Section 1 -

Atypical Mesothelial Proliferations Andrew Churg Department of Pathology University of British Columbia Vancouver, BC, Canada

Separation of Benign and Malignant Mesothelial Proliferations: Criteria for separating mesotheliomas from carcinomas or sarcomas metastatic to the pleura or peritoneum are now reasonably well established. However, deciding whether a mesothelial proliferation is a benign or malignant process has proved to be, in many respects, much more difficult. Given the dire prognosis of a diagnosis of malignant mesothelioma and the potential for the patient to receive highly toxic treatment or radical surgery that has a high morbidity and mortality, caution should be applied in equivocal cases: it is far preferable to label them as Aatypical mesothelial proliferation@ and to ask for another biopsy if the clinician deems it appropriate. This is not a trivial problem. In a series of more than 200 cases circulated to members of the US-Canadian Mesothelioma Panel, there was disagreement over the question of benign vs malignant in 22% of the cases; while this number is artificially high (straightforward cases were not circulated), it nonetheless indicates that this is an issue that needs more study. This issue is discussed and illustrated in detail in Churg et al 2000 and Churg et al 2006. Tables 1 and 2 provide some guidelines for making this separation. True stromal invasion is the best guide to malignancy. In the pleural cavity, mesothelial processes that invade deep into a thickened pleura are always malignant. Caution should be exercised in diagnosing mesothelioma if the apparent invasion is only superficial, since reactive processes may incorporate benign mesothelial cells just under the surface. Care should also be taken in examining small tissue fragments, because it is often difficult to determine whether one is dealing with invasion or an en face cut. The diagnosis is clear if the mesothelial cells reach the fat or muscle. Invasion is also an indicator of malignancy in the peritoneal cavity, but reactive proliferations that trap mesothelial cells, particularly reactions to inflammatory processes around the uterine adnexae, may be confusing. Such proliferations typically show superficial linear arrays of cells (McFadden and Clement, 1986) formed by the laying down of an organizing effusion (ascites) which eventually is transformed to collagen. The same process can be seen in the pleura and in hydrocoeles. True malignant mesotheliomas are usually not linear, but rather randomly invasive, in their cell distribution. Examination of the distribution of proliferating mesothelial cells (zonation) is also useful in this context. Reactive mesothelial proliferations, whether primarily composed of epithelial type mesothelial cells or spindled mesothelial cells (e.g., organizing pleuritis) show greater cellularity toward the surface that contacts the effusion or ascites. Cellularity decreases away from this surface as the process organizes. The finding of a homogeneously dense proliferation of mesothelial cells, from the organizing side toward the stromal (chest wall, in the pleural cavity) side, or a proliferation that is more cellular away from the effusion is extremely worrisome. Sometimes one encounters proliferations of atypical mesothelial cells that are confined to the pleural surface. It has been proposed that such proliferations might represent mesothelioma in situ (Whitaker et al 1992). However, in the experience of the Mesothelioma Panel, there are no clear morphologic features that allow a diagnosis of mesothelioma in situ. This again is a situation in which the term atypical mesothelial proliferation should be applied. Desmoplastic mesotheliomas present a problem of separation from organizing pleuritis. (Table 2). Desmoplastic mesotheliomas show a combination of a paucicellular densely fibrotic stroma (often storiform, sometimes in the so-called Apatternless pattern@ of Stout), along with invasion of fat or other tissues, or bland necrosis, or frankly sarcomatous areas. Occasionally such tumors also present as metastases. Cytologic atypia is often not useful in separating benign from malignant proliferations, particularly when dealing with epithelial lesions. Mesothelial reactions can be quite atypical and mitoses are common, particularly just under effusions. On the other hand epithelial mesotheliomas are often remarkably homogeneous appearing, although they have prominent nucleoli, and the nuclei are actually quite large. Mitoses are often hard to find in epithelial mesotheliomas. Immunohistochemistry offers no help in this area. Both benign and malignant mesothelial reactions are usually keratin positive. Although some articles suggest that p53 and EMA are positive only in malignant