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The 2021 WHO Classification of Tumors of the Thymus and Mediastinum: What Is New in Thymic Epithelial, Germ Cell, and Mesenchymal Tumors?

Open AccessPublished:October 22, 2021DOI:https://doi.org/10.1016/j.jtho.2021.10.010

      Abstract

      This overview of the fifth edition of the WHO classification of thymic epithelial tumors (including thymomas, thymic carcinomas, and thymic neuroendocrine tumors [NETs]), mediastinal germ cell tumors, and mesenchymal neoplasms aims to (1) list established and new tumor entities and subtypes and (2) focus on diagnostic, molecular, and conceptual advances since publication of the fourth edition in 2015. Diagnostic advances are best exemplified by the immunohistochemical characterization of adenocarcinomas and the recognition of genetic translocations in metaplastic thymomas, rare B2 and B3 thymomas, and hyalinizing clear cell carcinomas. Advancements at the molecular and tumor biological levels of utmost oncological relevance are the findings that thymomas and most thymic carcinomas lack currently targetable mutations, have an extraordinarily low tumor mutational burden, but typically have a programmed death-ligand 1high phenotype. Finally, data underpinning a conceptual advance are illustrated for the future classification of thymic NETs that may fit into the classification scheme of extrathoracic NETs. Endowed with updated clinical information and state-of-the-art positron emission tomography and computed tomography images, the fifth edition of the WHO classification of thymic epithelial tumors, germ cell tumors, and mesenchymal neoplasms with its wealth of new diagnostic and molecular insights will be a valuable source for pathologists, radiologists, surgeons, and oncologists alike. Therapeutic perspectives and research challenges will be addressed as well.

      Keywords

      Introduction

      The fifth edition of the “WHO Classification of Thoracic Tumours”
      WHO Classification of Tumours Editorial Board
      Thoracic tumours.
      is largely a revision of the fourth edition that was entitled “WHO Classification of Tumours of the Lung, Pleura, Thymus and Heart” and published in 2015 under the editorship of William D. Travis, Elizabeth Brambilla, Allen Burke, Alexander Marx, and Andrew Nicholson.
      • Travis W.D.
      • Brambilla E.
      • Burke A.P.
      • et al.
      WHO Classification of Tumours of the Lung, Pleura, Thymus and Heart.
      Similar to the fourth edition, the fifth edition continues (1) to use the unique, globally accepted “type A, AB, B1-B3 thymoma” nomenclature originally introduced by the late Dr. Juan Rosai for the major thymoma types in the second edition of the WHO Classification in 1999; (2) to cover all tumor types together with clinical, pathologic, and genetic data in one book (introduced in the third edition); and (3) to stress the interdisciplinary “tumor board approach” in mediastinal oncology that has been prominently advocated and promoted by the International Thymic Malignancy Interest Group (ITMIG)
      • Huang J.
      • Ahmad U.
      • Antonicelli A.
      • et al.
      Development of the international thymic malignancy interest group international database: an unprecedented resource for the study of a rare group of tumors.
      and led to the involvement of clinical experts from radiology, thoracic surgery, and oncology as co-authors in the fourth edition. Along these lines, the incorporation of state-of-the-art computed tomography and positron emission tomography/computed tomography images and cytology has since been maintained. Furthermore, the broad, interdisciplinary and international consensus that underlies the fifth edition is reflected by a change in editorship: instead of five editors who were in charge of the third and fourth editions owing to their expertise in the fields of pulmonary, pleural, thymic, and cardiac pathology, the fifth edition is under the auspices of an editorial board comprising 16 experts from North America, Europe, and Asia with expertise in pathology, surgery, radiology, and oncology.
      The time period since the publication of the fourth edition has seen highly dynamic new developments in the fields of pathology, tumor biology, and medical oncology related to thymic epithelial tumors (TETs) and left their imprints on the fifth edition: First, a TNM staging system for thymomas, thymic carcinomas (TCs), and thymic neuroendocrine tumors (NETs) was approved and published by the Union for International Cancer Control (previously International Union Against Cancer)
      in 2017 on the basis of joint data from the International Association for the Study of Lung Cancer and ITMIG,
      • Detterbeck F.C.
      • Stratton K.
      • Giroux D.
      • et al.
      The IASLC/ITMIG Thymic Epithelial Tumors Staging Project: proposal for an evidence-based stage classification system for the forthcoming (8th) edition of the TNM classification of malignant tumors.
      prompting the editors of the fifth edition to adopt the TNM system as obligatory and the modified Masaoka-Koga system as optional for the staging of TETs.
      • Ruffini E.
      • Fang W.
      • Guerrera F.
      • et al.
      The International Association for the Study of Lung Cancer thymic tumors staging project: the impact of the eighth edition of the Union for International Cancer Control and American Joint Committee on Cancer TNM stage classification of thymic tumors.
      Second, thymomas and TCs were the last cancer group investigated by the multiomics-based The Cancer Genome Atlas (TCGA) project, explaining why groundbreaking new insights were mainly achieved in thymomas and TC, including the realization that they exhibit an extreme paucity of targetable mutations.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      ,
      • Petrini I.
      • Meltzer P.S.
      • Kim I.K.
      • et al.
      A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors.
      Third, immune checkpoint inhibitors (ICIs) targeting the programmed cell death protein-1 or programmed death-ligand 1 (PD-L1), approved for treatment of several cancers since 2014,
      • Dobosz P.
      • Dzieciątkowski T.
      The intriguing history of cancer immunotherapy.
      have attracted the interest of medical oncologists and pathologists given the frequent expression of PD-L1 by TETs,
      • Inaguma S.
      • Wang Z.
      • Lasota J.
      • et al.
      Comprehensive immunohistochemical study of programmed cell death ligand 1 (PD-L1): analysis in 5536 cases revealed consistent expression in trophoblastic tumors.
      ,
      • Padda S.K.
      • Riess J.W.
      • Schwartz E.J.
      • et al.
      Diffuse high intensity PD-L1 staining in thymic epithelial tumors.
      resulting in initiation of clinical trials to evaluate ICIs for treatment of advanced TETs.
      • Rajan A.
      • Heery C.R.
      • Thomas A.
      • et al.
      Efficacy and tolerability of anti-programmed death-ligand 1 (PD-L1) antibody (avelumab) treatment in advanced thymoma.
      • Cho J.
      • Kim H.S.
      • Ku B.M.
      • et al.
      Pembrolizumab for patients with refractory or relapsed thymic epithelial tumor: an open-label phase II trial.
      • Giaccone G.
      • Kim C.
      • Thompson J.
      • et al.
      Pembrolizumab in patients with thymic carcinoma: a single-arm, single-centre, phase 2 study.
      The results, in turn, have sparked interest in paraneoplastic autoimmunity and biomarkers, because the occurrence of severe immune-related adverse events remains a major challenge.
      • Zhao C.
      • Rajan A.
      Immune checkpoint inhibitors for treatment of thymic epithelial tumors: how to maximize benefit and optimize risk?.
      In the chapters on germ cell tumors (GCTs) and soft tissue neoplasms, there are no changes in the concept or diagnostic criteria in the fifth compared with the fourth edition. Minor revisions concern new adaptation of nomenclature and definitions to the fifth WHO Classifications of Tumours—Soft Tissue and Bone Tumours
      WHO Classification of Tumours Editorial Board
      Soft tissue and bone tumours.
      and Tumours of the Urinary System and Male Genital Organs.
      “New” diagnostic pitfalls owing to overlapping immunohistochemical features between SMARCA4 thoracic tumors, NUT carcinomas, and GCTs are also mentioned.
      The subsequent review focuses on the differences between the fourth and fifth editions of the WHO classification of solid tumors of the thymus and mediastinum rather than providing comprehensive description of the tumors.

