switching promotes T cell central tolerance. Cell Rep. 2016;17(2):387–98. 25. Lu FT, et al. Thymic B cells promote thymus-derived regulatory T cell development and proliferation. J Autoimmun. 2015;61:62–72. 26. Surh CD, Sprent J.  T-cell apoptosis detected in situ during positive and negative selection in the thymus. Nature. 1994;372(6501): 100–3. 27. Wakimoto T, et  al. Identification and characterization of human thymic cortical dendritic macrophages that may act as professional scavengers of apoptotic thymocytes. Immunobiology. 2008;213(9-10):837–47. 28. Cosway EJ, et al. Formation of the intrathymic dendritic cell pool requires CCL21-mediated recruitment of CCR7(+) progenitors to the thymus. J Immunol. 2018;201(2):516–23.

A. Marx 29. Fergusson JR, et al. Maturing human CD127+ CCR7+ PDL1+ dendritic cells express AIRE in the absence of tissue restricted antigens. Front Immunol. 2018;9:2902. 30. Bockman DE.  Myoid cells in adult human thymus. Nature. 1968;218(5138):286–7. 31. Marx A, et al. A striational muscle antigen and myasthenia gravis-­ associated thymomas share an acetylcholine-receptor epitope. Dev Immunol. 1992;2(2):77–84. 32. Schluep M, et  al. Acetylcholine receptors in human thymic myoid cells in situ: an immunohistological study. Ann Neurol. 1987;22(2):212–22. 33. Marx A, et al. The different roles of the thymus in the pathogenesis of the various myasthenia gravis subtypes. Autoimmun Rev. 2013;12(9):875–84. 34. Van de Velde RL, Friedman NB. Thymic myoid cells and myasthenia gravis. Am J Pathol. 1970;59(2):347–68. 35. Garcia-Leon MJ, et al. Dynamic regulation of NOTCH1 activation and NOTCH ligand expression in human thymus development. Development. 2018;145:16. 36. Klein L, et  al. Positive and negative selection of the T cell repertoire: what thymocytes see (and don’t see). Nat Rev Immunol. 2014;14(6):377–91. 37. Gies V, et  al. B cells differentiate in human thymus and express AIRE. J Allergy Clin Immunol. 2017;139(3):1049–1052.e12. 38. Bacchetta R, Barzaghi F, Roncarolo MG.  From IPEX syndrome to FOXP3 mutation: a lesson on immune dysregulation. Ann N Y Acad Sci. 2018;1417(1):5–22. 39. Derbinski J, et  al. Promiscuous gene expression in medullary thymic epithelial cells mirrors the peripheral self. Nat Immunol. 2001;2(11):1032–9. 40. Munoz-Ruiz M, et al. Thymic determinants of gammadelta T cell differentiation. Trends Immunol. 2017;38(5):336–44. 41. Benlagha K, et  al. A thymic precursor to the NK T cell lineage. Science. 2002;296(5567):553–5. 42. Dooley J, Liston A.  Molecular control over thymic involution: from cytokines and microRNA to aging and adipose tissue. Eur J Immunol. 2012;42(5):1073–9. 43. Youm YH, et  al. Prolongevity hormone FGF21 protects against immune senescence by delaying age-related thymic involution. Proc Natl Acad Sci U S A. 2016;113(4):1026–31. 44. Hamazaki Y.  Adult thymic epithelial cell (TEC) progenitors and TEC stem cells: models and mechanisms for TEC development and maintenance. Eur J Immunol. 2015;45(11):2985–93. 45. Palmer S, et al. Thymic involution and rising disease incidence with age. Proc Natl Acad Sci U S A. 2018;115(8):1883–8. 46. Fessler J, et  al. The impact of aging on regulatory T-cells. Front Immunol. 2013;4:231.

2

Immunohistochemistry of Normal Thymus Maria Teresa Ramieri, Enzo Gallo, and Mirella Marino

2.1

Introduction

Immunohistochemistry contributed, among other specialized techniques, to characterize the two main compartments of the human thymus, cortex and medulla, and their constituent cells. Several antibodies have been raised which provide insight into the normal thymus as well as in the tumors derived from epithelial cells (ECs) in thymus, the thymoma and thymic carcinoma, collectively called thymic epithelial tumors (TETs) and variety of lymphoproliferative diseases occurring in the thymus. Table 2.1 shows a list of the immunohistochemical markers useful in the thymic microenvironment characterization by cell type: some are differentiation and/or cell lineage markers whereas others are functional biomarkers. Ki67 is a general proliferation marker [1–19]. Compartment-­ specific antibodies have also been raised, showing phenotypic differences between cortical epithelial cells (cEC) and medullary epithelial cells (mEC). A list of compartment-specific and of embryological regulatory markers is provided (Table 2.2) to the readers interested in specialized morphofunctional investigations [20–29].

2.2

Markers of the Epithelial Component

2.2.1 Cytoplasmic Markers 2.2.1.1 Cytokeratins Cytokeratins (CKs) are cytoplasmic intermediate-sized filaments (tonofilaments) expressed by epithelia. The complex

pattern of thymic CK staining includes almost all the CKs [1, 3, 30, 31]. The normal thymic EC of all compartments is CK19 positive; CK20, CK7, and CK14 are expressed by the subcapsular EC, mEC, and Hassall corpuscle (HC) EC, but not the cEC.  Only HC expresses consistently CK13 and CK10. CK8 and CK18 are expressed by mEC and HC [1]. The CK pattern complexity reflects the morphological and functional heterogeneity of EC in the thymus [32] (Fig. 2.1).

2.2.2 Nuclear Markers 2.2.2.1 p63 and p40 p63 is a member of the p53 tumor suppressor gene family with structural homology to p53. In the human thymus of different ages, it was found that all thymic epithelial components, including subcapsular, cortical, and medullary ECs, express the nuclear p63 and its DN-p63α isoform, also called p40, with lesser intensity in HC [4]. In Fig. 2.2a the nuclei of all EC react with anti-p63 antibody, both in the cortex and in the medulla; in Fig.  2.2b the p63-positive subcapsular EC layer is better seen. A similar pattern is seen by staining with anti-p40, which highlights the nuclei of all EC types (Fig. 2.2c); in the medulla, only few ECs in the external part of HC are stained (Fig. 2.2d). 2.2.2.2 PAX8 PAX8 is a member of the paired box (PAX) family of transcription/developmental genes. PAX8 is a tissue-specific transcription factor involved in the embryonic development of the kidney, Müllerian organs, and thyroid. Recent studies

M. T. Ramieri Anatomic Pathology Unit, Oncology Department and Specialist Medicines, San Camillo-Forlanini Hospitals, Rome, Italy E. Gallo · M. Marino (*) Department of Pathology, IRCCS Regina Elena National Cancer Institute, Rome, Italy e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_2

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Table 2.1  Routine markers for thymus characterization by immunohistochemistry Marker CK19 CK AE1/AE3 CK 5/6 P63 P40 PAX8 (paired box (PAX) family of transcription/ developmental genes Glut1 CD1a CD3 CD5 CD4 CD8 CD10 Terminal deoxynucleotidyl transferase (TdT) LIM only 2 domain (rhombotin-like 1) (LMO2) CD20 CD23 Desmin Ki67

Notes on specificity/characterization/expressing cells Cortical and medullary EC Pankeratin marker (acidic and basic) of EC Non-keratinizing epithelia; basal-type CK Nuclear marker of all thymic EC It represents a DN-p63α isoform; nuclear marker of all thymic EC Transcription factor during embryonic development; expressed weakly in thymus Member of the mammalian facilitative glucose transporter family of passive carriers Immature cortical thymocytes and DC in thymic medulla T cell lineage differentiation antigen Non-lineage specific pan-T cell antigen; aberrantly expressed in some thymic carcinomas T helper cells T cytotoxic cells Common acute lymphoblastic leukemia antigen (CALLA) Non-lineage specific immature T cell nuclear marker/reacting with T-Lb Negative in normal lymphoblasts, positive in T-Lb B cell lineage marker B cell lineage marker and follicular DC Myoid cells in thymic medulla All proliferating cells

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

EC epithelial cells, cEC cortical epithelial cells, mEC medullary epithelial cells, CK cytokeratin, CD cluster of differentiation, DC dendritic cells, DN Delta N, T-Lb T-lymphoblastic lymphoma

Table 2.2  Developmental and compartment-specific markers Marker FoxN1 CD205 Notch1

β5t proteasome CD40 CK10 Claudins (Cld)

Notes on role/characterization/expressing cells cEC differentiation regulator cEC and DC subset marker Transmembrane receptor regulating normal T cell development; expressed in T-ALL, not in normal T lymphoblasts cEC mEC Terminally differentiated mEC Membrane proteins involved in junctional complexes; mEC in thymus

References [20] [21] [22]

[23] [24] [25] [26]

cEC cortical epithelial cells, mEC medullary epithelial cells, CK cytokeratin, CD cluster of differentiation, DC dendritic cells, T-ALL T cell acute lymphoblastic leukemia

have shown that, among tumors, PAX8 is commonly expressed in epithelial tumors of the thyroid and parathyroid glands, kidney, thymus, and female genital tract [33]. The relevance of this marker in normal thymus is still to be established, whereas the anti-PAX8 antibody appears to be useful in the differential diagnosis of thymic carcinoma

versus poorly differentiated lung carcinoma [34]. In the normal thymus, PAX8 (if using polyclonal antibodies) stains scattered subcapsular EC in the thymic cortex (Fig. 2.2e) and scattered EC in the thymic medulla, whereas ECs of HC are weak positive or mostly negative (Fig. 2.2f).

2.2.3 Other Markers 2.2.3.1 Glut-1 Glut-1 is a member of the mammalian facilitative glucose transporter family of passive carriers that function as an energy independent system for glucose transport down a concentration gradient [35]. In TET, Glut-1 expression was found to be dependent on histological subtypes [36]. The Glut-1 stain should be considered in the differential diagnosis between B3 thymoma and thymic carcinoma [37]. In the normal thymus Glut-1 is not expected to stain ECs significantly. However, in the examples shown here, faintly stained EC cytoplasm in clusters at the border of a peritumoral thymus or in the medulla itself (Fig. 2.3a) or at the border of a thymic cyst (Fig. 2.3b) are seen. This finding is of unknown significance.

2  Immunohistochemistry of Normal Thymus

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a

b

c

d

Fig. 2.1  Cytokeratin staining of normal thymus. A low magnification view of normal thymus stained with the anticytokeratin AE1/AE3 showing all the ECs, at the cortical border as well as into the cortex and in the medulla; a prominent HC is seen, mainly reacting at its periphery (a, 100×); at higher magnification (b, 200×) the subcapsular ECs are

a

Fig. 2.2  p63, p40, and PAX8: (a, 100×), the nuclei of all EC react with anti-p63 antibody, both in C and in M; in (b, 200×), the p63-positive subcapsular EC layer is better seen. (c, 50×), A similar pattern is seen by staining with anti-p40, which highlights the nuclei of all EC types (d, 200×); in M, only few ECs in the external part of HC are stained.

highlighted by the stain; a network of positive EC is seen in the cortex. The anticytokeratin antibody 5/6, marker of a basal layer type of CK, stains a very rich EC network (c, 100×); CK5/6 stains the cortical lobules as well as the mEC, both as a network and as single EC (d, 200×)

b

(e, 200×), in the normal thymus Pax 8 stained scattered subcapsular EC in the thymic C and scattered EC in the thymic M, whereas (f, 200x) ECs of HC are weak positive. C cortex, M medulla, EC epithelial cells, HC Hassall corpuscles

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c

d

e

f

Fig. 2.2 (continued)

a

b

Fig. 2.3  Glut-1 staining. Faintly stained EC in clusters at the periphery of a peritumoral thymus and in the medulla (a, 200×) or at the border of a thymic cyst (b, 400×)

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a

b

c

d

Fig. 2.4  Immune checkpoint inhibitors. With PD1 only faint scattered cells seen positive in the thymic M (a, 200×) and, more distinctly, at the periphery of HC (b, 200×). In the normal thymus, distinct PD-L1 posi-

tivity is observed both in EC in cortex and in medulla (c, 200×, and d, 200×). In HC, only few cells reacted. EC Epitheial Cells, HC Hassall corpuscles

2.2.4 Immune Checkpoint Inhibitors

expressed exclusively in thymic cEC in mice and humans [23, 41, 42]. The thymoproteasome appears to play a central role in thymic immunologic function such as the generation of the MHC class-I-associated peptides and a restricted CD8+ T cell repertoire [23]. Among the other compartment-specific markers, several “medullary” EC markers are available: CD40, CK10, and Claudins (Cld family). The proliferation and differentiation of mEC is dependent on the tumor necrosis factor (TNF) superfamily cytokines, such as RANKL and CD40L, produced by lymphoid cells [25]. The CD40 signaling controls the development of mEC, which play critical roles in preventing autoimmunity [24, 43]. Moreover, terminally differentiated mEC express cytokeratin 10 (CK10) and involucrin [25]. The Claudin (Cld) family includes tetraspanning membrane proteins playing a crucial role in junctional complexes. In the adult thymus, diffuse and membranous expression of Cld 3 and 4 is retained in mEC [26]. Cld 3 and 4-positive ECs are scattered exclusively in the medullary region and have globular cell morphology in humans; moreover, Cld are expressed quite strongly in the HC, representing terminally maturated mEC. In the adult thymus, most Cld3 and 4+/high EC represent mature mEC. In the adult thymus, claudin-4 is preferentially expressed in mEC surrounding HC [21, 25, 32].

In the normal thymus, the PD-1/PD-L1 interaction is relevant for thymocyte selection [38] and in regulatory T cell (Treg) function and prevention of autoimmunity [39]. There is limited information regarding PD-L1 and PD-1 expression in normal thymus and in nonneoplastic thymic lesions. Marchevsky and Walts described a predominantly medullary distribution of PD-1 positivity in several settings of normal thymi (normal juvenile thymus, thymus with follicular hyperplasia, atrophic and peritumoral thymus). The reactivity was membranous and/or cytoplasmic. PD-L1 positivity was not found or only limited in cells of HC [40]. In the normal thymus, only faint positivity is seen for PD-1 in scattered cells in the thymic medulla (Fig 2.4a) and, more distinctly, at the periphery of Hassall corpuscles (Fig. 2.4b), whereas distinct PD-L1 positivity is seen both in EC in cortex and in medulla (Fig. 2.4c and 2.4d).

2.2.5 Compartment-Specific Antibodies Recently, compartment-specific thymic EC markers were described in thymus: the proteasome β5t subunit (β5t), a component of the thymoproteasome, was found to be

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a

b

c

d

Fig. 2.5  Compartment-specific antibodies. The β5t is a cortical marker highlighting the EC network (a), whereas CD40 stains scattered cells in the thymic medulla (b). CK10 is a strong marker of HC (c), whereas the

Claudin 4-R56 stains scattered EC in the medulla and peripheral EC in HC (d). (a, b, c, and d, 50×). (Courtesy of Professor A.  Marx, Mannheim, Germany). EC Epitheial Cells, HC Hassall corpuscles

All the compartment-specific antibodies listed are cytoplasmic markers. In Fig. 2.5a, the β5t appears as a clear-cut cortical marker highlighting the EC network, whereas CD40 stains scattered cells in the thymic medulla (Fig. 2.5b). CK10 is a strong marker of HC (Fig.  2.5c), whereas the Claudin 4-R56 stains scattered EC in the medulla and peripheral EC in HC (Fig. 2.5d).

mic anlage to a functional cortical and medullary EC meshwork [46]. In TETs, FoxN1 was found to be expressed as nuclear staining in most thymoma subtypes, whereas the staining was generally focal in thymic carcinoma [47]

2.2.6 FoxN1 and CD205 2.2.6.1 FoxN1 Among regulatory markers, FoxN1 (forkhead box protein n1) is a winged-helix transcription factor involved in the initial development of cEC from bipotent progenitors [44, 45]. FoxN1 is a master regulator in the cEC lineage specification; in fact it promotes transcription of genes, which, in turn, regulate EC differentiation [20], and it is required for thymic epithelial patterning and differentiation from the early thy-

2.2.6.2 CD205 DEC-205, also known as CD205, a known marker of cEC in the adult thymus [21], is an integral membrane protein homologous to the macrophage mannose receptor and related receptors which are able to bind carbohydrates and mediate endocytosis [48]. It is an endocytic receptor, a type I C-type lectin-like molecule expressed at high level by cEC and by dendritic cell (DC) subsets [49]. It acts via FoxN1-dependent mechanism. cEC show constitutive expression of CD205 [18]. CD205 shows cytoplasmic expression in the form of coarse granular staining with membranous accentuation in most thymomas, whereas it stains thymic carcinomas focally with variable intensity [47].

2  Immunohistochemistry of Normal Thymus

2.3

Markers of the Lymphoid Component

2.3.1 CD3 CD3 is a T cell lineage differentiation antigen first appearing in the cytoplasm of developing T cells as cytoplasmic CD3 (cCD3), then appearing on the surface of the mature T cells in cortex and medulla (Fig. 2.6 a). On the T- cell membrane the CD3 antigen is composed of four distinct subunits, and it is associated with the T-cell receptor (TCR) [9]. The staining pattern is membranous and cytoplasmic.

a

17

2.3.2 CD4 and CD8 Coexpression of both CD4 and CD8 is seen in early T cell precursors in the thymic cortex (double positive, DP). During maturation to mature T cell, the thymocytes become either CD4 T helper cells or CD8 cytotoxic T cells, by losing the other receptor (single positive, SP). The staining pattern for CD4 and CD8 in lymphocytes is both cytoplasmic and membranous (Fig. 2.6b, c).

2.3.3 CD5 CD5 is a pan T cell antigen, a membrane 67-kDa type I glycoprotein playing a role in the crosslinking of the T cell receptor (TCR) and its antigen on the major histocompatibility complex (MHC) of antigen-presenting cells (APCs). CD5 is an immunotyrosine-based inhibition motif-bearing receptor that antagonizes the overt T cell receptor activation response by recruiting inhibitory intracellular mediators which modulate the overall response [10]

2.3.4 CD10 b

CD10 (common acute lymphoblastic leukemia antigen (CALLA)) is a 749-amino acid type II integral membrane-­ bound zinc metalloendopeptidase (24.11-enkephalinase) enzyme thought to play a role in lymphoid development and/ or function [13]. The staining pattern is cytoplasmic. In the thymus it is expressed in the cortex.

2.3.5 CD1a

c

The CD1a family is part of the third family of antigen-­presenting molecules that bind bacterial and autologous lipid antigens for presentation to T cells [50]. The anti-CD1a antibody stains immature cortical thymocytes (Fig.  2.7a). In thymic medulla and perivascular spaces, the CD1a is expressed on cells with dendritic morphology, corresponding to dendritic cells (DC).

2.3.6 Terminal Deoxynucleotidyl Transferase

Fig. 2.6  Cluster of differentiation antigens (CD). (a, 100×): both cortical and medullary lymphocytes are CD3 positive; CD4 (b, 100×) and CD8 (c, 100×) stains do not discriminate between DP CD4+CD8+ lymphocytes present in the cortex from single-positive CD4+ or CD8+ cells present in the medulla. DP Double positive

Terminal deoxynucleotidyl transferase (TdT) is a non-­ lineage-­specific immature marker; it is an unusual deoxynucleotide polymerizing enzyme, exclusively localized in the nucleus [14] of cortical thymocytes during human life [51]. Despite thymic physiological involution TdT is expressed in the major subpopulation of cortical thymocytes [52], although there is an age-related change in activity [14] (Fig. 2.7b).

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LMO2 is a master gene regulator exerting a dysfunctional control on differentiation following chromosomal translocations [54]. In fact, LMO2 is activated by two translocations: t(11;14)p13;q11 or t(7;11)q35;p13, the latter seen predominantly in precursor T acute lymphoblastic leukemia [55]. However, it was found that upregulation of LMO2 may occur by a mechanism different from chromosomal rearrangements [15]. The LMO2 antibody clone SP51 has a nuclear staining pattern; since thymic ECs express LMO2 weakly, it is advisable to adopt a panel of lymphoid markers in the characterization of a suspicious lymphoid population. A cutoff of >30% expression by cells is used by most to define positivity for this marker. The LMO2 protein has recently been reported to be expressed in a large proportion of T cell acute lymphoblastic leukemias (T-ALLs), while it is absent from thymocytes in normal thymuses and thymomas [15] (Fig. 2.7c).

2.3.8 CD20

c

CD20 is a 33- to 37-kDa non-glycosylated phosphoprotein expressed on the surface of naive B cells that reach the thymus from the bone marrow and the blood. The B cells in thymus act by several mechanisms as antigen-presenting cells (APCs) for central tolerance [56]. They show a distinctive phenotype in comparison to other B cell subsets [17] and represent a unique population of B lymphocytes that reside at the corticomedullary junction (CMJ) [57]. Moreover, along with mEC, they express the autoimmune regulator (AIRE) gene, a transcription factor that controls the negative selection of selfreactive T cells and the complex T cell development [16]. Among the heterogeneous population of thymic B cells, a subpopulation of B cells with dendritic features expressing CD20 but also CD23 (Fig. 2.8) has been referred to as “asteroid” B cells. A further population of B lymphocytes together with frequent lymph follicles is present within the extrathymic perivascular spaces [17].

2.3.9 CD23 Fig. 2.7  Other lymphoid cell markers. (a, 200×): T lymphocytes of cortical immature phenotype CD1a+ are seen in C, whereas M is negative; T cortical lymphocytes are also positive with TdT; scattered cEC and macrophages are also seen (b, nuclear staining, 200×); (c), the LMO2 staining highlights few scattered cells in the medulla (nuclear staining, 200×). (Photograph courtesy Prof. S.  Ascani, Perugia University, Ospedale S. Maria, Terni, Italy)

2.3.7 LMO2 The LIM only 2 domain (rhombotin-like 1; also called LMO2) is a transcription factor required for hematopoiesis, involved in erythroid differentiation and angiogenesis [53].

CD23 is a low-affinity immunoglobulin (IgE) receptor widely distributed on the surface of various human cells including IgD-positive B lymphocytes, follicular dendritic cells (FDC), and airway smooth muscle cells. CD23 is related to B cell proliferation, thymocyte proliferation, antigen presentation, and rescue of germinal center B cells from apoptosis. Cells which positively stain for CD20 and CD23 (Fig.  2.8) were proposed as the possible normal counterpart of primary mediastinal B cell lymphoma (PMBL) [58]. Subsequently, a population of large thymic B cells resembling PMBL cells, with dendritic features and strong expression of CD23, were described and referred to as “asteroid cells” [17].

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Conflicts of Interest  The authors have no conflicts of interest to declare. Funding  The authors received no funding for this project.

References

Fig. 2.8  CD23: Among the heterogeneous population of thymic B cells, a subpopulation of B cells with dendritic features expressing CD23 (200×) which has been referred as “asteroid” B cells is shown. These large cells are present in the medulla and surround HC. HC Hassall corpuscles

2.3.10 Notch1 Notch1 is a gene encoding a transmembrane receptor that regulates normal T cell development and is among the main transcriptional players of T cell differentiation [27]. The role of NOTCH in normal thymic epithelium is less established, although NOTCH pathway activation has been shown in thymic EC [59, 60].

2.4

Conclusions

In the last three decades, immunohistochemistry has changed the diagnostic surgical pathology practice by providing a tool to visualize molecules in tissue sections. Previously mainly available on frozen tissue sections, with the development of the monoclonal antibody and of specialized platforms, immunohistochemistry allowed an extensive and sensitive tissue characterization in formalin-fixed paraffin-­ embedded sections also. Immunohistochemistry of normal thymus, alongside with other specialized laboratory techniques, contributed to clarify the complex spatial and functional interplay between its major cellular components, epithelial and lymphoid, the stromal cells, and other accessory cells, both during development/organogenesis and in the postnatal life. Moreover, the knowledge of the thymic microenvironment and cells is fundamental in the pathologic assessment of thymic malignancies and of thymic-related immune/autoimmune diseases. Acknowledgments The authors thank the Library of the IRCCS Regina Elena National Cancer Institute for the bibliographic assistance and Prof. S. Ascani, Perugia University, Ospedale S. Maria, Terni, Italy, for providing photographs of the LMO2 staining.

