Histopathologic studies of undifferentiated carcinoma of the parotid gland.

Five-hundred-fifty-five primary epithelial tumors of parotid gland origin were examined. Eighteen cases could be defined undifferentiated carcinomas, constituting 3.2% of the total number or 10.2% of 176 malignant tumors. The undifferentiated carcinomas could be divided on the basis of the cell size into two types: (1) Small-cell type (12/18; 66.7%); the tumor cells were as large as, or slightly larger than lymphocytes. The cytoplasm was scanty with dark chromatin. Mitotic figures were numerous. The tumor cells in the periphery of the cell clusters exhibited a palisading arrangement. Electron microscopy revealed that these cells were rich in tonofilaments and connected by desmosomes. (2) Large-cell type (6/18; 33.3%); the carcinoma cells were roughly twice as large as those of the small-cell carcinoma. The tumor cells were spheroidal or spindle in shape with rich and clear cytoplasm. Many of the tumor-cell clusters showed macroalveolar structures, suggesting the possibility that this type occurred as a variant of poorly differentiated adenocarcinoma or epidermoid carcinoma. Clinicopathologically, there was no sex difference in the total number of the undifferentiated carcinomas, but the large-cell carcinomas were frequently encountered in females (83.3%). As to the age distribution, the carcinomas were found frequently in middle-aged patients from 30-49 years (66.7%). Many of the tumors were 2-4.9 cm at their greatest diameter (55.6%). Tumor mass less than 1.9 cm could not be found.