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The 2021 WHO Classification of Tumors of the Pleura: Advances Since the 2015 Classification

Published:January 13, 2022DOI:https://doi.org/10.1016/j.jtho.2021.12.014

      Abstract

      Substantial changes in the 2021 WHO Classification of Tumors of the Pleura and Pericardium since the 2015 WHO Classification include the following: (1) pleural and pericardial tumors have been combined in one chapter whereas in the 2015 WHO, pericardial tumors were classified with cardiac tumors; (2) well-differentiated papillary mesothelioma has been renamed well-differentiated papillary mesothelial tumor given growing evidence that these tumors exhibit relatively indolent behavior; (3) localized and diffuse mesothelioma no longer include the term “malignant” as a prefix; (4) mesothelioma in situ has been added to the 2021 classification because these lesions can now be recognized by loss of BAP1 and/or MTAP by immunohistochemistry and/or CDKN2A homozygous deletion by fluorescence in situ hybridization; (5) the three main histologic subtypes (i.e., epithelioid, biphasic, and sarcomatoid) remain the same but architectural patterns and cytologic and stromal features are more formally incorporated into the 2021 classification on the basis of their prognostic significance; (6) nuclear grading for epithelioid diffuse mesothelioma is introduced, and it is recommended to record this and other histologically prognostic features in pathology reports; (7) BAP1, EZH2, and MTAP immunohistochemistry have been found to be useful in separating benign mesothelial proliferations from mesothelioma; (8) biphasic mesothelioma can be diagnosed in small biopsies having both epithelioid and sarcomatoid components even if the amount of one component is less than 10%; and (9) the most frequently altered genes in diffuse pleural mesothelioma include BAP1, CDKN2A, NF2, TP53, SETD2, and SETDB1.

      Keywords

      Introduction

      The 2021 (fifth edition) WHO Classification of Thoracic Tumors has recently been published.
      WHO Classification of Tumours Editorial Board
      WHO Classification of Tumours: Thoracic Tumours.
      Pleural and pericardial tumors have been combined in one chapter whereas in the 2015 WHO, pericardial tumors were classified with cardiac tumors.
      • Travis W.D.
      • Brambilla E.
      • Burke A.P.
      • Marx A.
      • Nicholson A.G.
      Introduction to the 2015 World Health Organization classification of tumors of the lung, pleura, thymus, and heart.
      The objective of this review is to summarize changes in the classification of pleural and pericardial tumors that represent advances in our understanding and affect the diagnosis of these tumors.
      Although the three main histologic subtypes (i.e., epithelioid, biphasic, and sarcomatoid) remain, there have been significant changes in the classification of mesothelial tumors since 2015 (Table 1). For each entity, the 2021 classification provides essential and desirable diagnostic criteria, which may be useful to the practicing pathologist (Table 2). The prefix “malignant” has been omitted from localized and diffuse mesothelioma because all mesotheliomas are regarded as malignant now that well-differentiated papillary mesothelioma (WDPM) has been renamed as WDPM tumor (WDPMT), given its relatively indolent behavior.
      • Butnor K.J.
      • Sporn T.A.
      • Hammar S.P.
      • Roggli V.L.
      Well-differentiated papillary mesothelioma.
      ,
      • Galateau-Sallé F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      Therefore, the term malignant is not needed to separate mesotheliomas from WDPMT. Localized mesothelioma remains distinct from diffuse mesothelioma because localized mesotheliomas have been found to be associated with better prognosis when completely resected.
      • Marchevsky A.M.
      • Khoor A.
      • Walts A.E.
      • et al.
      Localized malignant mesothelioma, an unusual and poorly characterized neoplasm of serosal origin: best current evidence from the literature and the International Mesothelioma Panel.
      Table 12021 WHO Classification of Tumors of the Pleura and Pericardium: ICD-O Coding and Terminology
      TumorsICD-O Code
      Morphology codes are taken from the ICD-O, third edition, second revision (ICD-O-3.2) (REF 1256). 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.
      Mesothelial tumors
      Benign and preinvasive mesothelial tumors
       Adenomatoid tumor9054/0
       Well-differentiated papillary mesothelial tumor9052/1
      Change in terminology of a previous code.
       Mesothelioma in situ9050/2
      This new code was approved by the International Agency for Research on Cancer/WHO Committee for the ICD-O at its meeting in October 2020.
      Mesothelioma
       Localized mesothelioma9050/3
      Change in terminology of a previous code.
       Diffuse mesothelioma, NOS9050/3
      Change in terminology of a previous code.
      Sarcomatoid mesothelioma9051/3
      Epithelioid mesothelioma9052/3
      Mesothelioma, biphasic9053/3
      ICD-O, International Classification of Diseases for Oncology.
      Reprinted with permission from WHO Classification of Tumours, 5th edition; volume 5, Tumours of the pleura and pericardium, Page 4, Copyright (2021).
      a Morphology codes are taken from the ICD-O, third edition, second revision (ICD-O-3.2) (REF 1256). 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.
      b Change in terminology of a previous code.
      c This new code was approved by the International Agency for Research on Cancer/WHO Committee for the ICD-O at its meeting in October 2020.
      Table 22021 WHO Classification of Tumors of the Pleura and Pericardium: Essential and Desirable Diagnostic Criteria
      DiagnosisEssential CriteriaDesirable Criteria
      Benign and preinvasive mesothelial tumors
      Adenomatoid tumor
      • Focal proliferation of tubular spaces or vacuoles lined by flattened or cuboidal mesothelial cells in a fibrous stroma
      • Lack of diffuse or multifocal spread along pleura and absence of malignant histologic features, such as invasive growth into underlying stroma, cytologic atypia, necrosis, or sarcomatoid patterns
      • Immunohistochemistry for mesothelial markers, if needed
      • Immunohistochemical staining for L1CAM, a marker of TRAF7 mutation, may be useful
      • BAP1 expression retained and absence of homozygous deletion of CDKN2A
      Well-differentiated papillary mesothelial tumor
      • Papillary stromal formations covered by bland mesothelium
      • No stromal invasion
      • Immunohistochemical staining for mesothelial markers
      • BAP1 expression retained
      Mesothelioma in situ
      • Pleural effusions (nonresolving)
      • No thoracoscopic or imaging evidence of tumor
      • Single layer of mesothelial cells (with or without atypia) on pleural surface
      • No histologic features of invasive growth
      • Loss of BAP1 and/or MTAP by immunohistochemistry and/or CDKN2A homozygous deletion by FISH
      • Multidisciplinary discussion of diagnosis
      • Papillary stromal formations covered by bland mesothelium
      • No stromal invasion
      Mesothelioma
      • Localized pleural mesothelioma
      • Presentation as a solitary localized mass by imaging, surgical findings, and histology
      • Examination of a surgical resection specimen revealing lack of invasion beyond the circumscribed borders of the tumor
      • Histologic features of diffuse mesothelioma
      • Immunohistochemical evidence of mesothelial origin
      • Multidisciplinary discussion to confirm the diagnosis
      • Diffuse pleural mesothelioma
      • Diffuse pleural thickening by a malignant neoplasm with epithelioid, sarcomatoid, or biphasic histology
      • Invasion of adjacent structures (i.e., adipose tissue, skeletal muscle, and/or lung parenchyma), tumor necrosis, or formation of unequivocal malignant tumor nodules
      • Desmoplastic mesothelioma is characterized by dense collagenized tissue separated by malignant mesothelial cells arranged in a storiform or so-called patternless pattern, which must be present in ≥50% of the tumor in definitive resection specimens
      • Biphasic mesothelioma is mesothelioma revealing ≥10% each of epithelioid and sarcomatoid patterns in definitive resection specimens or any percentage of each component in smaller biopsy and cytology specimens
      • Immunohistochemistry confirming mesothelial origin
      • Loss of BAP1 and/or MTAP by immunohistochemistry, and/or CDKN2A loss by FISH
      • Or mutations in BAP1 or CDKN2A demonstrated by next-generation sequencing
      FISH, fluorescence in situ hybridization.
      Reprinted with permission from WHO Classification of Tumours, 5th Edition; volume 5, Tumours of the pleura and pericardium, pages 193–226, Copyright (2021).
      The 2015 WHO classification
      • Galateau-Salle F.
      • Churg A.
      • Roggli V.
      • Travis W.D.
      World Health Organization Committee for Tumors of the Pleura
      The 2015 World Health Organization classification of tumors of the pleura: advances since the 2004 classification.
      recognized promising advances in the field of mesothelioma pathology, including histologic features with prognostic significance, nuclear grading of epithelioid diffuse pleural mesothelioma, and the use of BAP1 immunohistochemistry (IHC) and homozygous deletion of CDKN2A (9p21; encoding p16) by fluorescence in situ hybridization (FISH) in the separation of mesothelioma from reactive mesothelial proliferations, but these advances were not thoroughly incorporated into the classification until 2021.
      WHO Classification of Tumours Editorial Board
      WHO Classification of Tumours: Thoracic Tumours.
      Recent advances in the understanding of genomics of mesothelioma have led to increased recognition of a new entity, mesothelioma in situ (MIS), which was not formally recognized previously. Criteria for MIS have now been established and are included in the 2021 WHO classification.
      Mesenchymal tumors of the pleura have been moved to a new chapter titled “Mesenchymal tumours of the thorax.” An additional section in the “Metastases” chapter titled “Metastasis to the pleura” was added to the 2021 classification. The 2021 WHO Classification of Thoracic Tumors also includes a new chapter “Genetic tumour syndromes involving the thorax,” which includes a section titled “BAP1 tumour predisposition syndrome.” A review to address updates in hematolymphoid tumors of the pleura and pericardium will be published separately.