1. Kuo T. Cytokeratin profiles of the thymus and thymomas: histogenetic correlations and proposal for a histological classification of thymomas. Histopathology. 2000;36(5):403–14. 2. Jablonska-Mestanova V, Sisovsky V, Danisovic L, Polak S, Varga I. The normal human newborns thymus. Bratisl Lek Listy. 2013;114(7):402–8. 3. Laster AJ, Itoh T, Palker TJ, Haynes BF.  The human thymic microenvironment: thymic epithelium contains specific keratins associated with early and late stages of epidermal keratinocyte maturation. Differentiation. 1986;31(1):67–77. 4. Chilosi M, Zamò A, Brighenti A, Malpeli G, Montagna L, Piccoli P, et  al. Constitutive expression of DeltaN-p63alpha isoform in human thymus and thymic epithelial tumours. Virchows Arch. 2003;443(2):175–83. https://doi.org/10.1007/s00428-003-0857-4. 5. Dotto J, Pelosi G, Rosai J. Expression of p63 in thymomas and normal thymus. Am J Clin Pathol. 2007;127(3):415–20. https:// doi.org/10.1309/2gaykpddm85p2vew. 6. Weissferdt A, Moran CA.  Pax8 expression in thymic epithelial neoplasms: an immunohistochemical analysis. Am J Surg Pathol. 2011;35(9):1305–10. https://doi.org/10.1097/ PAS.0b013e3182260735. 7. Liu Y.  Characterization of thymic lesions with F-18 FDG PET-CT: an emphasis on epithelial tumors. Nucl Med Commun. 2011;32(7):554–62. https://doi.org/10.1097/ MNM.0b013e328345b984. 8. Mosser DD, Duchaine J, Martin LH.  Biochemical and developmental characterization of the murine cluster of differentiation 1 antigen. Immunology. 1991;73(3):298–303. 9. Huang J, Meyer C, Zhu C. T cell antigen recognition at the cell membrane. Mol Immunol. 2012;52(3-4):155–64. https://doi. org/10.1016/j.molimm.2012.05.004. 10. Bamberger M, Santos AM, Goncalves CM, Oliveira MI, James JR, Moreira A, et  al. A new pathway of CD5 glycoprotein-­ mediated T cell inhibition dependent on inhibitory phosphorylation of Fyn kinase. J Biol Chem. 2011;286(35):30324–36. https:// doi.org/10.1074/jbc.M111.230102. 11. Orazi A, Foucar K, Knowles DM, Weiss LE.  Knowles neoplastic hematopathology. Philadelphia: Wolters Kluwer Health / Lippincott Williams & Wilkins; 2014. 12. Miceli MC, Parnes JR. Role of CD4 and CD8 in T cell activation and differentiation. Adv Immunol. 1993;53:59–122. 13. Shipp MA, Vijayaraghavan J, Schmidt EV, Masteller EL, D'Adamio L, Hersh LB, et al. Common acute lymphoblastic leukemia antigen (CALLA) is active neutral endopeptidase 24.11 ("enkephalinase"): direct evidence by cDNA transfection analysis. Proc Natl Acad Sci U S A. 1989;86(1):297–301. 14. Deibel MR Jr, Riley LK, Coleman MS, Cibull ML, Fuller SA, Todd E.  Expression of terminal deoxynucleotidyl transferase in human thymus during ontogeny and development. J Immunol. 1983;131(1):195–200. 15. Jevremovic D, Roden AC, Ketterling RP, Kurtin PJ, McPhail ED.  LMO2 Is a specific marker of T-lymphoblastic leukemia/ lymphoma. Am J Clin Pathol. 2016;145(2):180–90. https://doi. org/10.1093/ajcp/aqv024. 16. Cepeda S, Cantu C, Orozco S, Xiao Y, Brown Z, Semwal MK, et al. Age-associated decline in thymic B cell expression of aire

20 and aire-dependent self-antigens. Cell Rep. 2018;22(5):1276–87. https://doi.org/10.1016/j.celrep.2018.01.015. 17. Fend F, Nachbaur D, Oberwasserlechner F, Kreczy A, Huber H, Müller-Hermelink HK. Phenotype and topography of human thymic B cells. An immunohistologic study. Virchows Arch B Cell Pathol Incl Mol Pathol. 1991;60(6):381–8. 18. Ströbel P, Hartmann E, Rosenwald A, Kalla J, Ott G, Friedel G, et  al. Corticomedullary differentiation and maturational arrest in thymomas. Histopathology. 2014;64(4):557–66. https://doi. org/10.1111/his.12279. 19. Kanavaros P, Stefanaki K, Rontogianni D, Papalazarou D, Sgantzos M, Arvanitis D, et  al. Immunohistochemical expression of p53, p21/waf1, rb, p16, cyclin D1, p27, Ki67, cyclin A, cyclin B1, bcl2, bax and bak proteins and apoptotic index in normal thymus. Histol Histopathol. 2001;16(4):1005–12. https://doi. org/10.14670/hh-16.1005. 20. Romano R, Palamaro L, Fusco A, Giardino G, Gallo V, Del Vecchio L, et al. FOXN1: a master regulator gene of thymic epithelial development program. Front Immunol. 2013;4:187. https:// doi.org/10.3389/fimmu.2013.00187. 21. Shakib S, Desanti GE, Jenkinson WE, Parnell SM, Jenkinson EJ, Anderson G. Checkpoints in the development of thymic cortical epithelial cells. J Immunol. 2009;182(1):130–7. 22. Jegalian AG, Bodo J, Hsi ED.  NOTCH1 intracellular domain immunohistochemistry as a diagnostic tool to distinguish T-lymphoblastic lymphoma from thymoma. Am J Surg Pathol. 2015;39(4):565–72. https://doi.org/10.1097/ PAS.0000000000000358. 23. Murata S, Sasaki K, Kishimoto T, Niwa S, Hayashi H, Takahama Y, et  al. Regulation of CD8+ T cell development by thymus-­ specific proteasomes. Science. 2007;316(5829):1349–53. https:// doi.org/10.1126/science.1141915. 24. Akiyama T, Shinzawa M, Akiyama N. TNF receptor family signaling in the development and functions of medullary thymic epithelial cells. Front Immunol. 2012;3:278. https://doi.org/10.3389/ fimmu.2012.00278. 25. Hamazaki Y, Sekai M, Minato N.  Medullary thymic epithelial stem cells: role in thymic epithelial cell maintenance and thymic involution. Immunol Rev. 2016;271(1):38–55. https://doi. org/10.1111/imr.12412. 26. Hamazaki Y, Fujita H, Kobayashi T, Choi Y, Scott HS, Matsumoto M, et  al. Medullary thymic epithelial cells expressing Aire represent a unique lineage derived from cells expressing claudin. Nat Immunol. 2007;8(3):304–11. https://doi.org/10.1038/ ni1438. 27. Famili F, Wiekmeijer AS, Staal FJ.  The development of T cells from stem cells in mice and humans. Future Sci OA. 2017;3(3):FSO186. https://doi.org/10.4155/fsoa-2016-0095. 28. Farley AM, Morris LX, Vroegindeweij E, Depreter ML, Vaidya H, Stenhouse FH, et  al. Dynamics of thymus organogenesis and colonization in early human development. Development. 2013;140(9):2015–26. https://doi.org/10.1242/dev.087320. 29. Manley NR, Condie BG.  Transcriptional regulation of thymus organogenesis and thymic epithelial cell differentiation. Prog Mol Biol Transl Sci. 2010;92:103–20. https://doi.org/10.1016/ s1877-1173(10)92005-x. 30. Masunaga A, Sugawara I, Nakamura H, Yoshitake T, Itoyama S.  Cytokeratin expression in normal human thymus at different ages. Pathol Int. 1997;47(12):842–7. 31. Shezen E, Okon E, Ben-Hur H, Abramsky O. Cytokeratin expression in human thymus: immunohistochemical mapping. Cell Tissue Res. 1995;279(1):221–31. 32. Hamazaki Y. Adult thymic epithelial cell (TEC) progenitors and TEC stem cells: models and mechanisms for TEC development and maintenance. Eur J Immunol. 2015;45(11):2985–93. https:// doi.org/10.1002/eji.201545844.

M. T. Ramieri et al. 33. Ordóñez NG. Value of PAX 8 immunostaining in tumor diagnosis: a review and update. Adv Anat Pathol. 2012;19(3):140–51. https:// doi.org/10.1097/PAP.0b013e318253465d. 34. Asirvatham JR, Esposito MJ, Bhuiya TA.  Role of PAX-8, CD5, and CD117  in distinguishing thymic carcinoma from poorly differentiated lung carcinoma. Appl Immunohistochem Mol Morphol. 2014;22(5):372–6. https://doi.org/10.1097/PAI.0b013e318297cdb5. 35. Olson AL, Pessin JE.  Structure, function, and regulation of the mammalian facilitative glucose transporter gene family. Annu Rev Nutr. 1996;16:235–56. https://doi.org/10.1146/annurev. nu.16.070196.001315. 36. Thomas de Montpréville V, Quilhot P, Chalabreysse L, De Muret A, Hofman V, Lantuéjoul S, et al. Glut-1 intensity and pattern of expression in thymic epithelial tumors are predictive of WHO subtypes. Pathol Res Pract. 2015;211(12):996–1002. https://doi. org/10.1016/j.prp.2015.10.005. 37. Kojika M, Ishii G, Yoshida J, Nishimura M, Hishida T, Ota SJ, et  al. Immunohistochemical differential diagnosis between thymic carcinoma and type B3 thymoma: diagnostic utility of hypoxic marker, GLUT-1, in thymic epithelial neoplasms. Mod Pathol. 2009;22(10):1341–50. https://doi.org/10.1038/ modpathol.2009.105. 38. Keir ME, Latchman YE, Freeman GJ, Sharpe AH. Programmed death-1 (PD-1):PD-ligand 1 interactions inhibit TCR-mediated positive selection of thymocytes. J Immunol. 2005;175(11): 7372–9. 39. Kumar P, Bhattacharya P, Prabhakar BS. A comprehensive review on the role of co-signaling receptors and treg homeostasis in autoimmunity and tumor immunity. J Autoimmun. 2018;95:77–99. https://doi.org/10.1016/j.jaut.2018.08.007. 40. Marchevsky AM, Walts AE.  PD-L1, PD-1, CD4, and CD8 expression in neoplastic and nonneoplastic thymus. Hum Pathol. 2017;60:16–23. https://doi.org/10.1016/j.humpath.2016.09.023. 41. Takahama Y, Ohigashi I, Murata S, Tanaka K. Thymoproteasome and peptidic self. Immunogenetics. 2019;71(3):217–21. https:// doi.org/10.1007/s00251-018-1081-3. 42. Tomaru U, Ishizu A, Murata S, Miyatake Y, Suzuki S, Takahashi S, et al. Exclusive expression of proteasome subunit {beta}5t in the human thymic cortex. Blood. 2009;113(21):5186–91. https:// doi.org/10.1182/blood-2008-11-187633. 43. Sun L, Li H, Luo H, Zhao Y.  Thymic epithelial cell development and its dysfunction in human diseases. Biomed Res Int. 2014;2014:206929. https://doi.org/10.1155/2014/206929. 44. Gordon J, Bennett AR, Blackburn CC, Manley NR.  Gcm2 and Foxn1 mark early parathyroid- and thymus-specific domains in the developing third pharyngeal pouch. Mech Dev. 2001;103(1-2):141–3. 45. Gordon J, Manley NR. Mechanisms of thymus organogenesis and morphogenesis. Development. 2011;138(18):3865–78. https:// doi.org/10.1242/dev.059998. 46. Palamaro L, Romano R, Fusco A, Giardino G, Gallo V, Pignata C.  FOXN1  in organ development and human diseases. Int Rev Immunol. 2014;33(2):83–93. https://doi.org/10.3109/08830185.2 013.870171. 47. Nonaka D, Henley JD, Chiriboga L, Yee H. Diagnostic utility of thymic epithelial markers CD205 (DEC205) and Foxn1  in thymic epithelial neoplasms. Am J Surg Pathol. 2007;31(7):1038–44. https://doi.org/10.1097/PAS.0b013e31802b4917. 48. Jiang W, Swiggard WJ, Heufler C, Peng M, Mirza A, Steinman RM, et al. The receptor DEC-205 expressed by dendritic cells and thymic epithelial cells is involved in antigen processing. Nature. 1995;375(6527):151–5. https://doi.org/10.1038/375151a0. 49. Shrimpton RE, Butler M, Morel AS, Eren E, Hue SS, Ritter MA. CD205 (DEC-205): a recognition receptor for apoptotic and necrotic self. Mol Immunol. 2009;46(6):1229–39. https://doi. org/10.1016/j.molimm.2008.11.016.

2  Immunohistochemistry of Normal Thymus 50. Zajonc DM.  The CD1 family: serving lipid antigens to T cells since the Mesozoic era. Immunogenetics. 2016;68(8):561–76. https://doi.org/10.1007/s00251-016-0931-0. 51. Wirt DP, Grogan TM, Nagle RB, Copeland JG, Richter LC, Rangel CS, et  al. A comprehensive immunotopographic map of human thymus. J Histochem Cytochem. 1988;36(1):1–12. https:// doi.org/10.1177/36.1.2961798. 52. Steinmann GG, Muller-Hermelink HK.  Immunohistological demonstration of terminal transferase (TdT) in the age-involuted human thymus. Immunobiology. 1984;166(1):45–52. https://doi. org/10.1016/s0171-2985(84)80142-4. 53. Matthews JM, Lester K, Joseph S, Curtis DJ. LIM-domain-only proteins in cancer. Nat Rev Cancer. 2013;13(2):111–22. https:// doi.org/10.1038/nrc3418. 54. Chambers J, Rabbitts TH. LMO2 at 25 years: a paradigm of chromosomal translocation proteins. Open Biol. 2015;5(6):150062. https://doi.org/10.1098/rsob.150062. 55. Boehm T, Spillantini MG, Sofroniew MV, Surani MA, Rabbitts TH. Developmentally regulated and tissue specific expression of mRNAs encoding the two alternative forms of the LIM domain

21 oncogene rhombotin: evidence for thymus expression. Oncogene. 1991;6(5):695–703. 56. Yamano T, Nedjic J, Hinterberger M, Steinert M, Koser S, Pinto S, et al. Thymic B cells are licensed to present self antigens for central T cell tolerance induction. Immunity. 2015;42(6):1048–61. https://doi.org/10.1016/j.immuni.2015.05.013. 57. Perera J, Huang H.  The development and function of thymic B cells. Cell Mol Life Sci. 2015;72(14):2657–63. https://doi. org/10.1007/s00018-015-1895-1. 58. Moller P, Moldenhauer G, Momburg F, Lammler B, Eberlein-­ Gonska M, Kiesel S, et  al. Mediastinal lymphoma of clear cell type is a tumor corresponding to terminal steps of B cell differentiation. Blood. 1987;69(4):1087–95. 59. Laky K, Fowlkes BJ.  Notch signaling in CD4 and CD8 T cell development. Curr Opin Immunol. 2008;20(2):197–202. https:// doi.org/10.1016/j.coi.2008.03.004. 60. Masuda K, Itoi M, Amagai T, Minato N, Katsura Y, Kawamoto H. Thymic anlage is colonized by progenitors restricted to T, NK, and dendritic cell lineages. J Immunol. 2005;174(5):2525–32.

3

Radiology of Normal Thymus, Thymic Lesions, and Tumors Manisha Jana and Ashu Seith Bhalla

3.1

Introduction

Thymus is a lymphoid organ in the anterior mediastinum which has a key role in cell-mediated as well as humoral immunity. The size of thymus is large at newborn period and infancy and undergoes progressive involution with age. Thymic size also varies significantly with different diseases and stress. The shape of thymus is also variable and may often mimic anterior mediastinal mass or lymphadenopathy.

3.2

Imaging Modalities

Imaging modalities used for the imaging of thymus include plain radiograph, ultrasonography (USG), computed tomography (CT), and magnetic resonance imaging (MRI). Plain radiograph often is the first imaging modality. However, for arriving at a final diagnosis often other imaging is required. USG is an inexpensive and easy imaging modality, which has no risk of ionizing radiation. It is especially useful in cases of evaluation of an “anterior mediastinal mass” on chest radiograph. The advantage of distinction of a solid versus cystic mass is one major use of ultrasound. CT, with intravenous contrast, is the workhorse of imaging in anterior mediastinal masses and thymic mass lesions. Routinely, a venous phase acquisition is sufficient. MRI is used in cases of doubtful/contradictory imaging findings on other imaging modalities. Routine spin-echo T1W and T2W sequences should be complemented with

chemical shift imaging (in- and opposed-phase images) and diffusion-weighted imaging [1].

3.3

Normal Thymus on Imaging

• Chest radiograph (CXR): the appearance of normal thymus on CXR changes with age of the subject. In neonates and small infants, thymus appears larger. It involutes with age and is hardly appreciated on radiograph as a “mass” in a healthy adult. • The appearance of an enlarged normal/hyperplastic thymus has been described with multiple imaging signs. –– “Sail sign”: triangular lateral extension of a normal thymus. –– “Wave sign”: wavy undulated outline of a normal thymus, created by impression of anterior ends of ribs (Fig. 3.1). • USG: a normal thymus has a typical bright speckled appearance: “starry sky pattern” which is very characteristic (Fig. 3.2). • CT: similar to CXR, the appearance of thymus changes on CT with age. Normal thymus in a child has a homogeneous soft tissue attenuation (Fig.  3.3), but is almost entirely fatty in the third/fourth decade. • MRI: a normal thymus is usually hypointense on T1W images and intermediate to mildly hyperintense on T2W image. Normal thymus does not show diffusion restriction (Fig. 3.4).

M. Jana · A. S. Bhalla (*) Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi, India e-mail: [email protected]; [email protected]

© Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_3

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M. Jana and A. S. Bhalla

24

Fig. 3.1  Normal thymus in an infant. Note the indentation by the anterior ends of the ribs (arrows). In spite of the large size (which is normal for age), this indentation (wave sign) is suggestive of a normal thymus

Fig. 3.3  CECT of normal thymus. Note the homogeneous attenuation and smooth margins

• The most important association of follicular thymic hyperplasia is with myasthenia gravis. Other associations include collagen vascular and autoimmune disorders like SLE, rheumatoid arthritis, scleroderma, Behcet disease, etc. Common thymic lesions on imaging are enlisted in Table 3.1.

3.5

Imaging Features of Specific Entities

3.5.1 Thymic Hyperplasia (Fig. 3.5)

Fig. 3.2  USG of normal thymus. Note the typical speckled/starry sky pattern

3.4

Thymic Lesions

• Many clinical conditions can be associated with thymic lesions. Thymic hyperplasia and thymoma are most important of them. A large number of conditions are associated with thymoma; a few significant clinical associations are myasthenia gravis, pure red cell aplasia, hypogammaglobulinemia, systemic lupus erythematosus (SLE), and rheumatoid arthritis. • Thymic hyperplasia can either be true thymic hyperplasia or follicular hyperplasia [see Chap. 5].

• Both true and follicular hyperplasia presents with a homogeneous enlargement of thymus. • Diffuse and symmetric enlargement is a key feature to differentiate it from a neoplasm. • The attenuation on CT is homogeneous and shows a homogeneous enhancement. • On chemical shift MRI, there can be signal drop in the opposed-phase images, which may further help in differentiating it from neoplasm.

3.5.2 Thymic Cyst (Fig. 3.6) • Often asymptomatic; can be congenital or acquired. • Acquired cysts: can be seen after radiotherapy for Hodgkin lymphoma, within a thymic tumor or in association with autoimmune disorders.

3  Radiology of Normal Thymus, Thymic Lesions, and Tumors

25

a

b

Fig. 3.4 (a, b) MRI of normal thymus. It is of intermediate signal intensity on T2W images (arrow in a) and balanced gradient echo images (arrow in b). Note the homogeneous attenuation and smooth margins

Table 3.1  Benign and malignant thymic lesions Benign Hyperplasia Cyst Lymphangioma Teratoma Abscess Thymolipoma

Malignant Thymoma Thymic carcinoid Thymic carcinoma Lymphoma

Fig. 3.5  CECT of thymic hyperplasia (asterisk) with myasthenia gravis. Note the homogeneous attenuation and smooth margins

a

b

Fig. 3.6 (a, b) Thymic cyst. CECT (a) reveals a homogeneous hypoattenuating focal lesion (asterisk) in the left lobe of thymus. T2W MRI (b) axial image of another patient shows a tiny hyperintense lesion in the right lobe of thymus (arrow)

26

• On imaging, they are brightly hyperintense on T2W sequences and hypointense on T1W images and are usually well circumscribed [1].

M. Jana and A. S. Bhalla

Fat appears hypodense on non-contrast CT, hyperintense on both T1 and T2W images. • Cystic teratoma appears as well-defined cystic lesion with foci of fat attenuation and internal soft tissue attenuation areas.

3.5.3 Thymic Lymphatic Malformation • Usually seen in association with cervical lymphatic malformation which has an anterior mediastinal extension. • On USG, they appear as anechoic cystic lesion with thin septations. If infected, can show internal echogenic debris [1, 2]. • CECT shows hypodense (fluid attenuation) lesion with enhancing thin septae. Attenuation may be higher if infected or complicated by hemorrhage.

3.5.4 Dermoid Cyst/Teratoma (Figs. 3.7 and 3.8) • Mature germ cell tumors can develop within or near the thymus. • Mature germ cell tumor/teratoma is usually asymptomatic. Unusual complications include rupture into tracheobronchial tree or pleural cavity. • Benign mature teratoma on imaging shows a mass lesion with internal soft tissue, fat attenuation, and calcification.

a

Fig. 3.8  CECT of thymic teratoma. Note the heterogeneous anterior mediastinal mass with soft tissue (arrow) and fat (asterisk) attenuation

b

Fig. 3.7 (a, b) Thymic dermoid cyst. CECT images reveal a heterogeneous hypoattenuating focal lesion (asterisk) in the left lobe of thymus, having non-enhancing soft tissue component (arrow in a) and fat attenuation (arrow in b)

3  Radiology of Normal Thymus, Thymic Lesions, and Tumors

3.5.5 Thymic Carcinoid • Neuroendocrine tumors may involve thymus. • Can present with Cushing syndrome due to ACTH secretion or SIADH (syndrome of inappropriate ADH secretion). • With Cushing syndrome often the lesion is small and difficult to detect. • Hyperenhancing mass on CECT with heterogeneous enhancement and occasional presence of calcification [3]. Local invasion may be seen. • MRI shows hypointense mass on T1W, hyperintense on T2W images.

3.5.6 Thymolipoma (Fig. 3.9)

27

• Soft well-circumscribed tumors, sometimes can be very large. • CT shows large tumor with hypodense fatty content and enhancing soft tissue components. • MRI reveals hyperintense fatty component on T1W and T2W images.

3.5.7 Thymoma (Figs. 3.10 and 3.11) • Usually tumors of malignant potential. • May be asymptomatic or may have association with myasthenia gravis, pure red cell aplasia, hypogammaglobulinemia, and connective tissue diseases. • Imaging shows homogeneous soft tissue attenuation mass in anterior mediastinum.

• Rare tumors, seen in second–third decades. a

b

Fig.3.9 (a, b) Thymolipoma. (a) CECT reveals a solid enhancing mass lesion with low attenuation (arrow) and enhancing septae. (b) On T1W MRI image it is hyperintense due to presence of fat

a

b

Fig. 3.10 (a, b) Thymoma. CXR (a) reveals an anterior mediastinal mass (arrow) obscuring the right heart border. CECT (b) shows the mass to be solid and heterogeneously enhancing

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M. Jana and A. S. Bhalla

a

b

Fig. 3.11 (a, b) Invasive thymoma. CECT reveals a solid enhancing mass lesion in anterior mediastinum (arrow) with involvement of the pericardium and encasement of superior vena cava (SVC). Right-sided pleural effusion with enhancing pleural deposit is also present (block arrow in a)

a

b

Fig. 3.12 (a, b) Thymic carcinoma. CECT sagittal MPR image reveals an anterior mediastinal mass (asterisk). Nodular pleural deposits (arrows in b) and pleural effusion suggest metastases. Note enlarged pericardial lymph node (block arrow in b)

• May show local invasion, manifested on imaging by irregular tumor margins, encasement of mediastinal structures, and nodular pleural thickening.

3.5.8 Thymic Carcinoma (Figs. 3.12 and 3.13) • Usually present in fifth–sixth decades of life. • Aggressive tumor and can have metastases at presentation.

• Various histologic subtypes exist. • Imaging may show a large mass with lobular contour and heterogeneous internal content [4]. Necrosis, hemorrhage, and calcification may be seen. Adjacent organ invasion, lymphadenopathy, and distant metastases are features that point toward an advanced thymoma or thymic carcinoma over early stages of thymoma.