proliferations, my experience is that both are commonly positive in benign reactions as well, and that p53 staining is technically difficult and poorly reproducible from lab to lab (Cagle et al 1994; Mangano 1998; Mayall 1992; Walts 1994). A recent paper examining a large number of cases found no differences in staining frequency for p53 and EMA between benign and malignant processes (Roberts et al 2001). Desmin has also been suggested as a stain that provides separation (Davidson et al 2001), but this claim is refuted in other studies (Hurliman 1994, Gonzalez-Lois 2001). Table 1: Separation of Reactive from Malignant Mesothelial Proliferations Benign Mesothelial Reactions Malignant Mesothelial Neoplasms No true invasion of stroma (but superficial entrapment may be present in areas of organization) Invasion of stroma (the deeper, the more definitive) May be densely cellular in the pleural space, but not in the stroma Dense cellularity (non inflammatory) in stroma favors malignancy Process becomes more fibrotic toward chest wall (Azonation@) No zonation to process; often more cellular away from effusion Cytologic atypia confined to present in any area, but area of organizing effusion Cytologic atypia mesotheliomas are deceptively bland and relatively monotonous Necrosis rare Necrosis usually a sign of malignancy Mitoses may be plentiful Many mesotheliomas show very few mitoses (but atypical mitoses favor malignancy) No nodular expansion of stroma Nodular expansions of stroma sometimes present Storiform pattern absent or minimal Extensive storiform pattern favors malignancy (desmoplastic mesothelioma) but is not sufficient by itself (see Table 2) Benign reactions may be keratin, p53, and EMA positive Mesotheliomas are almost always keratin and often EMA and p53 positive Table 2: Separation of Desmoplastic Mesothelioma from Organizing pleuritis Organizing Pleuritis Desmoplastic Mesothelioma 1 Cellularity greatest immediately under effusion; becomes more fibrotic away from effusion (ie, shows A zonation@ ) No zonation. Bulk of lesion is paucicellular. May have abrupt transitions to cellular, frankly sarcomatous foci anywhere in the lesion. Cells immediately under effusion may be very atypical Cytologic atypia often hard to discern Capillaries perpendicular to pleural surface Capillaries inconspicuous No stromal invasion Stromal invasion No necrosis Bland necrosis No sarcomatous foci Sarcomatous foci No nodular expansion of stroma Nodular expansions of stroma sometimes present 1Combinatio of a paucicellular fibrotic pattern plus stromal invasion or bland necrosis or sarcomatous foci required for diagnosis References Cagle PT, Brown RW, Lebovitz RM: p53 immunostaining in the differentiation of reactive processes from malignancy in pleural biopsy specimens. Human Pathol 1994; 25: 443-448. Churg A et al: The separation of benign and malignant mesothelial proliferations. Amer J Surg Pathol 2000; 24: 1183-1200. Churg A: Separation of benign and malignant mesothelial proliferations. In, Churg A, Roggli V, Cagle P: Tumors of the Serosal Membranes. Atlas of Tumor Pathology, Fourth Series. Washington DC, Armed Forces Institute of Pathology, 2006. Pages 83-101. Davidson B et al: The role of desmin and N-cadherin in effusion cytology: a comparative study using established markers of mesothelial and epithelial cells. Am J Surg Pathol 2001; 25:1405-1412. Gonzalez-Lois C et al: Combined use of novel epithelial (MOC-31) and mesothelial (HBME-1) immunohistochemical markers for optimal first line diagnostic distinction between mesothelioma and metastatic carcinoma in pleura. Histopathology 2001; 38: 528-534. Hurlimann J: Desmin and neural marker expression in mesothelial cells and mesotheliomas. Hum Pathol 1994; 25: 753-757. McFadden DE, Clement PB: Peritoneal inclusions cysts with mural mesothelial proliferation. Amer J Surg Pathol 1986; 10: 844-854. Mangano WE, Cagle PT, Churg A, Vollmer RT, Roggli VL: The diagnosis of desmoplastic malignant mesothelioma and its distinction from fibrous pleurisy. Amer J Clin Pathol 1998; 110: 191-199. Mayall FG, Goddard H, Gibbs AR: p53 immunostaining in the distinction between benign and malignant mesothelial proliferations using formalin-fixed paraffin sections. J Pathol 1992; 168: 377-381. Roberts F et al Immumohistochemical analysis still has a limited role in the diagnosis of malignant mesothelioma. Amer J Clin Pathol 2001;116: 253-262. Walts A, Said JW, Koeffler HP: Is immunoreactivity for p53 useful in distinguishing benign from malignant effusions? Localization of p53 gene product in benign mesothelial and adenocarcinoma cells. Mod Pathol 1994; 7: 462-466. Whitaker D, Henderson DW, Shilkin KB: The concept of mesothelioma in situ: implications for diagnosis and histogenesis. Sem Diag Pathol 1992; 9: 151-161.