Light microscopic, ultrastructural and immunocytochemical spectrum of malignant lacrimal and salivary gland tumors, including malignant mixed tumors.

Ten malignant myoepithelial tumors of the salivary glands and one of lacrimal gland origin were studied by light, electron microscopy and immunocytochemistry. The light microscopic appearance of the tumors varied from primarily spindle cell neoplasms (two cases), to others with predominantly epithelial components (four cases) and mixed varieties (five cases). Therefore, they can be confused with other epithelial and mesenchymal neoplasms. The electron microscopic spectrum varied from tumors with widespread and typical myoepithelial differentiation (i.e. myofilament bundles at the cell periphery, attachment plaques and intercellular junctions) to some with diffusely distributed filaments, without associated spindle densities but with attachment plaques, and others with evidence of duct formation and containing scattered cells showing intracytoplasmic tonofilaments. Often the tumors revealed mixed ultrastructural features; the relative numbers of the different cellular components was variable. The eleven neoplasms were S-100 protein, actin and keratin positive, either focally or diffusely, with varying degrees of intensity. Ten of the eleven tumors were positive for vimentin and nine of ten tested expressed carcinoembryonic antigen. Only two of nine were focally positive for glial fibrillary acidic protein. The study emphasizes the variable light microscopic appearances of these neoplasms and their immunocytochemical and ultrastructural spectrum. Accurate determination of myoepithelial differentiation sometimes requires careful evaluation of the light, ultrastructural and immunocytochemical findings. If all three diagnostic modalities are not utilized, it is likely that some of these neoplasms will be improperly classified.