      Thymoma

      Features Maintained

      The concept of the classification, nomenclature, diagnostic criteria, and reporting strategies for thymomas and their interpretation as malignant tumors (except for micronodular thymoma with lymphoid stroma) have been maintained (Table 1).
      International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
      Thymomas are classified as type A thymoma (including an atypical variant), AB thymoma, type B thymoma (separated into B1, B2, and B3 thymomas), micronodular thymoma with lymphoid stroma, and metaplastic thymoma by histologic features and, rarely, immunohistochemistry, such as immature T cell content. The high frequency of the GTF2I (p.L424H) mutation in type A and AB thymomas
      • Petrini I.
      • Meltzer P.S.
      • Kim I.K.
      • et al.
      A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors.
      was confirmed,
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      although the low prevalence in type B thymomas and TCs is less consistently reported.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      ,
      • Petrini I.
      • Meltzer P.S.
      • Kim I.K.
      • et al.
      A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors.
      ,
      • Higuchi R.
      • Goto T.
      • Hirotsu Y.
      • et al.
      Primary driver mutations in GTF2I specific to the development of thymomas.
      Table 1WHO Classification of Thymic Epithelial Tumors, Including Thymomas, Thymic Carcinomas, and Neuroendocrine Tumors
      ICD-O Morphology and Behavior Codes
      Epithelial tumors

      Thymomas
      8580/3Thymoma, NOS
      8581/3Thymoma, type A
      Including atypical variant.
      8582/3Thymoma, type AB
      8583/3Thymoma, type B1
      8584/3Thymoma, type B2
      8585/3Thymoma, type B3
      8580/1Micronodular thymoma with lymphoid stroma
      8580/3Metaplastic thymoma
      9010/0Lipofibroadenoma
      Squamous carcinomas
      8070/3Squamous cell carcinoma, NOS
      8123/3Basaloid carcinoma
      8082/3Lymphoepithelial carcinoma
      Previously labeled lymphoepithelioma-like carcinoma.
      Adenocarcinomas
      8140/3Adenocarcinoma, NOS
      8260/3Low-grade papillary adenocarcinoma
      Previously labeled papillary adenocarcinoma.
      8200/3Thymic carcinoma with adenoid cystic carcinoma-like features
      8144/3Adenocarcinoma, enteric type
      Newly delineated mucinous or nonmucinous adenocarcinoma with expression of at least one intestinal marker, CK20, CDX2, or MUC2.
      Adenosquamous carcinomas
      8560/3Adenosquamous carcinoma
      NUT carcinomas
      8023/3NUT carcinoma
      Salivary gland-like carcinomas
      8430/3Mucoepidermoid carcinoma
      8310/3Clear cell carcinoma
      Including hyalinizing clear cell carcinoma.
      8033/3Sarcomatoid carcinoma
      8980/3Carcinosarcoma
      Subtype of sarcomatoid carcinoma.
      Undifferentiated carcinomas
      8020/3Carcinoma, undifferentiated, NOS
      Thymic carcinomas
      8586/3Thymic carcinoma, NOS
      Including hepatoid carcinoma, rhabdoid carcinoma, undifferentiated large cell carcinoma associated with Castleman disease-like reaction, and sebaceous carcinoma.
      Thymic neuroendocrine neoplasms
      Neuroendocrine tumors
      8240/3Carcinoid tumor, NOS/neuroendocrine tumor, NOS
      8240/3Typical carcinoid/neuroendocrine tumor, grade 1
      8249/3Atypical carcinoid/neuroendocrine tumor, grade 2
      Neuroendocrine carcinomas
      8041/3Small cell carcinoma
      8045/3Combined small cell carcinoma
      8013/3Large cell neuroendocrine carcinoma
      Note: These morphology codes are from the International Classification of Diseases for Oncology, third edition, second revision (ICD-O-3.2) (IACR, 2019).
      International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
      Behavior is coded /0 for benign tumors; /1 for unspecified, borderline, or uncertain behavior; /2 for carcinoma in situ and grade III intraepithelial neoplasia; /3 for malignant tumors, primary site; and /6 for malignant tumors, metastatic site. Behavior code /6 is not generally used by cancer registries. This classification is modified from the previous WHO classification, taking into account changes in our understanding of these lesions.
      IACR, International Association of Cancer Registries; NOS, not otherwise specified.
      Reprinted from WHO Classification of Tumours Editorial Board. Thoracic Tumours. Lyon, France: International Agency for Research on Cancer; 2021 (WHO Classification of Tumours Series, 5th ed.; vol. 5, page 7, Copyright; 2021).
      a Including atypical variant.
      b Previously labeled lymphoepithelioma-like carcinoma.
      c Previously labeled papillary adenocarcinoma.
      d Newly delineated mucinous or nonmucinous adenocarcinoma with expression of at least one intestinal marker, CK20, CDX2, or MUC2.
      e Including hyalinizing clear cell carcinoma.
      f Subtype of sarcomatoid carcinoma.
      g Including hepatoid carcinoma, rhabdoid carcinoma, undifferentiated large cell carcinoma associated with Castleman disease-like reaction, and sebaceous carcinoma.

      What Is New?

      A formal new feature throughout the book is the introduction of paragraphs termed “essential and desirable diagnostic criteria” where obligatory and optional morphologic and molecular features of a given tumor entity are listed as exemplified for each TET in the Supplementary Table 1.
      Two lesions previously listed as thymoma types are no longer included in the fifth edition: Microscopic thymomas are now considered nodular epithelial hyperplasias because of persistent lack of evidence for their progressive potential, and sclerosing thymomas are now considered to represent conventional thymomas with regressive changes.
      The classification of thymomas and TCs is strongly reinforced by new molecular findings. First, the TCGA study
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      revealed that type A and AB thymomas on one hand and B1 to B3 thymomas on the other hand each belong to a spectrum of tumors, with minimal overlap between them. Both groups were genomically completely distinct from TCs (Fig. 1). Second, gain-of-function mutations of HRAS, such as the oncogenic GTF2I mutation, segregated with type A and AB thymomas, but loss-of-function mutations of TP53 were typical of type B thymomas (and TCs).
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Third, chromosomal translocations (gene fusions) have been newly recognized in thymomas: metaplastic thymomas had a unique YAP1-MAML2 translocation,
      • Vivero M.
      • Davineni P.
      • Nardi V.
      • Chan J.K.C.
      • Sholl L.M.
      Metaplastic thymoma: a distinctive thymic neoplasm characterized by YAP1-MAML2 gene fusions.
      whereas novel KMT2A-MAML2 translocations were restricted to 6% of pretreated aggressive types B2 and B3 and a combined TC and B3 thymoma, but were not found in other thymomas and “pure” TCs.
      • Massoth L.R.
      • Hung Y.P.
      • Dias-Santagata D.
      • et al.
      Pan-Cancer landscape analysis reveals recurrent KMT2A-MAML2 gene fusion in aggressive histologic subtypes of thymoma.
      Both translocations are thought to be oncogenic drivers. Together with the well-established MAML2 translocation of thymic mucoepidermoid carcinomas, there are now three different TET types implicating oncogenic involvement of the MAML2 gene, suggesting the possibility that the thymic niche could be conducive to growth of clones harboring fusions involving the MAML2 gene. Fourth, micro-RNA profiles (including overexpression of the micro-RNA cluster miC19MC on chromosome 19) sharply separate type A and AB thymomas from type B thymomas and TCs.
      • Enkner F.
      • Pichlhöfer B.
      • Zaharie A.T.
      • et al.
      Molecular profiling of thymoma and thymic carcinoma: genetic differences and potential novel therapeutic targets.
      ,
      • Radovich M.
      • Solzak J.P.
      • Hancock B.A.
      • et al.
      A large microRNA cluster on chromosome 19 is a transcriptional hallmark of WHO type A and AB thymomas.
      Figure thumbnail gr1
      Figure 1Molecular thymoma subtypes (A-like, AB-like, B-like, and “C-like” TCs) derived from integrative unsupervised clustering based on five data platforms largely reflect WHO histotypes.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Key features derived from single platforms (e.g., the overexpressed C19MC micro-RNA cluster in the A-like and AB-like cluster) are listed above the thin line, whereas results of multiplatform analysis that integrate copy number alteration and RNA expression profiles are listed below the thin line. MNT, micronodular thymoma with lymphoid stroma; TC, thymic carcinoma. Reproduced from of Ref. 7 with permission from Cancer Cell.
      With respect to the enigmatic triggering factors of thymomas, the observation of an enrichment of C>T mutations within CpG di-nucleotides is the first hint to an aging-related pathogenesis.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Unexpectedly, the TCGA study on thymomas did not reveal oncogenic driver mutations amenable to currently available targeted therapies.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Nevertheless, the small molecule inhibitor, everolimus, is clinically active and used for treatment of recurrent thymomas.
      • Zucali P.A.
      • De Pas T.
      • Palmieri G.
      • et al.
      Phase II study of everolimus in patients with thymoma and thymic carcinoma previously treated with cisplatin-based chemotherapy.
      Furthermore, thymomas were found to exhibit the lowest tumor mutational burden among all adult human cancers, which might limit the value of currently available ICIs.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Nevertheless, thymomas count among human cancers to have the highest prevalence of extensive and strong PD-L1 expression in tumor cells
      • Inaguma S.
      • Wang Z.
      • Lasota J.
      • et al.
      Comprehensive immunohistochemical study of programmed cell death ligand 1 (PD-L1): analysis in 5536 cases revealed consistent expression in trophoblastic tumors.
      ,
      • Padda S.K.
      • Riess J.W.
      • Schwartz E.J.
      • et al.
      Diffuse high intensity PD-L1 staining in thymic epithelial tumors.
      (Fig. 2A and B) , a predictive biomarker of response to ICIs. It is the high frequency of ICI-induced severe immune-mediated toxicity that currently prohibits adoption of immunotherapy, including ICIs for the management of thymomas.
      • Zhao C.
      • Rajan A.
      Immune checkpoint inhibitors for treatment of thymic epithelial tumors: how to maximize benefit and optimize risk?.
      Whether aneuploidy and particular transcriptomic profiles that were found to be associated with the presence of autoimmune myasthenia gravis in thymomas
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      ,
      • Yamada Y.
      • Weis C.A.
      • Thelen J.
      • et al.
      Thymoma associated myasthenia gravis (TAMG): differential expression of functional pathways in relation to MG status in different thymoma histotypes.
      may serve as predictive biomarkers of immune-mediated adverse events associated with ICI-based interventions has not been evaluated.
      Figure thumbnail gr2
      Figure 2PD-L1 expression in neoplastic epithelial cells of a type B3 thymoma (A and B) and undifferentiated thymic carcinoma (C and D). (A) Conventional histologic features of type B3 thymoma. (B) Strong, membranous PD-L1 expression in type B3 thymoma (IHC). (C) Bone metastasis of an undifferentiated thymic carcinoma. (D) Strong, membranous PD-L1 expression in apparently all tumors cells (IHC, TPS 100%). IHC, immunohistochemistry, using immunoperoxidase; PD-L1, programmed death-ligand 1; TPS, tumor proportion score.