      Significant Advances in Pleural Mesothelioma

      WDPM Has Been Renamed WDPMT

      WDPM of the pleura is very rare.
      • Butnor K.J.
      • Sporn T.A.
      • Hammar S.P.
      • Roggli V.L.
      Well-differentiated papillary mesothelioma.
      Given their relatively indolent behavior, WDPM has been redesignated as WDPMT in the 2021 WHO classification to distinguish them from diffuse mesothelioma.
      WHO Classification of Tumours Editorial Board
      WHO Classification of Tumours: Thoracic Tumours.
      ,
      • Galateau-Sallé F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      The definition of WDPMT is now restricted to tumors consisting of papillary formations covered by a single layer of bland mesothelial cells that lack stromal invasion (Table 2; Fig. 1A and B). Accurate diagnosis of this rare tumor requires histologic examination of the entire lesion to exclude the possibility of superficial sampling from a component of an invasive diffuse mesothelioma. WDPMT typically appears as an arborescent mass or nodularity on the visceral or parietal pleura. Patients often present with dyspnea owing to recurrent pleural effusions. Asbestos exposure has been documented in some cases of WDPMT, but there are insufficient data to implicate causation owing to the rarity of this tumor.
      • Butnor K.J.
      • Sporn T.A.
      • Hammar S.P.
      • Roggli V.L.
      Well-differentiated papillary mesothelioma.
      ,
      • Galateau-Sallé F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      The papillary cores in WDPMT are paucicellular, ranging from myxoid to fibrovascular or hyalinized, and they lack inflammation (Fig. 1C). The thin mesothelial surface lining consists of flat to cuboidal cells with inconspicuous nucleoli, negligible mitotic activity, and immunoreactivity for conventional mesothelial markers.
      Figure thumbnail gr1
      Figure 1Well-differentiated papillary mesothelial tumor. (A) Arborescent fibrovascular papillary cores, covered by a thin layer of (B—high power) cytologically bland mesothelial cells that lack stromal invasion. (C) Hyalinized fibrovascular papillary cores in this example highlight the histologic spectrum of WDPMT. WDPMT, well-differentiated papillary mesothelioma tumor.
      Low-grade epithelioid diffuse mesothelioma can have florid papillary growth that appears deceptively similar to WDPMT if biopsies are superficial and do not sample deeper areas of invasive growth. Papillae can also occasionally form in reactive pleuritis, but they tend to be more broad based and have reactive cytologic changes and conspicuous accompanying inflammation, in contrast to WDPMT.
      Pleural WDPMT can have variable behavior, including a protracted clinical course.
      • Butnor K.J.
      • Sporn T.A.
      • Hammar S.P.
      • Roggli V.L.
      Well-differentiated papillary mesothelioma.
      Data on tumors with architectural features of WDPMT that have focal invasion are too sparse to permit definitive classification.
      • Galateau-Sallé F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      CDKN2A homozygous deletion has not been reported in WDPMT, and BAP1 is usually retained. BAP1 loss has only been reported in exceptional cases with synchronous or subsequent diffuse mesothelioma,
      • Lee H.E.
      • Molina J.R.
      • Sukov W.R.
      • Roden A.C.
      • Yi E.S.
      BAP1 loss is unusual in well-differentiated papillary mesothelioma and may predict development of malignant mesothelioma.
      raising the possibility of diffuse papillary mesothelioma mimicking WDPMT.

      MIS

      MIS is now recognized as a distinct entity and is regarded as a precursor to invasive mesothelioma. The diagnosis requires multidisciplinary correlation among histologic, immunohistochemical and/or molecular, clinical, and radiologic findings (Table 2). MIS is defined as a single layer of relatively bland mesothelial cells growing along the pleural surface (Fig. 2A) that have lost BAP1 (Fig. 2B) or MTAP by IHC or homozygous deletion of CDKN2A by FISH (Fig. 2B).
      • Churg A.
      • Galateau-Salle F.
      • Roden A.C.
      • et al.
      Malignant mesothelioma in situ: morphologic features and clinical outcome.
      • Churg A.
      • Hwang H.
      • Tan L.
      • et al.
      Malignant mesothelioma in situ.
      • Minami K.
      • Jimbo N.
      • Tanaka Y.
      • et al.
      Malignant mesothelioma in situ diagnosed by methylthioadenosine phosphorylase loss and homozygous deletion of CDKN2A: a case report.
      Most often, the cells have a cuboidal shape and inconspicuous nucleoli. In occasional cases, the cells form simple papillary structures, but still with a single layer of covering cells.
      Figure thumbnail gr2
      Figure 2Mesothelioma in situ. (A) Typical appearance of fairly bland cuboidal cells growing along the pleural surface. (B) BAP1 immunohistochemistry reveals loss of nuclear staining. Note the presence of positive internal control of nuclear staining in stromal and inflammatory cells.
      Invasive mesothelioma must be absent to make a diagnosis of MIS. A layer of markedly atypical large mesothelial cells or mesothelial proliferations consisting of piled up cells growing along the pleural surface or forming exophytic papillary structures protruding from the pleural surface is more likely to be surface spread from a concurrent invasive mesothelioma than MIS. Pleural fluid cytology specimens in isolation of other data cannot separate MIS from invasive mesothelioma. The latter determination requires information on the presence or absence of invasive tumor, which may be obtained from direct observation of the pleura or imaging in conjunction with a pleural biopsy.
      Most patients with MIS present with recurrent pleural effusions of unknown cause. By definition, there must be no evidence of tumor on imaging or by direct visual inspection of the pleura. The diagnosis requires multiple samples (ideally 100–200 mm2) from different areas of the pleura in addition to clinical and radiologic information.
      How long MIS can persist without development of an invasive mesothelioma is unclear, although progression 12 to 92 (median: 60) months after a biopsy diagnosis of MIS has been reported.
      • Churg A.
      • Galateau-Salle F.
      • Roden A.C.
      • et al.
      Malignant mesothelioma in situ: morphologic features and clinical outcome.

