HYBRID TUMORS OR SALIVARY-GLAND TUMORS SHARING COMMON DIFFERENTIATIONPATHWAYS - REEXAMINING ADENOID CYSTIC AND EPITHELIAL-MYOEPITHELIAL CARCINOMAS

Three adenoid cystic carcinomas and two epithelial-myoepithelial carci nomas, which focally shared common histologic features, were studied t o examine the common differentiation pathways manifested by these tumo rs and to discuss criteria for hybrid salivary gland tumors. Regions o f the adenoid cystic carcinomas had cellular features ranging from sim ple clear cell change of basal/myoepithelial cells to combined clear c ells and prominent ductal structures mimicking epithelial-myoepithelia l carcinoma. Conversely, two epithelial-myoepithelial carcinomas had a denoid cystic carcinoma-like regions caused by the formation of ''pseu docysts''; this resulted in a focal cribriform pattern. Electron micro scopy of two additional but typical epithelial-myoepithelial carcinoma s revealed both excess basal lamina at the margins of cellular nests a nd widened intercellular spaces containing reduplicated basal lamina a nd accumulations of glycosaminoglycans; these ultrastructural features were identical to those seen in adenoid cystic carcinomas. The five c urrent cases are not examples of hybrid tumors, but they demonstrate t he effects of gene expression and the resulting differentiation of syn thetic products and tumor cells that are generally restricted to one o r the other of these two tumor types by as-yet-unknown means. To avoid misdiagnosis and its prognostic implications, adenoid cystic carcinom a-like regions in epithelial-myoepithelial carcinoma and epithelial-my oepithelial-like regions in adenoid cystic carcinoma should be recogni zed simply as anomalous differentiation.