      Reporting of Heterogeneous TETs

      Although thymomas composed of more than one histologic type are common, other combinations of TETs are rare.
      • Chan J.K.C.
      • Detterbeck F.
      • Marino M.
      • et al.
      Thymic carcinoma: introduction.
      Examples include combined thymic squamous cell carcinoma and type B2 or B3 thymoma, combined TC and micronodular thymoma with lymphoid stroma, combined low-grade papillary adenocarcinoma and type A or AB thymoma, combined sarcomatoid carcinoma and type A or metaplastic thymoma, combined small cell carcinoma and thymoma, and combined small cell carcinoma and TC. The combinations usually occur not as random events but are likely clonally related, owing to transformation of a lower-grade neoplasm to a more aggressive neoplasm or bidirectional differentiation of the neoplasm. In the fifth edition of the WHO Classification of Thoracic Tumours, the nomenclature is more streamlined (Fig. 3). Whereas the reporting of heterogeneous thymomas is exclusively based on the prevalence of the different thymoma components, the reporting of other combined TETs takes the aggressiveness of the various components into account, when applicable.
      Figure thumbnail gr3
      Figure 3Reporting scheme for thymic epithelial tumors with heterogeneous components. Top: Any combinations of thymoma, thymic carcinoma, carcinoid, small cell carcinoma, and large cell neuroendocrine carcinoma can occur, as indicated by the connecting lines. Such tumors are termed “combined tumor 1 and tumor 2,” with the more aggressive component being listed first. ∗In line with the nomenclature of pulmonary neuroendocrine neoplasms, small cell carcinoma combined with more than or equal to 10% large cell neuroendocrine carcinoma is termed “combined small cell carcinoma and large cell neuroendocrine carcinoma,” whereas the combined tumor is only termed “small cell carcinoma” if the large cell neuroendocrine carcinoma component is less than 10%. Middle: Thymoma composed of two or more types are termed “thymoma,” with listing of the components in 10% increments. Bottom: Thymic carcinomas composed of two different types are termed “combined thymic carcinoma,” with the components listed in 10% increments.
      Open questions are addressed in the subsequent texts together with those related to TCs.

      Thymic Carcinoma

      Features Maintained

      The nomenclature and diagnostic criteria of most TCs have remained unchanged. Nevertheless, changes concern not only molecular aspects but also new histologic types and subtypes (Table 1 and Supplementary Table 1), diagnostic refinement through new immunohistologic criteria, and new names for old entities to better convey tumor biology or streamline nomenclature across thoracic cancers.
      • Chan J.K.C.
      • Detterbeck F.
      • Marino M.
      • et al.
      Thymic carcinoma: introduction.

      What Is New?

      The group of TCs now includes several new subtypes (Table 1), which are as follows: (1) micronodular TC with lymphoid hyperplasia is an apparently less aggressive subtype of thymic squamous cell carcinoma with “non-organotypic” lymphoid stroma that otherwise mimics “micronodular thymoma with lymphoid stroma”
      • Weissferdt A.
      • Moran C.A.
      Micronodular thymic carcinoma with lymphoid hyperplasia: a clinicopathological and immunohistochemical study of five cases.
      (Fig. 4A–C), (2) hyalinizing clear cell carcinoma that resembles its salivary gland analogue in terms of histologic featues and EWSR1 rearrangement
      • Porubsky S.
      • Rudolph B.
      • Rückert J.C.
      • et al.
      EWSR1 translocation in primary hyalinising clear cell carcinoma of the thymus.
      (Fig. 4D–F), and (3) thymic sebaceous carcinoma (Fig. 4G–I) that mimics its cutaneous counterpart
      • Porubsky S.
      • Jessup P.
      • Kee D.
      • et al.
      Potentially actionable FGFR2 high-level amplification in thymic sebaceous carcinoma.
      and is currently grouped with other rare TCs among the new, heterogeneous group of “Thymic carcinoma NOS.”
      Figure thumbnail gr4
      Figure 4New thymic carcinoma (sub)types in the new WHO classification. (A–C) Micronodular thymic carcinoma with lymphoid hyperplasia revealing islands of cohesive, atypical epithelial cells in a lymphoid stroma without desmoplasia; inset highlights cytologic atypia of tumor cells. (B) Expression of CD117 (also called KIT) is common (IHC). (C) Absence of TdT(+) immature T cells on IHC is distinctive compared with micronodular thymoma with lymphoid stroma (IHC). (D–F) Hyalinizing clear cell carcinoma of the thymus: single or grouped clear cells in a collagen-rich stroma. (E) Diffuse p40 expression (IHC). (F) Labeling of CK5/6(+) basal cells in the periphery of clear tumor cell groups (IHC). (G–I) Sebaceous carcinoma of the thymus revealing multivacuolated clear cells (sebocytes) associated with basophilic squamoid cells and focal calcification. (H) Nuclear expression of androgen receptor (IHC). (I) Adipophilin expression in “sebocytes.” IHC, immunohistochemistry using immunoperoxidase.
      New nomenclature and refined immunohistologic criteria concern “papillary adenocarcinoma” that is now labeled “low-grade papillary adenocarcinoma” to convey its bland histologic features and mostly indolent clinical course. High-grade adenocarcinomas with papillary features are classified as “adenocarcinomas NOS” instead. There have also been changes in nomenclature to align with lung cancers, with the renaming of “lymphoepithelioma-like carcinoma” as “lymphoepithelial carcinoma.” “Mucinous adenocarcinomas” are now either reclassified as “enteric-type adenocarcinomas”
      • Moser B.
      • Schiefer A.I.
      • Janik S.
      • et al.
      Adenocarcinoma of the thymus, enteric type: report of 2 cases, and proposal for a novel subtype of thymic carcinoma.
      if there is expression of one or more enteric markers, CK20, CDX2, or MUC2, or are assigned to the new, morphologically heterogeneous group of “adenocarcinomas NOS.” Of note, enteric-type adenocarcinomas can also be nonmucinous.
      The TCGA study has revealed molecular pathogenesis of TCs distinct from thymomas.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      The identification of only two combined TCs and B3 thymomas in an independent cohort of more than 600 type B2 and B3 thymomas and TCs
      • Massoth L.R.
      • Hung Y.P.
      • Dias-Santagata D.
      • et al.
      Pan-Cancer landscape analysis reveals recurrent KMT2A-MAML2 gene fusion in aggressive histologic subtypes of thymoma.
      supports this conclusion. Furthermore, the most common abnormality identified in TCs, loss of chromosome 16q, was absent from thymomas, and the tumor mutational burden was higher in TCs, particularly in the rare TC case with an inactivating MLH1 mutation and mismatch repair deficiency (microsatellite instability).
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Another novel disease mechanism in TETs, chromoplexy, a single “catastrophic event” resulting in multiple rearrangements, has been discovered to result in formation of NUT-fusion oncoproteins, the sole drivers of NUT carcinoma growth.
      • Lee J.K.
      • Louzada S.
      • An Y.
      • et al.
      Complex chromosomal rearrangements by single catastrophic pathogenesis in NUT midline carcinoma.
      Nevertheless, apart from long-known rare actionable KIT mutations, TCs, such as thymomas, lack recurrently mutated genes that are currently targetable.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      Nevertheless, phase 2 clinical trials have revealed the efficacy of the targeted therapies, sunitinib, lenvatinib, and everolimus against some recurrent TCs.
      • Zucali P.A.
      • De Pas T.
      • Palmieri G.
      • et al.
      Phase II study of everolimus in patients with thymoma and thymic carcinoma previously treated with cisplatin-based chemotherapy.
      ,
      • Thomas A.
      • Rajan A.
      • Berman A.
      • et al.
      Sunitinib in patients with chemotherapy-refractory thymoma and thymic carcinoma: an open-label phase 2 trial.
      ,
      • Sato J.
      • Satouchi M.
      • Itoh S.
      • et al.
      Lenvatinib in patients with advanced or metastatic thymic carcinoma (REMORA): a multicentre, phase 2 trial.
      The underlying mechanisms of action of these drugs remain to be clarified.
      It seems promising that TCs have strong PD-L1 expression in tumor cells (Fig. 2C and D) almost as often as in thymomas.
      • Padda S.K.
      • Riess J.W.
      • Schwartz E.J.
      • et al.
      Diffuse high intensity PD-L1 staining in thymic epithelial tumors.
      ,
      • Sakane T.
      • Murase T.
      • Okuda K.
      • et al.
      A comparative study of PD-L1 immunohistochemical assays with four reliable antibodies in thymic carcinoma.
      Indeed, early clinical trials have revealed a correlation between PD-L1 expression in TCs and the response to the ICI, pembrolizumab.
      • Cho J.
      • Kim H.S.
      • Ku B.M.
      • et al.
      Pembrolizumab for patients with refractory or relapsed thymic epithelial tumor: an open-label phase II trial.
      ,
      • Giaccone G.
      • Kim C.
      • Thompson J.
      • et al.
      Pembrolizumab in patients with thymic carcinoma: a single-arm, single-centre, phase 2 study.
      Nevertheless, severe ICI-induced immune-mediated toxicity can occur quite often (∼20%), even though TCs are far less susceptible to develop paraneoplastic autoimmunity compared with thymomas.
      • Zhao C.
      • Rajan A.
      Immune checkpoint inhibitors for treatment of thymic epithelial tumors: how to maximize benefit and optimize risk?.