      Histologic Features With Prognostic Significance in Pleural Diffuse Epithelioid Mesothelioma

      Historically, the major histologic types of diffuse mesothelioma have been the main histologic indicators of prognosis. It is well known that patients with sarcomatoid and biphasic tumors have significantly worse overall survival compared with patients with epithelioid tumors.
      • Curran D.
      • Sahmoud T.
      • Therasse P.
      • van Meerbeeck J.
      • Postmus P.E.
      • Giaccone G.
      Prognostic factors in patients with pleural mesothelioma: the European Organization for Research and Treatment of Cancer experience.
      • Flores R.M.
      • Routledge T.
      • Seshan V.E.
      • et al.
      The impact of lymph node station on survival in 348 patients with surgically resected malignant pleural mesothelioma: implications for revision of the American Joint Committee on Cancer staging system.
      • Meyerhoff R.R.
      • Yang C.F.
      • Speicher P.J.
      • et al.
      Impact of mesothelioma histologic subtype on outcomes in the Surveillance, Epidemiology, and End Results database.
      • Rusch V.W.
      Extrapleural pneumonectomy and extended pleurectomy/decortication for malignant pleural mesothelioma: the Memorial Sloan-Kettering Cancer Center approach.
      • Rusch V.W.
      • Giroux D.
      Do we need a revised staging system for malignant pleural mesothelioma? Analysis of the IASLC database.
      • Rusch V.W.
      • Giroux D.
      • Kennedy C.
      • et al.
      Initial analysis of the international association for the study of lung cancer mesothelioma database.
      • Rusch V.W.
      • Rimner A.
      • Krug L.M.
      The challenge of malignant pleural mesothelioma: new directions.
      • Rusch V.W.
      • Venkatraman E.S.
      Important prognostic factors in patients with malignant pleural mesothelioma, managed surgically.
      • van Meerbeeck J.P.
      • Gaafar R.
      • Manegold C.
      • et al.
      Randomized phase III study of cisplatin with or without raltitrexed in patients with malignant pleural mesothelioma: an intergroup study of the European Organisation for Research and Treatment of Cancer Lung Cancer Group and the National Cancer Institute of Canada.
      In recent years, there has been increased recognition of histologic factors with prognostic significance that could improve risk stratification of patients with epithelioid diffuse pleural mesothelioma and inform clinical management decisions.
      • Nicholson A.G.
      • Sauter J.L.
      • Nowak A.K.
      • et al.
      EURACAN/IASLC proposals for updating the histologic classification of pleural mesothelioma: towards a more multidisciplinary approach.
      Although architectural patterns were discussed in the 2015 classification, architectural patterns, cytologic features, and stromal features are more formally incorporated in the 2021 classification (Table 3 and Fig. 3).
      • Nicholson A.G.
      • Sauter J.L.
      • Nowak A.K.
      • et al.
      EURACAN/IASLC proposals for updating the histologic classification of pleural mesothelioma: towards a more multidisciplinary approach.
      In addition, a nuclear grading system for epithelioid diffuse pleural mesothelioma (Table 4) has been developed, and its prognostic significance has been validated in multiple studies.
      • Kadota K.
      • Suzuki K.
      • Colovos C.
      • et al.
      A nuclear grading system is a strong predictor of survival in epitheloid diffuse malignant pleural mesothelioma.
      ,
      • Rosen L.E.
      • Karrison T.
      • Ananthanarayanan V.
      • et al.
      Nuclear grade and necrosis predict prognosis in malignant epithelioid pleural mesothelioma: a multi-institutional study.
      Table 32021 World Health Organization Classification of Diffuse Pleural Mesothelioma
      TypeDescriptionFeatures/PatternsFavorableUnfavorableReporting
      Epithelioid mesotheliomaComposed of round, epithelioid cells, usually with cohesive architecture, but single cells within a fibrous stroma may also be seenArchitectural patterns:

      Tubulopapillary

      Trabecular

      Adenomatoid

      Solid

      Micropapillary

      Cytologic features:

      Rhabdoid

      Deciduoid
      These cytologic features carry no prognostic significance but are important to recognize to avoid misdiagnosis with other entities in the differential diagnosis.


      Small cell
      These cytologic features carry no prognostic significance but are important to recognize to avoid misdiagnosis with other entities in the differential diagnosis.


      Clear cell
      These cytologic features carry no prognostic significance but are important to recognize to avoid misdiagnosis with other entities in the differential diagnosis.


      Signet ring
      These cytologic features carry no prognostic significance but are important to recognize to avoid misdiagnosis with other entities in the differential diagnosis.


      Lymphohistiocytoid

      Pleomorphic

      Stromal features:

      Myxoid
      Architectural patterns:

      Tubulopapillary

      Trabecular

      Adenomatoid

      Cytologic features:

      Lymphohistiocytoid

      Low nuclear grade
      See Table 4 for nuclear grading.


      Stromal features:

      Myxoid (if predominant, i.e., when ≥50% of tumor with <50% solid pattern contains myxoid stroma)
      Architectural patterns:

      Solid (≥50%)

      Micropapillary

      Cytologic features:

      Rhabdoid

      Pleomorphic

      High nuclear grade
      See Table 4 for nuclear grading.


      Necrosis (included in grading)
      Grade (high or low), architectural patterns present (and in definitive resection specimens, such as EPD and EPP, percentages of each pattern; for all other specimens, indicate “with … patterns/features”)
      Sarcomatoid mesothelioma, including desmoplastic patternComposed of elongated/spindle cells (>2 times longer than wide) arranged in solid sheets or within a fibrous stromaCytologic features:

      Lymphohistiocytoid

      Transitional

      Pleomorphic

      Stromal features:

      Desmoplastic

      With heterologous differentiation
      Cytologic features:

      Lymphohistiocytoid
      Cytologic features:

      Transitional
      Biphasic mesotheliomaComposed of both epithelioid and sarcomatoid components (in definitive resection specimens, namely EPD and EPP, ≥10% of each component is required for diagnosis); for smaller samples, including biopsy and cytology specimens, the diagnosis of biphasic mesothelioma can be rendered regardless of percentages of each component presentPercentage of sarcomatoid component should be reported regardless of specimen type
      EPD, extended pleurectomy/decortication; EPP, extrapleural pneumonectomy.
      Reprinted with permission from WHO Classification of Tumours, 5th Edition; volume 5, Tumours of the pleura and pericardium, page 208, Copyright (2021).
      a These cytologic features carry no prognostic significance but are important to recognize to avoid misdiagnosis with other entities in the differential diagnosis.
      b See Table 4 for nuclear grading.
      Figure thumbnail gr3
      Figure 3Poor prognostic histologic features in epithelioid diffuse pleural mesothelioma include (A) solid architectural pattern, (B) pleomorphic cytologic features, and (C) micropapillary architectural pattern. Pleural mesotheliomas with (D) transitional cytologic features are now classified as sarcomatoid mesothelioma because transitional cytologic features are associated with worse prognosis than epithelioid and biphasic mesothelioma. Pleural mesotheliomas with (E) lymphohistiocytoid cytologic features can be classified as epithelioid or sarcomatoid mesothelioma, and the presence of lymphohistiocytoid cytologic features is associated with better prognosis when seen in an otherwise sarcomatoid mesothelioma. The presence of (F) abundant myxoid stroma in more than or equal to 50% of an epithelioid mesothelioma with less than 50% solid pattern is associated with better prognosis.
      Table 4Nuclear Grading of Pleural Diffuse Epithelioid Mesothelioma
      Nuclear grade
      Nuclear atypia score1 for mild
      2 for moderate
      3 for severe
      Mitotic count score1 for low (≤1 mitosis/2 mm2)
      2 for intermediate (2–4 mitoses/2 mm2)
      3 for high (≥5 mitoses/2 mm2)
      Sum2 or 3 = nuclear grade I
      4 or 5 = nuclear grade II
      6 = nuclear grade III
      NecrosisPresent/absent
      Overall grade
       Low gradeNuclear grades I and II without necrosis
       High gradeNuclear grade II with necrosis

