Malignant myoepithelioma of the parotid gland: cytologic and histologic features

A case of malignant myoepithelioma of the parotid gland in a 34-year-old female is presented. In the fine-needle aspiration material, there was predominance of poorly cohesive polygonal cells with marked nuclear pleomorphism; no mitotic figures were observed. Focally, fragments of myxoid metachromatic intercellular material were also present. Histologically, the tumor was encepsulated, showing focal invasion of the capsule and tumor thrombi in the capsular vessels. The tumor was predominantly solid and myxoid, composed of cells with epithelioid features, marked anisonucleosis and a low mitotic activity. Immunohistochemically, the cells revealed positive staining for S-100 protein, vimentin, cytokeratins, glial acidic fibrillary protein and carcinoembryonic antigen; only several cell groups expressed smooth muscle actin and desmin; muscle specific actin was uniformly negative. In differential diagnosis, it was important to distinguish malignant myoepitelioma mainly from pleomorphic adenoma (mixed tumor), benign myoepithelioma, carcinoma ex pleomorphic adenoma and malignant melanoma. The criteria of malignancy in myoepithelial tumors are discussed.