      Open Questions in Relation to Thymomas and TCs

      The pathogenesis of most thymomas and TC remains unknown, largely precluding targeted interventions. Epigenetic, noncoding RNA-related and metabolic mechanisms need further elucidation.
      • Enkner F.
      • Pichlhöfer B.
      • Zaharie A.T.
      • et al.
      Molecular profiling of thymoma and thymic carcinoma: genetic differences and potential novel therapeutic targets.
      ,
      • Radovich M.
      • Solzak J.P.
      • Hancock B.A.
      • et al.
      A large microRNA cluster on chromosome 19 is a transcriptional hallmark of WHO type A and AB thymomas.
      ,
      • Soejima S.
      • Kondo K.
      • Tsuboi M.
      • et al.
      GAD1 expression and its methylation as indicators of malignant behavior in thymic epithelial tumors.
      ,
      • Yamada Y.
      • Simon-Keller K.
      • Belharazem-Vitacolonnna D.
      • et al.
      A tuft cell-like signature is highly prevalent in thymic squamous cell carcinoma and delineates new molecular subsets among the major lung cancer histotypes.
      No biomarkers are available to predict the response to chemotherapy or kinase inhibitors except for rare KIT, and even rarer PI3K mutations.
      • Alberobello A.T.
      • Wang Y.
      • Beerkens F.J.
      • et al.
      PI3K as a potential therapeutic target in thymic epithelial tumors.
      A key challenge in drug development for TETs is the paucity of thymoma and TC cell lines
      • Gökmen-Polar Y.
      • Sanders K.L.
      • Goswami C.P.
      • et al.
      Establishment and characterization of a novel cell line derived from human thymoma AB tumor.
      ,
      • Ehemann V.
      • Kern M.A.
      • Breinig M.
      • et al.
      Establishment, characterization and drug sensitivity testing in primary cultures of human thymoma and thymic carcinoma.
      and unavailability of more representative preclinical models (e.g., xenografts or organoids) for functional studies and high-throughput screens. To better exploit the strong expression of PD-L1 in TCs, new biomarkers are needed to better predict both the response to ICIs and the risk for development of severe immune-mediated toxicity, including strategies to mitigate ICI toxicity while maintaining therapeutic efficacy.

      Thymic Neuroendocrine Neoplasms

      Features Maintained

      The nomenclature and diagnostic criteria of NETs of the thymus (TNETs) and lung have remained unchanged (Table 1 and Supplementary Table 1). TNETs are classified into typical carcinoids, atypical carcinoids (ACs), large cell neuroendocrine carcinomas (LCNECs), and small cell carcinomas (SCCs). The separation of these tumors is based on morphologic features, presence of necrosis, and mitotic counts. Although the proliferation marker Ki67 is very useful to exclude LCNEC or SCC in crushed samples,
      • Pelosi G.
      • Rodriguez J.
      • Viale G.
      • Rosai J.
      Typical and atypical pulmonary carcinoid tumor overdiagnosed as small-cell carcinoma on biopsy specimens: a major pitfall in the management of lung cancer patients.
      it is unsuitable for the distinction between typical carcinoid and AC (Fig. 5).
      Figure thumbnail gr5
      Figure 5The group of atypical carcinoids with elevated mitotic counts (“NET G3”) shares morphologic, immunohistochemical, and molecular features with TC and ACs and cases with relapses or metastases revealed that these tumors form a continuum, whereas it remains to be found if such tumors can progress further to high-grade carcinomas (LCNEC and SCC). NET G3 and LCNEC can be discriminated using a panel of immunohistochemical markers, notably chromogranin A and EZH2. Ki67 staining is not helpful in individual cases owing to large overlap between the single entities (horizontal lines: range; vertical lines: mean). AC, atypical carcinoid; LCNEC, large cell neuroendocrine carcinoma; mut, mutated; neg, negative; SCC, small cell carcinoma; TC, typical carcinoid; WT, wild-type. Adapted from Dinter et al.
      • Dinter H.
      • Bohnenberger H.
      • Beck J.
      • et al.
      Molecular classification of neuroendocrine tumors of the thymus.

      What Is New?

      Emerging data reveal that pulmonary and thymic NETs fit into the common classification framework of neuroendocrine neoplasms elsewhere in the body
      • Rindi G.
      • Klimstra D.S.
      • Abedi-Ardekani B.
      • et al.
      A common classification framework for neuroendocrine neoplasms: an International Agency for Research on Cancer (IARC) and World Health Organization (WHO) expert consensus proposal.
      and fall into two main groups—low-grade (typical carcinoid, AC) and high-grade (LCNEC, SCC) tumors. Similar to grade 3 NET in the pancreas, recent data convincingly reveal the existence of a group of thymic tumors with carcinoid morphologic features and elevated mitotic counts and Ki67 index.
      • Dinter H.
      • Bohnenberger H.
      • Beck J.
      • et al.
      Molecular classification of neuroendocrine tumors of the thymus.
      Although these tumors are designated as LCNEC according to current WHO criteria by virtue of high mitotic activity, they seem more closely aligned with the low-grade group of tumors, that is, typical carcinoid and AC. Such “NET G3” tumors can be separated from true LCNEC by a panel of immunohistochemical markers, including chromogranin, EZH2, TP53, RB, and SSTR2A (Fig. 5).

      Open Questions in Relation to NETs

      Although the WHO classification of TNETs has revealed prognostic relevance,
      • Ströbel P.
      • Zettl A.
      • Shilo K.
      • et al.
      Tumor genetics and survival of thymic neuroendocrine neoplasms: a multi-institutional clinicopathologic study.
      it is currently unknown which of the findings mentioned previously should have an impact on clinical management. For example, it is not known whether an “NET G3” with a Ki67 index of 45% should be treated differently from a true high-grade LCNEC with a Ki67 index of 55%. Despite significant molecular overlap between low- and high-grade NETs, the current paradigm remains that low-grade TNETs generally do not progress to high-grade TNETs. It is unclear which molecular pathways drive (and separate) these two tumor groups and whether the essential oncogenic drivers in TNETs are different from NETs of the lung. It is also currently not possible to discriminate between primary TNETs and pulmonary NETs metastatic to the mediastinum by immunohistochemistry.

