Advertisement
Surgical Pathology Clinics

Benign and Malignant Mesothelial Proliferation

  • E. Handan Zeren
    Correspondence
    Corresponding author. Department of Pathology, Faculty of Medicine, Çukurova University, Adana 01330, Turkey.
    Affiliations
    Department of Pathology, Faculty of Medicine, Çukurova University, Adana 01330, Turkey

    Department of Pathology, Acıbadem Medical Group, Maslak Hospital, Büyükdere Caddesi 40, Istanbul 34457, Turkey
    Search for articles by this author
  • Funda Demirag
    Affiliations
    Department of Pathology, Atatürk Chest Diseases and Chest Surgery Education and Research Hospital, Ankara 06280, Turkey
    Search for articles by this author

      Keywords

      Mesothelial cells lining the pleural, pericardial, and peritoneal cavities are specialized coelomic epithelial cells and are, therefore, mesodermal in origin. Embryonic mesothelial cells contribute to the coronary and intestinal vasculature by differentiating into endothelial cells, vascular smooth muscle cells, and pericytes. This differentiation process is an epithelial-mesenchymal transition.
      • Witowski J.
      • Ksiazek K.
      • Jorres A.
      New insights into the biology of peritoneal mesothelial cells: the roles of epithelial-to-mesenchymal transition and cellular senescence.
      Embryonic and adult lungs are both covered by a thin layer of mesothelial cells. During development, the mesothelium plays an important role in regulating the overall size and morphology of the lung through interactions with submesothelial mesenchyma.
      • Que J.
      • Wilm B.
      • Hasegawa H.
      • et al.
      Mesothelium contributes to vascular smooth muscle and mesenchyme during lung development.
      Adult mesothelial cells in the pleura perform vital functions, such as contributing a smooth and lubricated surface, initiating and resolving inflammation, repairing tissue, and secreting and reabsorbing electrolytes and pleural fluid.
      • Ji H.L.
      • Nie H.G.
      Electrolyte and fluid transport in mesothelial cells.
      Malignant mesothelioma (MM) is a tumor of mesothelial cells that is predominantly associated with asbestos exposure. The diagnosis of this rare tumor is challenging in most instances and should be supported by clinical and radiologic information. In contrast, reactive mesothelial cells can mimic a variety of malignancies, including MM and other types of cancer, in cytologic specimens and pleural biopsies. During the past few decades, several histologic criteria as well as many immunohistochemical markers have been proposed in differentiating the diagnosis of MM from benign mesothelial hyperplasia and metastatic adenocarcinomas of various sites.
      • Marchevsky A.M.
      Application of immunohistochemistry to the diagnosis of malignant mesothelioma.
      • Churg A.
      • Colby T.V.
      • Cagle P.
      • et al.
      The separation of benign and malignant mesothelial proliferations.
      Key Features
      • Mesothelial cell proliferation and many malignant and benign tumors may mimic pleural MM.
      • Benign mesothelial proliferation, simple or atypical, primarily occurs as a result of infection, collagen vascular disease, pulmonary infarct, drug reaction, pneumothorax, subpleural lung carcinoma, surgery, or trauma.
      • Well-differentiated papillary mesothelioma (WDPM) is a localized lesion with a good prognosis. It is composed of fibrovascular papillary projections covered by a single layer of bland mesothelial cells.
      • MM presents as firm, gray-white thickening of the pleura; however, nodular and localized gross features can be observed.
      • MM can be divided into 3 basic histologic types: epithelial, biphasic, and sarcomatous. Several uncommon histologic subtypes have been noted, including deciduoid, small cell, clear cell, and signet ring cell variants of epithelioid MM and desmoplastic and lymphohistiocytic variants of sarcomatous MM. The desmoplastic histologic variant can also exhibit epithelial features focally.
      • Sarcomatous mesothelioma is primarily composed of fibroblast-like spindle cells; however, it may exhibit highly anaplastic features. Pleural fluid examination is not diagnostic for sarcomatous mesothelioma because the tumor cells are never shed into the pleural cavity.
      • Immunohistochemistry is a useful tool in the diagnosis of MM; however, none of the mesothelial cell markers is 100% specific and highly sensitive. The most commonly used mesothelial cell markers are calretinin, keratin 5/6, WT1, HBME-1, thrombomodulin, mesothelin, and podoplanin (D2-40). Caveolin-1 and tenascin-X recently have been proposed.
      • Epithelial histology and extrapleural pneumonectomy are independent predictors of prolonged survival. Homozygous deletion of 9p21 and loss of p16 immunoreactivity are also associated with shorter survival.

      Gross features

      Macroscopically, MM is observed as a dense thickening of the pleura with firm white-yellow tissue (Fig. 1). Sometimes, it may show cystic areas containing mucoid material. The tumor often invades the diaphragm, fissures, and underlying atelectatic lung and, in advanced cases, mediastinal and pericardial involvement can be seen. It is also associated with pleural effusion. MM may present as multiple tumor nodules or, occasionally, as a solid and localized mass (localized MM). The parietal pleura is often more extensively involved. The majority of MM cases are unicavitary; however, simultaneous involvement of the pleura and peritoneum can occur.
      • Attanoos R.L.
      • Gibbs A.R.
      Pathology of malignant mesothelioma.
      At autopsy, distant metastases are identified in up to 50% of all cases.
      Diffuse serosal growth is not a specific feature of MM. Nonmesothelioma neoplasms, so-called pseudomesotheliomas, demonstrating a diffuse growth pattern have been described. Pulmonary and nonpulmonary carcinomas, sarcomas, and lymphomas may demonstrate this growth pattern.
      • Attanoos R.L.
      • Gibbs A.R.
      ‘Pseudomesotheliomatous’ carcinomas of the pleura: a 10-year analysis of cases from the Environmental Lung Disease Research Group, Cardiff.
      A diffuse pleural growth pattern is not a consistent feature of MM, however. In localized MM, the tumor appears as a circumscribed pleural-based mass, with a size of 2.2 to 15 cm, without any evidence of pleural spread.
      • Allen T.C.
      • Cagle P.T.
      • Churg A.M.
      • et al.
      Localized malignant mesothelioma.
      WDPM is a distinct mesothelial tumor with a good outcome. Its presentation consists of unilateral pleural effusion and superficial spreading of stout papillary formations with myxoid cores. It can also present with only pleural effusion and without any nodularity or mass lesion. Galateau-Salle and colleagues
      • Galateau-Salle F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      detected thin, focal pleural thickening in 9 of 24 cases (37.5%).
      Local spread, primarily through the chest wall, lungs, and mediastinum, is a more common feature of MM. Distant metastases are rare in malignant pleural mesothelioma (MPM). Occasionally, lymph node metastases are noted as the initial presentation of the tumor. Observations reported in the literature suggest that mediastinal nodes may be the primary sites of metastatic disease; however, hilar lymph node involvement occurs almost only after the lung invasion.
      • Abdel Rahman A.R.M.
      • Gaafar A..M.
      • Baki H.A.
      • et al.
      Prevalence and pattern of lymph node metastasis in malignant pleural mesothelioma.
      Tumor nodules and tumor-like pleural thickening are not observed in BMP; however, inflammation and effusion are frequently present. Diffuse pleural fibrosis is usually bilateral, and the thickness may exceed 5 mm in many areas. Extensive adhesions between the visceral pleura and the chest wall may occur and, radiologically, this feature can mimic MM.
      • Hasleton P.
      Pleural disease.

