Histopathologic studies on adenocarcinoma of the parotid gland

Of 683 cases of primary epithelial tumors arising in the parotid gland, 20 cases could be defined as adenocarcinomas, constituting 2.9% of the total number or 10.0% of 201 malignant tumors. These adenocarcinomas were characterized by a marked diversity in the histologic manifestations, presenting tubular, papillary, pseudoadenoid cystic, cribriform, trabecular, sarcomatoid, and other patterns. Basically, however, they could be classified into two major subtypes, i.e., tubular and papillary variants on the basis of their predominant histological patterns. The tumor cells showed a distinctive sign of high grade malignancy such as the occurrence of many mitotic figures and involvements of lymphatic vessels and/or blood vessels. Ultrastructurally, the tumor cells consisted of two basic varieties, resembling myoepithelial cells and duct epithelium. It is thus suggested that immature tumor cells could differentiate into a structure mimicking an intercalated duct. Clinicopathologically, the tumors were seen with a higher frequency in middle-aged males (average 44 years of age).