      Need for Global Cooperation to Resolve Open Questions in Thymomas, TCs, and Thymic NETs

      Research to identify molecular biomarkers, especially those with therapeutic impact, has been hampered by the paucity of TETs
      WHO Classification of Tumours Editorial Board
      Thoracic tumours.
      with current evidence mostly arising from limited, single-institutional studies. Multi-institutional investigations driven by international organizations such as International Association for the Study of Lung Cancer and ITMIG were invaluable in building a staging system
      • Huang J.
      • Ahmad U.
      • Antonicelli A.
      • et al.
      Development of the international thymic malignancy interest group international database: an unprecedented resource for the study of a rare group of tumors.
      • Detterbeck F.C.
      • Stratton K.
      • Giroux D.
      • et al.
      The IASLC/ITMIG Thymic Epithelial Tumors Staging Project: proposal for an evidence-based stage classification system for the forthcoming (8th) edition of the TNM classification of malignant tumors.
      • Ruffini E.
      • Fang W.
      • Guerrera F.
      • et al.
      The International Association for the Study of Lung Cancer thymic tumors staging project: the impact of the eighth edition of the Union for International Cancer Control and American Joint Committee on Cancer TNM stage classification of thymic tumors.
      and uncovering molecular properties of TETs.
      • Radovich M.
      • Pickering C.R.
      • Felau I.
      • et al.
      The integrated genomic landscape of thymic epithelial tumors.
      In the future, more global collaborations are warranted to collect sufficient samples to further improve tumor classification (e.g., in TNETs) and discover clinically actionable biomarkers.

      Mediastinal GCTs

      Features Maintained

      The concepts, nomenclature, histologic, and immunohistologic criteria defining the main GCT categories are maintained in the fifth edition of the WHO classification (Table 2).
      International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
      Table 22021 WHO Classification of Mediastinal GCTs
      ICD-O Morphology and Behavior Codes
      9061/3Seminoma
      9070/3Embryonal carcinoma
      9071/3Yolk sac tumor
      9100/3Choriocarcinoma
      9080/0Mature teratoma
      9080/1Immature teratoma
      9085/3Mixed germ cell tumor
      9084/3Teratoma with somatic-type malignancies
      9086/3Germ cell tumor with associated hematological malignancy
      Note: These morphology codes are from the International Classification of Diseases for Oncology, third edition, second revision (ICD-O-3.2) (IACR, 2019).
      International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
      Behavior is coded /0 for benign tumors; /1 for unspecified, borderline, or uncertain behavior; /2 for carcinoma in situ and grade III intraepithelial neoplasia; and /3 for malignant tumors, primary site.
      GCT, germ cell tumors; IACR, International Association of Cancer Registries.
      Reprinted from WHO Classification of Tumours Editorial Board. Thoracic Tumours. Lyon, France: International Agency for Research on Cancer; 2021 (WHO Classification of Tumours Series, 5th ed.; vol. 5, page 8, Copyright; 2021).

      What Is New?

      The term “endodermal sinus tumor” has been discontinued and is not recommended as a synonym for yolk sac tumor.
      New insights have been gained into the pathogenesis of mediastinal GCTs: type I GCTs encompass infantile teratomas and yolk sac tumors.
      • Oosterhuis J.W.
      • Looijenga L.H.J.
      Germ cell tumors from a developmental perspective: cells of origin, pathogenesis, and molecular biology (emerging pattern).
      Somatic mutations are usually not identified in type I teratomas. Type II GCTs are malignant, include seminomas and nonseminomatous GCT, occur in adolescents and adult men, and characteristically have gain of chromosome 12p, most often as an isochromosome. A recent study further suggests that mediastinal teratomas possibly develop through two different pathways.
      • Kao C.S.
      • Bangs C.D.
      • Aldrete G.
      • Cherry A.M.
      • Ulbright T.M.
      A clinicopathologic and molecular analysis of 34 mediastinal germ cell tumors suggesting different modes of teratoma development.
      Teratomas in children, women, and a subset of men may arise from a benign pluripotent cell, lack 12p copy number alterations and cytologic atypia, often have organoid morphologic features, and behave in a benign manner. Pure teratomas resected in a subset of postpubertal men after chemotherapy for mixed GCTs may derive from a malignantly transformed precursor cell, most often harbor 12p copy number gains and cytologic atypia, only rarely exhibit organoid morphologic features, and behave in a malignant fashion. The latter teratoma likely develops through differentiation from a malignant component of a mixed GCT. After chemotherapy that eradicates the primitive GCT component, persistence of the teratoma component as malignant teratoma will result. Although one study failed to confirm 12p copy number gains in mature teratomas of postpubertal men,
      • Lee T.
      • Seo Y.
      • Han J.
      • Kwon G.Y.
      Analysis of chromosome 12p over-representation and clinicopathological features in mediastinal teratomas.
      none of these patients underwent presurgical chemotherapy.
      Some caveats are worth mentioning in connection with “old” immunohistochemical GCT markers: thoracic SMARCA4-deficient undifferentiated tumors
      • Rekhtman N.
      • Montecalvo J.
      • Chang J.C.
      • et al.
      SMARCA4-deficient thoracic sarcomatoid tumors represent primarily smoking-related undifferentiated carcinomas rather than primary thoracic sarcomas.
      ,
      • Le Loarer F.
      • Watson S.
      • Pierron G.
      • et al.
      SMARCA4 inactivation defines a group of undifferentiated thoracic malignancies transcriptionally related to BAF-deficient sarcomas.
      that most often invade the mediastinum often have SALL4, SOX2, and vimentin expression, may or may not have keratin positivity and, therefore, may be confused with GCTs. Another pitfall is misinterpretation of SALL4- and AFP-expressing NUT carcinomas as genuine GCTs,

      Agaimy A, Haller F, Renner A, Niedermeyer J, Hartmann A, French CA. Misleading germ cell phenotype in pulmonary NUT carcinoma harboring the ZNF532-NUTM1 fusion [e-pub ahead of print]. Am J Surg Pathol. https://doi.org/10.1097/PAS.0000000000001774. Accessed July 8, 2021.

      and conversely, the frequent expression of wild-type NUT protein in GCTs that can lead to the misdiagnosis of NUT carcinoma.
      • Haack H.
      • Johnson L.A.
      • Fry C.J.
      • et al.
      Diagnosis of NUT midline carcinoma using a NUT-specific monoclonal antibody.

      Open Questions

      An unresolved problem is lack of an official, meaningful staging system for primary mediastinal GCTs.

      Mesenchymal Tumors of the Mediastinum

      Features Maintained

      Nomenclature and diagnostic criteria of all entities are largely maintained (Table 3).
      International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
      The molecular pathology of mediastinal mesenchymal tumors corresponds to those occurring elsewhere. The current edition re-emphasizes the importance of immunohistochemical and molecular testing in establishing the correct diagnosis. No conceptual, diagnostic, or molecular changes have been published on desmoid fibromatosis and most lipomatous and malignant vascular tumors.
      Table 32021 WHO Classification of Mesenchymal Tumors of the Thorax
      ICD-O Morphology and Behavior Codes
      Adipocytic tumors
      8850/0Lipoma, NOS
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8850/0Thymolipoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8850/3Liposarcoma, NOS
      8851/3Liposarcoma, well-differentiated
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8852/3Myxoid liposarcoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8854/3Pleomorphic liposarcoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8858/3Dedifferentiated liposarcoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      Fibroblastic and myofibroblastic tumors
      8821/1Desmoid-type fibromatosis
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8815/1Solitary fibrous tumor, NOS
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      8817/0Calcifying fibrous tumor
      8825/1Inflammatory myofibroblastic tumor
      8811/3Myxofibrosarcoma
      Vascular tumors
      9120/0Hemangioma, NOS
      9121/0Cavernous hemangioma
      9122/0Venous hemangioma
      9132/0Intramuscular hemangioma
      9123/0Arteriovenous hemangioma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      9170/0Lymphangioma, NOS
      9173/0Cystic lymphangioma
      9133/3Epithelioid hemangioendothelioma
      9120/3Angiosarcoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      Skeletal muscle tumors
      8900/3Rhabdomyosarcoma, NOS
      8910/3Embryonal rhabdomyosarcoma
      8912/3Spindle cell rhabdomyosarcoma
      8920/3Alveolar rhabdomyosarcoma
      8901/3Pleomorphic rhabdomyosarcoma
      Peripheral nerve sheath and neural tumors
      8693/3Extra-adrenal paraganglioma
      9580/0Granular cell tumor
      9580/3Granular cell tumor, malignant
      9560/0Schwannoma
      9540/3Malignant peripheral nerve sheath tumor
      9490/0Ganglioneuroma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      9490/3Ganglioneuroblastoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      9500/3Neuroblastoma
      Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      Tumors of uncertain differentiation
      9040/3Synovial sarcoma, NOS
      9041/3Synovial sarcoma, spindle cell
      9042/3Synovial sarcoma, epithelioid cell
      9043/3Synovial sarcoma, biphasic
      9364/3Ewing sarcoma
      9367/3
      Codes marked with an (b) were approved by the International Agency for Research on Cancer/WHO Committee for ICD-O at its meeting in October 2020.
      CIC-rearranged sarcomaI
      9368/3
      Codes marked with an (b) were approved by the International Agency for Research on Cancer/WHO Committee for ICD-O at its meeting in October 2020.
      Sarcoma with BCOR genetic alterations
      9366/3
      Codes marked with an (b) were approved by the International Agency for Research on Cancer/WHO Committee for ICD-O at its meeting in October 2020.
      Round cell sarcoma with EWSR1-non-ETS fusions
      Note: These morphology codes are from the International Classification of Diseases for Oncology, third edition, second revision (ICD-O-3.2) (IACR, 2019).
      International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
      Behavior is coded /0 for benign tumors; /1 for unspecified, borderline, or uncertain behavior; /2 for carcinoma in situ and grade III intraepithelial neoplasia; and /3 for malignant tumors, primary site.
      IACR, International Associaton of Cancer Registries; NOS, not otherwise specified.
      Reprinted from WHO Classification of Tumours Editorial Board. Thoracic Tumours. Lyon, France: International Agency for Research on Cancer; 2021. (WHO Classification of Tumours Series, 5th ed.; vol. 5, page 6, Copyright; 2021)
      a Tumors that most often occur in the mediastinum are labeled (a). Mesenchymal tumors that are specific for the lung and heart are described separately in the respective sections.
      b Codes marked with an (b) were approved by the International Agency for Research on Cancer/WHO Committee for ICD-O at its meeting in October 2020.

