Myoepithelial tumors of salivary glands: a clinicopathologic, immunohistochemical, ultrastructural, and flow-cytometric study

Abstract

Myoepitheliomas of the salivary glands remain a controversial entity. To contribute to the knowledge of this entity, 16 myoepithelial tumors of the salivary glands were studied: 12 benign myoepitheliomas (BME) and 4 malignant myoepitheliomas (MME). The clinical and the histologic findings of each case were studied Immunohistochemistry and flow-cytometry analysis were performed from the paraffin-embedded material in 15 cases. An electron-microscopy study was performed in 8 cases. The myoepithelial tumors affected patients of both sexes equally. The mean age of the patients with BME was 54 years, and the mean age of patients with MME was 62 years. Eight cases of BME originated in the parotid gland and 4 cases originated in the minor salivary glands. All the MME developed from a benign preexistent tumor: two developed from a pleomorphic adenoma in the parotid gland, and the other two MME developed in the minor salivary gland from a BME. The myoepithelial tumors were composed of epithelioid, plasmacytoid, spindle, or clear cell types, and they showed a solid or a myxoid pattern of growth. Immunohistochemical studies revealed marked and diffuse positivity to cytokeratins, vimentin, and S-100 protein in all cases. Glial fibrillary acidic protein was positive in 8 cases (53%), and muscle-specific actin and smooth-muscle actin were positive in only 3 cases (20%); they were all cases of BME. Desmin was negative in all tumors. Ultrastructural studies showed the presence of basal membrane, tight junctions, intermediate filaments, and microvilli as well as actin-like filaments lacking focal densities in all cases. But actin-like filaments with focal densities were not identified. Flow cytometry determined that all BME were diploid with a mean proliferative index of 7.73%. Two of the MME were diploid and the other two MME were aneuploid. The mean proliferative index of MME was 11.93%. In conclusion, BME and MME originated in major and minor salivary glands can display different histologic patterns and cellular features. Some immunohistochemical and ultrastructural characteristics have been found in all these neoplasms, which supports the idea that myoepitheliomas are composed by neoplastic modified myoepithelial cells, not fully differentiated. These techniques can be useful for the diagnosis of these tumors.