      Nuclear grade III with or without necrosis
      Adapted with permission from Nicholson et al.
      • Nicholson A.G.
      • Sauter J.L.
      • Nowak A.K.
      • et al.
      EURACAN/IASLC proposals for updating the histologic classification of pleural mesothelioma: towards a more multidisciplinary approach.
      Copyright 2020, with permission from Elsevier.
      The 2021 WHO classification also discusses cytologic features that can be found in epithelioid diffuse pleural mesothelioma that do not carry prognostic significance but should be recognized to avoid misdiagnosis with other entities in the differential diagnosis, including deciduoid, small cell, clear cell, and signet ring features. Use of the term “small cell mesothelioma” is discouraged to avoid confusion with small cell carcinoma.
      • Mayall F.G.
      • Gibbs A.R.
      The histology and immunohistochemistry of small cell mesothelioma.
      ,
      • Ordóñez N.G.
      Mesotheliomas with small cell features: report of eight cases.
      Histologic features seen in epithelioid diffuse mesothelioma that are associated with better prognosis include tubulopapillary, trabecular, or adenomatoid architectural patterns, lymphohistiocytoid cytologic features, or the presence of myxoid stroma
      • Alchami F.S.
      • Attanoos R.L.
      • Bamber A.R.
      Myxoid variant epithelioid pleural mesothelioma defines a favourable prognosis group: an analysis of 191 patients with pleural malignant mesothelioma.
      ,
      • Shia J.
      • Qin J.
      • Erlandson R.A.
      • et al.
      Malignant mesothelioma with a pronounced myxoid stroma: a clinical and pathological evaluation of 19 cases.
      (when predominant, defined as present in more than or equal to 50% of a tumor with less than 50% solid pattern). Myxoid stroma is seen in rare cases of mesothelioma and is characterized by epithelioid tumor cells that typically have no more than mild atypia floating in a matrix of loose mucoid stroma (Fig. 3F). Unfavorable histologic features in epithelioid diffuse mesothelioma include micropapillary pattern (Fig. 3C), solid pattern (Fig. 3A) when present in more than or equal to 50% of a tumor, rhabdoid or pleomorphic cytologic features, or the presence of necrosis.
      • Rosen L.E.
      • Karrison T.
      • Ananthanarayanan V.
      • et al.
      Nuclear grade and necrosis predict prognosis in malignant epithelioid pleural mesothelioma: a multi-institutional study.
      ,
      • Kadota K.
      • Suzuki K.
      • Sima C.S.
      • Rusch V.W.
      • Adusumilli P.S.
      • Travis W.D.
      Pleomorphic epithelioid diffuse malignant pleural mesothelioma: a clinicopathological review and conceptual proposal to reclassify as biphasic or sarcomatoid mesothelioma.
      • Ordóñez N.G.
      Mesothelioma with rhabdoid features: an ultrastructural and immunohistochemical study of 10 cases.
      • Ordóñez N.G.
      Pleomorphic mesothelioma: report of 10 cases.
      Rhabdoid features are characterized by tumor cells with cytoplasmic globules, resembling rhabdomyoblastic tumors, but express cytokeratins. Mesotheliomas with rhabdoid features should be myogenin negative and do not have the molecular abnormalities observed in rhabdomyosarcoma.
      • Ordóñez N.G.
      Mesothelioma with rhabdoid features: an ultrastructural and immunohistochemical study of 10 cases.
      Pleomorphic cytologic features in mesothelioma are characterized by tumor cells with prominent anaplastic, bizarre nuclei, and/or multinucleated tumor giant cells (Fig. 3B). In the 2015 WHO classification, mesotheliomas with pleomorphic features were classified as epithelioid tumors, but in the 2021 classification, these tumors can be classified as epithelioid, biphasic, or sarcomatoid on the basis of coexistent tumor cell morphology.
      Lymphohistiocytoid cytologic features can reveal morphologic features that mimic lymphoma or lymphoepithelial carcinoma. Lymphohistiocytoid features are characterized by marked lymphoid infiltrates composed of CD8-positive lymphocytes obscuring polygonal malignant mesothelial cells that have histiocytoid morphology (Fig. 3E). Lymphohistiocytoid features do not simply represent prominent lymphoid infiltration in an epithelioid mesothelioma.
      • Galateau-Sallé F.
      • Attanoos R.
      • Gibbs A.R.
      • et al.
      Lymphohistiocytoid variant of malignant mesothelioma of the pleura: a series of 22 cases.
      • Henderson D.W.
      • Attwood H.D.
      • Constance T.J.
      • Shilkin K.B.
      • Steele R.H.
      Lymphohistiocytoid mesothelioma: a rare lymphomatoid variant of predominantly sarcomatoid mesothelioma.
      • Kawai T.
      • Hiroi S.
      • Nakanishi K.
      • et al.
      Lymphohistiocytoid mesothelioma of the pleura.
      • Yao D.X.
      • Shia J.
      • Erlandson R.A.
      • Klimstra D.S.
      Lymphohistiocytoid mesothelioma: a clinical, immunohistochemical and ultrastructural study of four cases and literature review.
      Although mesotheliomas with lymphohistiocytoid cytologic features were previously classified under epithelioid mesothelioma, the 2021 WHO classification allows these tumors to be classified as epithelioid, biphasic, or sarcomatoid on the basis of tumor cell morphology.
      Mesotheliomas with transitional cytologic features are characterized by elongated yet plump and cohesive tumor cells that seem intermediate between epithelioid and sarcomatoid in morphology and have a sheet-like growth pattern (Fig. 3D). In the 2021 WHO classification, tumors with the presence of transitional pattern are now classified as sarcomatoid owing to recent studies revealing the presence of transitional features to be associated with worse prognosis.
      • Churg A.
      • Nabeshima K.
      • Ali G.
      • Bruno R.
      • Fernandez-Cuesta L.
      • Galateau-Salle F.
      Highlights of the 14th international mesothelioma interest group meeting: pathologic separation of benign from malignant mesothelial proliferations and histologic/molecular analysis of malignant mesothelioma subtypes.
      • Courtiol P.
      • Maussion C.
      • Moarii M.
      • et al.
      Deep learning-based classification of mesothelioma improves prediction of patient outcome.
      • Dacic S.
      • Le Stang N.
      • Husain A.
      • et al.
      Interobserver variation in the assessment of the sarcomatoid and transitional components in biphasic mesotheliomas.
      • Galateau Salle F.
      • Le Stang N.
      • Tirode F.
      • et al.
      Comprehensive molecular and pathologic evaluation of transitional mesothelioma assisted by deep learning approach: a multi-institutional study of the international mesothelioma panel from the MESOPATH Reference Center.
      Therefore, if transitional features are seen in an otherwise epithelioid tumor, the tumor could be classified as biphasic mesothelioma depending on specimen type and percentage of transitional features seen (see terminology and criteria for reporting in the subsequent texts).
      Because most patients with sarcomatoid mesothelioma have a very poor prognosis, there are fewer histologic features that are prognostically significant in these tumors. Nevertheless, sarcomatoid tumors with lymphohistiocytoid cytologic features have been found to have better prognosis.
      • Galateau-Sallé F.
      • Attanoos R.
      • Gibbs A.R.
      • et al.
      Lymphohistiocytoid variant of malignant mesothelioma of the pleura: a series of 22 cases.
      ,
      • Kawai T.
      • Hiroi S.
      • Nakanishi K.
      • et al.
      Lymphohistiocytoid mesothelioma of the pleura.

