Hybrid tumors or salivary gland tumors sharing common differentiation pathways? Reexamining adenoid cystic and epithelial-myoepithelial carcinomas.

Three adenoid cystic carcinomas and two epithelial-myoepithelial carcinomas, which focally shared common histologic features, were studied to examine the common differentiation pathways manifested by these tumors and to discuss criteria for hybrid salivary gland tumors. Regions of the adenoid cystic carcinomas had cellular features ranging from simple clear cell change of basal/myoepithelial cells to combined clear cells and prominent ductal structures mimicking epithelial-myoepithelial carcinoma. Conversely, two epithelial-myoepithelial carcinomas had adenoid cystic carcinoma-like regions caused by the formation of "pseudocysts"; this resulted in a focal cribriform pattern. Electron microscopy of two additional but typical epithelial-myoepithelial carcinomas revealed both excess basal lamina at the margins of cellular nests and widened intercellular spaces containing reduplicated basal lamina and accumulations of glycosaminoglycans; these ultrastructural features were identical to those seen in adenoid cystic carcinomas. The five current cases are not examples of hybrid tumors, but they demonstrate the effects of gene expression and the resulting differentiation of synthetic products and tumor cells that are generally restricted to one or the other of these two tumor types by as-yet-unknown means. To avoid misdiagnosis and its prognostic implications, adenoid cystic carcinoma-like regions in epithelial-myoepithelial carcinoma and epithelial-myoepithelial-like regions in adenoid cystic carcinoma should be recognized simply as anomalous differentiation.