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3  Radiology of Normal Thymus, Thymic Lesions, and Tumors

Fig. 3.14  A case of thymic lymphoma in which CECT reveals a large transcompartmental solid mass (arrow)

3.6

Conclusion

Fig. 3.13  Chest X-ray shows neuroendocrine carcinoma of thymus. (Photograph courtesy Dr. Mark R Wick, Department of Pathology, UVA School of Medicine)

3.5.9 Lymphoma (Fig. 3.14)

Thymic imaging often requires multiple imaging modalities; and the choice of imaging modality depends on the age of the patient as well as the clinical suspicion. In early infancy, normal thymus may mimic anterior mediastinal masses or lymphadenopathy; and USG alone can prove to be crucial in establishing the diagnosis. Thymic tumors, to the contrary, trans-­ require CECT/MRI for detailed extent assessment.

• May present as anterior mediastinal or compartmental mass. • CT shows an enhancing mass with homogeneous soft tissue attenuation; necrosis or calcification is unusual. • MRI displays hypointense mass on T1W and intermediate signal intensity on T2W images and shows marked restriction on diffusion-weighted images. • May cause encasement of vessels and airway and result in superior mediastinal syndrome.

References 1. Manchanda S, Bhalla AS, Jana M, Gupta AK. Imaging of the pediatric thymus: clinicoradiologic approach. World J Clin Pediatr. 2017;6:10–23. 2. Nishino M, Ashiku SK, Kocher ON, Thurer RL, Boiselle PM, Hatabu H. The thymus: a comprehensive review. Radiographics. 2006;26:335–48. 3. Shimamoto A, Ashizawa K, Kido Y, Hayashi H, Nagayasu T, Kawakami A et al. CT and MRI findings of thymic carcinoid. Br J Radiol. 2017;90:20150341. 4. Jung KJ, Lee KS, Han J, Kim J, Kim TS, Kim EA. Malignant thymic epithelial tumors: CT-pathologic correlation. Am J Roentgenol. 2001;176:433–9.

4

Surgical Approach to Thymic Lesions Manjunath Bale and Rajinder Parshad

4.1

History

The first thymectomy was performed in 1911 by Sauer Bruch via a transcervical route [1]. Nearly two decades later Alfred Blalock and colleagues performed the first trans-sternal thymectomy in a case of thymoma with myasthenia gravis (MG) and they noted the resolution of myasthenia [2]. Subsequently the indication of thymectomy was extended to even non-thymomatous patients of MG with encouraging results and remission rates of up to 15–50%, thus establishing surgery as a potential therapeutic modality for MG [3, 4]. The next major paradigm shift happened with the description of ectopic thymic tissue in mediastinal fat beyond the confines of thymic gland by Masaoka et al. [5]. This brought in the concept of extended thymectomy as a preferred surgical option. This entails complete removal of thymus along with mediastinal fat. Subsequently even more radical maximal thymectomy was described by Jaretzki et al. [6] which includes removal of even cervical fat through an additional incision in the neck. The advent of minimal invasive surgery has prompted the surgeons to adapt these techniques in the management of thymic diseases which minimize the morbidity associated with open techniques. Currently, minimal invasive techniques are preferred and increasing number of studies have confirmed the safety and efficacy of these techniques in the treatment of MG [7, 8, 9]. The evolution of surgical approaches to thymus has been summarized in Table  4.1. The first video-assisted thoracoscopic surgery (VATS) thymectomy was performed by Sugarbaker from Boston and the Belgium group in 1993 [10]. VATS thymectomy has become increasingly popular due to low procedural

morbidity and mortality, improved cosmesis, and lesser degree of access trauma. There is increasing data to support equal efficacy with VATS thymectomy when compared to open thymectomy for non-thymomatous MG [11]. Francesco Paolo Caronia et al. did the first uniportal VATS thymectomy in 2015 [12].

4.2

Indications for Thymectomy

Patients with thymoma with or without MG warrant a thymectomy. MG in general is an indication for thymectomy, with early onset myasthenia patients benefitting the most [13, 14]. Conditions like AChR antibody-negative, anti-­ MUSK antibody-positive, pure ocular MG and late-onset non-thymomatous MG have poor response to thymectomy [11, 15]. Other rare indications for thymectomy are thymic cyst, thymic carcinoid, thymic carcinoma, ectopic intrathymic parathyroid, symptomatic thymic hyperplasia, and pure red cell aplasia. Table 4.1  Evolution of surgical approaches to thymus Approaches Transcervical [1] Transsternal [2] Extended thymectomy [5] Maximal thymectomy [6] VATS thymectomy [10] VATET [11] Uniportal thymectomy [12]

Year 1911 1939 1975 1988 1993 1996 2015

Author Sauerbruch F. Alfred Blalock et al. Masaoka et al. Alfred Jaretzki et al. Sugarbaker et al. Scelsi et al. Francesco Paolo Caronia et al.

M. Bale · R. Parshad (*) Department of Surgical Disciplines, All India Institute of Medical Sciences (AIIMS), New Delhi, India e-mail: [email protected]

© Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_4

31

32

4.3

M. Bale and R. Parshad

Surgical Anatomy of Thymus

ITMIG (international thymic malignancy interest group) system classifies the mediastinum into three compartments, the anterior prevascular compartment, the middle visceral compartment, and the posterior prevertebral compartment. The thymus lies in the prevascular compartment which is bounded superiorly by the thoracic inlet, inferiorly by the diaphragm, anteriorly by the posterior border of the sternum, laterally by the mediastinal pleura, and posteriorly, the anterior aspect of the pericardium [16]. The thymus gland has an H-shaped configuration which consists of elongated left and right lobes that join at their central portions just caudal to the left innominate vein (Fig. 4.1). The cephalad ends of each lobe become thin and are generally well defined, whereas the caudal ends of each lobe are thicker and less easily definable as they fade into surrounding mediastinal fat as one approaches the diaphragm. The gland is enveloped in a fibrous capsule which allows the surgeon to apply traction and also helps in differentiating from the surrounding fat. The gland itself, by virtue of its capsule, has a smoother, more lobulated appearance while the fat has a less coalesced, less solid appearance. Making this differentiation is particularly challenging during the most caudal portion of a thymic

Fig. 4.1  Anatomy of thymus. RIMA Right internal mammary artery, RIMV Right internal mammary vein, LIMA Left internal mammary artery, LIMV Left internal mammary vein, SVC Superior vena cava

Superior vena cava

dissection as one approaches the diaphragm and above the innominate vein. The presence of mediastinal ectopic foci of thymic tissue may be observed in a range from 20% up to 72% [5, 6, 17]. Ectopic foci of thymic tissue have been found in the neck, either fused or close to the parathyroid and thyroid glands (Fig. 4.2). Ectopic foci of thymus have also been reported in the base of skull, main bronchi, and in the parathyroid gland. Thus, total and complete removal of the thymic tissue to achieve a favorable clinical outcome is theoretically impossible, and it just indicates that ectopic thymic tissue is a marker for poor outcome after surgery. The blood supply to the body of the thymus is derived from small branches which arise from the superior and inferior thyroid arteries, the internal mammary vessels, and less importantly from the pericardiophrenic vessels. The venous drainage, on the other hand, is primarily through 1–3 larger branches that drain directly off of the posterior aspect of the gland into the left innominate vein. Lymph Node Dissection in Thymic Carcinoma  Lymph node dissection is not very well described in thymic malignancies as compared to other thoracic malignancies. The incidence of nodal metastases in patients with thymic carcinoma ranges from 26.8% to 41% [18–20].

Left brachiocephalic vein

Right subclavian vein Right internal mammary artery Right internal mammary vein

Left subclavian vein Left internal mammary artery Left internal mammary vein

Left phrenic nerve

Right phrenic nerve

Pericardial fat

Lower horns of thymus

4  Surgical Approach to Thymic Lesions Fig. 4.2  Areas of potential ectopic thymic tissue. RIMA Right internal mammary artery, RIMV Right internal mammary vein, LIMA Left internal mammary artery, LIMV Left internal mammary vein, SVC Superior vena cava and AP Aorto-pulmonary

33

Left brachiocephalic vein

Superior vena cava

Thymus in AP window

Right subclavian vein

Left subclavian vein

Right internal mammary artery

Left internal mammary artery

Right internal mammary vein

Left internal mammary vein

Left phrenic nerve

Right phrenic nerve

Lower horns of thymus

Pericardial fat with ectopic thymic tissues

The first ITMIG recommendations were given in 2011 [21]. They recommended that any suspicious lymph node should be removed. In thymomas with adjacent organ involvement, routine removal of anterior mediastinal lymph nodes and a systematic sampling of intrathoracic lymph nodes is recommended. For thymic carcinoma, the systematic removal of anterior mediastinal, intrathoracic, supraclavicular, and lower cervical lymph nodes was recommended. There has been a renewed interest in lymph node dissection and according to the new ITMIG/IASLC (2014) [16] lymph node map they are classified as the anterior region (N1) including the lower anterior cervical, peri-thymic, prevascular, para-aortic, ascending aortic, superior phrenic, supradiaphragmatic, inferior phrenic, and pericardial node groups. The deep region (N2) includes the deep cervical, supraclavicular, upper and lower paratracheal, subaortic, subcarinal, hilar, and internal mammary node groups. All nodes outside the anterior and deep regions were regarded as M components. By defining node areas both anatomically and on CT images, clinical and pathological staging is improved. Involved nodes should be classified as in either the “anterior region” or “deep region” according to the boundaries described; if possible, the specific location of the node should be recorded as well.

4.4

Principle of Surgery

Aim is to achieve complete clearance of mediastinal fat along with complete excision of thymus in non-­thymomatous myasthenia gravis. Thymectomy is done in patients of thymoma, although some recent studies are promoting thymomectomy in early-stage thymoma [22].

4.5

Surgical Approaches

The surgical decision is influenced by the extent of the planned resection (simple or extended thymectomy), status of thymus (atrophic, normal, hyperplasia, or thymoma), location of the gland (left or right), and involvement of major vascular structures. Other factors which influence surgical approach are the general and respiratory condition of the patient. Thymus extends in body cavities (neck and thoracic cavity) and approaches to thymus have been described from both regions. Additionally, subxiphoid and subcostal approaches are also described (Fig. 4.3). The role of VATS thymectomy in large thymomas is still debatable. The involvement of phrenic nerve, innominate vein, or other major vascular structures is a contraindica-

34

M. Bale and R. Parshad

(not en bloc) resection, or disruption of the tissues exposing the tumor. 5. The access incision for retrieval of the thymoma should be large enough to prevent specimen disruption. 6. Exploration of pleura should be done if the thymoma invades the mediastinal pleura. 7. Retrieval in the bag. 8. Examination of the removed specimen to assess for completeness of the resection is required. 9. Communication with pathologist about suspicious areas is essential. The issues are orientation of the specimen, marking of several routine areas both on the specimen and in the patient, and identification of areas of tissue disruption that were not “closed” during the dissection.

4.6

Transcervical route Sub xiphoid route

Right and left VATS Median strenotomy

Fig. 4.3  Various surgical approaches to thymus

tion for VATS, either diagnosed preoperatively or intraoperatively [21]. According to the European Society for Medical Oncology (ESMO) guidelines on thymic tumors [23], the standard surgical approach in resectable disease remains median sternotomy. Resectable tumors include all stage I/II disease (according to Masaoka-Koga stage) and selected stage III tumors. The ESMO guidelines however recognize that MIT (minimally invasive thymectomy) is an option for presumed stage I and II “in the hands of appropriately trained surgeons.” According to the ITMIG policies, a minimally invasive thymectomy should comprise of the following [21]. 1. A minimally invasive resection of a thymic malignancy should involve no rib spreading or sternal cutting. The intent should be to perform a complete resection, and a significant portion should be done with visualization on a video monitor. 2. Resection should involve the thymoma, thymus, and mediastinal fat. 3. Dissection and visualization of innominate vein and both phrenic nerves should be done. 4. Conversion to open is required if oncologic principles are being compromised or violated: e.g., perforation of the capsule, incomplete resection, risk of a discontinuous

VATS Thymectomy

VATS thymectomy has been performed in a left lateral, lateral semi-supine, and supine position. Figure 4.4 shows patient positioning for VATS thymectomy in supine position. The first port is placed in the fourth or fifth intercostal space. Once the first port is placed, a combination of carbon dioxide insufflation and lung isolation is used for creation of space. The rest of the ports are placed under direct vision. Port position is as shown in Fig.  4.5 for left VATS thymectomy. Once the thoracic cavity is entered a thorough assessment of the lesion is done along with assessment for any pleural deposits. The view from the right and left lateral ports is shown in Figs. 4.6 and 4.7. Step 1: Identification of phrenic nerve and internal mammary vessels.

Fig. 4.4  Patient positioning for VATS thymectomy. Arm tucked below the level of the table exposing the lateral chest wall for port placement and manipulation

4  Surgical Approach to Thymic Lesions

C

35

B

A

A-Camera port (10 mm) B-5 mm working port C-10 mm working port, also used for specimen extraction

Initially dissection starts by dividing the mediastinal pleura medial and parallel to the phrenic nerve as shown in Fig. 4.8. Step 2: The pleural incision is extending from lateral to medial as depicted in Fig. 4.9 toward the opposite pleura. Step 3: The next part of dissection begins at the inferior border at the pericardial reflection just medial to the phrenic nerve and proceeds from lateral to medial (Fig.  4.10). The end point of this dissection is identification of contralateral pleura and the contralateral phrenic nerve. Step 4: Identification of thymic veins draining into the innominate vein (Fig.  4.11). Once the veins are identified they are dissected, ligated using polymer or titanium clips, and divided (Fig. 4.12). Step 5: Taking down both the thymic horns is an important part of dissection. A series of blunt and sharp dissection is used to bring the thymic horns down (Figs.  4.13 and 4.14).

Fig. 4.5  Port placement for VATS thymectomy

Fig. 4.8  View of left VATS. Dissection starts by dividing the mediastinal pleura medial to phrenic nerve along the dotted lines

Fig. 4.6  View of anterior mediastinum from right VATS

Fig. 4.7  View of anterior mediastinum from left VATS

Fig. 4.9  Extension of the pleural incision from lateral to medial in the direction of the dotted lines toward the opposite pleura

36

Fig. 4.10  Thymus dissected from its attachments to the pericardium

M. Bale and R. Parshad

Fig. 4.14  Dissection of the right thymic horn going above the brachiocephalic vein. White arrow Right thymic horn going above the brachiocephalic vein, Blue dotted lines brachiocephalic vein

Fig. 4.11  Identification of thymic veins. White arrow thymic vein, Black arrow Brachiocephalic vein

Fig. 4.15  Dissection of the right thymic horn going under the brachiocephalic vein. Black arrow Right thymic horn going under the brachiocephalic vein, Blue arrow brachiocephalic vein Fig. 4.12  Identification of thymic veins and clipping them with polymer clips. White arrow Thymic vein clipped with polymer clips, Black arrow Brachiocephalic vein

Fig. 4.13  Dissection of the left thymic horn going above the brachiocephalic vein. White arrow Left thymic horn going above the brachiocephalic vein, Black arrow Clipped thymic vein

Caution should be taken as sometimes the thymic horns can go underneath the innominate vein as shown in Fig. 4.15. (Note: the order of steps 3, 4, and 5 is not fixed and is interchangeable depending on the anatomy, underlying disease, and body habitus of the patient.) Step 6: Clearance of the cardiophrenic fat pad ensuring clearance of all anterior mediastinal fat (Fig. 4.16). In some cases, the tumors infiltrate the adjacent organs as shown in Figs.  4.17 and 4.18. An R0 resection is recommended even in such conditions. The specimen is retrieved in a bag (Fig. 4.19).

4  Surgical Approach to Thymic Lesions

37

Initial view showing thymic region with mediastinal fat

Final view showing post thymectomy view with complete clearance of fat

Fig. 4.16  View of the anterior mediastinum after completion of thymectomy

Fig. 4.17  Thymoma with adjacent organ involvement. A Pericardium, B tumour invading pericardium, C Cardiac chambers

Fig. 4.18  Thymoma with adjacent organ involvement. A Adjacent lung parenchyma, B tumour invading lung, C Phrenic nerve

38

M. Bale and R. Parshad

Fig. 4.20  Mediastinal board in use for specimen orientation

Fig. 4.19  Specimen retrieved in bag

4.7

Table 4.2 Prognostic factors of thymectomy outcome in MG [23–26] Preoperative

Age of onset of MG Duration of disease Stage of myasthenia Medications

Gender Antibody status Postoperative

Histopathological stage Clinical outcome over time

40%) of IgG4-positive plasma cells (HE, a, b; immunoperoxidase, kappa and lambda light chains)

58

A. Marx

Differential Diagnosis  Diffuse tumor infiltrates accompanied by desmoplasia can mimic FM.

masses were detected fortuitously. Autoimmune phenomena were not observed.

Prognosis  The historically dismal prognosis due to 30% perioperative mortality has improved significantly during the last 2 decades, but bilateral mediastinal involvement remains a risk factor [92]. Infectious etiologies need antibiotic or antimycotic treatment but responses are often limited. Corticosteroids are the first choice in IgG4-related FM. In case of stenosis of large vessels due to FM, open surgery still appears advantageous compared to angioplasty and/or stenting [95].

Etiology and Pathogenesis  The trigger(s) of granuloma formation and mechanisms of coalescence are unknown. No longitudinal observations are available to document growth of the lesions. Infectious agents or “burnt-out” neoplasms have not been identified.

5.9

 umoral Cholesterol Granulomas T (“Cholesteroloma”) of the Thymus

Macroscopy  The lesions were resected from the anterior and rarely superior mediastinal region and described as tumor-like masses with maximal diameters of 2–6 cm.

Epidemiology  There are less than ten published cases of “cholesteroloma” [97–102]. Interestingly, all patients with available data have been men aged 58–75 years [97, 98].

Histology  Multiple large foreign body granulomas that are cell poor and delineated by fibrous septa from each other and from the mediastinal fat are characteristic (Fig. 5.12a). In other, apparently more florid cases, granulomas are made of more dispersed cholesterol clefts that are accompanied by debris and numerous multinucleated foreign body giant cells and rare lymphoid cells (Fig. 5.12b, c). This later form often occurs in the vicinity of thymic cysts. Calcification of granulomas is common (Fig. 5.12d). In the few published cases, adjacent thymic tissue did not show follicular hyperplasia or inadequate regressive changes.

Clinical Findings  A rare patients showed cough and dyspnea related to the mass [97]. In the others, the respective

Prognosis  Recurrences after complete removal or fatal outcomes have not been reported.

Cholesterol granulomas are a common sign of spontaneous or induced regressive changes of many neoplastic and reactive processes in the mediastinum [96]. Tumoral masses that are completely compose of cholesterol granulomas and detectable by imaging are rare.

a

Fig. 5.12  “Cholesteroloma” (tumoral cholesterol granulomas) (a). Confluent cholesterol needles forming large stacks that are separated from each other and from the mediastinal fat by cell-poor fibrous septae; paucity of foreign body giant cells; involuted thymus indicated by

b

black arrow (b, c). Cholesterol granuloma showing more dispersed cholesterol needles accompanied by debris and an active foreign body giant cell reaction (d). Calcification in a cholesterol granuloma (HE, a–d)

5  Pathology of Nonneoplastic Thymic Lesions

c

59

d

Fig. 5.12 (continued)

References 1. Kozu Y, et al. Single institutional experience with primary mediastinal cysts: clinicopathological study of 108 resected cases. Ann Thorac Cardiovasc Surg. 2014;20(5):365–9. 2. Esme H, et al. Primary mediastinal cysts: clinical evaluation and surgical results of 32 cases. Tex Heart Inst J. 2011;38(4):371–4. 3. Aydin Y, et al. Surgical treatment of mediastinal cysts: report on 29 cases. Acta Chir Belg. 2012;112(4):281–6. 4. Kelleher CM, et  al. Case records of the Massachusetts general hospital. Case 10-2012. A 16-year-old boy with epigastric pain and a mediastinal mass. N Engl J Med. 2012;366(13):1241–9. 5. Takeda S, et al. Clinical spectrum of primary mediastinal tumors: a comparison of adult and pediatric populations at a single Japanese institution. J Surg Oncol. 2003;83(1):24–30. 6. Nam JG, et al. Age- and gender-specific disease distribution and the diagnostic accuracy of CT for resected anterior mediastinal lesions. Thorac Cancer. 2019;10(6):1378–87. 7. Rosai J, Levine GD. Tumors of the thymus. In: Firminger HI, editor. Atlas of tumor pathology, vol. 13. 2nd ed. Washington, D.C.: Armed Forces Institute of Pathology; 1976. p. 228. 8. Haro Estarriol M, et al. Spontaneous resolution of a primary thymic cyst. An Med Interna. 2003;20(10):552–3. 9. Schweigert M, et al. Thymoma within a giant congenital thymic cyst. Interact Cardiovasc Thorac Surg. 2011;13(4):442–3. 10. Kitami A, et  al. Thymoma with intracystic dissemination arising in a unilocular thymic cyst. Gen Thorac Cardiovasc Surg. 2007;55(7):281–3. 11. Travis WD, et  al. WHO classification of tumours of the lung, pleura, thymus and heart. In: Bosman FT, et  al., editors. World health classification of tumours. 4th ed. Lyon: IARC; 2015. 12. Nakamura S, et  al. Multilocular thymic cyst associated with thymoma: a clinicopathologic study of 20 cases with an emphasis on the pathogenesis of cyst formation. Am J Surg Pathol. 2012;36(12):1857–64. 13. Asma B, et al. Acute respiratory failure revealing a multilocular thymic cyst in an infant: a case report. Cases J. 2009;2:9109. 14. Mishalani SH, Lones MA, Said JW. Multilocular thymic cyst. A novel thymic lesion associated with human immunodeficiency virus infection. Arch Pathol Lab Med. 1995;119(5):467–70.

15. Nonaka D, Klimstra D, Rosai J.  Thymic mucoepidermoid carcinomas: a clinicopathologic study of 10 cases and review of the literature. Am J Surg Pathol. 2004;28(11): 1526–31. 16. Matsumoto S, et al. Multilocular thymic cyst associated with rheumatoid arthritis. Kyobu Geka. 2012;65(3):205–8. 17. Tamagno M, et  al. Giant multilocular thymic cyst in an HIV-­ infected adolescent. J Pediatr Surg. 2011;46(9): 1842–5. 18. Suster S, Rosai J. Multilocular thymic cyst: an acquired reactive process. Study of 18 cases. Am J Surg Pathol. 1991;15(4):388–98. 19. Izumi H, et al. Multilocular thymic cyst associated with follicular hyperplasia: clinicopathologic study of 4 resected cases. Hum Pathol. 2005;36(7):841–4. 20. Suster S, et al. Multilocular thymic cysts with pseudoepitheliomatous hyperplasia. Hum Pathol. 1991;22(5):455–60. 21. Osaki T, Nakagawa M. Multilocular mediastinal cyst with rim calcification: report of a case. Surg Today. 2008;38(1):52–5. 22. Shen X, et  al. Thymoma and thymic carcinoma associated with multilocular thymic cyst: a clinicopathologic analysis of 18 cases. Diagn Pathol. 2018;13(1):41. 23. Weissferdt A, Moran CA. Thymic carcinoma associated with multilocular thymic cyst: a clinicopathologic study of 7 cases. Am J Surg Pathol. 2011;35(7):1074–9. 24. Moran CA, et  al. Carcinomas arising in multilocular thymic cysts of the neck: a clinicopathological study of three cases. Histopathology. 2004;44(1):64–8. 25. Huang WL, Tseng YL.  Rare presentation of giant isolated enteric cyst in anterior mediastinum. Ann Thorac Surg. 2019;107(6):e417–9. 26. Salyer DC, Salyer WR, Eggleston JC. Benign developmental cysts of the mediastinum. Arch Pathol Lab Med. 1977;101(3):136–9. 27. Schweigert M, et  al. The tale of spring water cysts: a historical outline of surgery for congenital pericardial diverticula and cysts. Tex Heart Inst J. 2012;39(3):330–4. 28. Park JG, et  al. Mediastinal lymphangioma: Mayo clinic experience of 25 cases. Mayo Clin Proc. 2006;81(9):1197–203. 29. Kadota Y, et  al. Lymphatic and venous malformation or “lymphangiohemangioma” of the anterior mediastinum: case report and literature review. Gen Thorac Cardiovasc Surg. 2011;59(8):575–8.