      What Is New?

      In contrast to the previous WHO classification of thoracic tumors, the fifth edition discusses only the most common mesenchymal tumors and those characteristically encountered in the thorax (Table 3). Thymolipoma as the only thymus-specific mesenchymal tumor is included in this section. The full repertoire of soft tissue tumors is covered in the WHO classification of soft tissue and bone tumor volume.
      WHO Classification of Tumours Editorial Board
      Soft tissue and bone tumours.
      Lipomas and all types of liposarcoma have been reported in the mediastinum and can occur in all mediastinal compartments, particularly in the anterior (prevascular) and posterior (paravertebral) compartments. MDM2 and CDK4 nuclear expression or evidence of MDM2 gene (12p15) amplification is desirable diagnostic criterion for well-differentiated and dedifferentiated liposarcomas. In myxoid liposarcoma, DDIT3 immunohistochemistry is a new option,
      • Baranov E.
      • Black M.A.
      • Fletcher C.D.M.
      • Charville G.W.
      • Hornick J.L.
      Nuclear expression of DDIT3 distinguishes high-grade myxoid liposarcoma from other round cell sarcomas.
      ,
      • Scapa J.V.
      • Cloutier J.M.
      • Raghavan S.S.
      • Peters-Schulze G.
      • Varma S.
      • Charville G.W.
      DDIT3 immunohistochemistry is a useful tool for the diagnosis of myxoid liposarcoma.
      and confirmation of DDIT3 gene rearrangement or detection of specific FUS-DDIT3 or EWSR1-DDIT3 gene fusion is desirable criterion in selected cases.
      Solitary fibrous tumor can rarely occur in the mediastinum. Immunohistochemistry to reveal nuclear STAT6 resulting from a NAB2-STAT6 gene fusion is most helpful in establishing the diagnosis.
      • Robinson D.R.
      • Wu Y.M.
      • Kalyana-Sundaram S.
      • et al.
      Identification of recurrent NAB2-STAT6 gene fusions in solitary fibrous tumor by integrative sequencing.
      Rather than classifying solitary fibrous tumor as benign, malignant, or of uncertain malignant potential, the fifth edition recommends reporting the risk according to the proposed three- and four-variable risk stratification models
      • Demicco E.G.
      • Wagner M.J.
      • Maki R.G.
      • et al.
      Risk assessment in solitary fibrous tumors: validation and refinement of a risk stratification model.
      ,
      • Demicco E.G.
      • Park M.S.
      • Araujo D.M.
      • et al.
      Solitary fibrous tumor: a clinicopathological study of 110 cases and proposed risk assessment model.
      (Table 4).
      Table 4Solitary Fibrous Tumor: New Four-Variable Risk Model for the Prediction of Metastatic Risk
      • Huang J.
      • Ahmad U.
      • Antonicelli A.
      • et al.
      Development of the international thymic malignancy interest group international database: an unprecedented resource for the study of a rare group of tumors.
      Risk FactorCutoff ValuePoints Assigned
      Age (y)<550
      >551
      Mitoses/2 mm200
      1–31
      ≥42
      Tumor size (mm)0–490
      50–991
      100–1492
      >1503
      Tumor necrosis<10%0
      >10%1
      RiskLow0–2 points
      Intermediate3–4 points
      High5–6 points
      Note: Compared with the historic three-variable model, the new model takes necrosis into account.
      Primary intrathoracic synovial sarcoma is uncommon, and its diagnosis is facilitated by the demonstration of one of the characteristic SS18-SSX fusions. A novel antibody against the SS18-SSX fusion protein, not available at the time of publication of the WHO classification blue book, is a welcome addition to aid in routine diagnosis of synovial sarcoma owing to its high sensitivity and specificity.
      • Baranov E.
      • McBride M.J.
      • Bellizzi A.M.
      • et al.
      A novel SS18-SSX fusion-specific antibody for the diagnosis of synovial sarcoma.
      ,
      • Perret R.
      • Velasco V.
      • Le Guellec S.
      • Coindre J.M.
      • Le Loarer F.
      The SS18-SSX antibody has perfect specificity for the SS18-SSX fusion protein: a validation study of 609 neoplasms including 2 unclassified tumors with SS18-non-SSX fusions.
      Nevertheless, rare cases stain negative and may still need SS18-SSX translocation analysis and exclusion of mediastinal spindle cell tumor mimics through immunohistochemistry and molecular testing (Supplementary Table 2).
      Benign vascular tumors such as hemangiomas occur in the thymus and slightly more often in the anterior rather than the posterior mediastinum. Many lesions previously described as cavernous or venous hemangiomas are now considered to be venous malformations as defined by the classification of the International Society for the Study of Vascular Anomalies.
      • Wassef M.
      • Blei F.
      • Adams D.
      • et al.
      Vascular anomalies classification: recommendations from the International Society for the study of vascular anomalies.
      Peripheral neuroblastic tumors (neuroblastoma, ganglioneuroblastoma, ganglioneuroma) occur in the posterior mediastinum. Rarely, neuroblastoma and ganglioneuroma occur in the thymus of adults presenting with the syndrome of inappropriate secretion of antidiuretic hormone or with a thymic cyst. It is desirable to perform molecular testing for MYCN, ALK, TERT, and ATRX status, DNA index and segmental chromosomal aberrations for predicting clinical behavior.
      • Peifer M.
      • Hertwig F.
      • Roels F.
      • et al.
      Telomerase activation by genomic rearrangements in high-risk neuroblastoma.

      Relevant Findings Since the Publication of the WHO Classification

      Frequent expression of cancer testis antigens, specifically MAGE-A, MAGE-C1, NY-ESO-1, SAGE, and GAGE7, was described in up to 43% of thymomas and TCs, and expression of SAGE and GAGE7 was associated with a poor prognosis in type B2 and B3 thymomas.
      • Sakane T.
      • Murase T.
      • Okuda K.
      • Masaki A.
      • Nakanishi R.
      • Inagaki H.
      Expression of cancer testis antigens in thymic epithelial tumors.
      In TCs, higher expression of GAD1 in relation to GAD1 hypermethylation was associated with an adverse clinical course.
      • Soejima S.
      • Kondo K.
      • Tsuboi M.
      • et al.
      GAD1 expression and its methylation as indicators of malignant behavior in thymic epithelial tumors.
      These findings might have an immunotherapeutic perspective. Finally, 80% of thymic squamous cell carcinomas, including all KIT(+) cases, were just described as POU2F3-positive “tuft cell-like cancers,” which share a unique gene expression profile not only with normal chemosensory tuft cells
      • Yamada Y.
      • Simon-Keller K.
      • Belharazem-Vitacolonnna D.
      • et al.
      A tuft cell-like signature is highly prevalent in thymic squamous cell carcinoma and delineates new molecular subsets among the major lung cancer histotypes.
      but also with 20% of small cell lung cancers
      • Huang Y.H.
      • Klingbeil O.
      • He X.Y.
      • et al.
      POU2F3 is a master regulator of a tuft cell-like variant of small cell lung cancer.
      and smaller subset of NSCLC.
      • Yamada Y.
      • Simon-Keller K.
      • Belharazem-Vitacolonnna D.
      • et al.
      A tuft cell-like signature is highly prevalent in thymic squamous cell carcinoma and delineates new molecular subsets among the major lung cancer histotypes.
      Because almost all “tuft cell-like cancers” lack targetable mutations, this hints to a currently unknown carcinogenic mechanism in tuft cell-like cancers, including thymic squamous cell carcinomas.
      • Yamada Y.
      • Simon-Keller K.
      • Belharazem-Vitacolonnna D.
      • et al.
      A tuft cell-like signature is highly prevalent in thymic squamous cell carcinoma and delineates new molecular subsets among the major lung cancer histotypes.

