Lymphohistiocytoid mesothelioma. An often misdiagnosed variant of sarcomatoid malignant mesothelioma.

Three cases of lympho-histiocytoid mesothelioma, a rare variant of pleural sarcomatoid malignant mesothelioma, are described. Histologically, the neoplasms were characterized by a diffuse discohesive proliferation of atypical histiocytoid cells intermixed with a marked lymphocytic and lesser plasmacytic infiltrate. One case initially was misdiagnosed as a ganglioneuroma, a second case was misinterpreted as malignant lymphoma, and a third case was sent in consultation with the differential diagnosis of inflammatory pseudotumor vs mesothelioma. Immunohistochemical studies showed strong and generalized expression of cytokeratins and vimentin by the neoplastic histiocytoid cells in all 3 cases. Two cases were positive for calretinin, one of which also was positive for HBME-1, thrombomodulin, and LeuM1. None of the cases stained with the epithelial glycoprotein markers carcinoembryonic antigen, B72.3, and Ber-EP4, or the blood group antigen, BG-8. The immunophenotype of the lymphoplasmacytic infiltrate revealed predominantly reactive, mature T cells, with fewer polytypic plasma cells, histiocytes, and B cells. In lymphohistiocytoid mesothelioma, as in the usual examples of sarcomatoid mesothelioma, the demonstration of cytokeratin expression by the neoplastic cells is the most useful diagnostic finding that allows exclusion of other neoplasms with which this entity may be confused.