      Microscopic features

      Mesothelial cells have predominantly an elongated and flattened morphology and are inconspicuous microscopically (Fig. 2). Irritation of the pleural surface, however, leads to simple hyperplasia of the mesothelial cell layer. The mesothelial cells in simple hyperplasia form a regular layer of relatively bland cuboidal cells along the surface without any evidence of invasion. Distinct nucleoli may sometimes be present (Fig. 3). Resting mesothelial cells with cuboidal morphology have been identified in the spleen, liver, and diaphragm.
      • Yung S.
      • Chan T.M.
      Mesothelial cells.
      Figure thumbnail gr2
      Fig. 2Normal mesothelial cells with flat and inconspicious morphology (HE ×200).
      Figure thumbnail gr3
      Fig. 3Benign mesothelial cell proliferation shows bland cuboidal cells (HE ×400).
      In atypical mesothelial hyperplasia (AMH), which is a more florid mesothelial proliferation along the pleural surface than simple hyperplasia, the mesothelial cells are cytologically atypical with large nuclei, conspicuous nucleoli, and pleomorphism or form papillary projections. If these 2 features are noted simultaneously, the morphologic picture may raise concern for malignancy. Some cases of AMH, however, may represent the noninvasive stage of MM tumorigenesis and should be regarded as MM in situ.
      • Cagle P.T.
      • Churg A.
      Differential diagnosis of benign and malignant mesothelial proliferations on pleural biopsies.
      According to the 2004 World Health Organization classification, MM can be divided into 3 basic histologic types: epithelial, biphasic, and sarcomatous. The epithelial subtype is the most common histologic type and can exhibit a wide range of histologic variations. The most common patterns include tubulopapillary, adenomatoid (microglandular), acinar, and solid patterns, which are composed of atypical mesothelial cells that are usually bland and closely resemble reactive mesothelial cells (Fig. 4, Fig. 5, Fig. 6). Clear cell, deciduoid, small cell, and pleomorphic patterns are less common epithelial patterns (Fig. 7, Table 1).
      • Travis W.
      • Brambilla E.
      • Müller-Hermelink H.K.
      • et al.
      Pathology and genetics of tumours of the lung, pleura, thymus, and heart in World Health Organization Classification of Tumours.
      Figure thumbnail gr4
      Fig. 4Tubulopapillary pattern of MM (HE ×200).
      Figure thumbnail gr5
      Fig. 5Adenomatoid pattern of MM (HE ×400).
      Figure thumbnail gr6
      Fig. 6Solid pattern consists of bland looking neoplastic mesothelial cells (HE ×200).
      Figure thumbnail gr7a
      Fig. 7Small cell variant of MM composed of small cells with scanty cytoplasms (A [HE ×40], B [HE ×100], C [HE ×400]).
      Figure thumbnail gr7b
      Fig. 7Small cell variant of MM composed of small cells with scanty cytoplasms (A [HE ×40], B [HE ×100], C [HE ×400]).
      Figure thumbnail gr7c
      Fig. 7Small cell variant of MM composed of small cells with scanty cytoplasms (A [HE ×40], B [HE ×100], C [HE ×400]).
      Table 1The histologic patterns of MPM and differential diagnosis
      Data from Cagle PT, Churg A. Differential diagnosis of benign and malignant mesothelial proliferations on pleural biopsies. Arch Pathol Lab Med 2005;129:1421–7.
      Mesothelioma SubtypeDifferential Diagnosis
      Epithelial mesothelioma
      TubulopapillaryAdenocarcinoma, benign mesothelial proliferation
      PapillaryAdenocarcinoma, benign mesothelial proliferation
      TubularAdenocarcinoma
      AcinarAdenocarcinoma
      Adenomatoid (microglandular)Metastatic adenoid cystic carcinoma, adenocarcinoma
      Solid, well-differentiatedBenign mesothelial proliferation
      Solid, pleomorphicLymphoma, anaplastic large cell carcinoma
      DeciduoidSquamous cell carcinoma, anaplastic large cell lymphoma
      Small cellSmall cell carcinoma, small round cell tumor
      Clear cellRenal cell carcinoma, clear cell carcinoma of the lung
      Clear cell sarcoma, malignant melanoma
      Signet ring cellAdenocarcinoma
      Adenoid cysticAdenocarcinoma
      LymphangiomatoidLymphangioma
      Sarcomatoid mesotheliomaMetastatic sarcoma, sarcomatoid carcinoma
      LymphohistiocyticLymphoma
      Desmoplastic mesotheliomaFibrous pleuritis
      Biphasic mesotheliomaCarcinosarcoma, pulmonary blastoma, synovial sarcoma
      Localized MMMetastatic and primary sarcoma and carcinoma
      Well-differentiated papillary mesotheliomaBenign mesothelial proliferation, noninvasive areas of tubulopapillary pattern
      The tubulopapillary pattern may consist of branching tubules, papillae with fibrovascular cores, and trabeculae covered by a monolayer of flattened to cuboidal cells. Psammoma bodies may be seen in approximately 5% to 10% of cases.
      • Churg A.
      • Colby T.V.
      • Cagle P.
      • et al.
      The separation of benign and malignant mesothelial proliferations.
      Bland, flat to cuboidal cells are arranged in small gland-like structures in the adenomatoid pattern (Fig. 8). The acinar pattern is characterized by acinar or glandular structures, lined by uniform and bland cuboidal cell.
      • Allen T.C.
      Recognition of histopathologic patterns of diffuse malignant mesothelioma in differential diagnosis of pleural biopsies.
      Figure thumbnail gr8
      Fig. 8Tubulopapillary/adenoid pattern of MM invading the pleura (HE ×100).
      MM may exhibit clear cell, signet ring cell, adenoid cystic, or lymphangiomatoid patterns as well as glomeruloid structures and squamoid features.
      • Ordonez N.G.
      Mesothelioma with clear cell features: an ultrastructural and immunohistochemical study of 20 cases.
      • Ruangchira-urai R.
      • Mark E.J.
      Lymphangiomatoid pattern in diffuse malignant mesothelioma of the pleura: a report of six cases.
      The tumor cells in a deciduoid pattern have abundant eosinophilic, glassy cytoplasm with well-defined cytoplasmic borders, which give the tumor a deciduoid appearance. Nuclei are oval or rounded with mild to moderate pleomorphism (Fig. 9).
      • Shanks J.H.
      • Harris M.
      • Banerjee S.S.
      • et al.
      Mesotheliomas with deciduoid morphology. A morphologic spectrum and a variant not confined to young females.
      Rarely, MM is composed of sheets of monotonous, small, hyperchromatic, uniform cells with a high nucleocytoplasmic ratio.
      • Mayall F.G.
      • Gibbs A.R.
      The histology and immunohistochemistry of small cell mesothelioma.
      The stroma of epithelial mesotheliomas may vary from scanty to highly cellular and sometimes myxoid (Fig. 10).
      • Shia J.
      • Qin J.
      • Erlandson R.A.
      • et al.
      Malignant mesothelioma with a pronounced myxoid stroma: a clinical and pathological evaluation of 19 cases.
      Figure thumbnail gr10
      Fig. 10This case of MM has a scanty but myxoid stroma (HE ×100).
      Biphasic mesothelioma consists of epithelioid and sarcomatous areas. The combination of these 2 components in same area is referred to as a transitional pattern (Fig. 11).
      • Allen T.C.
      Recognition of histopathologic patterns of diffuse malignant mesothelioma in differential diagnosis of pleural biopsies.
      Sarcomatous mesothelioma may resemble any one of the typical soft tissue counterpart tumors. Osteoid and cartilaginous metaplasia has been observed (Figs. 12 and 13).
      • Klebe S.
      • Mahar A.
      • Henderson D.W.
      • et al.
      Malignant mesothelioma with heterologous elements: clinicopathological correlation of 27 cases and literature review.
      The lymphohistiocytic variant is characterized by mesothelial cells with a histiocyte-like appearance and an associated dense lymphoid infiltrate.
      • Galateau-Salle F.
      • Attanous R.
      • Gibbs A.R.
      • et al.
      Lymphohistiocytoid variant of malignant mesothelioma of the pleura: a series of 22 cases.
      Desmoplastic mesothelioma is characterized by dense, paucicellular, hyalinized collagen and spindle cells arranged in a storiform pattern (Fig. 14).
      Figure thumbnail gr12
      Fig. 12Pleomorphic cells in sarcomatoid MM (HE ×200).
      Figure thumbnail gr13
      Fig. 13Osteoid metaplasia in sarcomatoid MM (HE ×100).
      Figure thumbnail gr14
      Fig. 14Desmoplastic MM which is highly hyalinized. Invasion of the fat tissue is prominent, however (HE ×100).
      WDPM is associated with a relatively uniform, superficial spreading of papillary projections with limited or no invasion. The papillae have thin or stout myxoid fibrovascular cores, which are covered by a single layer of flattened or cuboidal mesothelial cells (Fig. 15).
      • Galateau-Salle F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      Figure thumbnail gr15a
      Fig. 15(A) WDPM. Papillary projections are covered by a single layer of uniform mesothelial cells (HE ×100). (B) HBME-1 immunostaining of WDPM (HBME-1 ×100).
      Figure thumbnail gr15b
      Fig. 15(A) WDPM. Papillary projections are covered by a single layer of uniform mesothelial cells (HE ×100). (B) HBME-1 immunostaining of WDPM (HBME-1 ×100).