      CRediT Authorship Contribution Statement

      The authors are “responsible authors” of chapters that deal with thymic epithelial tumors, mediastinal germ cell tumors, and mediastinal mesenchymal neoplasms in the “WHO Classification of Tumours Editorial Board. Thoracic Tumours. Lyon (France): International Agency for Research on Cancer; 2021. (WHO classification of tumours, fifth ed.; vol. 5).” Specific contributions were as follows:
      Alexander Marx: Conceptualization, Original draft preparation, Visualization (submission of images for figure preparation), and Reviewing and editing.
      John K.C. Chan: Conceptualization, Original draft preparation, Visualization, and Reviewing and editing.
      Lara Chalabreysse, Frank Detterbeck, Christopher A. French, Jason L. Hornick, Hiroshi Inagaki, Deepali Jain, Alexander J. Lazar, Mirella Marino, Andre L. Moreira, Andrew G. Nicholson, Masayuki Noguchi, Daisuke Nonaka, Mauro G. Papotti, Lynette M. Sholl, Hisashi Tateyama, Vincent Thomas de Montpréville: Reviewing and editing.
      Sanja Dacic, Edith M. Marom, Arun Rajan, Anja C. Roden: Original draft preparation, Reviewing and editing.
      Stefan Porubsky: Visualization, Reviewing and editing.
      William D. Travis: Conceptualization, Visualization, Reviewing and editing.
      Philipp Ströbel: Original draft preparation, Visualization, Reviewing and editing.

      Acknowledgments

      We wish to thank Professor Ian Cree (Editorial Board Chair), all members of the Editorial Board for the Thoracic Tumor Book, and staffs from the International Agency for Research on Cancer involved in the WHO Classification of Tumours, fifth edition series, for their contribution to the completion of the fifth edition book.
      Table 1, Table 2, Table 3 are reprinted with permission from WHO Classification of Tumours Editorial Board. Thoracic Tumours. Lyon, France: International Agency for Research on Cancer; 2021. (WHO Classification of Tumours series, fifth ed.; vol. 5). In addition, with permission, Supplementary Table 1 summarizing the essential and diagnostic criteria is listed from this same WHO classification. Figure 1 is reprinted with permission from Cancer Cell.

      References

        • WHO Classification of Tumours Editorial Board
        Thoracic tumours.
        5th ed. WHO Classification of Tumours. 5. International Agency for Research on Cancer, Lyon, France2021
        • Travis W.D.
        • Brambilla E.
        • Burke A.P.
        • et al.
        WHO Classification of Tumours of the Lung, Pleura, Thymus and Heart.
        4th ed. WHO Classification of Tumours. 7. International Agency for Research on Cancer, Lyon, France2015
        • Huang J.
        • Ahmad U.
        • Antonicelli A.
        • et al.
        Development of the international thymic malignancy interest group international database: an unprecedented resource for the study of a rare group of tumors.
        J Thorac Oncol. 2014; 9: 1573-1578
      1. Brierley J.D. Gospodarowicz M.K. Wittekind C. TNM Classification of Malignant Tumours. John Wiley & Sons, Ltd, Chichester, United Kingdom2017
        • Detterbeck F.C.
        • Stratton K.
        • Giroux D.
        • et al.
        The IASLC/ITMIG Thymic Epithelial Tumors Staging Project: proposal for an evidence-based stage classification system for the forthcoming (8th) edition of the TNM classification of malignant tumors.
        J Thorac Oncol. 2014; 9: S65-S72
        • Ruffini E.
        • Fang W.
        • Guerrera F.
        • et al.
        The International Association for the Study of Lung Cancer thymic tumors staging project: the impact of the eighth edition of the Union for International Cancer Control and American Joint Committee on Cancer TNM stage classification of thymic tumors.
        J Thorac Oncol. 2020; 15: 436-447
        • Radovich M.
        • Pickering C.R.
        • Felau I.
        • et al.
        The integrated genomic landscape of thymic epithelial tumors.
        Cancer Cell. 2018; 33: 244-258.e10
        • Petrini I.
        • Meltzer P.S.
        • Kim I.K.
        • et al.
        A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors.
        Nat Genet. 2014; 46: 844-849
        • Dobosz P.
        • Dzieciątkowski T.
        The intriguing history of cancer immunotherapy.
        Front Immunol. 2019; 10: 2965
        • Inaguma S.
        • Wang Z.
        • Lasota J.
        • et al.
        Comprehensive immunohistochemical study of programmed cell death ligand 1 (PD-L1): analysis in 5536 cases revealed consistent expression in trophoblastic tumors.
        Am J Surg Pathol. 2016; 40: 1133-1142
        • Padda S.K.
        • Riess J.W.
        • Schwartz E.J.
        • et al.
        Diffuse high intensity PD-L1 staining in thymic epithelial tumors.
        J Thorac Oncol. 2015; 10: 500-508
        • Rajan A.
        • Heery C.R.
        • Thomas A.
        • et al.
        Efficacy and tolerability of anti-programmed death-ligand 1 (PD-L1) antibody (avelumab) treatment in advanced thymoma.
        J Immunother Cancer. 2019; 7: 269
        • Cho J.
        • Kim H.S.
        • Ku B.M.
        • et al.
        Pembrolizumab for patients with refractory or relapsed thymic epithelial tumor: an open-label phase II trial.
        J Clin Oncol. 2019; 37: 2162-2170
        • Giaccone G.
        • Kim C.
        • Thompson J.
        • et al.
        Pembrolizumab in patients with thymic carcinoma: a single-arm, single-centre, phase 2 study.
        Lancet Oncol. 2018; 19: 347-355
        • Zhao C.
        • Rajan A.
        Immune checkpoint inhibitors for treatment of thymic epithelial tumors: how to maximize benefit and optimize risk?.
        Mediastinum. 2019; 3: 35
        • WHO Classification of Tumours Editorial Board
        Soft tissue and bone tumours.
        5th ed. WHO Classification of Tumours. 3. International Agency for Research on Cancer, Lyon, France2020
      2. 4th ed. WHO Classification of Tumours of the Urinary System and Male Genital Organs. WHO Classification of Tumours. 8. International Agency for Research on Cancer, Lyon, France2016
      3. International Association of Cancer Registries (IACR) [Internet]. Lyon (France): International Agency for Research on Cancer; 2019. ICD-O-3.2.
        • Higuchi R.
        • Goto T.
        • Hirotsu Y.
        • et al.
        Primary driver mutations in GTF2I specific to the development of thymomas.
        Cancers. 2020; 12: 2032
        • Vivero M.
        • Davineni P.
        • Nardi V.
        • Chan J.K.C.
        • Sholl L.M.
        Metaplastic thymoma: a distinctive thymic neoplasm characterized by YAP1-MAML2 gene fusions.
        Mod Pathol. 2020; 33: 560-565
        • Massoth L.R.
        • Hung Y.P.
        • Dias-Santagata D.
        • et al.
        Pan-Cancer landscape analysis reveals recurrent KMT2A-MAML2 gene fusion in aggressive histologic subtypes of thymoma.
        JCO Precis Oncol. 2020; 4: 109-115
        • Enkner F.
        • Pichlhöfer B.
        • Zaharie A.T.
        • et al.
        Molecular profiling of thymoma and thymic carcinoma: genetic differences and potential novel therapeutic targets.
        Pathol Oncol Res. 2017; 23: 551-564
        • Radovich M.
        • Solzak J.P.
        • Hancock B.A.
        • 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: 477-484
        • Zucali P.A.
        • De Pas T.
        • Palmieri G.
        • et al.
        Phase II study of everolimus in patients with thymoma and thymic carcinoma previously treated with cisplatin-based chemotherapy.
        J Clin Oncol. 2018; 36: 342-349
        • Yamada Y.
        • Weis C.A.
        • Thelen J.
        • et al.
        Thymoma associated myasthenia gravis (TAMG): differential expression of functional pathways in relation to MG status in different thymoma histotypes.
        Front Immunol. 2020; 11: 664
        • Chan J.K.C.
        • Detterbeck F.
        • Marino M.
        • et al.
        Thymic carcinoma: introduction.
        in: WHO Classification of Tumours Editorial Board Thoracic Tumours. 5. International Agency for Research on Cancer (IARC), Lyon (France)2021: 351-353
        • Weissferdt A.
        • Moran C.A.
        Micronodular thymic carcinoma with lymphoid hyperplasia: a clinicopathological and immunohistochemical study of five cases.
        Mod Pathol. 2012; 25: 993-999
        • Porubsky S.
        • Rudolph B.
        • Rückert J.C.
        • et al.
        EWSR1 translocation in primary hyalinising clear cell carcinoma of the thymus.
        Histopathology. 2019; 75: 431-436
        • Porubsky S.
        • Jessup P.
        • Kee D.
        • et al.
        Potentially actionable FGFR2 high-level amplification in thymic sebaceous carcinoma.
        Virchows Arch. 2020; 476: 323-327
        • Moser B.
        • Schiefer A.I.
        • Janik S.
        • 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: 541-548
        • Lee J.K.
        • Louzada S.
        • An Y.
        • et al.
        Complex chromosomal rearrangements by single catastrophic pathogenesis in NUT midline carcinoma.
        Ann Oncol. 2017; 28: 890-897
        • Thomas A.
        • Rajan A.
        • Berman A.
        • et al.
        Sunitinib in patients with chemotherapy-refractory thymoma and thymic carcinoma: an open-label phase 2 trial.
        Lancet Oncol. 2015; 16: 177-186
        • Sato J.
        • Satouchi M.
        • Itoh S.
        • et al.
        Lenvatinib in patients with advanced or metastatic thymic carcinoma (REMORA): a multicentre, phase 2 trial.
        Lancet Oncol. 2020; 21: 843-850
        • Sakane T.
        • Murase T.
        • Okuda K.
        • et al.
        A comparative study of PD-L1 immunohistochemical assays with four reliable antibodies in thymic carcinoma.
        Oncotarget. 2018; 9: 6993-7009
        • Soejima S.
        • Kondo K.
        • Tsuboi M.
        • et al.
        GAD1 expression and its methylation as indicators of malignant behavior in thymic epithelial tumors.
        Oncol Lett. 2021; 21: 483
        • Yamada Y.
        • Simon-Keller K.
        • Belharazem-Vitacolonnna D.
        • et al.
        A tuft cell-like signature is highly prevalent in thymic squamous cell carcinoma and delineates new molecular subsets among the major lung cancer histotypes.
        J Thorac Oncol Off Publ Int Assoc Study Lung Cancer. 2021; 16: 1003-1016
        • Alberobello A.T.
        • Wang Y.
        • Beerkens F.J.
        • et al.
        PI3K as a potential therapeutic target in thymic epithelial tumors.
        J Thorac Oncol. 2016; 11: 1345-1356
        • Gökmen-Polar Y.
        • Sanders K.L.
        • Goswami C.P.
        • et al.
        Establishment and characterization of a novel cell line derived from human thymoma AB tumor.
        Lab Investig. 2012; 92: 1564-1573
        • Ehemann V.
        • Kern M.A.
        • Breinig M.
        • et al.
        Establishment, characterization and drug sensitivity testing in primary cultures of human thymoma and thymic carcinoma.
        Int J Cancer. 2008; 122: 2719-2725
        • Pelosi G.
        • Rodriguez J.
        • Viale G.
        • Rosai J.
        Typical and atypical pulmonary carcinoid tumor overdiagnosed as small-cell carcinoma on biopsy specimens: a major pitfall in the management of lung cancer patients.
        Am J Surg Pathol. 2005; 29: 179-187
        • Rindi G.
        • Klimstra D.S.
        • Abedi-Ardekani B.
        • et al.
        A common classification framework for neuroendocrine neoplasms: an International Agency for Research on Cancer (IARC) and World Health Organization (WHO) expert consensus proposal.
        Mod Pathol. 2018; 31: 1770-1786
        • Dinter H.
        • Bohnenberger H.
        • Beck J.
        • et al.
        Molecular classification of neuroendocrine tumors of the thymus.
        J Thorac Oncol. 2019; 14: 1472-1483
        • Ströbel P.
        • Zettl A.
        • Shilo K.
        • et al.
        Tumor genetics and survival of thymic neuroendocrine neoplasms: a multi-institutional clinicopathologic study.
        Genes Chromosomes Cancer. 2014; 53: 738-749
        • Oosterhuis J.W.
        • Looijenga L.H.J.
        Germ cell tumors from a developmental perspective: cells of origin, pathogenesis, and molecular biology (emerging pattern).
        in: Pathology and Biology of Human Germ Cell Tumors. 201. Springer, Berlin, Germany2017: 23-129
        • Kao C.S.
        • Bangs C.D.
        • Aldrete G.
        • Cherry A.M.
        • Ulbright T.M.
        A clinicopathologic and molecular analysis of 34 mediastinal germ cell tumors suggesting different modes of teratoma development.
        Am J Surg Pathol. 2018; 42: 1662-1673
        • Lee T.
        • Seo Y.
        • Han J.
        • Kwon G.Y.
        Analysis of chromosome 12p over-representation and clinicopathological features in mediastinal teratomas.
        Pathology. 2019; 51: 62-66
        • Rekhtman N.
        • Montecalvo J.
        • Chang J.C.
        • et al.
        SMARCA4-deficient thoracic sarcomatoid tumors represent primarily smoking-related undifferentiated carcinomas rather than primary thoracic sarcomas.
        J Thorac Oncol. 2020; 15: 231-247
        • Le Loarer F.
        • Watson S.
        • Pierron G.
        • et al.
        SMARCA4 inactivation defines a group of undifferentiated thoracic malignancies transcriptionally related to BAF-deficient sarcomas.
        Nat Genet. 2015; 47: 1200-1205
      4. Agaimy A, Haller F, Renner A, Niedermeyer J, Hartmann A, French CA. Misleading germ cell phenotype in pulmonary NUT carcinoma harboring the ZNF532-NUTM1 fusion [e-pub ahead of print]. Am J Surg Pathol. https://doi.org/10.1097/PAS.0000000000001774. Accessed July 8, 2021.