      Grading of Pleural Diffuse Epithelioid Mesothelioma

      Although preliminary data regarding prognostic significance of nuclear grading in epithelioid diffuse mesothelioma were mentioned in the 2015 WHO classification, a formal grading system was not included. Nevertheless, a two-tier nuclear grading system that incorporates nuclear atypia (Fig. 4A-C), mitoses, and the presence or absence of necrosis is now included in the 2021 classification for epithelioid diffuse mesothelioma.
      On the basis of an investigation of a large series of epithelioid diffuse malignant pleural mesothelioma, Kadota et al.
      • Kadota K.
      • Suzuki K.
      • Colovos C.
      • et al.
      A nuclear grading system is a strong predictor of survival in epitheloid diffuse malignant pleural mesothelioma.
      first proposed a nuclear grading system combining the two independent prognostic factors on multivariate analysis—nuclear atypia and mitotic count. After this study, Rosen et al.
      • Rosen L.E.
      • Karrison T.
      • Ananthanarayanan V.
      • et al.
      Nuclear grade and necrosis predict prognosis in malignant epithelioid pleural mesothelioma: a multi-institutional study.
      revealed the value of nuclear grade and necrosis in predicting overall survival in epithelioid diffuse pleural mesothelioma in a multi-institutional study that included many experts in mesothelioma pathology. They also revealed that the addition of necrosis to nuclear grade further stratified overall survival, allowing classification of epithelioid diffuse pleural mesothelioma into the following four distinct prognostic groups: nuclear grade I tumors without necrosis (29 mo), nuclear grade I tumors with necrosis and grade II tumors without necrosis (16 mo), nuclear grade II tumors with necrosis (10 mo), and nuclear grade III tumors (8 mo).
      • Rosen L.E.
      • Karrison T.
      • Ananthanarayanan V.
      • et al.
      Nuclear grade and necrosis predict prognosis in malignant epithelioid pleural mesothelioma: a multi-institutional study.
      On the basis of these results, a multidisciplinary group supported by European Reference Network for rare solid adult cancers (EURACAN)/International Association for the Study of Lung Cancer (IASLC) proposed a two-tier system of low and high grade for epithelioid diffuse pleural mesothelioma, based on a combination of nuclear features, mitotic rate, and the presence or absence of necrosis (Fig. 4).
      • Nicholson A.G.
      • Sauter J.L.
      • Nowak A.K.
      • et al.
      EURACAN/IASLC proposals for updating the histologic classification of pleural mesothelioma: towards a more multidisciplinary approach.
      According to the grading system described in the EURACAN/IASLC proposal and adopted by the 2021 WHO classification, areas revealing the highest-grade features should be used to assign tumors to low grade or high grade. All nuclear grade 1 tumors (with or without necrosis) and nuclear grade 2 tumors without necrosis are classified as low grade, and nuclear grade 2 tumors with necrosis and any nuclear grade 3 tumors are classified as high grade.
      • Nicholson A.G.
      • Sauter J.L.
      • Nowak A.K.
      • et al.
      EURACAN/IASLC proposals for updating the histologic classification of pleural mesothelioma: towards a more multidisciplinary approach.
      The 2021 WHO classification recommends routine reporting of the EURACAN/IASLC nuclear grade in both biopsy and resection specimens of epithelioid diffuse pleural mesothelioma to help identify tumors that may behave more aggressively. Zhang et al.
      • Zhang Y.Z.
      • Brambilla C.
      • Molyneaux P.L.
      • et al.
      Utility of nuclear grading system in epithelioid malignant pleural mesothelioma in biopsy-heavy setting: an external validation study of 563 cases.
      validated this two-tiered grading system in a large study of more than 500 biopsy specimens.
      Figure thumbnail gr4
      Figure 4The 2021 WHO two-tiered nuclear grading incorporates nuclear atypia. Examples of diffuse pleural mesothelioma with nuclear atypia scores of (A) mild, 1; (B) moderate, 2; and (C) severe, 3, are found.

      IHC in Diagnosis of Mesothelioma

      Claudin 4 with membranous and cytoplasmic staining has emerged as a reliable carcinoma marker in the differential diagnosis of mesothelioma and metastatic carcinoma, and it has been found to perform with a higher sensitivity (77%–100%) and specificity (99%–100%) than conventional carcinoma markers.
      • Jo V.Y.
      • Cibas E.S.
      • Pinkus G.S.
      Claudin-4 immunohistochemistry is highly effective in distinguishing adenocarcinoma from malignant mesothelioma in effusion cytology.
      • Patel A.
      • Borczuk A.C.
      • Siddiqui M.T.
      Utility of Claudin-4 versus BerEP4 and B72.3 in pleural fluids with metastatic lung adenocarcinoma.
      • Soini Y.
      • Kinnula V.
      • Kahlos K.
      • Pääkkö P.
      Claudins in differential diagnosis between mesothelioma and metastatic adenocarcinoma of the pleura.
      BAP1 (Fig. 5A) and EZH2 IHC and CDKN2A homozygous deletion by FISH (Fig. 5C and D) have emerged as reliable markers for the separation of benign mesothelial proliferation versus mesothelioma , but they are not appropriate for distinguishing mesothelioma from other malignant tumors.
      • Ang P.P.
      • Tan G.C.
      • Karim N.
      • Wong Y.P.
      Diagnostic value of the EZH2 immunomarker in malignant effusion cytology.
      • Bott M.
      • Brevet M.
      • Taylor B.S.
      • et al.
      The nuclear deubiquitinase BAP1 is commonly inactivated by somatic mutations and 3p21.1 losses in malignant pleural mesothelioma.
      • Churg A.
      • Galateau-Salle F.
      The separation of benign and malignant mesothelial proliferations.
      • Hwang H.C.
      • Pyott S.
      • Rodriguez S.
      • et al.
      BAP1 Immunohistochemistry and p16 FISH in the diagnosis of sarcomatous and desmoplastic mesotheliomas.
      • Monaco S.E.
      • Shuai Y.
      • Bansal M.
      • Krasinskas A.M.
      • Dacic S.
      The diagnostic utility of p16 FISH and GLUT-1 immunohistochemical analysis in mesothelial proliferations.
      • Shinozaki-Ushiku A.
      • Ushiku T.
      • Morita S.
      • Anraku M.
      • Nakajima J.
      • Fukayama M.
      Diagnostic utility of BAP1 and EZH2 expression in malignant mesothelioma.
      • Testa J.R.
      • Cheung M.
      • Pei J.
      • et al.
      Germline BAP1 mutations predispose to malignant mesothelioma.
      • Yoshimura M.
      • Kinoshita Y.
      • Hamasaki M.
      • et al.
      Highly expressed EZH2 in combination with BAP1 and MTAP loss, as detected by immunohistochemistry, is useful for differentiating malignant pleural mesothelioma from reactive mesothelial hyperplasia.
      In addition, cytoplasmic loss of MTAP expression by IHC (Fig. 5B) occurs in approximately 90% of tumors with homozygous deletion of CDKN2a because these two genes reside within close proximity on the 9p21 region.
      • Illei P.B.
      • Ladanyi M.
      • Rusch V.W.
      • Zakowski M.F.
      The use of CDKN2A deletion as a diagnostic marker for malignant mesothelioma in body cavity effusions.
      Therefore, MTAP IHC is increasingly used in clinical practice as a surrogate marker for CDKN2A homozygous deletion.
      • Berg K.B.
      • Dacic S.
      • Miller C.
      • Cheung S.
      • Churg A.
      Utility of methylthioadenosine phosphorylase compared with BAP1 immunohistochemistry, and CDKN2A and NF2 fluorescence in situ hybridization in separating reactive mesothelial proliferations from epithelioid malignant mesotheliomas.
      It is important to also note that these markers have variable sensitivity for diffuse pleural mesothelioma; therefore, the analysis of retained expression of BAP1, EZH2, or MTAP by IHC or the absence of CDKN2a homozygous deletion by FISH does not exclude the diagnosis of diffuse pleural mesothelioma.
      Figure thumbnail gr5
      Figure 5Analysis of loss of (A) BAP1 and (B) MTAP expression by immunohistochemistry (note the presence of internal positive controls) and (D and C) homozygous deletion of CDKN2A detected by FISH are reliable in supporting a diagnosis of mesothelioma when benign mesothelial proliferations are in the differential diagnosis. These ancillary studies can be performed on cytology specimens. BAP1 immunohistochemistry found here was performed on cell block material. FISH, fluorescence in situ hybridization.
      The 2021 WHO classification emphasizes the importance of rigorous validation of ancillary studies, particularly with BAP1 IHC, given the increased acceptance of their utility and increasingly widespread use for the diagnosis of mesothelioma. Reliable validation of nonpredictive marker assays requires 10 positive and 10 negative samples, with results comparable with published data on sensitivities and specificities.
      • Fitzgibbons P.L.
      • Bradley L.A.
      • Fatheree L.A.
      • et al.
      Principles of analytic validation of immunohistochemical assays: guideline from the College of American Pathologists Pathology and Laboratory Quality Center.
      These markers have also been found to be useful in effusion specimens when proper validation of these antibodies on cytology material is performed in individual laboratories.
      • Fitzgibbons P.L.
      • Bradley L.A.
      • Fatheree L.A.
      • et al.
      Principles of analytic validation of immunohistochemical assays: guideline from the College of American Pathologists Pathology and Laboratory Quality Center.
      • Sauter J.L.
      • Grogg K.L.
      • Vrana J.A.
      • Law M.E.
      • Halvorson J.L.
      • Henry M.R.
      Young investigator challenge: validation and optimization of immunohistochemistry protocols for use on cellient cell block specimens.
      • Buonocore D.J.
      • Konno F.
      • Jungbluth A.A.
      • et al.
      CytoLyt fixation significantly inhibits MIB1 immunoreactivity whereas alternative Ki-67 clone 30-9 is not susceptible to the inhibition: critical diagnostic implications.
      • Jain D.
      • Nambirajan A.
      • Borczuk A.
      • et al.
      Immunocytochemistry for predictive biomarker testing in lung cancer cytology.
      If a laboratory uses fixatives other than buffered formalin, a common practice for cytology specimens, the laboratory is obligated to validate the performance of the test in samples fixed with the alternative fixative against samples fixed in buffered formalin because some antibodies do not perform well after alcohol fixation of specimens.
      • Sauter J.L.
      • Grogg K.L.
      • Vrana J.A.
      • Law M.E.
      • Halvorson J.L.
      • Henry M.R.
      Young investigator challenge: validation and optimization of immunohistochemistry protocols for use on cellient cell block specimens.
      ,
      • Wolff A.C.
      • Hammond M.E.
      • Hicks D.G.
      • et al.
      Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update.

