Localized fibrous tumor of the serosal cavities. Immunohistochemical, electron-microscopic, and flow-cytometric DNA study.

Localized fibrous tumor of the serosal cavities (localized fibrous mesothelioma) is a generally benign spindle cell neoplasm of uncertain histogenesis. Fourteen histologically similar primary tumors from different mesothelium-lined sites (11 pleural and 1 each in the pericardium, peritoneal cavity, and pouch of Douglas) and 2 recurrences of those tumors (pericardium and pouch of Douglas) were examined histopathologically and by flow cytometry to relate histologic features and DNA ploidy to biologic behavior (follow-up, 48-255 months among 13 patients). All 16 tumors (14 primaries and 2 recurrences) displayed diploid DNA pattern, and none of 13 patients died of disease (1 patient was lost to follow-up). To elucidate the histogenesis, seven primary tumors were examined for vimentin and keratin immunostaining and six primary tumors were assessed by electron microscopy. All cases exhibited spindle-fibroblastic cell proliferation with a prominent hemangiopericytic pattern. All cases so examined had positive results for vimentin and negative results for keratin. Ultrastructurally, the tumor cells showed mesenchymal-fibroblastic features. These results support a mesenchymal origin, most likely from submesothelial fibroblasts. Further, this neoplasm may recur but retain its basic histologic features, diploidy, and benign outcome.