        • Haack H.
        • Johnson L.A.
        • Fry C.J.
        • et al.
        Diagnosis of NUT midline carcinoma using a NUT-specific monoclonal antibody.
        Am J Surg Pathol. 2009; 33: 984-991
        • Baranov E.
        • Black M.A.
        • Fletcher C.D.M.
        • Charville G.W.
        • Hornick J.L.
        Nuclear expression of DDIT3 distinguishes high-grade myxoid liposarcoma from other round cell sarcomas.
        Mod Pathol. 2021; 34: 1367-1372
        • Scapa J.V.
        • Cloutier J.M.
        • Raghavan S.S.
        • Peters-Schulze G.
        • Varma S.
        • Charville G.W.
        DDIT3 immunohistochemistry is a useful tool for the diagnosis of myxoid liposarcoma.
        Am J Surg Pathol. 2021; 45: 230-239
        • Robinson D.R.
        • Wu Y.M.
        • Kalyana-Sundaram S.
        • et al.
        Identification of recurrent NAB2-STAT6 gene fusions in solitary fibrous tumor by integrative sequencing.
        Nat Genet. 2013; 45: 180-185
        • Demicco E.G.
        • Wagner M.J.
        • Maki R.G.
        • et al.
        Risk assessment in solitary fibrous tumors: validation and refinement of a risk stratification model.
        Mod Pathol. 2017; 30: 1433-1442
        • Demicco E.G.
        • Park M.S.
        • Araujo D.M.
        • et al.
        Solitary fibrous tumor: a clinicopathological study of 110 cases and proposed risk assessment model.
        Mod Pathol. 2012; 25: 1298-1306
        • Baranov E.
        • McBride M.J.
        • Bellizzi A.M.
        • et al.
        A novel SS18-SSX fusion-specific antibody for the diagnosis of synovial sarcoma.
        Am J Surg Pathol. 2020; 44: 922-933
        • Perret R.
        • Velasco V.
        • Le Guellec S.
        • Coindre J.M.
        • Le Loarer F.
        The SS18-SSX antibody has perfect specificity for the SS18-SSX fusion protein: a validation study of 609 neoplasms including 2 unclassified tumors with SS18-non-SSX fusions.
        Am J Surg Pathol. 2021; 45: 582-584
        • Wassef M.
        • Blei F.
        • Adams D.
        • et al.
        Vascular anomalies classification: recommendations from the International Society for the study of vascular anomalies.
        Pediatrics. 2015; 136: e203-e214
        • Peifer M.
        • Hertwig F.
        • Roels F.
        • et al.
        Telomerase activation by genomic rearrangements in high-risk neuroblastoma.
        Nature. 2015; 526: 700-704
        • Sakane T.
        • Murase T.
        • Okuda K.
        • Masaki A.
        • Nakanishi R.
        • Inagaki H.
        Expression of cancer testis antigens in thymic epithelial tumors.
        Pathol Int. 2021; 71: 471-479
        • Huang Y.H.
        • Klingbeil O.
        • He X.Y.
        • et al.
        POU2F3 is a master regulator of a tuft cell-like variant of small cell lung cancer.
        Genes Dev. 2018; 32: 915-928