      Diagnostic Criteria for Reporting Pleural Diffuse Mesothelioma by Specimen Type

      The 2021 WHO classification includes specific recommendations for the reporting of pleural diffuse mesothelioma on the basis of specimen type. The classification provides reporting recommendations and templates to assist in reporting diagnoses in definitive resection specimens (i.e., extended pleurectomy/decortication [EPD] and extrapleural pneumonectomy [EPP]) and all other smaller specimens (i.e., smaller biopsy specimens and cytology) (Table 5).
      Table 5Examples of Pathology Reporting a Diffuse Pleural Mesothelioma in Biopsy and Resection Specimens (i.e., Extended Pleurectomy/Extrapleural Pneumonectomy)
      Specimens
      Small specimens (i.e. biopsy and cytology specimens):
      Tumor site, specimen type:
       Histologic type (epithelioid, biphasic,
      When a diagnosis of biphasic mesothelioma is made, a comment should be included to indicate the percentage of sarcomatoid component present.
      or sarcomatoid; if desmoplastic features are present, include “with desmoplastic features”)
       High/low grade (use only for epithelioid)
       List all architectural patterns (do not give a percentage) and any cytologic or stromal features present (do not give a percentage)
      Example of a pathology report for a biopsy specimen:
      Pleura (biopsy): epithelioid mesothelioma, high grade. Solid pattern and with rhabdoid cytologic features
      Resection specimens (i.e., extended pleurectomy/extrapleural pneumonectomy):
      Tumor site, specimen type:
       Histologic type (epithelioid, biphasic,
      When a diagnosis of biphasic mesothelioma is made, a comment should be included to indicate the percentage of sarcomatoid component present.
      or sarcomatoid/desmoplastic)
       High/low grade (use only for epithelioid)
       List all architectural patterns present (give a predominant pattern and percentages for each pattern listed) and any cytologic and/or stromal features present
       Staging
      Using the TNM staging system.
      Example of a pathology report for a resection specimen:
      Extended pleurectomy: Epithelioid mesothelioma, high grade. Predominantly tubulopapillary pattern (80%), also with micropapillary pattern (20%) and pleomorphic features (20%).
      AJCC
      Using the TNM staging system.
      stage (eighth edition): pT1pN0
      AJCC, American Joint Committee on Cancer.
      Adapted with permission from Nicholson et al.
      • Nicholson A.G.
      • Sauter J.L.
      • Nowak A.K.
      • et al.
      EURACAN/IASLC proposals for updating the histologic classification of pleural mesothelioma: towards a more multidisciplinary approach.
      Copyright 2020, with permission from Elsevier.
      a When a diagnosis of biphasic mesothelioma is made, a comment should be included to indicate the percentage of sarcomatoid component present.
      b Using the TNM staging system.
      A minimum of 10% of either epithelioid or sarcomatoid component remains the criterion for the diagnosis of biphasic mesothelioma in definitive resection specimens (i.e., EPD/EPP) but is no longer required for the diagnosis of biphasic mesothelioma in smaller specimens, including biopsy and cytology specimens. The percentage of sarcomatoid component should be reported for biphasic mesotheliomas regardless of specimen type. Similarly, desmoplastic mesothelioma can be diagnosed in definitive resection specimens (i.e., EPD/EPP) if more than 50% of the tumor has desmoplastic features, characterized by spindle cells with minimal atypia arranged in a haphazard pattern within a dense/hyalinized stroma. In small biopsy specimens, the designation “with desmoplastic features” is recommended when these morphologic features are present.
      For epithelioid diffuse mesothelioma, the grade (high or low) and any architectural patterns and/or cytologic and/or stromal features present in the tumor should be reported. In definitive resection specimens (i.e., EPD/EPP), the percentages of each pattern/feature should be reported. For all other specimen types, reporting should include “with … patterns/features.”
      Staging should be included in pathology reports for definitive resection specimens (i.e., EPD/EPP) using the TNM staging system.