      Differential diagnosis

      The histologic heterogeneity of MM contributes to a broad differential diagnosis (see Table 1). Malignant mesothelial proliferations confined to the pleural surface should be distinguished from BMP and superficial spreading carcinoma. BMP consists of bland cuboidal cells line along the pleural surface without invasion (Fig. 16). Mesothelial inclusions, however, entrapped by granulation tissue, as a result of organizing pleuritis, may mimic invasive mesothelioma (Fig. 17). Entrapped cells are often located parallel to the surface and never extend into the deeper layers of the pleura and surrounding tissues. The proliferating cells of BMP sometimes exhibit distinct nucleoli. Varying degrees of cytologic atypia and the presence of papillary tufts without invasion support the diagnosis of AMH (Table 2).
      • Cagle P.T.
      • Churg A.
      Differential diagnosis of benign and malignant mesothelial proliferations on pleural biopsies.
      It is important to distinguish AMH from WDPM. Galateau-Salle and colleagues
      • Galateau-Salle F.
      • Vignaud J.M.
      • Burke L.
      • et al.
      Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
      reported that epithelial membrane antigen (EMA) is a reliable marker for the distinction of WDPM and AMH. Immunohistochemical expression of different markers, such as GLUT-1, Ki-67, restrictedly expressed proliferation-associated protein of 86 kDa (repp86), p53, EMA, Bcl-2, transforming growth factor α (TGF-α), and desmin, has been reported in benign and malignant mesothelial proliferations (Table 3).
      • Kato Y.
      • Tsuta K.
      • Seki K.
      • et al.
      Immunohistochemical detection of GLUT-1 can discriminate between reactive mesothelium and malignant mesothelioma.
      • Kimura F.
      • Kawamura J.
      • Watanabe J.
      • et al.
      Significance of cell proliferation markers (Minichromosome maintenance protein 7, topoisomerase IIalpha and Ki-67) in cavital fluid cytology: can we differentiate reactive mesothelial cells from malignant cells?.
      • Taheri Z.M.
      • Mehrafza M.
      • Mohammadi F.
      • et al.
      The diagnostic value of Ki-67 and repp86 in distinguishing between benign and malignant mesothelial proliferations.
      • Cai Y.C.
      • Roggli V.
      • Mark E.
      • et al.
      Transforming growth factor alpha and epidermal growth factor receptor in reactive and malignant mesothelial proliferations.
      • Attanoos R.L.
      • Griffin A.
      • Gibbs A.R.
      The use of immunohistochemistry in distinguishing reactive from neoplastic mesothelium. A novel use for desmin and comparative evaluation with epithelial membrane antigen, p53, platelet-derived growth factor-receptor, P-glycoprotein and Bcl-2.
      • Saad R.S.
      • Cho P.
      • Liu Y.L.
      • et al.
      The value of epithelial membrane antigen expression in separating benign mesothelial proliferation from malignant mesothelioma: a comparative study.
      • Shen J.
      • Pinkus G.S.
      • Deshpande V.
      • et al.
      Usefulness of EMA, GLUT-1, and XIAP for the cytologic diagnosis of malignant mesothelioma in body cavity fluids.
      • Tigrani D.Y.
      • Weydert J.A.
      Immunohistochemical expression of osteopontin in epithelioid mesotheliomas and reactive mesothelial proliferations.
      GLUT-1 positivity may be helpful in the diagnosis of epithelioid and sarcomatoid mesothelioma; however, recent study demonstrated lack of specificity and sensitivity (Figs. 18 and 19).
      • Shen J.
      • Pinkus G.S.
      • Deshpande V.
      • et al.
      Usefulness of EMA, GLUT-1, and XIAP for the cytologic diagnosis of malignant mesothelioma in body cavity fluids.
      • Husain A.N.
      • Colby T.V.
      • Ordonez G.N.
      • et al.
      Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group.
      Recent studies demonstrated a potential diagnostic usefulness of p16/CDKN2A deletion in separating benign from malignant mesothelial proliferations in the body cavity effusion specimens and formalin-fixed paraffin embedded surgical specimens.
      • Illei P.B.
      • Ladanyi M.
      • Rusch V.W.
      • et al.
      The use of CDKN2A deletion as a diagnostic marker for malignant mesothelioma in body cavity effusions.
      • Chiosea S.
      • Krasinkas A.
      • Cagle P.T.
      • et al.
      Diagnostic importance of 9p21 homozygous deletion in malignant mesotheliomas.
      • Flores-Staino C.
      • Darai-Ramqvist E.
      • Dobra K.
      • et al.
      Adaptation of a commercial fluorescent in situ hybridization test to the diagnosis of malignant cells in effusions.
      • Savic S.
      • Franco N.
      • Grilli B.
      • et al.
      Fluorescence in situ hybridization in the definitive diagnosis of malignant mesothelioma in effusion cytology.
      Homozygous deletion can be detected by dual-color fluorescence in situ hybridization probe targeting chromosome 9p21. p16/CDKN2A deletion can be detected in 70% of MPMs and the frequency of the deletion depends on histologic subtype (100% sarcomatoid, 70% biphasic, and 60% epithelioid). None of benign reactive mesothelial proliferations were ever reported as positive for p16 deletion. The presence of this deletion, however, in morphologically atypical mesothelial proliferations is a strong indicator of malignancy and can be used to triage patients for more aggressive follow-up.
      Figure thumbnail gr16
      Fig. 16Benign mesothelial proliferation limited to the pleural surface (keratin ×400).
      Figure thumbnail gr17
      Fig. 17Entrapment of mesothelial cells (HE ×200).
      Table 2Histopathologic features of benign mesothelial proliferation and MM
      Benign Mesothelial ProliferationMalignant Mesothelioma
      ArchitectureNoncomplex papillary structureComplex papillary structure
      InvasionEntrapmentTrue invasion (ie, into fat and lung)
      Capillaries in biopsyParallel to the pleural surfacePerpendicular to the pleural surface
      Fibrin deposition with inflammationOftenRare
      Cellular nodules with stromaAbsentFrequent
      Cytologic atypiaIf present, confined to area of organizing effusionSometimes, can be present in any area
      Atypical mitosesAbsentSometimes
      Bland necrosisAbsentSometimes
      Table 3Immunohistochemistry to distinguish MM from benign mesothelial proliferation
      AntibodyBenign Mesothelial Proliferation (Positive Cases/Total Number of the Studied Cases)Malignant Mesothelioma (Positive Cases/Total Number of the Studied Cases)References
      GLUT-10/40 (%0)40/40 (100%)Kato
      TGF-α14/20 (70%)29/38 (76%)Cai
      EGFR1/30 (3%)17/39 (45%)Cai
      Osteopontin19/20 (95%)7/7 (100%)Tigrani
      EMA cloneMc512/20 (60%)14/20 (70%)Saad
      EMA cloneE290/20 (0%)15/20 (75%)Saad
      Desmin34/40 (85%)6/60 (10%)Attonoos
      EMA clone E298/40 (20%)48/60 (80%)Attanoos
      p530/40 (0%)27/60 (45%)Attanoos
      Bcl-20/15 (0%)0/15 (0%)Attanoos
      P-glycoprotein0/15 (0%)2/15 (13%)Attanoos
      PDGF-β6/15 (40%)15/15 (100%)Attanoos
      repp86 (LI>%21)0/22 (0%)20/36 (55%)Taheri
      Ki-67 (LI>%25)0/22 (0%)14/36 (38%)Taheri
      Desmin47/56 (84%)1/12 (8%)Davidson
      N-cadherin48/56 (86%)12/12 (100%)Davidson
      Abbreviation: PDGF-β, platelet derived growth factor-β.
      Figure thumbnail gr18
      Fig. 18GLUT-1 positivity in epithelioid MM (GLUT-1 ×400).
      Figure thumbnail gr19
      Fig. 19GLUT-1 positivity in sarcomatoid MM (GLUT-1 ×200).
      When malignancy is suspected, an open or video-assisted thoracoscopic surgery (VATS) pleural biopsy should be submitted for pathologic evaluation. In addition to histopathologic and immunohistochemical findings, conventional cytomorphologic assessment may be helpful. Cell ball formation, cell-in-cell engulfment, and cell monolayers, a feature of BMP, are useful to differentiate MM from BMP (Table 4).
      • Cakir E.
      • Demirag F.
      • Aydin M.
      • et al.
      Cytopathologic differential diagnosis of malignant mesothelioma, adenocarcinoma and reactive mesothelial cells: a logistic regression analysis.
      The main morphologic features that support the diagnosis of MM are invasion of the stroma and surrounding tissues, complex papillae and tubules, severe atypia and atypical mitoses, necrosis, expansile nodules, and disorganized growth pattern (Figs. 20 and 21).
      Table 4Cytologic features of MM, adenocarcinoma and benign mesothelial proliferation (listed in order of decreasing frequency with respect to MM)
      Data from Cakir E, Demirag F, Aydin M, et al. Cytopathologic differential diagnosis of malignant mesothelioma, adenocarcinoma and reactive mesothelial cells: a logistic regression analysis. Diagn Cytopathol 2009;37:4–10.
      FeaturesMalignant Mesothelioma (%)Adenocarcinoma (%)Mesothelial Proliferation (%)
      Uniform Cell Population10035100
      Centrally Placed Nucleus92.575100
      Cell Ball87.5453.3
      Binucleation82.557.586.7