60 30. Moran CA, Suster S. Thymoma with prominent cystic and hemorrhagic changes and areas of necrosis and infarction: a clinicopathologic study of 25 cases. Am J Surg Pathol. 2001;25(8):1086–90. 31. Suster S, Rosai J. Thymic carcinoma. A clinicopathologic study of 60 cases. Cancer. 1991;67(4):1025–32. 32. Suster S, Rosai J. Cystic thymomas. A clinicopathologic study of ten cases. Cancer. 1992;69(1):92–7. 33. Rieker RJ, et  al. Cystic thymoma. Pathol Oncol Res. 2005;11(1):57–60. 34. Brown JG, et al. Thymic basaloid carcinoma: a clinicopathologic study of 12 cases, with a general discussion of basaloid carcinoma and its relationship with adenoid cystic carcinoma. Am J Surg Pathol. 2009;33(8):1113–24. 35. Moser B, et  al. Adenocarcinoma of the thymus, enteric type: report of 2 cases, and proposal for a novel subtype of thymic carcinoma. Am J Surg Pathol. 2015;39(4):541–8. 36. Weissferdt A, Kalhor N, Moran CA.  Cystic well-differentiated squamous cell carcinoma of the thymus: a clinicopathological and immunohistochemical study of six cases. Histopathology. 2016;68(3):333–8. 37. Lindholm KE, Moran CA. Cystic and encapsulated atypical thymoma (World Health Organization Type B3). Am J Clin Pathol. 2019;152(4):512–6. 38. Betts G, Beckett E, Nonaka D. GATA3 shows differential immunohistochemical expression across thyroid and parathyroid lesions. Histopathology. 2014;65(2):288–90. 39. Tulay CM.  Primary mediastinal hydatid cysts. Ann Thorac Cardiovasc Surg. 2014;20(4):316–9. 40. Sabzi F, Faraji R.  A rare case of anterior mediastinal mass caused by Brucella infection. Asian Cardiovasc Thorac Ann. 2017;25(3):222–5. 41. Mao JC, et al. Craniocervical necrotizing fasciitis with and without thoracic extension: management strategies and outcome. Am J Otolaryngol. 2009;30(1):17–23. 42. Strobel P, et al. The ageing and myasthenic thymus: a morphometric study validating a standard procedure in the histological workup of thymic specimens. J Neuroimmunol. 2008;201–202:64–73. 43. Middleton G, Schoch EM.  The prevalence of human thymic lymphoid follicles is lower in suicides. Virchows Arch. 2000;436(2):127–30. 44. Marx A, et  al. Thymus pathology observed in the MGTX trial. Ann N Y Acad Sci. 2012;1275:92–100. 45. Alpert LI, et al. A histologic reappraisal of the thymus in myasthenia gravis. A correlative study of thymic pathology and response to thymectomy. Arch Pathol. 1971;91(1):55–61. 46. Koneczny I, et al. Characterization of the thymus in Lrp4 myasthenia gravis: four cases. Autoimmun Rev. 2019;18(1):50–5. 47. Zisimopoulou P, et al. A comprehensive analysis of the epidemiology and clinical characteristics of anti-LRP4 in myasthenia gravis. J Autoimmun. 2014;52:139–45. 48. Leite MI, et  al. Fewer thymic changes in MuSK antibody-­ positive than in MuSK antibody-negative MG.  Ann Neurol. 2005;57(3):444–8. 49. Dalla Costa M, Mangano FA, Betterle C.  Thymic hyperplasia in patients with Graves’ disease. J Endocrinol Investig. 2014;37(12):1175–9. 50. Hofmann WJ, Moller P, Otto HF.  Thymic hyperplasia. II. Lymphofollicular hyperplasia of the thymus. An immunohistologic study. Klin Wochenschr. 1987;65(2):53–60. 51. Meyer A, Levy Y.  Geoepidemiology of myasthenia gravis [corrected]. Autoimmun Rev. 2010;9(5):A383–6. 52. Maniaol AH, et  al. Late onset myasthenia gravis is associated with HLA DRB1∗15:01 in the Norwegian population. PLoS One. 2012;7(5):e36603.

A. Marx 53. Seldin MF, et  al. Genome-wide association study of late-onset myasthenia gravis: confirmation of TNFRSF11A and identification of ZBTB10 and three distinct HLA associations. Mol Med. 2016;21(1):769–81. 54. Chuang WY, et  al. Late-onset myasthenia gravis  - CTLA4(low) genotype association and low-for-age thymic output of naive T cells. J Autoimmun. 2014;52:122–9. 55. Giraud M, et  al. An IRF8-binding promoter variant and AIRE control CHRNA1 promiscuous expression in thymus. Nature. 2007;448(7156):934–7. 56. Gregersen PK, et al. Risk for myasthenia gravis maps to a (151) Pro-->Ala change in TNIP1 and to human leukocyte antigen­B∗08. Ann Neurol. 2012;72(6):927–35. 57. Renton AE, et al. A genome-wide association study of myasthenia gravis. JAMA Neurol. 2015;72(4):396–404. 58. Gradolatto A, et al. Both Treg cells and Tconv cells are defective in the Myasthenia gravis thymus: roles of IL-17 and TNF-alpha. J Autoimmun. 2014;52:53–63. 59. Vincent A, et al. In-vitro synthesis of anti-acetylcholine-receptor antibody by thymic lymphocytes in myasthenia gravis. Lancet. 1978;1(8059):305–7. 60. Wakkach A, et al. Expression of acetylcholine receptor genes in human thymic epithelial cells: implications for myasthenia gravis. J Immunol. 1996;157(8):3752–60. 61. Kao I, Drachman DB.  Thymic muscle cells bear acetylcholine receptors: possible relation to myasthenia gravis. Science. 1977;195(4273):74–5. 62. Leite MI, et  al. Myasthenia gravis thymus: complement vulnerability of epithelial and myoid cells, complement attack on them, and correlations with autoantibody status. Am J Pathol. 2007;171(3):893–905. 63. Hohlfeld R, Wekerle H. Reflections on the “intrathymic pathogenesis” of myasthenia gravis. J Neuroimmunol. 2008;201–202:21–7. 64. Wolfe GI, et al. Long-term effect of thymectomy plus prednisone versus prednisone alone in patients with non-thymomatous myasthenia gravis: 2-year extension of the MGTX randomised trial. Lancet Neurol. 2019;18(3):259–68. 65. Suster S, Moran CA.  Micronodular thymoma with lymphoid B-cell hyperplasia: clinicopathologic and immunohistochemical study of eighteen cases of a distinctive morphologic variant of thymic epithelial neoplasm. Am J Surg Pathol. 1999;23(8):955–62. 66. Weissferdt A, Moran CA.  Micronodular thymic carcinoma with lymphoid hyperplasia: a clinicopathological and immunohistochemical study of five cases. Mod Pathol. 2012;25(7):993–9. 67. Weissferdt A, Moran CA.  Thymic hyperplasia with lymphoepithelial sialadenitis (LESA)-like features: a clinicopathologic and immunohistochemical study of 4 cases. Am J Clin Pathol. 2012;138(6):816–22. 68. Marchevsky A, et  al. Policies and reporting guidelines for small biopsy specimens of mediastinal masses. J Thorac Oncol. 2011;6(Suppl 3):S1724–9. 69. Inagaki H, et al. Primary thymic extranodal marginal-zone B-cell lymphoma of mucosa-associated lymphoid tissue type exhibits distinctive clinicopathological and molecular features. Am J Pathol. 2002;160(4):1435–43. 70. Aoki M, et  al. A case of resected plasma cell type castleman’s disease with intramediastinal lymph nodes spread. Ann Thorac Cardiovasc Surg. 2014;20:682–5. 71. Tani T, et  al. A case of true thymic hyperplasia showing slow growth as revealed by chest X-ray. Nihon Kyobu Shikkan Gakkai Zasshi. 1994;32(2):194–8. 72. Judd RL. Massive thymic hyperplasia with myoid cell differentiation. Hum Pathol. 1987;18(11):1180–3.

5  Pathology of Nonneoplastic Thymic Lesions 73. Eifinger F, et al. True thymic hyperplasia associated with severe thymic cyst bleeding in a newborn: case report and review of the literature. Ann Diagn Pathol. 2007;11(5):358–62. 74. Strobel P, et al. Deficiency of the autoimmune regulator AIRE in thymomas is insufficient to elicit autoimmune polyendocrinopathy syndrome type 1 (APS-1). J Pathol. 2007;211(5):563–71. 75. Marx A, et al. The 2015 World Health Organization classification of tumors of the thymus: continuity and changes. J Thorac Oncol. 2015;10(10):1383–95. 76. Adam P, et  al. Thymoma with loss of keratin expression (and giant cells): a potential diagnostic pitfall. Virchows Arch. 2014;465(3):313–20. 77. Balbach ST, et al. Proposal of a genetic classifier for risk group stratification in pediatric T-cell lymphoblastic lymphoma reveals differences from adult T-cell lymphoblastic leukemia. Leukemia. 2016;30(4):970–3. 78. Burkhardt B, Hermiston ML.  Lymphoblastic lymphoma in children and adolescents: review of current challenges and future opportunities. Br J Haematol. 2019;185(6):1158–70. 79. Oschlies I, et  al. Diagnosis and immunophenotype of 188 pediatric lymphoblastic lymphomas treated within a randomized prospective trial: experiences and preliminary recommendations from the European childhood lymphoma pathology panel. Am J Surg Pathol. 2011;35(6):836–44. 80. Pilozzi E, et al. Gene rearrangements in T-cell lymphoblastic lymphoma. J Pathol. 1999;188(3):267–70. 81. den Bakker MA, Oosterhuis JW. Tumours and tumour-like conditions of the thymus other than thymoma; a practical approach. Histopathology. 2009;54(1):69–89. 82. Shimosato Y, Mukai K, Matsuno Y. Tumors of the mediastinum. Washington, DC: American Registry of Pathology; 2010. 83. Tabarin A, et  al. Paraneoplastic Cushing’s syndrome. Pseudotumors of the thymus occurring after correction of hypercorticism. 3 cases. Presse Med. 1993;22(38):1908–10, 1915. 84. Chertoff J, Barth RA, Dickerman JD.  Rebound thymic hyperplasia five years after chemotherapy for Wilms’ tumor. Pediatr Radiol. 1991;21(8):596–7. 85. Kissin CM, et al. Benign thymic enlargement in adults after chemotherapy: CT demonstration. Radiology. 1987;163(1):67–70. 86. Jeon TJ, et al. Rebound thymic hyperplasia detected by 18F-FDG PET/CT after radioactive iodine ablation therapy for thyroid cancer. Thyroid. 2014;24(11):1636–41. 87. Priola AM, et  al. Nonsuppressing normal thymus on chemical-­ shift MR imaging and anterior mediastinal lymphoma: differentiation with diffusion-weighted MR imaging by using the apparent diffusion coefficient. Eur Radiol. 2018;28(4):1427–37. 88. Richmond BW, et  al. Genome-wide association study of 58 individuals with Fibrosing Mediastinitis reveals possible

61 underlying genetic susceptibility. Am J Respir Crit Care Med. 2018;197(9):1219–20. 89. Flieder DB, Suster S, Moran CA.  Idiopathic fibroinflammatory (fibrosing/sclerosing) lesions of the mediastinum: a study of 30 cases with emphasis on morphologic heterogeneity. Mod Pathol. 1999;12(3):257–64. 90. Takanashi S, et  al. IgG4-related fibrosing mediastinitis diagnosed with computed tomography-guided percutaneous needle biopsy: two case reports and a review of the literature. Medicine (Baltimore). 2018;97(22):e10935. 91. Giorgadze T, et  al. Postradiation-associated sclerosing mediastinitis diagnosed in fine needle aspiration specimen: a cytological-­ pathological correlation. Ann Diagn Pathol. 2017;27:43–7. 92. Peikert T, et  al. Fibrosing mediastinitis: clinical presentation, therapeutic outcomes, and adaptive immune response. Medicine (Baltimore). 2011;90(6):412–23. 93. Zhou Y, et  al. Pulmonary vascular involvement of IgG4-related disease: case series with a PRISMA-compliant systemic review. Medicine (Baltimore). 2019;98(6):e14437. 94. Peikert T, et  al. Histopathologic overlap between Fibrosing Mediastinitis and IgG4-related disease. Int J Rheumatol. 2012;2012:207056. 95. Sfyroeras GS, et al. A review of open and endovascular treatment of superior vena cava syndrome of benign aetiology. Eur J Vasc Endovasc Surg. 2017;53(2):238–54. 96. Weissferdt A, Moran CA.  The impact of neoadjuvant chemotherapy on the histopathological assessment of thymomas: a clinicopathological correlation of 28 cases treated with a similar regimen. Lung. 2013;191(4):379–83. 97. Weissferdt A, Kalhor N, Moran C.  Primary thymic cholesteroloma: a clinicopathological correlation of four cases of an unusual benign lesion. Virchows Arch. 2015;467(5):609–11. 98. Ezzat TF, Alowami S. Cholesterol granuloma of the anterior mediastinum with osseous metaplasia. Rare Tumors. 2012;4(4):e47. 99. Krishnan TR, Sinha SK, Kejriwal NK.  A rare case of cholesterol granuloma in the anterior mediastinum. Heart Lung Circ. 2013;22(4):303–4. 100. Fujimoto K, et  al. Focal cholesterol granuloma in the anterior mediastinum: [18F]-fluoro-2-deoxy-D-glucose-positron emission tomography and magnetic resonance imaging findings. J Thorac Oncol. 2007;2(11):1054–6. 101. Luckraz H, Coulston J, Azzu A.  Cholesterol granuloma of the superior mediastinum. Ann Thorac Surg. 2006;81(4): 1509–10. 102. Hamza A, Weissferdt A.  Non-neoplastic and benign tumoral lesions of the thymic gland: a review and update. Adv Anat Pathol. 2019;26(4):257–69.

6

Gross Pathology of Lesions in the Thymic Region Mark R. Wick and Justin A. Bishop

For cases in which the first procedure pertaining to an anterior mediastinal lesion is an attempt at excision, the pathologist may use selected macroscopic characteristics to begin the process of differential diagnosis. Invasion into attached portions of lung, pericardium, or large blood vessels is generally linked to a potential for aggressive behavior, regardless of the histotype of the proliferation. Not all lesions with such features are malignant cytologically. For example, desmoid-­type fibromatosis, thymoma, and fibrosing mediastinitis may all demonstrate invasive growth. However, all of them do have the capacity to cause significant morbidity and even mortality. On the other hand, encapsulation is generically a property of biologically indolent anterior mediastinal processes. Benign cysts of thymic, lymphatic, parathyroid, mesothelial, and bronchogenic types usually have distinct capsules, as do many thymomas and teratomas. On the other hand, it is uncommon for thymic carcinomas, malignant germ cell tumors, neuroendocrine neoplasms, and sarcomas to be invested by fibrous tissue at their peripheries, and malignant lymphomas virtually never are encapsulated. With specific reference to cystic change, several lesions other than bona fide cysts may exhibit that attribute. These potentially include thymoma, thymic carcinoma arising in a thymic cyst, seminoma, Hodgkin lymphoma, and teratoma.

Other macroscopic findings are sometimes noteworthy points. Thymomas often are subdivided internally by broad fibrous bands that intersect one another at acute angles, whereas sclerosing lymphomas—which may sometimes otherwise simulate thymic epithelial neoplasms—exhibit indistinct fibrous trabeculation or stromal bands that connect with one another obliquely. Extensive intralesional hemorrhage and necrosis are also notable because they are generally uncommon in lymphomas and benign tumors of the mediastinum. An exception is represented potentially by thymoma, which can demonstrate extensive degenerative changes that simulate those of spontaneous necrosis. A uniformly firm, but not hard, white-tan “fish flesh” appearance of lesional cut surfaces is also important to record. It can be present in lymphomas, sarcomas, high-­ grade carcinomas (especially lymphoepithelioma-like thymic carcinoma), and peculiar nonneoplastic proliferations such as Rosai-Dorfman disease and Castleman disease. Marked stromal sclerosis is a property of fibrosing mediastinitis and desmoid-type fibromatosis and can also be present in thymoma, seminoma, selected large cell lymphomas, and carcinoid tumors. Friability and global hemorrhage are seen respectively in acute tumefactive mediastinitis and mediastinal hematoma.

M. R. Wick (*) Division of Surgical Pathology & Cytopathology, University of Virginia Medical Center, Charlottesvile, VA, USA e-mail: [email protected] J. A. Bishop Division of Surgical Pathology, University of Texas-Southwestern Medical Center, Dallas, TX, USA e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_6

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6.1

M. R. Wick and J. A. Bishop

 art I: Thymic Hyperplasias P (Figs. 6.1 and 6.2)

Fig. 6.2  Acquired thymic hyperplasia in adults also manifests with diffuse thymic enlargement but basic retention of a normal glandular profile, as shown here. It can be associated with totally normal histologic architecture or the presence of lymphoid hyperplasia with follicle formation. The latter finding is often seen in patients with myasthenia gravis

Fig. 6.1  True thymic hyperplasia is seen here in a newborn infant who died of sepsis. The thymus has a normal configuration but is much larger than it should be for age. Histologically, it showed a physiological composition. In the past, massive true thymic hyperplasia in infancy was termed “status thymicolymphaticus” and was thought to be a potential cause of respiratory embarrassment

6  Gross Pathology of Lesions in the Thymic Region

6.2

 art II: Cystic Lesions of the Thymic P Region and Anterior Mediastinum

Fig. 6.3  Unilocular (congenital) thymic cysts have only one cavity, which is filled with serous or grumous-keratinaceous material. The lining epithelium is simple and indistinct in most instances, although

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(Figs. 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 6.10, 6.11, 6.12, 6.13, and Table 6.1)

uncommon cases show proliferation of it, forming nodules. Cholesterol deposits may sometimes be seen in the cyst wall at a macroscopic level

Fig. 6.4  Multilocular (acquired) thymic cysts contain several cavities that are bounded by fibrous septa and lined by squamoid thymic epithelium. The cyst contents may be represented by clear serous or turbid fluid. Epithelial proliferation is again a possibility in a small minority of cases

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Fig. 6.5  Parathyroid cysts of the thymic region may be associated with clinical normocalcemia or hypercalcemia. In pure form, they have a single cavity filled with serous fluid and an attenuated but stratified lining epithelium composed of parathyroid chief cells

Fig. 6.7  Bronchogenic cysts, shown here, may be unilocular or multilocular. The cavitary spaces in such lesions contain turbid or viscous mucoid fluid, and their walls are heterogeneously solid. Microscopically,

M. R. Wick and J. A. Bishop

Fig. 6.6  Pericardial (mesothelial) cysts are macroscopically similar to the image described in Fig. 6.5, but their lining comprises a single layer of cytologically bland mesothelium. An attachment to the pericardium is apparent radiologically and at surgery

one sees a pseudostratified ciliated columnar epithelial lining, with smooth muscle or cartilage in the cyst wall

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Fig. 6.8  Lymphangiomas of the thymic region may be unilocular or multilocular. They are thin walled and lined by lymphatic-type endothelium admixed with lymphoid infiltrates histologically. Cyst contents are serous or slightly turbid

Fig. 6.9  Thymomas with cystic change may contain one or several loculated spaces. In the extreme, only a few mural nodules of thymomatous tissue are seen in the wall of a large cyst (left). Rarely, the tumor may be virtually entirely cystic, necrotic, and hemorrhagic (right), and

extensive sampling of the wall is necessary to document the presence of thymoma. The latter tumors do not behave any more adversely than ordinary thymomas do

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M. R. Wick and J. A. Bishop

Fig. 6.10  Thymic teratomas may be mature (left) or immature (right) histologically. Both forms of this lesion contain several internal cavities of variable sizes, as well as a range of solid tissue components. The

contents of the cysts are turbid or “cheesy,” and differentiated structures such as hair (left), bone, or teeth may be observed grossly

Fig. 6.11  Cystic thymic seminoma may contain one cavity or several, in which grumous, turbid, or serous fluid can be seen. The cavities contain variable amounts of solid mural tissue

Fig. 6.12  Thymic carcinoma may arise in an acquired multilocular thymic cyst, as shown here. The malignant component “overruns” the lesion and fills some cavities with solid tissue. Histologically, carcinomas in thymic cysts typically show a basaloid histologic constituency [see Chap. 9]

6  Gross Pathology of Lesions in the Thymic Region

69 Table 6.1  Macroscopic differential diagnosis of anterior mediastinal cystic lesions Type of cyst Parathyroid Thymic Bronchogenic Pericardial (mesothelial) Lymphangiomatous Cystic teratoma Cystic thymoma

Fig. 6.13  Cystic thymic carcinoid (shown here) is rare, and the locular spaces that appear in it are relatively small. They contain serous or turbid fluid. The remainder of the mass is composed of solid, non-­ trabeculated, pink-tan tissue which may contain areas of necrosis and hemorrhage

Contents Serous fluid Serous or turbid fluid Viscous or turbid fluid Serous fluid Serous or slightly turbid fluid Turbid or “cheesy” fluid Variable

Thymic carcinoma ex thymic cyst Cystic seminoma

Solid, multinodular Variable

Cystic carcinoid

Variable

Loculation Unilocular Unilocular or multilocular Unilocular or multilocular Unilocular Unilocular or multilocular Multilocular Unilocular or multilocular Multilocular Unilocular or multilocular Unilocular or multilocular

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6.3

M. R. Wick and J. A. Bishop

 art III: Encapsulated, Non-cystic P Lesions (Figs. 6.14 and 6.15, 6.16, and 6.17)

Figs. 6.14 and 6.15  Thymomas are often peripherally encapsulated, and they also show internal subdivision by fibrous septa, into lobules. Encapsulated thymomas may still occasionally recur

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Fig. 6.16  Parathyroid adenomas have a thin peripheral capsule, and they are constituted by solid tan-pink tissue

Fig. 6.17  Thymolipomas are peculiar tumors that generally retain the overall external configuration of the thymus, complete with a peripheral capsule. However, they are much larger than normal or even hyperplas-

tic thymuses, with a yellow color caused by abundant adipose tissue that is admixed with thymic parenchyma microscopically

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6.4

M. R. Wick and J. A. Bishop

 art IV: Unencapsulated, Solid Masses P Containing Multiple Foci of Hemorrhage and Necrosis (Figs. 6.18, 6.19, 6.20, 6.21, 6.22, 6.23, and 6.24)

Fig. 6.19  Plasmacytoma of the thymic region, comprising multiple solid nodules with internal hemorrhage and necrosis

Fig. 6.18  Mixed embryonal carcinoma-yolk sac carcinoma of the thymus, showing a heterogeneous solid consistency with several foci of hemorrhage and necrosis

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Fig. 6.20  Thymic carcinoid lacks internal septation and manifests areas of necrosis and hemorrhage

Fig. 6.21  Non-neuroendocrine thymic carcinomas have gross appearances which mirror those of thymic carcinoids. Clockwise from the top left of this photograph, they are represented by lymphoepithelioma-like

carcinoma, poorly differentiated squamous carcinoma (which may show a NUT gene mutation in some instances), sarcomatoid carcinoma, and papillary carcinoma

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M. R. Wick and J. A. Bishop

Fig. 6.22  Primary thymic choriocarcinoma is a markedly hemorrhagic and necrotic solid tumor

Fig. 6.24  This parathyroid carcinoma of the thymus contains several small foci of necrosis grossly. These tumors may also be widely infiltrative into perithymic soft issue in some cases

Fig. 6.23 Tumefactive tuberculous lymphadenitis of the thymic region demonstrates regional foci of necrosis in the context of lymphadenopathy

6  Gross Pathology of Lesions in the Thymic Region

6.5

 art V: Solid Masses with a P Homogeneous Cut Surface Resembling “Fish Flesh” (Figs. 6.25–6.34)

Other solid, unencapsulated masses in the thymic region have white-tan or tan-pink cut surfaces that resemble “fish flesh.” They are potentially represented by Castleman dis-

Fig. 6.25  Castleman disease of the thymic region

Fig. 6.26  Hodgkin lymphoma of the anterior mediastinum

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ease (Fig.  6.25), Hodgkin lymphoma (Fig.  6.26), non-­ Hodgkin lymphomas of the large B-cell type (Fig.  6.27) and lymphoblastic type (Fig.  6.28), neuroblastoma (Fig. 6.29), paraganglioma (Fig. 6.30), primitive neuroectodermal tumor (Fig. 6.31), rhabdomyosarcoma (Fig. 6.32), Rosai-Dorfman disease (Fig.  6.33), and seminoma (Fig. 6.34) .