      Differential Diagnoses of Diffuse Pleural Mesothelioma: Thoracic SMARCA4-Deficient Undifferentiated Tumor

      Thoracic SMARCA4-deficient undifferentiated tumor (TSDUT) is a recently described entity recognized in the 2021 WHO classification.
      • Hasegawa T.
      • Matsuno Y.
      • Shimoda T.
      • Umeda T.
      • Yokoyama R.
      • Hirohashi S.
      Proximal-type epithelioid sarcoma: a clinicopathologic study of 20 cases.
      • Le Loarer F.
      • Watson S.
      • Pierron G.
      • et al.
      SMARCA4 inactivation defines a group of undifferentiated thoracic malignancies transcriptionally related to BAF-deficient sarcomas.
      • Perret R.
      • Chalabreysse L.
      • Watson S.
      • et al.
      SMARCA4-deficient thoracic sarcomas: clinicopathologic study of 30 cases with an emphasis on their nosology and differential diagnoses.
      • 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.
      • Yoshida A.
      • Kobayashi E.
      • Kubo T.
      • et al.
      Clinicopathological and molecular characterization of SMARCA4-deficient thoracic sarcomas with comparison to potentially related entities.
      • Sauter J.L.
      • Graham R.P.
      • Larsen B.T.
      • et al.
      SMARCA4-deficient thoracic sarcoma: a distinctive clinicopathological entity with undifferentiated rhabdoid mrophology and aggressive behaviro.
      TSDUTs are in the differential diagnosis of diffuse pleural mesothelioma because they can involve the pleura. The typical morphologic features of TSDUTs include solid sheets of discohesive epithelioid tumor cells with or without rhabdoid-like features and often with tumor necrosis. The morphologic features of TSDUTs (Fig. 6A) can be similar to some epithelioid mesotheliomas (Fig. 6B). TSDUTs typically are negative for keratins, or have only very focal keratin expression, are often positive for stem cell markers, including CD34, SOX2, and SALL4, and have loss of BRG1 (SMARCA4) by IHC (Fig. 6C–F) or mutations in the SMARCA4 gene by sequencing. The use of mesothelial markers in the workup of pleural tumors should also help differentiate mesothelioma from TSDUTs.
      Figure thumbnail gr6
      Figure 6The 2021 WHO recognizes a new entity (A) TSDUT, which expands the differential diagnosis of (B) diffuse pleural mesothelioma because some pleural mesotheliomas have morphologic features similar to those of TSDUTs. TSDUTs have a distinct immunophenotype and are often negative or only focally positive for (C) cytokeratins (cytokeratin AE1/AE3 shown), have loss of (D) BRG1 (SMARCA4) expression, and can be positive for stem cell markers, such as (E) CD34 and (F) SALL4. TSDUT, thoracic SMARCA4-deficient undifferentiated tumor.