      Giant Atypical Mesothelial Cell62.57.520
      Cell-in-cell Engulfment57.532.53.3
      Multinucleation57.53556.7
      Hyperchromatic Nuclei52.592.520
      Eccentric Nuclei5087.530
      Papillae Formation459013.3
      Monolayer aggregates4012.596.7
      Balloon-like Vacuolization17.55513.3
      Acinar Structures1562.520
      Nuclear Pleomorphism7.5856.7
      Inflammatory Ground17.527.573.4
      Figure thumbnail gr20
      Fig. 20Nodular expansion of the tumor in a case of MM. This pattern is highly suggestive of malignancy (HE ×40).
      Figure thumbnail gr21
      Fig. 21Invasion of the fat tissue by neoplastic mesothelial cells.A characteristic feature of malignancy (HE ×200).
      Pulmonary adenocarcinoma and adenocarcinomas of other sites are the most frequent secondary tumors of the pleura. In distinguishing metastatic adenocarcinoma from epithelial-type MM, radiologic features, clinical information, histopathology, cytopathology of pleural fluid, and immunohistochemistry are all helpful. True acinar structures, composed of columnar cells with eccentric nuclei arranged around a central lumen, are typical of adenocarcinoma. The cells in adenocarcinoma exhibit eccentric nuclei, prominent nucleoli, vesicular chromatin pattern, and a high degree of nuclear overlap and pleomorphism. Intracytoplasmic vacuoles can be identified in adenocarcinoma and MM. Mesothelial vacuoles are usually degenerative in nature and appear loculated with indistinct borders. Psammoma bodies are more frequent in adenocarcinoma than MM.
      • Butnor K.J.
      My approach to the diagnosis of mesothelial lesions.
      Nuclear pleomorphism, papillae formation, and eccentric, hyperchromatic nuclei with peripherally located nucleoli are diagnostic for adenocarcinoma (Figs. 22 and 23).
      • Cakir E.
      • Demirag F.
      • Aydin M.
      • et al.
      Cytopathologic differential diagnosis of malignant mesothelioma, adenocarcinoma and reactive mesothelial cells: a logistic regression analysis.
      Epithelioid hemangioendothelioma of the pleura mimics mesothelioma and adenocarcinoma clinically, cytologically, and histologically.
      • Saqi A.
      • Nisbet L.
      • Gagneja P.
      • et al.
      Primary pleural epithelioid hemangioendothelioma with rhabdoid phenotype: report and review of the literature.
      Immunohistochemical application of an endothelial marker, such as CD31 or CD34, should be helpful in suspected cases.
      Figure thumbnail gr22
      Fig. 22Cell ball formation in MM in pleural fluid (Papanicolaou [PAP] ×200).
      Figure thumbnail gr23
      Fig. 23Eccentric nuclei, nuclear pleomorphism, and peripheral vacuole in adenocarcinoma (PAP ×400).
      Carcinomas other than adenocarcinoma, especially large cell and squamous cell carcinoma of the lung, are also included in the differential diagnosis of MM. In such cases, the antibody panel should be modified. P63 and MOC-31 are two of the most sensitive squamous cell markers. CK5/6 is also a good squamous cell marker; however, it is not the antibody of choice for this panel because it is also expressed by 75% to 100% of epithelial mesotheliomas.
      • Husain A.N.
      • Colby T.V.
      • Ordonez G.N.
      • et al.
      Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group.
      Biphasic mesothelioma should be differentiated from pulmonary pleomorphic carcinoma, pulmonary and nonpulmonary carcinosarcoma, biphasic pulmonary blastoma, and biphasic synovial sarcoma.
      • Allen T.C.
      Recognition of histopathologic patterns of diffuse malignant mesothelioma in differential diagnosis of pleural biopsies.
      Histopathologic features are more useful than immunohistochemical features for diagnosing biphasic mesothelioma. Pleomorphic carcinoma consists of non–small cell carcinoma and spindle or giant cells. Pulmonary blastoma is a biphasic tumor containing well-differentiated fetal adenocarcinoma and primitive mesenchyma. Distinguishing synovial sarcoma from biphasic MM may be difficult because synovial sarcoma may exhibit calretinin, keratin, and EMA positivity (Fig. 24).
      • Zeren H.
      • Moran C.A.
      • Suster S.
      • et al.
      Primary pulmonary sarcomas with features of monophasic synovial sarcoma: a clinicopathological, immunohistochemical and ultrastructural study of 25 cases.
      • Gaertner E.
      • Zeren E.H.
      • Fleming M.V.
      • et al.
      Biphasic synovial sarcomas arising in the pleural cavity. A clinicopathologic study of five cases.
      Epithelial markers, however, are usually focally and weakly positive in synovial sarcoma. Demonstration of the X:18 translocation of synovial sarcoma can be helpful.
      • Weinbreck N.
      • Vignaud J.M.
      • Begueret H.
      • et al.
      SYT-SSX fusion is absent in sarcomatoid mesothelioma allowing its distinction from synovial sarcoma of the pleura.
      Figure thumbnail gr24
      Fig. 24Biphasic synovial sarcoma (HE ×200).
      Sarcomatoid mesothelioma cells vary from fibroblast-like spindle cells to anaplastic cells. Therefore, the differential diagnosis of sarcomatous mesothelioma includes a wide range of benign and malignant lesions, such as fibrous pleuritis, primary and metastatic sarcoma, sarcomatoid carcinoma, and malignant melanoma (Fig. 25).
      • Lucas D.R.
      • Pass H.I.
      • Madan S.K.
      • et al.
      Sarcomatoid mesothelioma and its histological mimics: a comparative immunohistochemical study.
      Fibroblasts of fibrous pleuritis are also keratin positive; therefore, in most cases, this is not considered a useful marker (Fig. 26).
      Figure thumbnail gr25
      Fig. 25Monophasic synovial sarcoma. Spindle cells arranged in a patternless pattern (HE ×400).
      Figure thumbnail gr26
      Fig. 26Keratin positivity of fibroblasts in a case of fibrous pleuritis (HE ×100).
      The lymphohistiocytic variant of sarcomatoid MM, although rare, should be distinguished from malignant lymphoma and other neoplasms that contain lymphocytic cells, such as thymoma.
      Desmoplastic mesothelioma mimics fibrous pleuritis. Noninvasive areas and the lack of cellular pleomorphism cause diagnostic difficulties. The observation of a storiform pattern, necrosis, and zonation and extensive sampling to identify frankly malignant foci may resolve these difficulties. Zonation refers to the highest degree of cellularity and atypia near the serosal surface.
      • Butnor K.J.
      My approach to the diagnosis of mesothelial lesions.
      Differential Diagnosis
      • Some malignant tumors of the pleura, primary or metastatic or fibrous pleuritis with hyalinization, can increase the thickness of the pleura diffusely and mimic MM radiologically and clinically. Mesothelioma can present as a localized lesion, mimicking most metastatic tumors.
      • The main competing diagnosis of MM is benign mesothelial cell proliferation. Cytologic and histopathologic features have been identified to distinguish between these two entities.
      • Cytologically, neoplastic mesothelial cells can closely resemble reactive mesothelial cells. Atypia, multinucleation, vacuolization, and acinar structure are not helpful features in most cases. Cell ball formation, cell-in-cell engulfment, and atypical mitoses, however, are suggestive of a malignancy.
      • Invasion of the pleura and the surrounding structures is the main distinctive histopathologic feature of MM. Cellular nodules within the stroma and complex papillary structures also support the diagnosis of malignancy.
      • A broad differential diagnosis should be considered, depending on the histologic types of MM. Metastatic adenocarcinoma, primarily from the lungs, can present diagnostic difficulties, especially in small biopsy specimens. Cytologically, adenocarcinoma exhibits a higher degree of atypia, pleomorphism, and papillae formation.
      • Epithelial MM with solid and deciduoid pattern should be distinguished from squamous cell carcinoma, large cell carcinoma, lymphoma, small cell variant from small cell tumors, and signet ring variant from signet ring cell carcinoma. Sarcomatous MM can closely resemble all types of sarcomas, including osteosarcoma, chondrosarcoma, and sarcomatoid carcinoma. The main differential diagnosis for biphasic MM includes carcinosarcoma, biphasic synovial sarcoma, and pulmonary blastoma.
      • Immunohistochemistry is essential in many instances and should be applied as a panel of antibodies, including at least 2 mesothelial cell markers and 2 other appropriate antibodies for the lesion being considered. Carcinoembryonic antigen (CEA), MOC-31, BG8 (Lewis), Ber-EP4, B72.3, thyroid transcription factor-1 (TTF-1), and CD15 (Leu-M1) are the most reliable antibodies for adenocarcinoma. P63 is a useful marker for squamous cell carcinoma. CAM5.2 is the most sensitive marker in differentiating sarcomatous mesothelioma from true sarcomas. WT1 and AE1/AE3 are recommended in the routine work-up of sarcomatous mesothelioma.

