Malignant myoepithelial carcinoma (myoepithelioma) arising in a pleomorphic adenoma of the parotid gland. An immunohistochemical study and review of the literature.

A 66-year-old woman presented with a giant tumor of the parotid gland, which under a light microscope had a sarcomatous appearance with spindle-shaped myoepithelial cells (malignant myoepithelioma). Histochemical examination showed longitudinal fibrils in the cytoplasm of the myoepithelial cells. Immunohistochemical studies showed positive staining for S-100 protein, vimentin, and actin in the myoepithelial cells. Cytokeratin was seen in the ductular epithelial cells and in the periductular myoepithelial cells, but it was absent from malignant myoepithelial cells. Ultrastructurally, these cells contained several longitudinally oriented slender structures resembling myofilaments without dense bodies. A review of the literature confirms the rarity of malignant myoepithelial carcinoma (myoepithelioma) arising in the parotid gland, and we therefore add another case.