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Fig. 6.27  Large-cell B-cell lymphoma of the thymus

Fig. 6.28  Lymphoblastic lymphoma of the thymic region

M. R. Wick and J. A. Bishop

6  Gross Pathology of Lesions in the Thymic Region

Fig. 6.29  Neuroblastoma of the mediastinum, demonstrating internal foci of necrosis

Fig. 6.30  Paraganglioma of the anterior mediastinum

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Fig. 6.31  Primitive neuroectodermal tumor of the thymic region

Fig. 6.32  Alveolar rhabdomyosarcoma of the anterior mediastinum

M. R. Wick and J. A. Bishop

Fig. 6.33  Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy) of the thymic region

6  Gross Pathology of Lesions in the Thymic Region

Fig. 6.34  Seminoma of the thymic region

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6.6

M. R. Wick and J. A. Bishop

 art VI: Solid Unencapsulated Masses P with a “Gritty” or Fibrous Cut Surface (Figs. 6.35–6.38)

Anterior mediastinal lesions with this characteristic are potentially represented by desmoid-type fibromatosis

Fig. 6.35  Desmoid-type fibromatosis of the anterior mediastinum

Fig. 6.36  Solitary fibrous tumor of the thymic region

(Fig.  6.35), solitary fibrous tumor (Fig.  6.36), synovial sarcoma (Fig.  6.37), and fibrosing mediastinitis (Fig. 6.38).

6  Gross Pathology of Lesions in the Thymic Region

Fig. 6.37  Mediastinal synovial sarcoma

Fig. 6.38  Fibrosing (sclerosing) mediastinitis

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6.7

M. R. Wick and J. A. Bishop

 art VII: Anterior Mediastinal Masses P with Miscellaneous Appearances, Not Previously Listed (Fig. 6.39, 6.40, 6.41, 6.42, 6.43, and 6.44)

Fig. 6.39  The fibroinflammatory exudate in acute anterior mediastinitis may produce a mass. It is friable and whitish yellow in character and is relatively easily detached from the adjacent thymus and pericardium (autopsy specimen)

Fig. 6.41  Another peculiar variant of thymoma is diffusely white and sclerotic, owing to the presence of marked stromal fibrosis

Fig. 6.40  Thymoma may show wholesale invasion of perithymic soft tissue or adjacent organs such as the lungs, pericardium, and great vessels (left panel). Rarely, it may seed the pleural surfaces diffusely, imitating the gross appearance of mesothelioma (right panel)

6  Gross Pathology of Lesions in the Thymic Region

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Fig. 6.42  Liposarcomas of the thymic region are unencapsulated, with a variably firm, yellow cut surface

Fig. 6.43  Anterior mediastinal hematomas comprise clotted blood that is deep red and friable

Fig. 6.44  A macroscopically singular variant of thymic carcinoma is the mucinous type, with a translucent and gelatinous cut surface

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References 1. Detterbeck FC, Parsons AM.  Thymic tumors. Ann Thorac Surg. 2004;77:1860–9. 2. Ganesh Y, Yadala V, Nalini Y, Dal A, Raju AD.  Huge mediastinal mass with minimal symptoms: thymolipoma. BMJ Case Rep. 2011;24:2011. 3. Goldstein AJ, Oliva I, Honarpisheh H, Rubinowitz A. A tour of the thymus: a review of thymic lesions with radiologic and pathologic correlation. Can Assoc Radiol J. 2015;66:5–15. 4. Guimarães MD, Benveniste MF, Bitencourt AG, Andrade VP, Souza LP, Gross JL, Godoy MC.  Thymoma originating in a giant thymolipoma: a rare intrathoracic lesion. Ann Thorac Surg. 2013;96:1083–5. 5. Jia R, Sulentic P, Xu JM, Grossman AB. Thymic neuroendocrine neoplasms: Biological behavior and therapy. Neuroendocrinology. 2017;105:105–14. 6. Kim JH, Goo JM, Lee HJ, Chung MJ, Jung SI, Lim KY, Lee MW, Im JG.  Cystic tumors in the anterior mediastinum. Radiologic-­pathological correlation. J Comput Assist Tomogr. 2003;27:714–23. 7. Lack EE. Thymic hyperplasia with massive enlargement: report of two cases with review of diagnostic criteria. J Thorac Cardiovasc Surg. 1981;81:741–6.

M. R. Wick and J. A. Bishop 8. Luh SP, Kuo C, Liu WS, Wu TC, Koo CL, Chen JY.  Carcinoid tumor of the thymus: a clinicopathologic report of two cases with a review of the literature. Int Surg. 2005;90:270–4. 9. Miller Q, Moulton MJ, Pratt J.  Surgical treatment of thymoma. Curr Surg. 2002;59:101–5. 10. Quagliano PV.  Thymic carcinoma: case reports and review. J Thorac Imaging. 1996;11:66–74. 11. Rosado-de-Christenson ML, Galobardes J, Moran CA. Thymoma: radiologic-pathologic correlation. Radiographics. 1992;12:151–68. 12. Rosai J.  The pathology of thymic neoplasia. Monogr Pathol. 1987;29:161–83. 13. Sumner TE, Volberg FM, Kiser PE, Shaffner LD. Mediastinal cystic hygroma in children. Pediatr Radiol. 1981;11:160–2. 14. Tamura M, Ohta Y, Oda M, Watanabe G. Thymic carcinoid tumor. Jpn J Thorac Cardiovasc Surg. 2003;51:29–31. 15. Uematsu M, Kondo M. A proposal for treatment of invasive thymoma. Cancer. 1986;58:1979–84. 16. Walker AN, Mills SE, Fechner RE. Thymomas and thymic carcinomas. Semin Diagn Pathol. 1990;7:250–65. 17. Wasserman JK, Purgina B, Sekhon H, Gomes MM, Lai C.  The gross appearance of a NUT midline carcinoma. Int J Surg Pathol. 2016;24:85–8. 18. Wick MR, Rosai J.  Neuroendocrine neoplasms of the thymus. Pathol Res Pract. 1988;183:188–99.

7

Histomorphology of Thymomas Prerna Guleria and Deepali Jain

7.1

Introduction

Thymomas are rare but the most common anterior mediastinal masses with an incidence of 1.3–2.5/million per year. They show a wide age range, however uncommon in children and young adults, and do not have major sex predilection [1]. Over the years, different systems have been proposed for the histologic classification of thymomas. Due to a lack of consensus and the difficulties thereby faced by pathologists to subtype these tumors, in 1999 the World Health Organization (WHO) formed a panel of experts from different regions of the world to formulate a classification system of thymic epithelial tumors (TETs) [2].This classification system has been revised and the latest refinement of this classification was brought about in the 2015 edition of the WHO classification of tumors of lung, pleura, thymus, and heart [3].

7.2

WHO Classification of Thymomas

The 2015 WHO classification used an interdisciplinary approach to the diagnosis of TET with contributions from radiologists, oncologists, and thoracic surgeons [4]. The histomorphological and immunohistochemical features included in this classification were refined to increase reproducibility of thymoma subtypes as well as to simplify distinction between thymomas and thymic carcinomas. In this system, the existing subtypes of type A, AB, B1, B2, and B3 thymomas were retained with addition of certain obligatory and optional features for diagnosis [5]. These five main subtypes are broadly divided based on the neoplastic epithelial cells being spindled (A, AB) or epithelioid (B1–3) [6–8]. They are further subdivided depending on the content of neoplastic epithelial cells and the nonneoP. Guleria · D. Jain (*) Department of Pathology, All India Institute of Medical Sciences, New Delhi, India e-mail: [email protected]

plastic immature T-cells. One addition was the recognition of mixed patterns and a proposal was made to record these subtypes in 10% increments. Also, all thymomas were recognized to have malignant potential and were excluded from benign category except for micronodular thymoma with lymphoid stroma which has uncertain behavior. This new classification system also incorporated the molecular basis of thymomas including the genetic, epigenetic, and transcriptomic changes [4].

7.3

Masaoka-Koga Staging for Pathologists

The importance of staging TETs lies in the fact that it is the most important prognostic factor surpassing the histologic classification [9]. The staging of TET is based on invasion, implants, lymph node involvement, and/or distant metastases. Fourteen different staging systems have been proposed in literature, of which the Masaoka-Koga and the TNM staging systems are more commonly followed worldwide. The Masaoka staging system [10] was developed in 1981 keeping in mind that all thymomas may be potentially malignant and that their prognosis may be determined by their stage. Later on, Koga in 1994 [10] recommended a modification to this classification wherein a tumor invading into the capsule but not breaking through it was categorized as stage I whereas that infiltrating into normal thymic tissue as a result of transcapsular invasion was classified as stage II.  Also, tumors invading into the pleura or pericardium were categorized as stage III based on the fact that only a thin layer of fibrous tissue exists between the thymus and the mediastinal pleura or pericardium, making it difficult to discriminate invasion into the adjacent organs from fibrous adhesion to the pleura or pericardium. The modified Masaoka-Koga staging is represented in Table 7.1. In 2009, the International Thymic Malignancy Interest Group (ITMIG) and the International Association for the

© Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_7

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Table 7.1 Modified Masaoka-Koga staging of thymic epithelial tumors [10] Stage I Stage II  IIa  IIb

Stage III Stage IV  IVa  IVb

Grossly and microscopically completely encapsulated Microscopic transcapsular invasion Macroscopic invasion into thymic or surrounding fatty tissue or grossly adherent to (but not breaking through) mediastinal pleura or pericardium Macroscopic invasion into neighboring organ, i.e., pericardium, great vessels, or lung Pleural or pericardial metastasis Lymphatic or hematogenous metastasis

Table 7.2  TNM staging [12] Primary tumor (T) TX Primary tumor cannot be assessed T0 No evidence of primary tumor T1 Tumor encapsulated or extending into the mediastinal fat, may involve the mediastinal pleura  T1a •  Tumor with no mediastinal pleura involvement  T1b •  Tumor with direct invasion of mediastinal pleura T2 Tumor with direct invasion of the pericardium (either partial or full thickness) T3 Tumor with direct invasion into any of the following: lung, brachiocephalic vein, superior vena cava, phrenic nerve, chest wall, or extrapericardial pulmonary artery or veins T4 Tumor with invasion into any of the following: aorta (ascending, arch, or descending), arch vessels, intrapericardial pulmonary artery, myocardium, trachea, esophagus Regional lymph node (N) NX Regional lymph nodes cannot be assessed N0 No regional lymph nodes metastasis N1 Metastasis in anterior (perithymic) lymph nodes N2 Metastasis in deep intrathoracic or cervical lymph nodes Distant metastasis (M) M0 No pleural, pericardial, or distant metastasis M1 Pleural, pericardial, or distant metastasis  M1a Separate pleural or pericardial nodule(s)  M1b Pulmonary intraparenchymal nodule or distant organ metastasis

Study of Lung Cancer (IASLC) formulated a consistent staging system for thymic tumors which would be easily followed worldwide. The classification system proposed by them was eventually accepted by the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC)—the bodies responsible for defining stage classifications throughout the world [11, 12]. The TNM (tumor, node, metastasis) staging as proposed is described in Table 7.2. The AJCC prognostic stage grouping for the above TNM classification is given in Table 7.3.

Table 7.3  AJCC prognostic stage grouping [12] Stage I Stage II Stage IIIA Stage IIIB Stage IVA Stage IVB

7.4

T1a, T1b T2 T3 T4 Any T Any T Any T Any T

N0 N0 N0 N0 N1 N0, N1 N2 Any N

M0 M0 M0 M0 M0 M1a M0, M1a M1b

Type A Thymoma

7.4.1 Epidemiology and Clinical Features Type A thymoma is a thymic epithelial tumor comprising of bland spindled/oval cells with few admixed immature lymphocytes. It is an uncommon subtype of thymomas. It has a slight female predominance and has a wide age range from 8–88 years. Type A thymomas are proposed to have originated from a thymic epithelial precursor with a potential for cortico-medullary differentiation. Paraneoplastic syndromes are less frequent with this subtype with approximately 20% which have associated myasthenia gravis [13]. On imaging, these are smaller with smooth distinct borders and show low FDG uptake on FDG PET-CT [14]. They are commonly low stage (modified Masaoka-­ Koga stage I–II) tumors with good prognosis [15].

7.4.2 Pathological Features Grossly, type A thymomas are well circumscribed or encapsulated. They have a homogenous, light tan to white cut surface with some lobulations (Fig. 7.1). Microscopically, these tumors have an incomplete or complete capsule with thick fibrous bands separating the parenchyma into lobulations. The tumor cells are arranged in a variety of patterns including glandular, fascicular, storiform, and hemangiopericytoma like (Figs. 7.2, 7.3, 7.4, and 7.5). Rosettes, whorls, microcystic change, and occasionally papillary arrangement are also seen (Figs. 7.6, 7.7, 7.8, and 7.9) [14]. Due to variety of patterns seen in type A thymoma, the differential diagnosis may range from adenocarcinoma to carcinoid/neuroendocrine tumor to sarcomas if a clinical history and site of the biopsy is not provided. Hassall corpuscles are absent. The tumor cells are spindled to oval with bland nuclei, fine chromatin, and inconspicuous nucleoli (Fig.  7.10). Mitotic activity is low that is   =  4/10 high power fields), and focal areas of coagulative necrosis (Figs.  7.13, 7.14, 7.15, 7.16, 7.17, 7.18, and 7.19) [5, 14].

7.4.3 Atypical Type A Thymoma

7.5.1 Epidemiology and Clinical Features

In addition to the above described features, the atypical features present in atypical type A thymoma are ­hypercellularity,

Type AB thymoma is a thymic epithelial tumor composed of a dual population of spindled epithelial cells and immature T lym-

7.5

Type AB Thymoma

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Fig. 7.5  Photomicrograph of type A thymoma with a hemangiopericytoma-­ like pattern of arrangement of epithelial cells (H&E ×200). Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

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Fig. 7.8  Type A thymoma with a prominent reticular pattern of arrangement of epithelial cells (H&E ×40)

Fig. 7.6  Type A thymoma with pseudorosettes (H&E ×100)

Fig. 7.9  Type A thymoma with areas of pseudopapillary formations (H&E ×40). Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

phocytes in variable proportions. Its incidence worldwide ranges from 15 to 43%. The patients are younger than that of type A thymoma. Of these patients 18–20% present with myasthenia gravis [13]. Most type AB thymomas are lower-stage tumors (stages I and II) [17].

7.5.2 Pathological Features Fig. 7.7  Thymoma with formation of pseudorosettes. (H&E ×200). Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

Grossly, these are commonly encapsulated tumors with a nodular cut surface having tan-colored nodules of varying sizes (Fig. 7.20).

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Fig. 7.10  High power view of type A thymoma shows spindled epithelial cells with elongated oval nuclei, bland chromatin, and inconspicuous nucleoli (H&E ×400)

Fig. 7.12  Type A thymoma with adjacent focus of micronodular thymoma with lymphoid stroma (arrow) (H&E ×40)

Fig. 7.11  A type A thymoma with spindled epithelial cells in sheets at the right half of the figure along with presence of micronodular thymoma with lymphoid stroma at the left half comprising of tumor cells in small nodules separated by lymphocyte rich stroma (H&E ×40)

Fig. 7.13  Atypical type A thymoma shows a nodular arrangement of spindled epithelial cells separated by dense sclerotic stroma (H&E ×40)

Microscopically, type AB thymomas show a lobulated growth comprising of a variable mixture of lymphocyte-poor epithelial cells (type A-like) and a lymphocyte-rich type B-like area which may form separate nodules or may be intermingled (Fig. 7.21). The type A-like areas are composed of spindled epithelial cells in fascicles coursing around type B-like areas. The tumor cells of type B areas are small, oval to polygonal and have bland chromatin and inconspicuous nucleoli (Figs.  7.22, 7.23, 7.24, 7.25, 7.26, 7.27, 7.28, and 7.29). The lymphocytes are immature T-cells which are TdT + and are either difficult to count or countable in >10% of tumor area [18]. Hassall corpuscles are absent. Immunohistochemically, the type A cells are positive for p63, PAX8, and FOXN1 and negative for CD5 and CD117. They fre-

quently express CD20 focally. The epithelial cells present in the type B areas are CK14+. Ki-67 proliferation index is low.

7.6

 ype B1 Thymoma (Blue on Low T Magnification)

7.6.1 Epidemiological and Clinical Features Type B1 thymomas form approximately 17–20% of all thymomas and have a female predominance [19]. It is most commonly seen in the fifth to sixth decades. Clinically, about a third of the patients are asymptomatic. Others develop local symptoms such as chest pain, cough, and dyspnea. The incidence of myasthenia gravis is more than that in type A

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Fig. 7.14  Atypical type A thymoma with predominantly glandular pattern (H&E ×40)

Fig. 7.15  Atypical type A thymoma with epithelial cells arranged in a trabecular pattern (H&E ×100)

thymomas and occurs in about 44% patients [20]. About 50% of the tumors are in stage I. They are generally encapsulated and extension to adjacent structures and pleural dissemination are rare.

7.6.2 Pathological Features Grossly, the type B1 thymomas are usually encapsulated and have a nodular external surface. The cut surface is soft, smooth, and tan-pink in color (Fig. 7.30). Microscopically, type B1 thymomas have a thymus-like architecture where the cortical areas are the predominant

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Fig. 7.16  Atypical A thymoma showing nuclear atypia in the form of nuclear enlargement, vesicular chromatin, and mitotic activity (H&E ×200)

Fig. 7.17  Atypical A thymoma stained diffusely with p40 immunostain (strong nuclear positivity) (p40 ×40)

population. The lobules, if present, are larger than normal thymus and are separated by fibrous septae. The neoplastic epithelial cells are barely visible and are embedded in a nonneoplastic immature lymphoid population (Figs. 7.31, 7.32, and 7.33). The epithelial cell clusters, if seen, should be less than three contiguous epithelial cells to designate the lesion as type B1 thymoma. The epithelial cells have oval to rounded nuclei with pale chromatin and small conspicuous nucleoli. Pale nodular areas commensurate with medullary foci are always present (Figs.  7.34 and 7.35). These areas have increased B-cells and mature T-cells (Fig. 7.36). Hassall corpuscles are also seen in these areas. Perivascular spaces may also be found in this subtype of thymoma [21].

7  Histomorphology of Thymomas

Fig. 7.18  Atypical A thymoma shows paucity of TdT-positive (comprising of less than 10% of tumor area) lymphocytes (TdT x40)

Fig. 7.19  Ki-67 (MIB1 labeling index) staining of atypical A thymoma which shows increased proliferation with 10–20% labeling (Ki-­67 ×40)

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Fig. 7.20  Gross image of type AB thymoma showing a nodular cut surface having tan-colored nodules of varying sizes. Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

Fig. 7.21  Low power view of type AB thymoma with sheets of epithelial cells and admixed lymphocytes separated by thick fibrovascular septae (H&E ×40)

Distinctive features of type B1 thymoma are as follows: 1 . Close resemblance to normal thymus 2. Non-involuted thymic cortex 3. Presence of medullary islands Type B2 and B3 thymomas may be admixed with type B1 thymomas. Immunohistochemically, the epithelial cells are diffusely positive for CK19 in a delicate network pattern in both the medullary islands and cortical areas (Fig. 7.37). CK20 is negative. All cases express p63 and PAX8. Lymphocytes are mostly immature T-cells expressing TdT, CD3, CD1a, CD4, and CD8. The lymphocytes in the med-

ullary islands are CD3+, either CD4 or CD8 positive, and CD1a-; admixed with B-cells which are CD20+ and CD79a+.

7.7

Type B2 Thymoma

7.7.1 Epidemiology and Clinical Features Type B2 thymomas are lymphocyte-rich tumors composed of polygonal neoplastic cells, which form small clusters, the density of which is higher than type B1 thymomas, in a background of immature T-cells. These form approximately a

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Fig. 7.22  Type AB thymoma with the epithelial cells exhibiting a whorling pattern (arrows) (H&E ×40)

Fig. 7.23  Another case of type AB thymoma shows admixture of epithelial cells and lymphocytes (H&E ×40)

third of all thymomas [22]. They are found in adults. Clinical features vary from being asymptomatic to having local symptoms. Myasthenia gravis is a little more frequent in these thymomas (up to 54%) [20]. The type B2 thymomas are commonly seen infiltrating the surrounding fat as well as pleural space.

7.7.2 Pathological Features Grossly, these tumors may be encapsulated or may invade the adjacent structures. The cut surface is lobulated, soft to firm, and gray-white with areas of necrosis, cystic change, and/or hemorrhage (Fig. 7.38).

P. Guleria and D. Jain

Fig. 7.24  Type AB thymoma with an area of entrapped adipocytes (H&E ×40)

Fig. 7.25  Type AB thymoma with sheets of spindled thymic epithelial cells and thymocytes (thymic lymphocytes) separated by short thick bundles of fibrous septae (H&E ×40)

Microscopically, the type B2 thymomas have lobular architecture with abundance of lymphoid cells surrounded by a fibrous tumor capsule (Figs. 7.39, 7.40, 7.41, 7.42, 7.43, and 7.44). Interspersed among the lymphoid cells are epithelial cells arranged singly or in clusters of >  =  3 cells (Figs.  7.45, 7.46, and 7.47) [22]. The epithelial cells have round to oval nuclei with vesicular chromatin and small prominent nucleoli. Another typical feature of type B2 thymoma is the presence of perivascular spaces comprising of a central venule surrounded by a clear space containing proteinaceous fluid (Fig.  7.48). Hassall corpuscles are seen (Fig. 7.49). The medullary islands are not found. Associated areas of type B1 and B3 thymomas may be found.

7  Histomorphology of Thymomas

Fig. 7.26  A case of type AB thymoma with spindled thymic epithelial cells enmeshed within a fair number of lymphocytes (H&E ×40)

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Fig. 7.28  Closer view of thymic epithelial cells in a case of type AB thymoma. Cyst macrophages are also seen within the fluid-containing spaces. Lymphocytes were seen in some other areas of the tumor (H&E ×200)

Fig. 7.27  Type AB thymoma with areas of fluid-containing pseudo-­ glandular spaces (H&E ×40)

Immunohistochemically, the cytokeratin-positive network of epithelial cells is denser than type B1 (Fig.  7.50) surrounded by TdT+ immature T-cells.

7.8

 ype B3 Thymoma (Pink on Low T Magnification)

7.8.1 Epidemiology and Clinical Features Type B3 thymomas are thymic epithelial tumors composed predominantly of polygonal epithelial cells in solid sheets displaying mild to moderate atypia along with intermixed nonneoplastic immature T-cells. The incidence of these

Fig. 7.29  High power view of type AB thymoma showing thymic epithelial cells with clear cytoplasm and well-defined cell borders admixed with fair number of lymphocytes (H&E ×200)

tumors varies with geographical location, being more common in Asian countries (30%) as compared to the West (15– 17%) [19]. They have a mean age of presentation of 55 years and show a slight male predominance. Most patients have local symptoms or superior vena cava syndrome. Myasthenia gravis is seen in around 50% of the cases [20].

7.8.2 Pathological Features Grossly, these tumors are poorly circumscribed with extensions into the surrounding mediastinal fat and adjacent struc-

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Fig. 7.30  Type B1 thymoma with a lobulated cut surface which appears soft smooth and tan pink in color along with areas of hemorrhage. Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine Fig. 7.33  Type B1 thymoma showing immature lymphocytes admixed with foamy histiocytes indicating xanthomatous change (H&E ×100)

Fig. 7.31  Type B1 thymoma showing intermixed perivascular lakes containing lymph as well as areas of hemorrhage (H&E ×40) Fig. 7.34  Low power view of type B1 thymoma showing pale staining medullary islands surrounded by dark staining cortical regions (H&E ×40)

Fig. 7.32  Type B1 thymoma with perivascular spaces (empty spaces around blood vessels) (H&E ×100). Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

Fig. 7.35  Medium power view of type B1 thymoma showing pale medullary islands surrounded by dark cortical regions (H&E ×100)

7  Histomorphology of Thymomas

Fig. 7.36  High power view of type B1 thymoma shows predominantly lymphoid component and no easily identifiable epithelial cell clusters (H&E ×400)

Fig. 7.37  Immunohistochemical stain of pan-cytokeratin in a case of type B1 thymoma highlighting a delicate network of epithelial cells intermixed with lymphoid cells (Pan-CK ×100)

Fig. 7.38  Type B2 thymoma with a lobulated cut surface which is soft to firm and appear gray-white with areas of necrosis, cystic change, and/or hemorrhage. Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

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Fig. 7.39  Type B2 thymoma shows tumor present in lobules separated by fibroadipose tissue (H&E ×40)

Fig. 7.40  Section from type B2 thymoma shows nodules of epithelial and lymphoid cells separated by fibrous stroma (H&E ×40)

tures (Fig.  7.51). Rare encapsulated and cystic forms are recognized [23]. The cut surface appears firm, gray to yellow in color with nodular appearance. There may be associated necrosis and hemorrhage. Microscopically, the tumor shows a lobular architecture separated by fibrous septae and has pushing borders (Fig. 7.52). The tumor cells are present in solid sheets and are polygonal with eosinophilic to clear cytoplasm, round to oval nuclei with atypia and sometimes prominent nucleoli (Figs. 7.53, 7.54, 7.55, 7.56, 7.57, 7.58, 7.59, 7.60, and 7.61). There is paucity of intermixed immature lymphocytes. In addition, there are prominent perivascular spaces with epithelial palisading. Hassall corpuscles are rarely found. There may be coexisting areas of type B2 thymomas or thymic carcinomas [24].