      Genetics

      Several large-scale sequencing studies addressing molecular profiles of diffuse pleural mesothelioma have been published since the 2015 WHO classification advancing the understanding of the genomic landscape of diffuse mesothelioma. Most studies focused on improving the histologic classification and prognostic stratification of pleural mesothelioma and revealed intratumoral genomic heterogeneity.
      • Bueno R.
      • Stawiski E.W.
      • Goldstein L.D.
      • et al.
      Comprehensive genomic analysis of malignant pleural mesothelioma identifies recurrent mutations, gene fusions and splicing alterations.
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      • Blum Y.
      • Meiller C.
      • Quetel L.
      • et al.
      Dissecting heterogeneity in malignant pleural mesothelioma through histo-molecular gradients for clinical applications.
      • Alcala N.
      • Mangiante L.
      • Le-Stang N.
      • et al.
      Redefining malignant pleural mesothelioma types as a continuum uncovers immune-vascular interactions.
      • Quetel L.
      • Meiller C.
      • Assié J.B.
      • et al.
      Genetic alterations of malignant pleural mesothelioma: association with tumor heterogeneity and overall survival.
      • Meiller C.
      • Montagne F.
      • Hirsch T.Z.
      • et al.
      Multi-site tumor sampling highlights molecular intra-tumor heterogeneity in malignant pleural mesothelioma.
      Bueno et al.
      • Bueno R.
      • Stawiski E.W.
      • Goldstein L.D.
      • et al.
      Comprehensive genomic analysis of malignant pleural mesothelioma identifies recurrent mutations, gene fusions and splicing alterations.
      reported four cluster groups of mesotheliomas on the basis of expression patterns that mostly matched the 2015 WHO histologic classification and correlated with overall survival. Those clusters included sarcomatoid, epithelioid, biphasic-epithelioid, and biphasic-sarcomatoid, recapitulating epithelial-to-mesenchymal transition. Similarly, the TCGA cohort identified four distinct prognostic groups on the basis of genomic, transcriptomic, and epigenomic analysis
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      (Fig. 7). By combining transcriptome, methylome, and miRNome analysis, Blum et al.
      • Blum Y.
      • Meiller C.
      • Quetel L.
      • et al.
      Dissecting heterogeneity in malignant pleural mesothelioma through histo-molecular gradients for clinical applications.
      revealed that pleural mesotheliomas have different proportions of epithelioid and sarcomatoid components (E- score and S-score). Alcala et al.
      • Alcala N.
      • Mangiante L.
      • Le-Stang N.
      • et al.
      Redefining malignant pleural mesothelioma types as a continuum uncovers immune-vascular interactions.
      revealed the link between those scores and the mesothelioma microenvironment. Transcriptome analysis of previously recognized epithelioid pattern of transitional mesothelioma provided the rationale for its reclassification as a cytologic feature of sarcomatoid mesothelioma as it revealed genomic characteristics similar to sarcomatoid subtype.
      • Galateau Salle F.
      • Le Stang N.
      • Tirode F.
      • et al.
      Comprehensive molecular and pathologic evaluation of transitional mesothelioma assisted by deep learning approach: a multi-institutional study of the international mesothelioma panel from the MESOPATH Reference Center.
      Genomic analysis of pleomorphic mesothelioma revealed molecular characteristics shared with both epithelioid and sarcomatoid subtypes and therefore was reclassified from epithelioid pattern to a cytologic feature that could be associated with either sarcomatoid or epithelioid subtypes.
      • Roy S.
      • Galateau-Sallé F.
      • Le Stang N.
      • et al.
      Molecular characterization of pleomorphic mesothelioma: a multi-institutional study.
      Figure thumbnail gr7
      Figure 7Genomic features of pleural mesothelioma reported by The Cancer Genome Atlas. Reprinted with permission from Hmeljak et al.
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      INDEL, insertion or deletion; NA, not applicable; NOS, not otherwise specified; SCNA, somatic copy number alterations; Syn, synchronous.
      Large-scale comprehensive testing, such as next-generation sequencing, can be successfully performed on pleural fluid or tissue samples, but identification of druggable targets is very low.
      • Sneddon S.
      • Dick I.
      • Lee Y.C.G.
      • et al.
      Malignant cells from pleural fluids in malignant mesothelioma patients reveal novel mutations.
      ,
      • Lo Iacono M.
      • Monica V.
      • Righi L.
      • et al.
      Targeted next-generation sequencing of cancer genes in advanced stage malignant pleural mesothelioma: a retrospective study.
      Because of the lack of identifiable targets on most of the commercially available next-generation sequencing platforms, testing for predictive biomarkers of response is not recommended as a routine clinical practice at this time.
      • Kindler H.L.
      • Ismaila N.
      • Armato 3rd, S.G.
      • et al.
      Treatment of malignant pleural mesothelioma: American Society of Clinical Oncology Clinical Practice Guideline.
      ,
      • Nicolini F.
      • Bocchini M.
      • Bronte G.
      • et al.
      Malignant pleural mesothelioma: state-of-the-art on current therapies and promises for the future.
      Somatic mutation burden in mesothelioma is low, usually less than two nonsynonymous mutations per megabase, and with no difference among histologic subtypes.
      • Bueno R.
      • Stawiski E.W.
      • Goldstein L.D.
      • et al.
      Comprehensive genomic analysis of malignant pleural mesothelioma identifies recurrent mutations, gene fusions and splicing alterations.
      ,
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      Somatic copy number alterations, primarily deletions, and most frequently CDKN2A, are the most common genetic events.
      • Bueno R.
      • Stawiski E.W.
      • Goldstein L.D.
      • et al.
      Comprehensive genomic analysis of malignant pleural mesothelioma identifies recurrent mutations, gene fusions and splicing alterations.
      ,
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      CDKN2A homozygous deletions most frequently occur in sarcomatoid mesotheliomas, followed by biphasic and epithelioid, and are an established diagnostic marker of malignant mesothelial proliferations.
      • Churg A.
      • Galateau-Salle F.
      The separation of benign and malignant mesothelial proliferations.
      ,
      • Chiosea S.
      • Krasinskas A.
      • Cagle P.T.
      • Mitchell K.A.
      • Zander D.S.
      • Dacic S.
      Diagnostic importance of 9p21 homozygous deletion in malignant mesotheliomas.
      Most frequently mutated genes are BAP1, NF2, TP53, SETD2, DDX3X, ULK2, RYR2, CFAP45, SETDB1, and DDX51.
      • Bueno R.
      • Stawiski E.W.
      • Goldstein L.D.
      • et al.
      Comprehensive genomic analysis of malignant pleural mesothelioma identifies recurrent mutations, gene fusions and splicing alterations.
      ,
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      In the TCGA cohort, a subset of mesotheliomas with TP53 and SETDB1 co-mutations associated with genome-wide loss of heterozygosity which affects more than 80% of the genome (“genomic near-haploidization”) was identified mostly in young female patients.
      • Hmeljak J.
      • Sanchez-Vega F.
      • Hoadley K.A.
      • et al.
      integrative molecular characterization of malignant pleural mesothelioma.
      In addition, recurrent EWSR1/FUS-ATF1 fusions have been detected in mesotheliomas in younger adults.
      • Desmeules P.
      • Joubert P.
      • Zhang L.
      • et al.
      A subset of malignant mesotheliomas in young adults are associated with recurrent EWSR1/FUS-ATF1 fusions.
      Recent clinical trials revealed promising results for efficacy of immune checkpoint inhibitors in the treatment of pleural mesotheliomas, but no definitive predictive biomarker has been identified.
      • Menis J.
      • Pasello G.
      • Remon J.
      Immunotherapy in malignant pleural mesothelioma: a review of literature data.
      The predictive role of programmed death-ligand 1 (PD-L1) expression in pleural mesotheliomas remains controversial, and there is no defined predictive cutoff of response. Nevertheless, PD-L1–positive pleural mesotheliomas respond better to immune checkpoint inhibitors than those that are negative for PD-L1.
      • Disselhorst M.J.
      • Quispel-Janssen J.
      • Lalezari F.
      • et al.
      Ipilimumab and nivolumab in the treatment of recurrent malignant pleural mesothelioma (INITIATE): results of a prospective, single-arm, phase 2 trial.
      Several trials are ongoing to investigate the role and prognostic value of the immune microenvironment of mesotheliomas. Molecular S- and E-scores seem to reflect on mesothelioma microenvironment and may have predictive value.
      • Blum Y.
      • Meiller C.
      • Quetel L.
      • et al.
      Dissecting heterogeneity in malignant pleural mesothelioma through histo-molecular gradients for clinical applications.
      ,
      • Alcala N.
      • Mangiante L.
      • Le-Stang N.
      • et al.
      Redefining malignant pleural mesothelioma types as a continuum uncovers immune-vascular interactions.
      The S-score correlated with the presence of T-cells, monocytes, fibroblasts, and endothelial cells and high expression of PD-L1. The E-score was associated with infiltration of natural killer cells, complement pathway, and VISTA overexpression. These results are consistent with reports of frequent association of PD-L1 protein expression and sarcomatoid mesotheliomas, poor prognosis, and increased lymphocytic inflammation.
      • Wadowski B.
      • Bueno R.
      • De Rienzo A.
      Immune microenvironment and genetics in malignant pleural mesothelioma.
      • Napoli F.
      • Listì A.
      • Zambelli V.
      • et al.
      Pathological characterization of Tumor Immune Microenvironment (TIME) in malignant pleural mesothelioma.
      • Brcic L.
      • Klikovits T.
      • Megyesfalvi Z.
      • et al.
      Prognostic impact of PD-1 and PD-L1 expression in malignant pleural mesothelioma: an international multicenter study.
      • Fusco N.
      • Vaira V.
      • Righi I.
      • et al.
      Characterization of the immune microenvironment in malignant pleural mesothelioma reveals prognostic subgroups of patients.
      • Brosseau S.
      • Danel C.
      • Scherpereel A.
      • et al.
      Shorter survival in malignant pleural mesothelioma patients with high PD-L1 expression associated with sarcomatoid or biphasic histology subtype: a series of 214 cases from the Bio-MAPS cohort.
      • Sobhani N.
      • Roviello G.
      • Pivetta T.
      • et al.
      Tumour infiltrating lymphocytes and PD-L1 expression as potential predictors of outcome in patients with malignant pleural mesothelioma.
      • Combaz-Lair C.
      • Galateau-Sallé F.
      • McLeer-Florin A.
      • et al.
      Immune biomarkers PD-1/PD-L1 and TLR3 in malignant pleural mesotheliomas.
      • Muller S.
      • Victoria Lai W.
      • Adusumilli P.S.
      • et al.
      V-domain Ig-containing suppressor of T-cell activation (VISTA), a potentially targetable immune checkpoint molecule, is highly expressed in epithelioid malignant pleural mesothelioma.
      TRAF mutations have been described in localized pleural mesothelioma
      • Hung Y.P.
      • Dong F.
      • Dubuc A.M.
      • Dal Cin P.
      • Bueno R.
      • Chirieac L.R.
      Molecular characterization of localized pleural mesothelioma.
      and in adenomatoid tumors of the genital tract
      • Goode B.
      • Joseph N.M.
      • Stevers M.
      • et al.
      Adenomatoid tumors of the male and female genital tract are defined by TRAF7 mutations that drive aberrant NF-kB pathway activation.
      but have not yet been identified in thoracic adenomatoid tumors, although these are exceedingly rare with less than 20 reported in the thoracic region.
      • Kaplan M.A.
      • Tazelaar H.D.
      • Hayashi T.
      • Schroer K.R.
      • Travis W.D.
      Adenomatoid tumors of the pleura.
      • Kelly S.
      European drug regulation.
      • Minato H.
      • Nojima T.
      • Kurose N.
      • Kinoshita E.
      Adenomatoid tumor of the pleura.
      BAP1 mutations and genomic near haploidization have also been described in localized pleural mesothelioma.
      • Hung Y.P.
      • Dong F.
      • Dubuc A.M.
      • Dal Cin P.
      • Bueno R.
      • Chirieac L.R.
      Molecular characterization of localized pleural mesothelioma.

      Summary

      In summary, this review provides an update to the changes in the WHO Classification of Pleural and Pericardial Tumors on the basis of recent advances since the 2015 classification which have led to improvements in the diagnosis of these tumors.

      CRediT Authorship Contribution Statement

      Jennifer L. Sauter, William D. Travis: Conceptualization, Data curation, Writing - original draft, Writing - review & editing.
      Sanja Dacic, Kelly J. Butnor, Andrew Churg, and Kyuichi Kadota: Writing - original draft, Writing - review & editing.
      Francoise Galateau-Salle, Richard L. Attanoos, Aliya N. Husain, Andras Khoor, Andrew G. Nicholson, Victor Roggli, Fernando Schmitt, and Ming-Sound Tsao: Writing - review & editing.

      Acknowledgments

      The authors wish to thank Professor Ian Cree (Editorial Board Chair), all members of the Editorial Board for the Thoracic Tumor Book, and International Agency for Research on Cancer staff involved in the WHO Classification of Tumours, fifth edition series, for their contribution to the completion of the fifth edition book. The authors also thank Ms. Francis Bodd and Ms. Jessica Lopardo for their assistance with manuscript preparation.

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