      Diagnosis

      Correlation of clinical and radiologic information and cytologic and pathologic features is essential for the diagnosis of MM. Chest pain, diffuse pleural thickening and nodulation, and a history of asbestos exposure are all seen in MM; however, the tumor can present without pleural thickening or asbestos exposure. Additionally, asbestos exposure can cause benign pleural diseases, such as pleural thickening, benign mesothelial reactions, pleural effusions, pleural plaques, and rounded atelectasis. The International Mesothelioma Interest Group does not recommend using history of asbestos exposure to influence the diagnosis of MM.
      • Husain A.N.
      • Colby T.V.
      • Ordonez G.N.
      • et al.
      Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group.
      An adequate amount of tissue is essential for the diagnosis of MM. Diagnostic pleural biopsies can be obtained by blind or CT-guided needle biopsy, VATS, and open pleural biopsy. Attanoos and Gibbs
      • Attanoos R.L.
      • Gibbs A.R.
      Pathology of malignant mesothelioma.
      showed that open pleural biopsy produced the highest diagnostic accuracy. The size of the biopsy specimen is an important factor in making a definitive diagnosis. Specimens greater than 10 mm in size are more diagnostic than specimens less than 10 mm in size.
      • Attanoos R.L.
      • Gibbs A.R.
      The comparative accuracy of different pleural biopsy techniques in the diagnosis of malignant mesothelioma.
      Hematoxylin-eosin–stained (HE) sections are evaluated for histopathologic patterns. Benign mesothelial proliferations, adenocarcinomas, other carcinomas, biphasic tumors, and sarcomas must be considered in the differential diagnosis (discussed previously).
      There is currently no unique immunohistochemical mesothelial marker that provides 100% specificity and high sensitivity for the diagnosis of MM. A panel of antibodies, including mesothelial markers and markers for the other tumors, is recommended to obtain the correct diagnosis. A sensitivity of 80% or higher is recommended by the International Mesothelioma Interest Group for inclusion in the panel. Keratin staining should be performed because all MMs, except rare sarcomatoid cases, stain positively with the antibody. Currently used diagnostic panels include calretinin, keratin 5/6, WT1, HBME-1, thrombomodulin, mesothelin, and podoplanin (D2-40) as mesothelial cell markers (Figs. 27 and 28).
      • Husain A.N.
      • Colby T.V.
      • Ordonez G.N.
      • et al.
      Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group.
      Immunosensitivity for mesothelial cell markers differs according to the histologic pattern. Calretinin shows the highest sensitivity for epithelioid MM, with good expression in tubulopapillary, adenomatoid, solid, and pleomorphic patterns. For small cell mesothelioma, thrombomodulin seems to confer higher sensitivity.
      • Gumurdulu D.
      • Zeren E.H.
      • Cagle P.T.
      • et al.
      Specificity of MOC-31 and HBME-1 immunohistochemistry in the differential diagnosis of adenocarcinoma and malignant mesothelioma: a study on environmental malignant mesothelioma cases from Turkish villages.
      In some cases of metastatic adenocarcinoma, application of the CK7/CK20 panel can be helpful because mesothelial cells, including MM, are always positive for CK7 (Fig. 29).
      Figure thumbnail gr27
      Fig. 27Calretinin positivity in MM (HE ×400).
      Figure thumbnail gr28
      Fig. 28Keratin 5/6 positivity in MM (HE ×400).
      Figure thumbnail gr29
      Fig. 29Benign mesothelial cell proliferation in a case of metastatic adenocarcinoma. CK7 positivity is diffuse and strong in mesothelial cells, however, the cells of metastatic gastric carcinoma stain focally and weakly (CK7 ×200).
      Pathologists should be aware of the limitations of mesothelial markers. Mesothelioma markers may demonstrate immune reaction with different tumors, other than MM, with variable frequency. Calretinin expression is common in pulmonary giant cell carcinoma, small cell carcinoma, and large cell carcinoma. Breast and ovarian carcinoma, synovial sarcoma, epithelioid sarcoma, malignant melanoma, thymoma, and thymic carcinoma can also be positive for calretinin. Although keratin 5/6 is a useful marker in distinguishing epithelioid MM from pulmonary adenocarcinoma, it is also expressed in squamous cell, pulmonary large cell, small cell, ovarian, and breast and pancreatic carcinomas as well as thymoma. WT1 shows strong nuclear positivity in MM, but the majority of cases of ovarian and peritoneal serous carcinomas also express WT1. Membranous staining for HBME-1 is observed in epithelioid MM, but it is not reliable in the differential diagnosis of MM because almost half of adenocarcinomas are also positive for HBME-1.
      • Attanoos R.L.
      • Webbs R.
      • Dojcinov S.D.
      • et al.
      Malignant epithelioid mesothelioma: anti-mesothelial marker expression correlates with histological pattern.
      Thrombomodulin can be expressed in lung adenocarcinoma and vascular neoplasms.
      • Suster S.
      • Moran C.A.
      Applications and limitations of immunohistochemistry in the diagnosis of malignant mesothelioma.
      • Miettinen M.
      • Sarlomo-Rikala M.
      Expression of calretinin, thrombomodulin, keratin 5, and mesothelin in lung carcinomas of different types: an immunohistochemical analysis of 596 tumors in comparison with epithelioid mesotheliomas of the pleura.
      Two mesothelial markers and 2 appropriate markers for the other tumors can be used in the first step. The panel can be expanded if the results are inconvenient.
      • Marchevsky A.M.
      Application of immunohistochemistry to the diagnosis of malignant mesothelioma.
      • Husain A.N.
      • Colby T.V.
      • Ordonez G.N.
      • et al.
      Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group.
      The selected nonmesothelial markers change according to the suspected metastatic carcinoma. MOC-31, BG8 (Lewis), CEA, B72.3, Ber-EP4, TTF-1, and CD15 (Leu-M1) are favorable markers for pulmonary adenocarcinoma. MM can be separated from squamous cell carcinoma by its WT1 positivity and p63 negativity.
      • Pu R.T.
      • Pang Y.
      • Michael C.W.
      Utility of WT-1, p63, MOC31, mesothelin, and cytokeratin (K903 and CK5/6) immunostains in differentiating adenocarcinoma, squamous cell carcinoma, and malignant mesothelioma in effusions.
      Sarcomatoid mesothelioma mimics soft tissue sarcomas and sarcomatoid carcinoma. Kushitani and colleagues
      • Kushitani K.
      • Takeshima Y.
      • Amatya V.J.
      • et al.
      Differential diagnosis of sarcomatoid mesothelioma from true sarcoma and sarcomatoid carcinoma using immunohistochemistry.
      showed that CAM5.2 had the highest sensitivity and specificity for differentiating sarcomatoid mesothelioma from true sarcomas. The combination of CAM5.2, WT1, and AE1/AE3 is recommended for the routine pathologic diagnosis.
      Limitation of the available mesothelial markers promotes the search for a better marker. Caveolin-1 is a novel immunohistochemical marker for the differentiation of epithelioid mesothelioma from lung adenocarcinoma.
      • Amatya V.J.
      • Takeshima Y.
      • Kushitani K.H.
      • et al.
      Caveolin-1 is a novel immunohistochemical marker to differentiate epithelioid mesothelioma from lung adenocarcinoma.
      Tenascin-X may be a new diagnostic marker in the differential diagnosis of MM and serous carcinoma of the ovary and peritoneum.
      • Yuan Y.
      • Nymoen D.A.
      • Stavnes H.T.
      • et al.
      Tenascin-X is a novel diagnostic marker of malignant mesothelioma.

