Cellular differentiation and morphologic heterogeneity in polymorphous low-grade adenocarcinoma of minor salivary gland.

Histologic diversity is intrinsic to salivary gland tumors, but it is a particular feature of polymorphous low-grade adenocarcinoma as denoted by the terminology. Application of immunohistochemistry and electron microscopy to three examples allowed the study of some aspects of tumor cell differentiation in this minor salivary gland lesion. In one case where the tumor cells were cytologically of one type, immunohistochemistry clearly identified both luminal and nonluminal tumor cells, but the latter showed no evidence of myoepithelial cell differentiation. A second case revealed differentiation only of luminal-type tumor cells, while a third example was largely differentiated as myoepithelial cells but again immunohistochemistry confirmed focal formation of duct-like structures by luminal epithelium. These cases show a considerable range of tumor cell heterogeneity as well as variations in their organization. This variation in differentiation characteristics underlies the histology of polymorphous low-grade adenocarcinomas, and likely occurs in other salivary gland tumors. To establish specific and reliable diagnostic criteria for such tumors requires awareness of this neoplastic process. The limited malignant potential and excellent survival of patients with polymorphous low-grade adenocarcinoma is apparently little affected by patterns of differentiation in this particular neoplasm.