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Fig. 7.41  Type B2 thymoma displays nodules of epithelial cells with surrounding lymphoid stroma infiltrating into the surrounding adipose tissue (H&E ×40)

Fig. 7.43  Type B2 thymoma with capsular involvement (inked surface) (H&E ×40)

Fig. 7.42  Type B2 thymoma comprising of admixture of sheets of epithelial cells along with lymphocytes in lobular architecture separated by fibrous septae (H&E ×40)

Fig. 7.44  Type B2 thymoma with lobular arrangement of tumor cells comprising of epithelial and lymphoid cells admixed together and infiltrating into the peritumoral adipose tissue (H&E ×40)

Immunohistochemically, the tumor cells are positive for pancytokeratin (Fig. 7.62), CK19, CK5/6, and CK7 and negative for CK20. They are also positive for p63, for PAX8, and focally for EMA. They are consistently negative for CD20, CD5, and CD117. Immature T-cells, if present, are positive for TdT.

include all the histologic subtypes mentioning the predominant pattern followed by minor components in 10% increments. Most common combination is B3 and B2. The rule does not apply to type AB thymomas. In case thymomas accompany a thymic carcinoma, then the entity is labeled thymic carcinoma irrespective of percentage of carcinoma component. Though, it is advisable to write percentage of each histologic type in the pathology report. The existence of this heterogeneity emphasizes the need for extensive sampling of the tumors. Some cases of such ­combinations of different thymoma subtypes have been shown in Figs. 7.63, 7.64, 7.65, 7.66, and 7.67.

7.9

Heterogeneous Thymomas

Thymomas with more than one histological pattern were earlier labeled as combined thymomas, a term which is no longer recommended to be used [5]. The diagnosis should

7  Histomorphology of Thymomas

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Fig. 7.45  Type B2 thymoma shows intricate admixture of epithelial cells and lymphocytes. The epithelial cells are polygonal with round to oval nuclei, pale chromatin, and prominent nucleoli (H&E ×100)

Fig. 7.47  Type B2 thymoma with epithelial cells having round to slightly oval nuclei, distinct nuclear membrane, pale chromatin, and conspicuous nucleoli (H&E ×400)

Fig. 7.46  Type B2 thymoma demonstrates polygonal epithelial cells having round to oval nuclei with pale chromatin, distinct nuclear membrane, and conspicuous nucleoli (H&E ×400)

Fig. 7.48  Type B2 thymoma with presence of large perivascular spaces containing lymph-like proteinaceous material (H&E ×40)

7.10 Rare Types of Thymoma 7.10.1 Micronodular Thymoma 7.10.1.1 Epidemiology and Clinical Features Micronodular thymoma with lymphoid stroma is a rare thymic epithelial tumor composed of multiple small tumor islands of spindled or oval epithelial cells surrounded by a lymphocyte-rich stroma. It accounts for only about 1% of all thymic epithelial tumors [6]. They have a slightly male predominance. The patients are usually asymptomatic and the

tumor is generally detected incidentally. Most of them are localized and encapsulated.

7.10.1.2 Pathological Features Grossly, the tumors are well circumscribed and encapsulated. The cut surface is soft and friable, homogenous, and light tan in color. Microscopically, these tumors are characterized by multiple discrete small nests or solid islands of epithelial cells separated by a lymphocyte-rich stroma (Figs.  7.68, 7.69, 7.70, and 7.71). The lymphoid areas may even contain lymphoid follicles with or without germinal centers and plasma

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Fig. 7.49  Type B2 thymoma with admixture of epithelial and lymphoid cells along with a Hassall corpuscle in the center (H&E ×200)

Fig. 7.50  Type B2 thymoma with immunohistochemical stain of pan-­ cytokeratin depicting epithelial cell clusters amidst a background of lymphoid cells (Pan-CK ×400)

cells. The epithelial cells within the nodules are short ­spindled or oval with scant cytoplasm, elongated nuclei with dispersed chromatin, and inconspicuous nucleoli. Other associated findings may be micro- or macrocystic change, rosette-like structures, or glandular formation. However, a lobular architecture, Hassall corpuscles, or perivascular spaces are generally absent. There may be associated type A thymoma in about 30% cases [25]. Association with type AB and B2 thymomas is rare. Immunohistochemically, the epithelial nodules stain for pan-cytokeratin, CK5/6, and CK19 and lack CK20. The lymphoid cells are an admixture of mature B-cells

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Fig. 7.51  Gross photograph of type B3 thymoma having a variegated cut surface with nodules, areas of hemorrhage, and extension into the surrounding soft tissues. Photograph courtesy: Dr. Mark R Wick, Department of Pathology, UVA School of Medicine

Fig. 7.52  Type B3 thymoma shows predominantly epithelial sheets with surrounding fibrous stroma (H&E ×40)

(CD20 and CD79a positive), mature T-cells (CD3+, TdT−), and immature T-cells (CD3+, CD1a+, TdT+, CD99+). The TdT-­positive cells are scarce within the epithelial nodules which helps in distinguishing from type AB thymomas in which they are intermixed with the epithelial cells [5].

7.10.2 Sclerosing Thymoma Sclerosing thymoma does not seem to be a distinct subtype of thymoma. It represents extensive sclerosis and hyalinization in any type of above described thymomas [26].

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Fig. 7.53  Type B3 thymoma, a pink tumor, shows epithelial cells (H&E ×40)

Fig. 7.55  Type B3 thymoma shows sheets of polygonal epithelial cells with hyalinized blood vessels (H&E ×40)

Fig. 7.54  Another case of type B3 thymoma with sheets of polygonal epithelial cells and a paucity of lymphoid cells (H&E ×40)

Fig. 7.56  Type B3 thymoma with adjacent areas of fibrosis and cholesterol clefts (H&E ×40)

7.10.2.1 Pathological Features Grossly, the tumors are well circumscribed, with a light tan cut surface which is firm to hard in consistency (Fig. 7.72). Microscopically, the tumor is composed of a predominantly hyalinized, fibrosclerotic stroma. The neoplastic thymic epithelial cells may not be detected on H&E staining and there may be a paucity of immature T-cells. Occasionally, areas of conventional thymomas are seen (Figs.  7.73, 7.74, 7.75, 7.76, 7.77, 7.78, and 7.79) [27]. These tumors are associated with degenerative changes in the form of dystrophic calcification, cholesterol granulomas, and cystic change.

Immunohistochemically, the epithelial cells are pan-­ cytokeratin+ and the immature T-cells, if present, are TdT+.

7.10.3 Metaplastic Thymoma 7.10.3.1 Epidemiology and Clinical Features Metaplastic thymomas are extremely rare thymoma subtypes [28] which have a biphasic pattern comprising of solid epithelial areas in a background of spindle cell proliferation which appear bland. They are common in adults. These tumors are generally incidentally detected or the patients

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Fig. 7.57  Type B3 thymoma showing epithelial cells in sheets with sparsely distributed lymphocytes (H&E ×200)

Fig. 7.59  Type B3 thymoma shows epithelial cells with nuclear atypia in the form of nuclear enlargement and hyperchromasia (H&E ×400)

Fig. 7.58  Type B3 thymoma showing perivascular spaces filled with foamy histiocytes (H&E ×100)

Fig. 7.60  Type B3 thymoma shows focal clearing of cytoplasm in epithelial cells (H&E ×100)

may have localized symptoms. They are commonly lower-­ moid and have moderate eosinophilic cytoplasm with oval to stage tumors. rounded nuclei which may sometime exhibit pleomorphism. The spindle cell stroma is intermixed with the epithelial 7.10.3.2 Pathological Features islands and is seen in short intersecting fascicles (Figs. 7.80, Grossly, these tumors may be encapsulated or well circum- 7.81, 7.82, 7.83, 7.84, 7.85, and 7.86). There is an absence or scribed. They have a homogenous cut surface which appears paucity of lymphoid cells in the tumor [28, 29]. yellow to gray-white in color. Immunohistochemically, the epithelial cells are positive Microscopically, the tumor shows a biphasic pattern com- for epithelial membrane antigen (EMA), cytokeratin, and prising of epithelial and stromal components. The epithelial p63 and the spindled cells show positivity for vimentin. The component may be present in solid sheets, trabeculae, or spindle cells may show focal EMA or cytokeratin positivity anastomosing islands. The epithelial cells may appear squa- (Figs. 7.87, 7.88, 7.89, and 7.90).

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Fig. 7.61  Type B3 thymoma possesses epithelial cells with distinct cytoplasmic borders, clear cytoplasm, oval nuclei, vesicular chromatin, and prominent nucleoli (H&E ×400)

Fig. 7.63  A case of thymoma with type A areas (right half) and type B2 areas (left half) (H&E ×40)

Fig. 7.62  Immunohistochemical stain of pan-cytokeratin outlining the thymic epithelial cells in a case of type B3 thymoma (pan-cytokeratin ×200)

Fig. 7.64  High power view of thymoma showing spindled epithelial cells of type A area with adjacent focus of type B2 area (H&E ×100)

7.10.4 So-called Microscopic Thymoma or Nodular Hyperplasia of Thymic Epithelium These are multifocal unencapsulated non-neoplastic thymic epithelial proliferations of 50%), CD5/CD117 negative, clinical correlation (rapid growth, history of thyroid nodules). Poorly differentiated thyroid carcinoma: CD5 negative, thyroid follicular cell markers (TTF-1, thyroglobulin) positive. Most of the carcinomas from the differential diagnosis list, either thyroid origin or metastatic, are high-grade malignancies while ITC is a morphologically and clinically low-grade carcinoma. Ectopic thymoma: no pleomorphism and cytologic features of malignancy, abundant lymphocytic component, absent/rare expression of CD117.

11.3.4.2 Cytology • Poorly differentiated thyroid carcinoma: close mimicker, immunostaining (thyroid and thymic markers) can be helpful. • MTC: dyscohesive, salt-and-pepper chromatin, no lymphocytes, calcitonin. • Anaplastic thyroid carcinoma: prominent pleomorphism. • Metastatic SCC: more atypical. • Lymphoma: no epithelial clusters. • SETTLE: more spindled, no lymphocytes.

11.4 Spindle Epithelial Tumor with Thymus-Like Differentiation (SETTLE)

Fig. 11.10  Low-power view of ITC. The tumor (right) is demarcated from benign thyroid (left) but there is no capsule formation (a). Lobulated appearance due to irregular invasion and compression of the outer fibrous tissue (b). Sometimes necrosis can be prominent, up to geographic and confluent (c). Hematoxylin and eosin, ×10 (a–b: bar, 1 mm), ×40 (c)

Spindle epithelial tumor with thymus-like differentiation or elements (SETTLE) is a rare biphasic malignant tumor of the thyroid composed of spindle epithelial cells that merge into glandular structures [9]. Histogenesis of SETTLE is uncertain with speculations revolving around a branchial pouch or thymic origin. It is a low-grade malignancy manifested as thyroid or neck mass in children and adolescents, with a male predilection [6]. Cervical lymph nodes involvement is infrequent, reported in 10% of cases. It may also develop delayed blood-borne metastases with latency from few to 20–35 years after the primary diagnosis [17]. Surgical resection added by chemoradiotherapy for metastatic cases is usually effective in controlling disease and achieving long-term survival [18].

11  Pathology of Ectopic Thymic Tumors

Fig. 11.11  Histological features of ITC. Fibrous bands between solid nests of cancer cells (a). Invasion into the adjacent thyroid (b). Associated lymphoid infiltrate can be abundant, with the formation of

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lymphoid follicles (c). Perineural invasion (d). Local invasion (b, d) is a characteristic feature of ITC. Hematoxylin and eosin, ×100

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Fig. 11.12  High-power appearance of ITC. Solid epithelial growth in a background of lymphocyte-rich stroma (a). Enlarged nuclei with prominent nucleoli (b). Densely packed neoplastic epithelial cells (c). Cell appearance can be ranged from squamoid with abundant eosino-

philic cytoplasm (b) to basaloid (c). Coagulative necrosis (lower left) (d). Mitotic activity (arrowheads) with atypical mitoses (e). Cancer cells with pleomorphism (f). Hematoxylin and eosin, ×400 (a, e), ×600 (b–c), ×200 (d, f)

The tumor is grossly encapsulated, partially circumscribed, or infiltrative, with a mean size of 4  cm. The cut surface is firm, greyish white to tan, and vaguely lobulated (Fig. 11.16).

• Spindle cells arranged in interlacing fascicles or reticular structures are usually observed as a predominant population (Fig. 11.17a–c). • The glandular component appears as tubules, papillae, cords, cystic spaces, mucinous and glomeruloid glands (Fig. 11.17d–e). • Both components are blended imperceptibly. • Spindle epithelial cells are bland and monomorphic; glandular cells are variable in shape (flat, cuboidal, columnar) and are sometimes mucinous or ciliated (Fig. 11.17d–e).

11.4.1 Histopathology • Highly cellular biphasic tumor with fibrous septa producing a vaguely lobulated pattern.

11  Pathology of Ectopic Thymic Tumors

Fig. 11.13  Fine-needle aspiration cytology of ITC. Large trabecular cluster with no follicular or papillary patterns (a). The spindle-shaped carcinoma cells with sparsely scattered small lymphocytes in the background (b). Tightly arranged tumor cells with dense and wide cytoplasm suggesting squamous differentiation (c). Loosely cohesive tumor

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cells have small round nuclei and cyanophilic narrow cytoplasm (d). Papanicolaou stain, ×100 (a), ×400 (b–d). [Fig. 11.13b is reproduced from Fig. 41.3 in https://doi.org/10.1007/978-981-13-1897-9_41 published by Springer]

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Fig. 11.14  Main immunohistochemical markers of ITC. CD5 immunoreactivity well-appreciated on low magnification (a). A delicate membranous staining pattern with CD5 in carcinoma cells (b). CD117

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(c-Kit) revealed membranous expression (c). Nuclear positivity of cancer cells with p63 antibody (d). Immunohistochemistry, ×100 (a, d), ×400 (b), ×200 (c)

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Fig. 11.15 Extended immunophenotype of ITC.  High molecular weight cytokeratin antibody (clone 34 beta E12) revealed diffuse expression in carcinoma cells (a). Strong expression of bcl-2 in large neoplastic epithelial cells and small-sized lymphocytes (b). Focal

expression of EMA (c) and CEA (d). Heterogeneous nuclear expression of PAX8 (e). Diffuse positivity for p53 (f). Immunohistochemistry, ×100 (a–d, f), ×200 (e)

• Occasionally seen mitotic activity and squamous differentiation (Fig. 11.17f). • Stroma is hyalinized, with scant lymphocytes (Fig. 11.17b). • Most cases are biphasic, but monophasic variants (spindle or glandular) have been reported. • Separated from the adjacent thyroid tissue by a capsule or sclerotic rim (Fig. 11.17a). • Invasive growth with vascular invasion can be rarely identified (Fig. 11.17a).

11.4.2 Cytology • Cellular smear with large cohesive solid/trabecular cell clusters. • Spindle cells are less cohesive, with scanty cytoplasm and uniform elongated nuclei (Fig. 11.18). • Cohesive clusters of glandular/epithelial cells with moderate-­ to-abundant cytoplasm and variable-sized mildly pleomorphic oval nuclei.

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• Fine-needle aspiration biopsy is rarely diagnostic for SETTLE [18, 19].

11.4.3 Immunohistochemistry

Fig. 11.16  Gross appearance of SETTLE. A 4 cm large greyish white tumor occupied almost entire lobe of thyroid

Fig. 11.17  Histological features of SETTLE.  Encapsulated tumor demarcated from the adjacent thyroid tissue (left) (a). Stromal hyalinization (b). Monophasic spindle cell pattern resembling monophasic synovial sarcoma (c). Biphasic pattern with spindle cells and pseudo-

• Both spindle and glandular cells diffusely express high molecular weight cytokeratin and CK7, but only focally low molecular weight cytokeratin (Fig. 11.19) [6]. • Nuclear expression of p63 and p40, predominantly in spindle cells (Fig. 11.19b, d). • Variable expression of vimentin, CD5/CD117 (CD117 > CD5), myoepithelial and neuroendocrine markers. • Negative for thyroid-specific markers (TTF-1, thyroglobulin) and calcitonin. • Different series and case reports provide contradictory results regarding immunoexpression of ancillary (vimentin, CD5/CD117) and even main (cytokeratins, p63) markers

glandular structures (d). Uniform spindle cells without evident atypia (e). Tubular duct with squamous metaplasia (f). Hematoxylin and eosin, ×100 (a–c), ×200 (d–f). Figure 11.17 a, b, c are reproduced from Fig. 1 g, h, j in https://doi.org/10.1111/cyt.12742 Ref. [19]

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Fig. 11.17 (Continued)

Fig. 11.18  Cytologic appearance of SETTLE. Epithelial cells arranged in spindle-shaped and whorled formations. Elongated moderately enlarged nuclei with tiny nucleoli. Papanicolaou stain, ×400

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Fig. 11.19  Immunoprofile of SETTLE. Cytoplasmic expression of CK5/6 (a) and CK7 (c) accompanied by diffuse nuclear expression of p40 (b, d). Thyroid follicles are negative for p40 (d, bottom). Immunohistochemistry, ×100 (a, b, d), ×200 (c)

11  Pathology of Ectopic Thymic Tumors

11.4.4 Differential Diagnosis • Synovial sarcoma (main mimicker): high-grade morphology (pleomorphism, necrosis), frequent mast cells, no diffuse expression of cytokeratins, SS18-SSX translocation • Ectopic thymoma: prominent lobulation, abundant T lymphocytes • ITC: squamoid morphology, lymphocyte-rich background, higher mitotic activity, essentially CD5 positive • Ectopic hamartomatous thymoma: prominent anastomosing syringoma-like networks, squamoid features, myoepithelial component, admixed adipocytes • MTC, spindle cell variant: amyloid, expression of TTF-1, and calcitonin • Anaplastic thyroid carcinoma: high-grade morphology, high Ki-67 labeling index (> 50%), clinical correlation (rapid growth)

References 1. Prabhu AV, Kale HA, Branstetter BFT. Residual cervical thymus: a normal CT finding that may be present throughout patients’ lives. AJNR Am J Neuroradiol. 2015 Aug;36(8):1525–8. 2. Carpenter GR, Emery JL. Inclusions in the human thyroid. J Anat. 1976 Sep;122(Pt 1):77–89. 3. Fukushima T, Suzuki S, Ohira T, Shimura H, Midorikawa S, Ohtsuru A, et  al. Prevalence of ectopic intrathyroidal thymus in Japan: the Fukushima health management survey. Thyroid. 2015 May;25(5):534–7. 4. Kim HG, Kim MJ, Lee MJ.  Sonographic appearance of intrathyroid ectopic thymus in children. J Clin Ultrasound. 2012 Jun;40(5):266–71. 5. Weissferdt A, Moran CA.  The spectrum of ectopic thymomas. Virchows Archiv. 2016 Sep;469(3):245–54. 6. Lloyd RV, Osamura RY, Klöppel GK, Rosai J. WHO classification of tumours. Pathology and genetics of tumours of endocrine organs. 4th ed. Lyon: IARC Press; 2017. 7. Travis WD, Brambilla E, Burke AP, Marx A, Nicholson AG. WHO classification of tumours of the lung, pleura, thymus and heart. 4th ed. Lyon: IARC Press; 2015.

167 8. Ali SZ, Cibas ES. The Bethesda system for reporting thyroid cytopathology: definitions, criteria, and explanatory notes. 2nd ed. New York: Springer; 2018. 236 p. 9. Chan JK, Rosai J. Tumors of the neck showing thymic or related branchial pouch differentiation: a unifying concept. Hum Pathol. 1991 Apr;22(4):349–67. 10. Ge W, Yao YZ, Chen G, Ding YT.  Clinical analysis of 82 cases of carcinoma showing thymus-like differentiation of the thyroid. Oncol Lett. 2016 Feb;11(2):1321–6. 11. Ito Y, Miyauchi A, Nakamura Y, Miya A, Kobayashi K, Kakudo K.  Clinicopathologic significance of intrathyroidal epithelial thymoma/carcinoma showing thymus-like differentiation: a collaborative study with Member Institutes of The Japanese Society of Thyroid Surgery. Am J Clin Pathol. 2007 Feb;127(2):230–6. 12. Kakudo K, Bai Y, Ozaki T, Homma K, Ito Y, Miyauchi A.  Intrathyroid epithelial thymoma (ITET) and carcinoma showing thymus-like differentiation (CASTLE): CD5-positive neoplasms mimicking squamous cell carcinoma of the thyroid. Histol Histopathol. 2013 May;28(5):543–56. 13. Collins JA, Ping B, Bishop JA, Ali SZ. Carcinoma showing thymus-­ like differentiation (CASTLE): cytopathological features and differential diagnosis. Acta Cytol. 2016;60(5):421–8. 14. Hirokawa M, Kuma S, Miyauchi A. Cytological findings of intrathyroidal epithelial thymoma/carcinoma showing thymus-like differentiation: a study of eight cases. Diagn Cytopathol. 2012 May;40(Suppl 1):E16–20. 15. Rajeshwari M, Singh V, Nambirajan A, Mridha AR, Jain D. Carcinoma showing thymus like elements: report of a case with EGFR T790M mutation. Diagn Cytopathol. 2018 May;46(5):413–8. 16. Weissferdt A, Moran CA.  Immunohistochemistry in the diagnosis of thymic epithelial neoplasms. Appl Immunohistochem Mol Morphol. 2014 Aug;22(7):479–87. 17. Ippolito S, Bellevicine C, Arpaia D, Peirce C, Ciancia G, Vigliar E, et al. Spindle epithelial tumor with thymus-like differentiation (SETTLE): clinical-pathological features, differential pathological diagnosis and therapy. Endocrine. 2016 Mar;51(3):402–12. 18. Recondo G Jr, Busaidy N, Erasmus J, Williams MD, Johnson FM.  Spindle epithelial tumor with thymus-like differentiation: a case report and comprehensive review of the literature and treatment options. Head Neck. 2015 May;37(5):746–54. 19. Nambirajan A, Singh V, DVK I, Agarwal S, Jain D.  Spindle epithelial tumor with thymus-like differentiation of thyroid presenting with lymph node metastasis: an illustrative case report with review of literature. Cytopathology. 2019 Jun 18;30(6):657–61. https://doi. org/10.1111/cyt.12742.

12

Molecular Pathology of Thymic Epithelial Tumors Aruna Nambirajan, Varsha Singh, and Deepali Jain

12.1 Introduction

molecular genetic features of diagnostic, prognostic, or potential therapeutic significance in TETs.

Chromosome copy number alterations

Recurrent gene mutations

Integrated genomic analyses of thymic epithelial tumors (TETs) demonstrate significant genetic and molecular heterogeneity among the different histological subtypes [1]. Type A, type AB thymomas, type B thymomas, and thymic carcinomas segregate into distinct genetic/molecular entities on multiplatform omics studies with differences in the loci and frequency of their chromosomal copy number alterations, recurrent gene mutations, miRNA profiles, and, more recently, DNA methylation patterns. Gene fusions and expression of viral/bacterial antigens have not been identified in these tumors [1]. Overall, there is limited understanding of the molecular pathogenesis of thymic epithelial tumors and successful targeted therapies are yet to be discovered. In this chapter, we highlight select

12.2 R  ecurrent Gene Mutations in Thymic Epithelial Tumors Thymic epithelial tumors harbor one of the lowest rates of somatic mutations among adulthood onset cancers with estimated tumor mutation burden of approximately 0.48 mutations per mega base [1]. Whole genome sequencing studies have identified recurrent mutations in a very limited number of genes (Fig.  12.1), the most frequent being the General Transcription Factor 2I (GTF2I) gene, point mutations of which occur in ~39% of all TETs [1]. Recurrent mutations of

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A. Nambirajan · V. Singh · D. Jain (*) Department of Pathology, All India Institute of Medical Sciences, New Delhi, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 D. Jain et al. (eds.), Atlas of Thymic Pathology, https://doi.org/10.1007/978-981-15-3164-4_12

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genes involved in the EGFR signaling pathway such as RAS family, PIK3CA, AKT, EGFR, and TP53, also occur, albeit at much lower frequencies [1].