      Prognosis

      MM is a fatal cancer. The median survival time after diagnosis is approximately 6 to 12 months. Unexpectedly long survival with MM, however, has been reported by various investigators.
      • Demirag F.
      • Unsal E.
      • Tastepe I.
      Biphasic malignant mesothelioma cases with osseous differentiation and long survival: a review of the literature.
      • Wong C.F.
      • Fung S.L.
      • Yew W.W.
      • et al.
      A case of malignant pleural mesothelioma with unexpectedly long survival without active treatment.
      The majority of reports indicate that epithelial MM carries a better prognosis. Epithelial histologic subtype and extrapleural pneumonectomy were identified as independent predictors for 18-month survival.
      • Yan T.D.
      • Boyer M.
      • Sim J.
      • et al.
      Prognostic features of long-term survivors after surgical management of malignant pleural mesothelioma.
      Histomorphologic parameters were evaluated with respect to MM prognosis. The presence of tumor necrosis is a poor prognostic factor in MM.
      • Edwards J.G.
      • Swinson D.E.B.
      • Jones J.L.
      • et al.
      Tumor necrosis correlates with angiogenesis and is a predictor of poor prognosis in malignant mesothelioma.
      The lymphocytic cell infiltrate in the tumor has various functions, such as modulating local tumor immunity. The presence of high levels of CD8+ lymphocytes infiltrating the tumor is associated with a better prognosis in MM.
      • Anraku M.
      • Cunningham K.S.
      • Yun Z.
      • et al.
      Impact of tumor-infiltrating T cells on survival in patients with malignant pleural mesothelioma.
      Epithelial mesenchymal transition is characterized by the loss of cell adhesion and increased cell motility. This partial fibroblastic phenotype of MM is associated with invasiveness and chemoresistance. EMT, including periostin and phosphatase and tensin homolog (PTEN), were investigated in MM. The high expression of PTEN and low expression of cytosolic periostin could be independent prognostic factors for long survival.
      • Schramm A.
      • Opitz I.
      • Thies S.
      • et al.
      Prognostic significance of epithelial-mesenchymal transition in malignant pleural mesothelioma.
      Homozygous deletion of p16/CDKN2A is the most common genetic abnormality in MM. Presence of homozygous deletion of 9p21 and loss of p16 immunoreactivity is associated with shorter survival in MM.
      • Dacic S.
      • Kothmaier H.
      • Land S.
      • et al.
      Prognostic significance of p16/cdkn2a loss in pleural malignant mesotheliomas.
      • Kobayashi N.
      • Toyooka S.
      • Yanai H.
      • et al.
      Frequent p16 inactivation by homozygous deletion or methylation is associated with a poor prognosis in Japanese patients with pleural mesothelioma.
      • Lopez-Rios F.
      • Chuai S.
      • Flores R.
      • et al.
      Global gene expression profiling of pleural mesotheliomas: overexpression of aurora kinases and P16/CDKN2A deletion as prognostic factors and critical evaluation of microarray-based prognostic prediction.
      Antiapoptosis was up-regulated in short-term survivors by signal transducer and activator of transcription 1 and surviving and related molecules but not in long-term survivors.
      • Kothmaier H.
      • Quehenberger F.
      • Halbwedl I.
      • et al.
      EGFR and PDGFR differentially promote growth in malign epithelioid mesothelioma of short and long term survivors.
      Edwards and colleagues
      • Edwards J.G.
      • Swinson D.E.
      • Jones J.L.
      • et al.
      EGFR expression: association with outcome and clinicopathological variables in malignant pleural mesothelioma.
      showed that independent indicators of poor prognosis were nonepithelial cell type, weight loss, performance status, and white blood cell count greater than 8.3 × 109/L. Overexpression of epidermal growth factor receptor (EGFR) has been reported in MM; however, its correlation with histology, clinical characteristics, and prognosis are controversial.
      • Destro A.
      • Falleni M.
      • Zucalli P.A.
      • et al.
      EGFR overexpression in malignant pleural mesothelioma. An immunohistochemical and molecular study with clinico-pathological correlations.
      EGFR-targeted therapy for MM is still the subject of debate.
      • Garland L.L.
      • Rankin C.
      • Gandara D.R.
      • et al.
      Phase II study of erlotinib in patients with malignant pleural mesothelioma: a Southwest Oncology Group Study.
      • Govindan R.
      • Kratzke R.A.
      • Hendon J.E.
      • et al.
      Gefitinib in patients with malignant mesothelioma: a phase II study by the Cancer and Leukemia Group B.
      • Kawaguchi K.
      Combined inhibition of MET and EGFR suppresses proliferation of malignant mesothelioma cells.
      Vascular endothelial growth factor, known as an important angiogenic peptide, has been implicated in the development and progression of MM. It is also an independent prognostic factor for pleural MM.
      • Demirag F.
      • Ünsal E.
      • Yilmaz A.
      • et al.
      Prognostic significance of vascular endothelial growth factor, tumor necrosis, and mitotic activity index in malignant pleural mesothelioma.
      Pinton and colleagues
      • Pinton G.
      • Brunelli E.
      • Murer B.
      • et al.
      Estrogen receptor-beta affects the prognosis of human malignant mesothelioma.
      identified female gender as a positive prognostic factor, and the role of the estrogen receptor has been studied. In this study, immunohistochemical analysis showed intense nuclear estrogen receptor-beta (ERβ) staining in normal pleural samples, which was reduced in tumor tissues. In vitro studies showed that 17β-estradiol treatment has a suppressive effect on cell proliferation in MM, and, as demonstrated by G2/M-phase cell cycle arrest, cell growth was decreased by ERβ overexpression.
      Pitfalls
      • !
        The diagnosis of MM is difficult, especially in small tissue samples, and the interpretation of clinical, radiologic, and pathologic features is essential.
      • !
        Epithelial MM has a range of mimics, and the diagnosis of malignancy mostly depends on invasion.
      • !
        Entrapped reactive mesothelial cells can mimic invasion; however, they are situated mostly in the upper layers of the pleura.
      • !
        Severe atypia of the tumor cells is not a typical feature of MM and is highly suggestive of other malignant lesions; however, pleomorphic MMs may occur.
      • !
        Immunohistochemistry is a useful tool in the diagnosis of MM; however, the intensity and staining pattern may vary depending on the laboratory and the antibody clone, which need standardization.
      • !
        Immunohistochemistry should always be conducted with a panel of antibodies, and interpretation of the results should take histologic findings into account.
      • !
        Mesothelial cell markers are more effective for epithelial MM but can be positive in tumors other than mesothelioma. Keratin positivity is essential for diagnosis.
      • !
        Immunohistochemistry is not recommended in the differential diagnosis between benign and malignant mesothelial proliferations in the individual case.

