Diffuse malignant biphasic peritoneal mesothelioma with cystic areas.

UNLABELLED We report a case of peritoneal biphasic mesothelioma with cystic areas in a patient with professional exposure to asbestos. It showed focal epithelial glandular and papillary proliferations, also presenting fluid filled cysts, whose wall consisted of a proliferation of spindle cells. Atypia and mitoses were very scanty. EMA, vimentin, CK5/6, D2-40, calretinin and P53 were positive and desmin was negative in both epithelial and spindle areas, including the ones surrounding the cystic spaces. These findings gave an essential aid in the differential diagnosis with a benign cystic mesothelioma and with a cystic epithelial mesothelioma with secondary pseudosarcomatous myofibroblastic proliferation. The presence of cystic areas in a malignant mesothelioma could make difficult the diagnosis. A large amount of tumour tissue is necessary for confirming the biphasic histotype, an aggressive histotype, even in the presence of mild histological features and of some others favourable clinical prognostic indices as in this case. To our knowledge this is the first case of malignant peritoneal biphasic mesothelioma with cystic features reported in the literature. KEY WORDS Cystic Mesothelioma, Immunohistochemistry, Malignant Mesothelioma, Peritoneal Diseases, Mesothelial Neoplasms.