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Histopathologic studies on carcinoma in pleomorphic adenoma of the parotid gland

Five‐hundred‐eighteen cases of primary epithelial tumors of the parotid gland were examined. The incidence of carcinomas in pleomorphic adenoma was 9.3% (48 cases). In addition to cellular atypism, the following histological findings were thought to be important criteria for the diagnosis of carcinomas: (1) capsular invasion, (2) infiltration into adjacent organs, (3) proliferation of atypical cells within fibrous tissues and chondroid matrix in the area of pleomorphic adenoma, (4) vascular involvement, and (5) mitotic figures. Pleomorphic adenomas (282 cases) were found predominantly in females (70.8%), at the age of 30 to 49 (48.2%), whereas carcinomas in pleomorphic adenoma were more often found in males (70.8%) and at the ages over 50 (43.8%). Histologic types were composed mostly of undifferentiated carcinoma (56.2%) and adenocarcinoma (37.5%). The prognosis of the carcinomas, especially the undifferentiated type, was generally poor; about 45% of the cases either experienced recurrence or died within five years. Electron microscopy on this undifferentiated carcinoma revealed characteristic filamentous structure in the carcinoma cells, resembling that observed in myoepithelial cells. It was assumed that the undifferentiated carcinoma cells were of myoepithelial origin.