      References

        • Witowski J.
        • Ksiazek K.
        • Jorres A.
        New insights into the biology of peritoneal mesothelial cells: the roles of epithelial-to-mesenchymal transition and cellular senescence.
        Nephron Exp Nephrol. 2008; 108: 69-73
        • Que J.
        • Wilm B.
        • Hasegawa H.
        • et al.
        Mesothelium contributes to vascular smooth muscle and mesenchyme during lung development.
        Proc Natl Acad Sci U S A. 2008; 105: 16626-16630
        • Ji H.L.
        • Nie H.G.
        Electrolyte and fluid transport in mesothelial cells.
        J Epithel Biol Pharmacol. 2008; 1: 1-7
        • Marchevsky A.M.
        Application of immunohistochemistry to the diagnosis of malignant mesothelioma.
        Arch Pathol Lab Med. 2008; 132: 397-401
        • Churg A.
        • Colby T.V.
        • Cagle P.
        • et al.
        The separation of benign and malignant mesothelial proliferations.
        Am J Surg Pathol. 2000; 24: 1183-1200
        • Attanoos R.L.
        • Gibbs A.R.
        Pathology of malignant mesothelioma.
        Histopathology. 1997; 30: 403-418
        • Attanoos R.L.
        • Gibbs A.R.
        ‘Pseudomesotheliomatous’ carcinomas of the pleura: a 10-year analysis of cases from the Environmental Lung Disease Research Group, Cardiff.
        Histopathology. 2003; 43: 444-452
        • Allen T.C.
        • Cagle P.T.
        • Churg A.M.
        • et al.
        Localized malignant mesothelioma.
        Am J Surg Pathol. 2005; 29: 866-873
        • Galateau-Salle F.
        • Vignaud J.M.
        • Burke L.
        • et al.
        Well-differentiated papillary mesothelioma of the pleura: a series of 24 cases.
        Am J Surg Pathol. 2004; 28: 534-540
        • Abdel Rahman A.R.M.
        • Gaafar A..M.
        • Baki H.A.
        • et al.
        Prevalence and pattern of lymph node metastasis in malignant pleural mesothelioma.
        Ann Thorac Surg. 2008; 86: 391-395
        • Hasleton P.
        Pleural disease.
        in: Haselton P. Spencer's pathology of the lung. McGraw Hill, New York (NY)1996: 1131-1210
        • Yung S.
        • Chan T.M.
        Mesothelial cells.
        Perit Dial Int. 2007; 27: S110-S115
        • Cagle P.T.
        • Churg A.
        Differential diagnosis of benign and malignant mesothelial proliferations on pleural biopsies.
        Arch Pathol Lab Med. 2005; 129: 1421-1427
        • Travis W.
        • Brambilla E.
        • Müller-Hermelink H.K.
        • et al.
        Pathology and genetics of tumours of the lung, pleura, thymus, and heart in World Health Organization Classification of Tumours.
        IARC Press, Lyon (France)2004
        • Allen T.C.
        Recognition of histopathologic patterns of diffuse malignant mesothelioma in differential diagnosis of pleural biopsies.
        Arch Pathol Lab Med. 2005; 129: 1415-1420
        • Ordonez N.G.
        Mesothelioma with clear cell features: an ultrastructural and immunohistochemical study of 20 cases.
        Hum Pathol. 2005; 36: 465-473
        • Ruangchira-urai R.
        • Mark E.J.
        Lymphangiomatoid pattern in diffuse malignant mesothelioma of the pleura: a report of six cases.
        Virchows Arch. 2009; 455: 143-148
        • Shanks J.H.
        • Harris M.
        • Banerjee S.S.
        • et al.
        Mesotheliomas with deciduoid morphology. A morphologic spectrum and a variant not confined to young females.
        Am J Surg Pathol. 2000; 24: 258-294
        • Mayall F.G.
        • Gibbs A.R.
        The histology and immunohistochemistry of small cell mesothelioma.
        Histopathology. 1992; 20: 47-51
        • Shia J.
        • Qin J.
        • Erlandson R.A.
        • et al.
        Malignant mesothelioma with a pronounced myxoid stroma: a clinical and pathological evaluation of 19 cases.
        Virchows Arch. 2005; 447: 828-834
        • Klebe S.
        • Mahar A.
        • Henderson D.W.
        • et al.
        Malignant mesothelioma with heterologous elements: clinicopathological correlation of 27 cases and literature review.
        Mod Pathol. 2008; 21: 1084-1094
        • Galateau-Salle F.
        • Attanous R.
        • Gibbs A.R.
        • et al.
        Lymphohistiocytoid variant of malignant mesothelioma of the pleura: a series of 22 cases.
        Am J Surg Pathol. 2007; 31: 711-716
        • Kato Y.
        • Tsuta K.
        • Seki K.
        • et al.
        Immunohistochemical detection of GLUT-1 can discriminate between reactive mesothelium and malignant mesothelioma.
        Mod Pathol. 2007; 20: 215-220
        • Kimura F.
        • Kawamura J.
        • Watanabe J.
        • et al.
        Significance of cell proliferation markers (Minichromosome maintenance protein 7, topoisomerase IIalpha and Ki-67) in cavital fluid cytology: can we differentiate reactive mesothelial cells from malignant cells?.
        Diagn Cytopathol. 2010; 38: 161-167
        • Taheri Z.M.
        • Mehrafza M.
        • Mohammadi F.
        • et al.
        The diagnostic value of Ki-67 and repp86 in distinguishing between benign and malignant mesothelial proliferations.
        Arch Pathol Lab Med. 2008; 132: 694-697
        • Cai Y.C.
        • Roggli V.
        • Mark E.
        • et al.
        Transforming growth factor alpha and epidermal growth factor receptor in reactive and malignant mesothelial proliferations.
        Arch Pathol Lab Med. 2004; 128: 68-70
        • Attanoos R.L.
        • Griffin A.
        • Gibbs A.R.
        The use of immunohistochemistry in distinguishing reactive from neoplastic mesothelium. A novel use for desmin and comparative evaluation with epithelial membrane antigen, p53, platelet-derived growth factor-receptor, P-glycoprotein and Bcl-2.
        Histopathology. 2003; 43: 231-238
        • Saad R.S.
        • Cho P.
        • Liu Y.L.
        • et al.
        The value of epithelial membrane antigen expression in separating benign mesothelial proliferation from malignant mesothelioma: a comparative study.
        Diagn Cytopathol. 2005; 32: 156-159
        • Shen J.
        • Pinkus G.S.
        • Deshpande V.
        • et al.
        Usefulness of EMA, GLUT-1, and XIAP for the cytologic diagnosis of malignant mesothelioma in body cavity fluids.
        Am J Clin Pathol. 2009; 131: 516-523
        • Tigrani D.Y.
        • Weydert J.A.
        Immunohistochemical expression of osteopontin in epithelioid mesotheliomas and reactive mesothelial proliferations.
        Am J Clin Pathol. 2007; 127: 580-584
        • Husain A.N.
        • Colby T.V.
        • Ordonez G.N.
        • et al.
        Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group.
        Arch Pathol Lab Med. 2009; 133: 1317-1331
        • Illei P.B.
        • Ladanyi M.
        • Rusch V.W.
        • et al.
        The use of CDKN2A deletion as a diagnostic marker for malignant mesothelioma in body cavity effusions.
        Cancer. 2003; 99: 51-56
        • Chiosea S.
        • Krasinkas A.
        • Cagle P.T.
        • et al.
        Diagnostic importance of 9p21 homozygous deletion in malignant mesotheliomas.
        Mod Philol. 2008; 21: 742-747
        • Flores-Staino C.
        • Darai-Ramqvist E.
        • Dobra K.
        • et al.
        Adaptation of a commercial fluorescent in situ hybridization test to the diagnosis of malignant cells in effusions.
        Lung Cancer. 2010; 68: 39-43
        • Savic S.
        • Franco N.
        • Grilli B.
        • et al.
        Fluorescence in situ hybridization in the definitive diagnosis of malignant mesothelioma in effusion cytology.
        Chest. 2010; ([Epub ahead of print])
        • Cakir E.
        • Demirag F.
        • Aydin M.
        • et al.