• KRAS and NRAS mutations described in type B2 thymomas and thymic carcinomas [2].

12.2.3 TP53 Mutations

12.2.1 GTF2I (General Transcription Factor 2I) Mutations • GTF2I is localized on chromosomal region 7q11.23 and encodes for members of the transcription factor IIi, a group of ubiquitously expressed proteins involved in diverse signaling pathways. • Mutations are most frequent in type A (82–100%) and type AB (70–74%) thymomas, less common in type B thymomas (20–30%) and thymic carcinoma (7–8%) [1, 2]. • Characterized by a missense mutation in exon 15 at a single codon (L424H) resulting from T>A nucleotide substitution (Fig. 12.2) [3]. • Highly specific for thymic epithelial tumors; rare GTF2I mutations described in tumors other than thymomas involve other codons [1]. • GTF2I mutant tumors show higher expression of genes involved in cell morphogenesis, receptor tyrosine kinases, retinoic acid receptors, neuronal processes, and WNT and SHH signaling pathways [1]. • GTF2I mutation status not associated with myasthenia gravis [1]. • Within individual histological subtypes, GTF2I mutant tumors show better outcomes as compared to those wild type [4].

12.2.2 RAS Mutations • Activating mutations in HRAS (codon 12, 13, 117), NRAS (codon 61), or KRAS are the second most prevalent gene mutations in TETs [1]. • HRAS mutations reported predominantly in type A thymomas [1, 5, 6].

• Rare, mainly reported in thymic carcinomas and some type B thymomas [1, 2, 5]. • All mutations are pathogenic loss of function mutations.

12.2.4 KIT Mutations • Very rare, detected exclusively in thymic carcinomas (~7% incidence). • Reported KIT mutations include V560 deletion in exon 11, H697Y mutation in exon 14, L576P mutation in exon 11, and D820E mutation in exon 17, of which all except the last predict sensitivity to tyrosine kinase inhibition [7]. • KIT mutations do not correlate with KIT protein overexpression which is seen in more than 75% of thymic carcinomas [8].

12.2.5 Others • EGFR mutations are consistently absent in thymomas and very rarely described in thymic carcinomas [5] despite frequent EGFR protein overexpression and EGFR gene amplification [8]; thymic epithelial tumors generally do not respond to EGFR tyrosine kinase inhibitors [8]. • Mutations in other genes involved in EGFR signaling, namely, AKT1 and PIK3CA, have been reported in type B3 thymomas and thymic carcinomas [5]. • Rare example of a thymic carcinoma with microsatellite instability due to MLH1 somatic mutation and high tumor mutation burden has been reported [1].

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Fig. 12.2  Sanger sequencing chromatogram showing L424H point mutation (T > A nucleotide substitution) in exon 15 of GTF2I gene

12  Molecular Pathology of Thymic Epithelial Tumors

12.3 C  hromosome Copy Number Alterations Copy number gains and losses of multiple chromosomal regions are well described in thymic epithelial tumors with the frequency and complexity of the alterations increasing with the aggressiveness of the histological type (Fig. 12.1). The biological significance of a majority of these alterations is, however, yet to be determined.

12.3.1 6q25.2-25.3 Loss of Heterozygosity • Observed in most types of thymomas including thymic carcinomas. • FOXC1, a gene encoding for a transcription factor involved in normal thymus development, is located at this locus and is implicated as a tumor suppressor in the development of thymic epithelial tumors. • Tumors with reduced m-RNA and protein expression levels of FOXC1 associate with poor prognosis [9].

12.3.2 CDKN2A/B Alterations • Homozygous 9p21.3 (CDKN2A/B locus) copy number losses seen only in type B3 thymoma and thymic carcinoma [9]. • Loss of CDKN2A/p16 protein correlates with CDKN2A deletions and associates with poor prognosis in thymic carcinomas [10].

12.4 Epigenomic Alterations • Many miRNAs have been found to be differentially expressed in various histological subtypes of thymic epithelial tumors [11]. • A large microRNA cluster on chr19q13.42 activating the PI3K pathway has been identified as the transcriptional hallmark of type A and AB thymomas and is a potential actionable target [12]. • Altered expression levels of specific miRNAs have been correlated with tumor pathogenesis and prognosis in TETs [8, 13, 14]. • Recent studies have identified significant differences in the DNA methylation patterns among normal thymus, type A thymoma, type B thymoma, and thymic carcinoma with diagnostic and prognostic connotations [15, 16].

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References 1. Radovich M, Pickering CR, Felau I, Ha G, Zhang H, et  al. The integrated genomic landscape of thymic epithelial tumors. Cancer Cell. 2018;33:244–58. 2. Strobel P, Marx A, Badve S, Chan JKC, Chen G, Detterbeck F, et al. Thymomas Type A thymoma, including atypical variant. In: Travis WD, Brambilla E, Burke AP, Marx A, Nicholson AG, editors. WHO classification of tumours of lung, pleura, thymus and heart. 4th ed. Lyon: IARC; 2015. p. 187–92. 3. Petrini I, Meltzer PS, Kim IK, Lucchi M, Park KS, Fontanini G, et  al. A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors. Nat Genet. 2014;46:844–9. 4. Feng Y, Lei Y, Wu X, Huang Y, Rao H, Zhang Y, Wang F. GTF2I mutation frequently occurs in more indolent thymic epithelial tumors and predicts better prognosis. Lung Cancer. 2017;110:48–52. 5. Sakane T, Murase T, Okuda K, Saida K, Masaki A, Yamada T, et al. A mutation analysis of the EGFR pathway genes, RAS, EGFR, PIK3CA, AKT1, and BRAF, and TP53 gene in thymic carcinoma and thymoma type A/B3. Histopathology. 2019 Jun 10;75(5):755– 66. https://doi.org/10.1111/his.13936. 6. Enkner F, Pichlhöfer B, Zaharie AT, Krunic M, Holper TM, Janik S, et  al. Molecular profiling of thymoma and thymic carcinoma: genetic differences and potential novel therapeutic targets. Pathol Oncol Res. 2017;23:551–64. 7. Girard N, Shen R, Guo T, Zakowski MF, Heguy A, Riely GJ, et al. Comprehensive genomic analysis reveals clinically relevant molecular distinctions between thymic carcinomas and thymomas. Clin Cancer Res. 2009;15:6790–9. 8. Rajan A, Girard N, Marx A. State of the art of genetic alterations in thymic epithelial tumors. J Thorac Oncol. 2014;9:S131–6. 9. Petrini I, Wang Y, Zucali PA, Lee HS, Pham T, Voeller D, Meltzer PS, Giaccone G. Copy number aberrations of genes regulating normal thymus development in thymic epithelial tumors. Clin Cancer Res. 2013;19:1960–71. 10. Aesif SW, Aubry MC, Yi ES, Kloft-Nelson SM, Jenkins SM, Spears GM, Greipp PT, Sukov WR, Roden AC.  Loss of p16(INK4A) expression and homozygous CDKN2A deletion are associated with worse outcome and younger age in thymic carcinomas. J Thorac Oncol. 2017;12:860–71. 11. Ganci F, Vico C, Korita E, Sacconi A, Gallo E, Mori F, et  al. MicroRNA expression profiling of thymic epithelial tumors. Lung Cancer. 2014;85:197–204. 12. Radovich M, Solzak JP, Hancock BA, Conces ML, Atale R, Porter RF, et al. A large microRNA cluster on chromosome 19 is a transcriptional hallmark of WHO type A and AB thymomas. Br J Cancer. 2016;114:477–84. 13. Bellissimo T, Ganci F, Gallo E, Sacconi A, Tito C, De Angelis L, et  al. Thymic epithelial tumors phenotype relies on miR-145-5p epigenetic regulation. Mol Cancer. 2017;16:88. https://doi. org/10.1186/s12943-017-0655-2. 14. Wei J, Liu Z, Wu K, Yang D, He Y, Chen GG, Zhang J, Lin J. Identification of prognostic and subtype-specific potential miRNAs in thymoma. Epigenomics. 2017;9:647–57. 15. Bi Y, Meng Y, Niu Y, Li S, Liu H, He J, et al. Genome-wide DNA methylation profile of thymomas and potential epigenetic regulation of thymoma subtypes. Oncol Rep. 2019;41:2762–74. 16. Li S, Yuan Y, Xiao H, Dai J, Ye Y, Zhang Q, et al. Discovery and validation of DNA methylation markers for overall survival prognosis in patients with thymic epithelial tumors. Clin Epigenet. 2019;11:38. https://doi.org/10.1186/s13148-019-0619-z.

Lymphomas and Other Rare Tumors of the Thymus

13

Mirella Marino, Malgorzata Szolkowska, and Stefano Ascani

13.1 Introduction The incidence of mediastinal masses in the general population is estimated to be one case per 100,000 persons per year, malignancy being more frequent in anterior mediastinal masses [1–3]. The distribution of the wide variety of tumors and of nonneoplastic lesions varies according to age and gender [4]. Moreover the anterior mediastinum (also named prevascular compartment) is the most common location of neoplasia (68%) in adults, whereas the posterior mediastinum is more frequently affected in children (52%) [5]. The most common abnormalities encountered in the prevascular compartment include benign thymic lesions (cysts, hyperplasia) and intrathoracic goiter. Malignancies include thymoma, thymic carcinoma, neuroendocrine thymic tumors (all together collectively indicated also as thymic epithelial tumors, TET), lymphoma, mesenchymal tumors and germ cell tumors (GCT), and metastatic tumors [6]. Frequencies are provided for the main entities among anterior mediastinal masses: thymic malignancy occurs in approximately 35%, lymphoma in approximately 25% , thyroid and other endocrine tumors in approximately 15%, benign teratoma in approximately 10%, malignant GCT in approximately 10% (seminoma, 4%, and non-seminomatous germ cell tumors (NSGCT), 7%), and benign thymic lesions in approximately 5% [7]. However, several other cell types and ectopic tissues in the mediastinum may give rise to a variety of other tumors rarer than TET. Most of the tumors in the anterior mediastinum arise in the thymus. It may be very difficult to distinguish if an anterior

M. Marino (*) Department of Pathology, IRCCS Regina Elena National Cancer Institute, Rome, Italy e-mail: [email protected] M. Szolkowska Department of Pathology, National Tuberculosis and Lung Diseases Research Institute, Warsaw, Poland S. Ascani Pathology Unit, Perugia University, Terni, Italy

mediastinal tumor has an extrathymic or intrathymic origin, particularly for large masses. Most of the thymic tumors arise from its main cellular components, epithelial and lymphoid. Ectopic thymic tissue may be found along the embryonal route of descent as well as in different areas of the mediastinum/thoracic cavity which give rise to related tumors at these sites [8, 9]. Metastasis occurs in the thymus, although specific localization into thymus or anterior/middle mediastinal lymph nodes is difficult to demonstrate [10]. The metastases constitute the prevailing neoplastic pathology in the mediastinum [11].

13.2 Lymphoma of the Thymus/Anterior Mediastinum Lymphomas occurring in the thymus belong both to T and B cell lineage or are Hodgkin lymphomas. They account for approximately 25% (13% of cases are Hodgkin lymphomas (HL) and 12% are non-Hodgkin lymphomas (NHL) of mediastinal tumors. However, only approximately 3% of HL and 6% of NHL arise as primary mediastinal malignancies. In fact, about 50% of HL and 20% of NHL involve the mediastinum in the framework of a systemic involvement [7]. In patients with a mediastinal component of generalized NHL, the involvement is mostly seen in mediastinal and hilar lymph nodes [12]. The two most common histologic subtypes that present with localized mediastinal involvement, primary mediastinal B lymphoma (PMBL) and T-lymphoblastic lymphoma (T-Lb), appear to arise from thymic tissue [13, 14]. The age-­ related specific subtype incidence is discussed in several papers [2, 15, 16]. A detailed classification and morphological/clinical overview of the different lymphoma types and their diagnosis can be accessed from various references [17–23]. In the subsequent section, examples of the most common mediastinal/primary thymic lymphoma, their main immunohistochemical markers, and morphological ­peculiarity/diag-

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nostic pitfalls will be discussed. In addition, rare tumorlike lesions of relevant interest for their complex pathogenesis, heterogeneity of clinical features, constituting specific and rare biological entities such as Castleman disease occurring in the mediastinum [24–27] and in the thymus [28], and IgG4-related disease [29, 30] will be described.

13.3 P  rimary Lymphoma in the Anterior Superior Mediastinum Lymphomas of immature precursor cells (T and B) and of peripheral T and B cells occur in the thymus. However, Hodgkin lymphoma of classic type (cHL), particularly the nodular sclerosis (NS) subtype, represents the most frequent mediastinal lymphoma. Among the lymphomas occurring in the mediastinum, T-cell lymphomas of precursor type predominate in pediatric age, whereas, in the adults, the most frequent lymphomas are of B cell origin, primary cHL of the thymus being rarely observed [15, 16, 31]. Table 13.1 lists the main immunophenotypical markers to characterize a suspect lymphoma in the mediastinum/thymus. Specific lymphomas arising or considered to derive from the thymus itself are as follows. Table 13.1  Main immunohistochemical markers useful to characterize lymphomas and hematological neoplasias/pseudotumors in the mediastinum Immunohistochemical markers of mediastinal lymphomas Immature T-cell markers CD1a, CD10, CD99, Terminal deoxynucleotidyl transferase (TdT) Other T-cell markers CD3, CD5 CD4/CD8 CD2, CD7 Other markers of lymphoid cells CD45 LMO2 (LIM domain only 2) CD30 CD15 CD20 CD23 BCL2 PAX5 CD79a CD56 OCT-2/BOB.1 Mal (myelin and lymphocyte protein) Markers of germinal center differentiation: CD10, BCL6 Markers of activated B cells (ABC): MUM1 Epstein-Barr virus (LMP1)

Markers associated with hematological diseases/ neoplasms/pseudotumors CD34 Myeloperoxidase (MPO) Glycophorin CD61 Fact VIII CD68/PG-M1 CD68/KP1 LANA-1 (human herpesvirus 8 (HHV8) CD138 IgG4 IgM

Adapted with permission from AME Publishing Company [32]

13.3.1  T-Lymphoblastic Lymphoma and Other T-Cell Lymphomas The thymus is the main site of T-cell differentiation and education [17, 22]. During embryonic development, the T-cell lineage committed bone marrow progenitors enter the thymus via veins close to the corticomedullary junction and then migrate to the cortex. Thymopoiesis continues to occur in the thymus of human adults late in life despite the thymic involution [33]. Figure 13.1 shows the main characteristics of a T-Lymphoblastic Lymphoma. It should be noted that in the differential diagnosis of T-Lymphoblastic Lymphoma and a B1 thymoma, the evaluation of lymphoblast morphology is of little help, as they are similar in both tumors. Moreover, frequent mitosis and extensive necrosis are also seen in both tumors. Therefore, in an entirely necrotic tumor or in a small biopsy the diagnosis is often based on the evaluation of specific immunohistochemical markers targeting the neoplastic lymphoblasts, such as LMO2 [34, 35]. Previously, cyclin-dependent kinase-6 (CDK6) had been shown to stain only T-Lymphoblastic Lymphoma cells and subcapsular lymphoblasts in normal/hyperplastic thymuses [36]. Thymic remnant and epithelial cell (EC) networks, however, are very rarely found in T-Lymphoblastic Lymphomas, as the tumor growth is very destructive, in contrast to a B1 thymoma, where EC network may partially be seen even in necrotic tumors by appropriate immunohistochemical stains (cytokeratins). As other rare T-cell lymphomas, mature, peripheral T-cell lymphomas have also been reported in the thymus/mediastinal lymph nodes [37]

13.3.2  C  lassic Hodgkin Lymphoma (cHL) and B Cell Lymphoma in the Thymus Most lymphomas in the thymus derive from B cells. Thymic B cells, mainly located in the medulla or at the perivascular spaces, show a distinctive phenotype in comparison to other B cell subsets [38–40]; their relationship to peripheral B cell is still unclear [41]. These thymic B cells could give rise to B cell thymic lymphoma and to cHL of the thymus. In biopsies or surgical specimens of a lymphoid mass, it is difficult to find thymic remnants and other morphological findings that could indicate a thymic origin due to the fact that the neoplastic growth has a destructive action. However, when these remnants are seen they should be correctly r­ecognized as such. Figure  13.2 shows a rare example of early phase of cHL in its intrathymic development.

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Fig. 13.1  T-cell lymphoma, lymphoblastic (T-Lb lymphoma). (a) 22-year-old male, H&E, 200×; (b) H&E, 400×; (c) CD3, 400×; (d) Pax5, 200×; (e) CD1a, 400×; (f) CD10, 400×; (g) LMO2, 200×; (h) TdT, 400×. In the case shown, the monotonous, atypical neoplastic cells (a–b) are positive for CD3, CD1a, CD10, LMO2, and TdT (c–e–f–g–h)

and negative for PAX5 (d). LMO2 is an interesting antibody which reacts mainly with malignant T-lymphoblastic cells, useful in the differential diagnosis with cortical thymocytes (adapted with permission from Ame Publishing Company) [32]

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Fig. 13.2  Early phase of Hodgkin lymphoma (HL) in the thymus. These pictures derive from the peripheral part of a mass surgically removed and found to be a thymic HL localization. (a) H&E 100×, the image is mainly focused to thymic medulla (M); lobules of cortex (C) are seen on the right. In the M sparse large atypical cells are seen; (b) TdT 200×, the stain underlines the C which is devoid of infiltration by

13.3.2.1  Classic Hodgkin Lymphoma (cHL) The most frequent subtype of cHL in the mediastinum is the nodular sclerosis (NS) variant [42], constituting about 50–70% of primary mediastinal lymphoma [43]. cHL-NS of the thymus is predominantly a tumor of the young age and

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large atypical cells; (c) CK MNF116 200×, normal cytokeratin pattern in a normal thymus; (d) CD30 200×, large CD30+ Hodgkin cells in the M, low magnification; (e) CD30 400×, large CD30+ Hodgkin cells in the M, high magnification; (f) CD15 100×, the large atypical Hodgkin cells are also CD15+

primarily of females. In a sclerotic background with a polymorphic inflammatory population the demonstration of typical CD30+ cells, which are often very rare in the fibrous background (Fig. 13.3), is worthwhile to support the diagnosis. Reed-Sternberg cells (RS) are large with abundant

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Fig. 13.3  Classical Hodgkin lymphoma (cHL) of the thymus. (a) Polymorphic lymphoid cell population including scattered large atypical cells with the morphological features of Hodgkin’s cells, H&E, 200×; (b) CD30 staining of large atypical Hodgkin’s cells 200×; (c) macroscopy of a case of cHL in the thymus: mediastinal Hodgkin lym-

phoma is often cystic. In this case the mediastinal mass was adherent to lung parenchyma. Therefore extensive sampling of cystic mediastinal lesions is recommended (adapted with permission from Ame Publishing Company) [32]

eosinophilic cytoplasm, large double or multiple nuclei, and eosinophilic nucleoli; lacunar cells (LC) with small hyperlobated nuclei, small nucleoli, and clear, retracted cytoplasm are the cells more frequently associated to the NS subtype of cHL. As a specific pitfall, cHL induces reactive EC proliferation and/or cystic changes (Fig. 13.4) which may simulate a thymoma. Therefore, mediastinal cystic lesion should be extensively sampled because foci of cHL could be found in the wall [44–46]. The lymphoma usually forms large sclerotic masses with foci of necrosis and eosinophilic abscesses. cHL of the thymus is frequently mistaken with the primary mediastinal B cell lymphoma (PMBL), which also induces sclerotic reaction and may show RS-like cells. In fact, cHL-­NS and PMBL have the same (B) cell origin [47, 48] and similar morphology and may show a similar clinical presentation.

13.3.2.2  P  rimary Mediastinal B Cell Lymphoma (PMBL) This lymphoma usually forms bulky, solid masses of > 10 cm, with local symptoms of rapid growth, invasion, and compression of vital structures. Tumor development may represent a hematological emergency. At the time of primary diagnosis, the tumor is limited to the thorax with no involvement of lymph nodes or other lymphoid organs (only supraclavicular nodes are eventually reached). During ­ relapse, the subdiaphragmatic lymphoma extension is frequent. This lymphoma was first described as mediastinal B cell lymphoma with sclerosis, as it is associated with a distinctive fibrosis [49]. Neoplastic cells are of variable size, frequently of large or medium-large size, sometimes with pale clear cytoplasm in the central part of the tumor and

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Fig. 13.4  cHL panel: Hodgkin lymphoma in the thymus. The neoplastic lymphoid proliferation stimulates also EC network proliferation and therefore a thymoma. (a) Paracardiac anterior mediastinal mass occurring in a 22-year-old female, H&E staining, 100×; (b) H&E staining of the tumor, showing large cells with atypical nuclei in a lymphoid background, 200×; (c) IHC highlights CD30+ cells in the lym-

phoid background, 400×; (d) pankeratin immunostain reveals the presence of a disorganized EC network, 400× (courtesy of Prof. Lucia Anemona, Tor Vergata University, Rome, Italy). EC epithelial cells; IHC immunohistochemistry. (adapted with permission from Ame Publishing Company) [44]

peripherally distributed lymphocytes in the sclerotic background (Fig.  13.5). Neoplastic infiltrating CD20+ B cells have pleomorphic nuclei, ranging from regular, round nuclei to irregular, multilobulated forms [50]. In certain cases, neoplastic cells are large with prominent eosinophilic nucleoli, which resemble RS cells or variants. CD30 is weakly or focally expressed, with lesser intensity than in cHL but MAL, a protein involved in lymphocyte signal transduction, present on a minor subpopulation of thymic medullary B cells, is expressed by PMBL [51]. CD23 is also a PMBL marker [52]. However, in the differential diagnosis of these mediastinal lymphomas (cHL, PMBL, and the gray zone lymphoma (GZL)) use of a panel of antibodies is recommended [32, 53, 54].

other localizations. GZL is a lymphoma with intermediate morphological and immunohistochemical characteristics (Fig.  13.6) between cHL and PMBL [48, 57]. In the case showed in Fig. 13.6, the large neoplastic cells coexpressed CD20, CD79a, CD30, PAX5, and the transcriptional factors OCT2 and BOB1. CD45 is negative. The genetical and epigenetic alterations reported show similarities and differences with those reported for PMBL and cHL. Consensus studies and updated diagnostic criteria were recently reported by Pilichowska et al. [58] and by Sarkozy et al., in the framework of the lymphoma study association (LYSA) [59] and in the recently published WHO classification of hematological malignancies [23]. A diagnostic scoring system was also proposed [60]. Table 13.2 shows comparison of main immunophenotypical characteristics of neoplastic lymphoid cells in cHL, PMBL, and GZL.

13.3.2.3  Gray Zone Lymphoma (GZL) “Gray zone lymphoma” (GZL) or “B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma and classical Hodgkin lymphoma” [55] according to the 2017 WHO classification [23] originates in the mediastinum from a thymic medullary B cell [56]. GZL is a lymphoma of young patients (20 to 40 years) with a large anterior mediastinal mass, eventually involving the supraclavicular lymph nodes. Less frequently GZL occur in

13.3.3  MALT Lymphoma of the Thymus Among the peripheral B cell lymphomas occurring in the thymus, the most common is the mucosa-associated B cell lymphoma of MALT type or extranodal marginal zone lym-

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Fig. 13.5  Primary mediastinal B cell lymphoma (PMBL): (a) H&E, 400×; (b) H&E, 400×; (c) CD20, 200×; (d) CD30, 200×. In (a and b), the sclerotic background is seen, as well as the irregular nucleus and the clear cytoplasm of the neoplastic B cells in PMBL. CD20 positivity is

phoma of MALT type [61]. This rare lymphoma type, often cystic, usually develops in association with autoimmune diseases [62] and is characterized by scant residual thymic EC network, lymphoepithelial (LE) lesions involving Hassall corpuscle (HC), and a monotonous infiltrate of marginal zone B cells [63] (Fig. 13.7).

13.3.4  V  ery Rare B Cell Lymphoma in the Mediastinum In the setting of a systemic disease, B lymphoblastic lymphoma (B-Lb) may occur in the thymus. Rarely a primary mediastinal mass may also occur [64, 65], as shown in the Fig. 13.8, with B-Lb infiltrating the pericardium.

the hallmark of the disease, but CD30 is often coexpressed. A & B: courtesy of Prof. Fabio Menestrina (adapted with permission from Ame Publishing Company) [32]

13.4 N  eoplasms of Accessory Cells of the Lymphatic Tissue, with a Distinct Mention of Castleman Disease Castleman disease (CD) is a morphological and clinically heterogeneous group of nonneoplastic lymphoproliferative disorder of the lymphoid tissue [24, 25, 27], occasionally giving rise to neoplasms of the constituting cells (both ­lymphoid and “accessory”) [26, 66]. In the mediastinum, the most frequent type of CD is the hyaline vascular (HV) type. The reader is referred to several recent reports/reviews on this complex disease [67, 68]. The follicular dendritic cell sarcomas (FDCS), a neoplasm of accessory cells of the lymphoid tissue, may occur in the framework of CD [69].