        Cytopathologic differential diagnosis of malignant mesothelioma, adenocarcinoma and reactive mesothelial cells: a logistic regression analysis.
        Diagn Cytopathol. 2009; 37: 4-10
        • Butnor K.J.
        My approach to the diagnosis of mesothelial lesions.
        J Clin Pathol. 2006; 59: 564-574
        • Saqi A.
        • Nisbet L.
        • Gagneja P.
        • et al.
        Primary pleural epithelioid hemangioendothelioma with rhabdoid phenotype: report and review of the literature.
        Diagn Cytopathol. 2007; 35: 203-208
        • Zeren H.
        • Moran C.A.
        • Suster S.
        • et al.
        Primary pulmonary sarcomas with features of monophasic synovial sarcoma: a clinicopathological, immunohistochemical and ultrastructural study of 25 cases.
        Hum Pathol. 1995; 26: 474-480
        • Gaertner E.
        • Zeren E.H.
        • Fleming M.V.
        • et al.
        Biphasic synovial sarcomas arising in the pleural cavity. A clinicopathologic study of five cases.
        Am J Surg Pathol. 1996; 20: 36-45
        • Weinbreck N.
        • Vignaud J.M.
        • Begueret H.
        • et al.
        SYT-SSX fusion is absent in sarcomatoid mesothelioma allowing its distinction from synovial sarcoma of the pleura.
        Mod Pathol. 2007; 20: 617-621
        • Lucas D.R.
        • Pass H.I.
        • Madan S.K.
        • et al.
        Sarcomatoid mesothelioma and its histological mimics: a comparative immunohistochemical study.
        Histopathology. 2003; 42: 270-279
        • Attanoos R.L.
        • Gibbs A.R.
        The comparative accuracy of different pleural biopsy techniques in the diagnosis of malignant mesothelioma.
        Histopathology. 2008; 53: 340-344
        • Gumurdulu D.
        • Zeren E.H.
        • Cagle P.T.
        • et al.
        Specificity of MOC-31 and HBME-1 immunohistochemistry in the differential diagnosis of adenocarcinoma and malignant mesothelioma: a study on environmental malignant mesothelioma cases from Turkish villages.
        Pathol Oncol Res. 2002; 8: 188-193
        • Attanoos R.L.
        • Webbs R.
        • Dojcinov S.D.
        • et al.
        Malignant epithelioid mesothelioma: anti-mesothelial marker expression correlates with histological pattern.
        Histopathology. 2001; 39: 584-588
        • Suster S.
        • Moran C.A.
        Applications and limitations of immunohistochemistry in the diagnosis of malignant mesothelioma.
        Adv Anat Pathol. 2006; 13: 316-329
        • Miettinen M.
        • Sarlomo-Rikala M.
        Expression of calretinin, thrombomodulin, keratin 5, and mesothelin in lung carcinomas of different types: an immunohistochemical analysis of 596 tumors in comparison with epithelioid mesotheliomas of the pleura.
        Am J Surg Pathol. 2003; 27: 150-158
        • Pu R.T.
        • Pang Y.
        • Michael C.W.
        Utility of WT-1, p63, MOC31, mesothelin, and cytokeratin (K903 and CK5/6) immunostains in differentiating adenocarcinoma, squamous cell carcinoma, and malignant mesothelioma in effusions.
        Diagn Cytopathol. 2008; 36: 20-25
        • Kushitani K.
        • Takeshima Y.
        • Amatya V.J.
        • et al.
        Differential diagnosis of sarcomatoid mesothelioma from true sarcoma and sarcomatoid carcinoma using immunohistochemistry.
        Pathol Int. 2008; 58: 75-83
        • Amatya V.J.
        • Takeshima Y.
        • Kushitani K.H.
        • et al.
        Caveolin-1 is a novel immunohistochemical marker to differentiate epithelioid mesothelioma from lung adenocarcinoma.
        Histopathology. 2009; 55: 10-19
        • Yuan Y.
        • Nymoen D.A.
        • Stavnes H.T.
        • et al.
        Tenascin-X is a novel diagnostic marker of malignant mesothelioma.
        Am J Surg Pathol. 2009; 33: 1673-1682
        • Demirag F.
        • Unsal E.
        • Tastepe I.
        Biphasic malignant mesothelioma cases with osseous differentiation and long survival: a review of the literature.
        Lung Cancer. 2007; 57: 233-236
        • Wong C.F.
        • Fung S.L.
        • Yew W.W.
        • et al.
        A case of malignant pleural mesothelioma with unexpectedly long survival without active treatment.
        Respiration. 2002; 69: 166-168
        • Yan T.D.
        • Boyer M.
        • Sim J.
        • et al.
        Prognostic features of long-term survivors after surgical management of malignant pleural mesothelioma.
        Ann Thorac Surg. 2009; 87: 1552-1556
        • Edwards J.G.
        • Swinson D.E.B.
        • Jones J.L.
        • et al.
        Tumor necrosis correlates with angiogenesis and is a predictor of poor prognosis in malignant mesothelioma.
        Chest. 2003; 124: 1916-1923
        • Anraku M.
        • Cunningham K.S.
        • Yun Z.
        • et al.
        Impact of tumor-infiltrating T cells on survival in patients with malignant pleural mesothelioma.
        J Thorac Cardiovasc Surg. 2008; 135: 823-829
        • Schramm A.
        • Opitz I.
        • Thies S.
        • et al.
        Prognostic significance of epithelial-mesenchymal transition in malignant pleural mesothelioma.
        Eur J Cardiothorac Surg. 2010; 37: 566-572
        • Dacic S.
        • Kothmaier H.
        • Land S.
        • et al.
        Prognostic significance of p16/cdkn2a loss in pleural malignant mesotheliomas.
        Virchows Arch. 2008; 453: 627-635
        • Kobayashi N.
        • Toyooka S.
        • Yanai H.
        • et al.
        Frequent p16 inactivation by homozygous deletion or methylation is associated with a poor prognosis in Japanese patients with pleural mesothelioma.
        Lung Cancer. 2008; 62: 120-125
        • Lopez-Rios F.
        • Chuai S.
        • Flores R.
        • et al.
        Global gene expression profiling of pleural mesotheliomas: overexpression of aurora kinases and P16/CDKN2A deletion as prognostic factors and critical evaluation of microarray-based prognostic prediction.
        Cancer Res. 2006; 66: 2970-2979
        • Kothmaier H.
        • Quehenberger F.
        • Halbwedl I.
        • et al.
        EGFR and PDGFR differentially promote growth in malign epithelioid mesothelioma of short and long term survivors.
        Thorax. 2008; 63: 345-351
        • Edwards J.G.
        • Swinson D.E.
        • Jones J.L.
        • et al.
        EGFR expression: association with outcome and clinicopathological variables in malignant pleural mesothelioma.
        Lung Cancer. 2006; 54: 399-407
        • Destro A.
        • Falleni M.
        • Zucalli P.A.
        • et al.
        EGFR overexpression in malignant pleural mesothelioma. An immunohistochemical and molecular study with clinico-pathological correlations.
        Lung Cancer. 2006; 51: 207-215
        • Garland L.L.
        • Rankin C.
        • Gandara D.R.
        • et al.
        Phase II study of erlotinib in patients with malignant pleural mesothelioma: a Southwest Oncology Group Study.
        J Clin Oncol. 2007; 25: 2406-2413
        • Govindan R.
        • Kratzke R.A.
        • Hendon J.E.
        • et al.
        Gefitinib in patients with malignant mesothelioma: a phase II study by the Cancer and Leukemia Group B.
        Clin Cancer Res. 2005; 11: 2300-2304
        • Kawaguchi K.
        Combined inhibition of MET and EGFR suppresses proliferation of malignant mesothelioma cells.
        Carcinogenesis. 2009; 30: 1097-1105
        • Demirag F.
        • Ünsal E.
        • Yilmaz A.
        • et al.
        Prognostic significance of vascular endothelial growth factor, tumor necrosis, and mitotic activity index in malignant pleural mesothelioma.
        Chest. 2005; 128: 3382-3387
        • Pinton G.
        • Brunelli E.
        • Murer B.
        • et al.
        Estrogen receptor-beta affects the prognosis of human malignant mesothelioma.
        Cancer Res. 2009; 69: 4598-4604