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Fig. 13.6  Gray zone lymphoma (GZL) is a lymphoma with morphological and immunohistochemical characteristics intermediate between cHL and PMBL—(a) H&E, 100×; (b) H&E, 200×; (c) CD20, 200×; (d) CD20, 100×; (e) CD30, 100×; (f) PAX5, 200×; (g) MUM1, 200×; (h)

OCT2 200×. In the GZL case shown here neoplastic cells coexpress CD20, CD30, PAX5, and OCT2 (adapted with permission from Ame Publishing Company) [32]

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Table 13.2  Immunohistochemistry in the differential diagnosis of thymic cHL, PMBL, and GZL Antibodies CD30 CD15

cHL + (85–96%, Memb + Golgi area) + (75–85%)

PMBL >80%, weak and heterogeneous Usually −

GZL Usually + May be expressed

CD45 CD20 CD79a PAX5 OCT2 BOB1 MUM1 EBER (EBV) LMP-1 (EBV) MAL CD23 BCL6 Cyclin E P63 Fascin

− =50 IgG4 (+) cells/high power field (HPF) and obliterative phlebitis are among the diagnostic histological features of mediastinum and the other tissue localizations.

13.6.3  Mesenchymal Soft Tissue Tumors Most of the mesenchymal tumors occurring in the anterior mediastinum show a thymic origin [79]. Mesenchymal soft tissue tumors of the mediastinum are similar in morphology and molecular features with their counterparts occurring in other sites. They account for 2% of all tumors in the mediastinum [79, 80]. A list of the histotypes more frequent in the anterior mediastinum is provided in a recent review [81]. Among soft tissue tumors, mediastinal sarcomas may either develop de novo or they may arise as “somatic-type” malignancy in a mediastinal GCT.  The sarcomatous component develops more frequently in mediastinal GCT than in other sites [79]. Soft tissue tumors show a typical age and gender predilection or have specific associated diseases. Their diagnosis requires the use of immunohistochemistry by a specialized panel of antibodies, molecular testing, and the knowledge of the clinical setting [81].

13.6.3.1  Neoplasms with a Lipomatous Component and Fibroblastic Tumors Liposarcoma Lipomatous tumors account for up to 10% of mediastinal masses. Liposarcoma, in particular, is the most common primary malignant soft tissue tumor of the mediastinum.

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Fig 13.9  Langerhans cell histiocytosis involving thymic gland. (a) The thymic gland shows a patchy cellular infiltrate focally in a more fibrotic background (arrows). (b) These infiltrates are comprised of large atypical epithelioid cells characterized by grooved and folded

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Fig. 13.10 Follicular dendritic cell sarcoma (FDCS) arising in Castleman disease (CD). (a) H&E, 100×; (b) H&E, 100×; (c) H&E, 100×; (d) CD21, 100×; (e) EMA, 100×; (f) clusterin, 200×; (g) CD163, 200×; (h) S100. (a–c) Residual lymphoid follicles of CD surrounded by

nuclei, some with more conspicuous nucleoli. These cells are marked with CD1a and langerin (not shown). Scattered eosinophils and neutrophils are also noted. Magnification x 20 (a), x 200 (b). Courtesy of Dr. Anja C. Roden (Mayo Clinic Rochester, MN, USA)

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a polymorphic neoplastic population. (d–g) Neoplastic follicular dendritic cells are positive for CD21, EMA, clusterin, and CD163 and negative for S100 (h) (adapted with permission from Ame Publishing Company) [32]

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Fig. 13.10 (continued)

A case of well-differentiated liposarcoma (Fig.  13.14) forms a huge tumor of 23 cm in diameter. The tumor shows a sclerosing pattern with focal loss of lipocytic differentiation; mature and immature (multivesicular) adipocytes embedded in fibro-myxoid (basophilic) stroma are seen. The lipoblasts contain several small fat droplets in the cytoplasm. Thymolipoma These are rare mediastinal tumors with benign clinical course which are also called as lipothymomas or simply lipomas due to scant or absent thymic tissue respectively. Thymic tissue may

not be seen in small biopsies due to limited sampling. These are associated with MG and other autoimmune diseases. Grossly, as the name suggests, they are soft and yellow, well-outlined fleshy lobulated tumors. The size can be very large and weigh up to 2  kg. No necrosis or hemorrhage is present (Fig. 13.15). Microscopically, these show variable proportions of adipose tissue and thymic tissue. Thymic tissue shows admixture of epithelial and lymphocytic cells including HC. No atypia is seen in thymic or lipomatous components of the tumor (Figs.  13.16 and 13.17). Rare rhabdomyoblastic, fibrous, sebaceous, and smooth muscle differentiation is noted.

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Fig. 13.11 Mediastinal localization of blastic crisis in chronic myeloid leukemia (CML)—(a) H&E, 200×, sclerotic background infiltrated by an undifferentiated neoplasm; (b) CD45, 200×, the cells are CD45+; (c) CD34, 200×, positivity for CD34 is consistent with a blastic crisis occurring in the mediastinum in a male patient affected by CML; (d) FISH analysis, performed in FFPE sections. FISH BCR-ABL result in the mediastinal biopsy using LSI BCR-ABL dual color extra signal (Vysis, Abbott), 1000×:

Angiomyolipoma These are incidentally found rare mediastinal tumors, mostly reported in anterior mediastinum (Fig.  13.18). Mediastinal angiomyolipomas do not show close association with tuberous sclerosis. Grossly these are large, soft, and yellow-colored tumors (Fig.  13.19). Histomorphologically they are same as other angiomyolipomas which are constituted by variable proportion of adipose tissue, smooth muscle, and ectatic blood vessels (Fig. 13.20). Residual thymic tissue can be seen in the vicinity of the tumor. Lipofibroadenoma It is a benign tumor of the thymus with only six case reports available in the literature [14, 82, 83]. It bears resemblance to

the LSI BCR probe labeled with spectrum green and LSI ABL probe labeled with spectrum orange. The presence of the yellow BCR/ABL fusion signal confirms the presence of t(9;22) translocation between BCR gene located on chromosome 22 and ABL located on chromosome 9 (courtesy of Dr Roberta Merola) (adapted with permission from Ame Publishing Company) [32]. Chronic myeloid leukemia (CML); formalin-fixed paraffin-embedded (FFPE); fluorescence in situ hybridization (FISH)

fibroadenoma of the breast. They may arise de novo or in continuity with thymomas. Grossly, the tumors are well-circumscribed and have a solid gray-white cut surface. Microscopically, the tumor has fibrotic and hyalinized stroma with entrapped epithelial cells and few interspersed TdT-negative lymphocytes (Fig. 13.21). Although differentiation from thymolipoma is based on predominance of adipose tissue in the thymolipoma and fibrous tissue in lipofibroadenoma, both may be considered in the spectrum of the same disease.  olitary Fibrous Tumor and Malignant Solitary S Fibrous Tumor Among fibrous/spindle cell mesenchymal tumors, solitary fibrous tumor (SFT) [84, 85] is a prototype in mediastinum.

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SFT is, at present, considered a potentially malignant tumor even if morphology does not meet the criteria of the malignant SFT (Figs. 13.22 and 13.23). These tumors, considered to originate in the pleura, have a “patternless” a­ rchitecture with randomly distributed hypocellular and hypercellular areas, sometimes embedded in keloid-like collagen. The criteria are same in mediastinum for predicting their malignant behavior, which include a high mitotic count (>4 mitoses per 2  mm2), high cellularity, pleomorphism, and necrosis [86]. The SFT cells are cytologically benign spindle shaped with few mitoses and positive for CD34 (Fig. 13.22). In addition to CD34, the positivity for STAT6 (Fig. 13.23) is specific, as almost all SFTs harbor NAB2-STAT6 fusion gene [87].

Fig. 13.12  Photomicrograph shows solid nests of epithelial cells with sebaceous differentiation. The background shows lymphoid tissue (H&E ×400). Photograph courtesy: Dr Mark R Wick, Department of Pathology, UVA School of Medicine

Desmoid Tumors Desmoid tumors (Fig. 13.24) should be suspected in case of spindle cell, “benign-looking” tumor without mitotic activity and necrosis but with a diffuse infiltration of surrounding tissues (fat tissue or skeletal muscles). These tumors prevail in

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Fig. 13.13  IgG4-related disease (IgG4-RD), an idiopathic fibroinflammatory disorder associated with hypergammaglobulinemia and increased serum levels of IgG4, produces pseudotumors, which develop in different organs/systems, including mediastinum: (a) H&E, 100×, fibroinflammatory infiltrate, rich in plasma cells; (b) H&E, 200×, many

plasma cells are seen among the lymphoid cells, surrounding the reactive lymphoid follicle; (c) IgG, 100×, the plasma cells produce IgG; (d) IgG4, most plasma cells are IgG4 positive, 100× (adapted with permission from Ame Publishing Company) [32]

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Fig. 13.15  Gross photograph of thymolipoma which is a large, lobulated, soft, and yellow tumor. Photograph courtesy Dr Mark R Wick, Department of Pathology, UVA School of Medicine

cavernous) have been described in the mediastinum or in the thymus. Tumor size ranges from a few centimeters to 20 cm. In the series by Moran and Suster, associated histological features included fatty metaplasia, fibrosis, smooth muscle overgrowth, and inflammation [91].

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Fig. 13.14  Liposarcoma: male, aged 73, huge tumor of the anterior mediastinum (up to 23 cm in the largest diameter) with several smaller satellite lesions. (a) The tumor was composed of fat tissue cells and fibro-myxoid component. H&E, 40×). (b and c) The lipoblasts contain several small fat drops in the cytoplasm instead of single big one. The nucleus is compressed and often located in the center of a cell. H&E, 200× and 400×

woman under hormonal influence and are completely resected with difficulty, as they usually infiltrate much beyond macroscopic borders [88]. Desmoid tumors are driven by alterations of the Wnt/APC/β-catenin pathway [89]; mutations in the gene-encoding β-catenin, CTNNB1, are highly prevalent in sporadic desmoid tumors. Therefore, the β-catenin is an important marker in the diagnosis of these tumors [90].

13.6.3.2  Vascular Tumors Hemangioma The hemangioma depicted in Fig. 13.25 occurred in a patient with massive facial hemangiomatosis. Among benign vascular tumors, several types of hemangioma variants (capillary,

Epithelioid Hemangioendothelioma Epithelioid hemangioendothelioma comes under vascular tumors with intermediate malignancy. Endothelial tumors of intermediate grade (hemangioendotheliomas) are characterized by local infiltrative growth and rare metastases (Fig.  13.26). These tumors show a spectrum of features with cells having abundant eosinophilic cytoplasm showing prominent vacuolization and intracellular lumen formation, few mitotic figures and myxoid changes in the stroma or more pronounced cytologic atypia, increased mitotic activity, and necrotic areas [92, 93]. Epithelioid Angiosarcoma Epithelioid angiosarcoma (EAS) is a high-grade vascular neoplasm [94] characterized by high-grade cytology, necrosis, and abundance of mitosis (Fig. 13.27). Among vascular tumors, blood lakes, proliferation of slit-like vessels, and prominent nucleoli favor EAS.  In the EAS series from Anderson et al. [95] CAMTA1 rearrangement was negative in all cases, whereas a WWTR1 complex abnormality was found in rare cases. In EAS, the positivity of at least one vascular marker is reported, which allows differentiation from primary thoracic epithelial malignancies. However, as a potential pitfall with epithelial tumors, 25% of EAS show keratin expression. With regard to a possible thymic origin, in two cases from a series of angiosarcoma of the anterior mediastinum, Weissferdt reported the presence of a thymic tissue rim at the tumor periphery [96].

13.6.3.3  L  ymphangioma or Vascular Lymphatic Neoplasms/Malformations Lymphangioma Lymphangiomas are not considered true neoplasms but rather malformations of the lymphatic vasculature of

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Fig. 13.16  Thymolipoma. (a) At low magnification the lesion is largely comprised of benign adipose tissue intersected by strands of cellular and fibrotic tissue. (b) The intersecting tissue is comprised of small lymphocytes and usually small islands of bland-appearing epithe-

uncertain origin [97, 98]. A thymic example is shown here (Fig. 13.28). Kaposiform Lymphangiomatosis Mediastinal kaposiform lymphangiomatosis (KLA) is histologically similar to its soft tissue counterparts and is characterized by poorly circumscribed nodules of tightly packed small capillary-sized vessels, Kaposi sarcoma-like areas with spindled cells, and absence of human herpesvirus 8 (HHV-8) immunoreactivity. A component of larger lymphatic vessels is often seen (Fig.  13.29). These tumors are positive for CD31, CD34, and D2-40. In KLA, the spindle cells are distributed in sparse and poorly marginated clusters. Reported mediastinal KLA cases were usually seen in pediatric population, with almost equal sex distribution, and associated consumptive coagulopathy (Kasabach-Merritt syndrome) as a major cause of tumor-related fatality [99, 100].

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lial cells (arrow). Calcified Hassall corpuscle are also present. Magnification ×12.5 (a), ×100 (b). Courtesy of Dr. Anja C.  Roden (Mayo Clinic Rochester, MN, USA)

13.8 Germ Cell Tumors of the Mediastinum

13.7 A  damantinoma-Like Ewing Family of Tumors

The true incidence of mediastinal GCT is difficult to establish due to the rarity of these tumors, the scarcity of large series published, and the variable criteria chosen to consider this tumor group. The anterior mediastinum and retroperitoneum constitute the main sites of extragenital GCT development. It has been suggested that the GCT arise from ectopic germ cells diffused during embryogenesis [104] or that they derive from germ cells diffused through the body during embryogenesis to contribute to important regulatory, hematological, or immunological processes [70]. The hypotheses on the origin of extragonadal GCT have been extensively discussed by Oosterhuis et al. [105]. Recently the same researchers proposed a comprehensive developmental pathogenetic model for the origin of all GCT [106]. For a detailed description, the reader is referred to a recent review on GCT [71], other previous papers [107, 108], and on the data published by the 2015 WHO classification of tumors of the lung, thymus, and pleura [14].

The initial microscopic diagnosis of this case was that of thymic carcinoma with squamous cell differentiation and unusual expression of CD99. However, the translocation t(11;22) (q24;q12)EWSR1-FLI1+ was found positive in tumor cells. This tumor shares the features of Ewing sarcoma (CD99 positivity, morphology of poorly differentiated component, and translocation) and squamous cell carcinoma (cytokeratin and squamous markers positive) (Fig.  13.30). This tumor is part of the Ewing family of tumors [80, 101, 102]. The t(11;22)(q24;q12) chromosomal translocation (EWS-FLI1 gene fusion) is highly specific for ES/PNET, as >90% of the tumors show this gene rearrangement [103].

1. Immature teratoma with embryonal carcinoma component of the mediastinum: two biopsies from the mass of a young man are shown. In the small first sample (surgical biopsy tissue) structures of different epithelial differentiation (glandular and squamoid) in a highly cellular undifferentiated stroma suggested immature teratoma (Fig. 13.31a). The second sample contained only small amount of tissue containing gland-like structures with numerous mitotic figures and karyorrhexis (Figs. 13.31b and 13.32). Immunophenotype of the cells of this sample corresponded to embryonal carcinoma. Mediastinal teratoma arising in the thymus has been reported and discussed elsewhere [109, 110].

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Fig. 13.18  Gross photograph of angiomyolipoma. Photograph courtesy: Dr Mark R Wick, Department of Pathology, UVA School of Medicine

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Fig. 13.19  CT scan shows posterior mediastinal angiomyolipoma. Photograph courtesy: Dr Mark R Wick, Department of Pathology, UVA School of Medicine

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Fig. 13.17  Another case of thymolipoma at low (a, b), medium (c), and high magnification (d)

2. Seminoma: case of a young man with a mediastinal tumor. A monotonous infiltrate composed of large lymphocyte-­ like cells with associated granulomatous reaction is seen. The neoplastic cells were PLAP, CD117, and D2-40 positive (Fig. 13.33). Several series of seminomas occurring in the mediastinum have been reported and discussed [111–113]. 3. Yolk sac tumor: yolk sac tumor may present a variety of patterns including a pseudoadenocarcinoma pattern. Characteristic intracellular hyaline globules may be found in both yolk sac tumor and adenocarcinoma. A panel of immunohistochemical markers may allow the correct diagnosis. In this case the tumor cells were positive for

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b Fig. 13.20  Angiomyolipoma: the photomicrograph shows admixture of adipocytes, ectatic vessels with smooth muscle proliferation. H&E, ×400

AFP (alpha-fetoprotein), negative for mucin, and positive for SALL-4 [113]. Markers for embryonal carcinoma and choriocarcinoma (CD30 and beta-H‑CG) were negative (Fig. 13.34).

13.9 Metastases and Ectopic Tumors 13.9.1  Metastases The incidence of thymic metastases from extrathymic tumors is difficult to establish, as at times perithymic lymph nodes are involved and thymus is secondarily involved by local extension of the disease. However, metastatic tumors in the mediastinum are very frequent [11]. Metastatic lung carcinomas involve the mediastinal lymph nodes more frequently than other tumors. Teratoma metastasized from the testis to the mediastinal lymph nodes is shown in Fig.  13.35. Melanoma metastasis has been also reported in the thymus (Fig. 13.36). Immunohistochemistry plays an important role in the identification or confirmation of the primary site [114]. The p­ ossibility of very unusual primary thymic neoplasms should also be considered, as primary thymic melanomas have been reported [115].

13.9.2  Ectopic Tumors Among ectopic tumors, intrathymic ectopic parathyroid adenomas (Fig. 13.37) are reported. The occurrence of ectopic parathyroid tissue in the anterior mediastinum is rather

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Fig. 13.21  Lipofibroadenoma. (a) This lesion is comprised of benign adipose tissue intersected by strands of sclerosing fibrosis with a minimal cellular component. (b) In other areas the sclerosing and hyaline fibrosis is dominant with only small clusters of adipocytes and strands of cellular tissue. (c) The cellular areas are comprised of small lymphocytes, small vessels and some epithelioid cells. Hassall corpuscles are also present. Magnification ×12.5 (a), ×40 (b), ×200 (c). Courtesy of Dr. Anja C. Roden (Mayo Clinic Rochester, MN, USA)

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Fig. 13.22  Solitary fibrous tumor (SFT) in a female, aged 63. SFT is a spindle cell intrathoracic tumor often derived from the stromal cells of the pleura and simulating a mediastinal tumor. SFT themselves cannot be regarded as “benign,” as they are tumors of unknown malignant potential (or “potentially malignant”). (a) The tumor is composed of

benign-looking spindle cells. In this case in several fields (a, b, c, d, H&E, respectively, 40×, 100×, 200×, 200×) no sign of malignancy was found; (e) CD34 stains the spindle cells as a characteristic finding in SFT (100×); (f) the KI67 stain highlights very few scattered nuclei (200×)

frequent and cases of parathyroid adenoma have been reported [116]. These ectopic tumors can be responsible for primary hyperparathyroidism. A case of primary juvenile

sporadic hyperparathyroidism due to a parathyroid adenoma developing in a supernumerary fifth intrathymic parathyroid has been reported [117].

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Fig. 13.23 Malignant solitary fibrous tumor, female aged 65. According to WHO classification cellular-rich tumors with mitotic activity >4/10 HPF are named malignant SFT (MSFT). (a) Patternless architecture alternating hypocellular and hypercellular areas, 100×; (b)

high magnification, 200×; two mitosis are seen; (c) CD34, 200×; (d) positive nuclear reaction for STAT-6, 200×. STAT6 stain is specific for SFT, as almost all SFT harbor an NAB2-STAT6 fusion gene. S100 was also negative (not shown)

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Fig. 13.24  Desmoid tumor: female, aged 67. A tumor of the anterior mediastinum or pleura localized next to the pericardial sac without infiltration of the lung. Desmoids may resemble SFT. In the DD, IHC is decisive, as desmoids are positive for SMA and beta-catenin (nuclear reaction!) and may be positive for estrogen or progesterone receptors. The cells were also CD34 (-), CD99 (-), and Bcl-2 (-) (not shown). Despite “benign” cytology, prognosis is not good—desmoids usually infiltrate much beyond macroscopic borders. Complete resection is

very difficult, so the tumors relapse many times but do not metastasize. (a) This tumor diffusely infiltrated fat tissue of the mediastinum, H&E, 40×; (b) spindle, bland-looking cells without atypia, mitotic activity, and necrotic areas. H&E, 200×; (c) cells were focally positive for SMA, 200×; (d) β-catenin, nuclear staining, 200×. Solitary fibrous tumors (SFT), differential diagnosis (DD), immunohistochemistry (IHC), smooth muscle actin (SMA)

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Fig. 13.25  Hemangioma: female aged 73.This case occurred in a patient with massive facial hemangiomatosis and long-standing obstructive sleep apnea; the mediastinal tumor measured 2  cm in the largest dimension. The tumor was composed of vascular channels of

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different size, filled with blood, and lined by flat endothelial cells. The surrounding fibrotic stroma had hemosiderin deposits. (a) H&E, 100×; (b) H&E, 200×

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Fig. 13.26 Hemangioendothelioma, epithelioid; female, aged 63. Tumor of mediastinum, 5 cm in the largest diameter, well-­circumscribed, and encapsulated in MRI scans. Microscopic appearance on low magnification could suggest liposarcoma (dispersed adipocytic cells) but both higher magnification and immunohistochemistry results (CD31 +, CD34+, AE1AE3-, S-100 -, GLUT-1-, calretinin-) revealed that it was a vascular tumor. Atypia, diffuse, non-lobulated type of growth, elevated proliferation index, and negative reaction for GLUT-1 excluded hemangioma. Low mitotic index and lack of necrosis did not favor a diagnosis of angiosarcoma. Malignant pleural mesothelioma was con-

sidered in the DD but positive vascular markers and negative calretinin excluded this diagnosis. Proliferation index Ki-67 reached approximately 20% (not shown). (a) Infiltration of fat tissue by atypical neoplastic cells is seen. H&E, 40×. (b–c) On the medium and high magnification numerous small capillaries surrounded by epithelioid cells and filled with erythrocytes may be appreciated. Despite nuclear atypia neither necrosis nor increased mitotic activity was found (1 mitotic figure/10 HPF): H&E, (b) 200× and (c) 400×. (d) CD31 stained all cell membranes, 200×. Magnetic resonance imaging (MRI), differential diagnosis (DD)

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Fig. 13.27  Epithelioid angiosarcoma with a mediastinal localization in a 28-year-old man. Epithelioid angiosarcoma is a highly aggressive endothelial cell origin tumor. Here pleomorphic epithelioid cells, with abundant eosinophilic cytoplasm, vesicular nuclei, and prominent nucleoli, are seen. CD34 positivity ranges in different series of epitheli-

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oid angiosarcoma. Factor VIII, CD31, Fli-1, and vimentin are usually also positive. Pancytokeratins may be also positive—(a) H&E, 200×, highly atypical cell population, necrosis; (b) CD34 positivity of neoplastic cells, 200× (adapted with permission from Ame Publishing Company) [32]

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Fig. 13.28  Lymphangioma. (a) Macroscopical picture of the tumor, recapitulating the thymus shape; (B) H&E stain of the tumor, 50×; (c) CD34 stain of lymphatic endothelium, 100×; (d) TdT positivity in thymic remnants, 100× (adapted with permission from Ame Publishing Company) [32]

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Fig. 13.29 Kaposiform lymphangiomatosis (KLA): female, 21. Kaposiform lymphangiomatosis (KLA) is a newly classified clinicopathological entity—it is a variant of generalized lymphatic anomaly (GLA, previously named lymphangiomatosis) regarded as a malformation of lymphatic vessels with a concomitant failure of blood vessels, coagulopathy, and hemorrhages. The benign-looking spindle cells are positive for common vascular marker (CD34) and for markers of lymphatic endothelium: D2-40 or, e.g., PROX-1. Hemosiderin deposits are a proof of small hemorrhages. The tumor of mediastinum was associated to multiple additional lesions in the lungs and spleen and one osteolytic lesion in a sacral bone. The patient suffered from persistent

cough and reported an episode of hemoptysis two years before admission to the hospital. Blood analysis revealed elevated D-dimer level (>14,000 ng/mL [reference value: