Myoepithelial carcinoma of the salivary glands - A clinicopathologic study of 25 patients

Salivary gland carcinomas displaying exclusively myoepithelial differentiation (myoepithelial carcinoma) are considered rare. Their histopathologic features, immunohistochemical profile, and clinical behavior are not well characterized. The authors reviewed the clinicopathologic features of 25 salivary gland tumors fulfilling two fundamental histologic criteria: unequivocally malignant and exclusively myoepithelial. For most of these, the original diagnosis was malignant mixed tumor. Thirteen men and 12 women aged 24 to 77 years (mean age, 55 yrs) participated in the study, and most presented with a painless mass. The parotid gland was the most common site (n = 15). Tumors ranged from 2.1 to 5.5 cm, arising either in association with a benign mixed tumor (n = 15) or de novo (n = 10). Histologically, all the tumors displayed infiltrative growth and most had a characteristic multinodular architecture with a cellular periphery and central necrotic/myxoid zones. Epithelioid, hyaline, spindle, clear, or mixed cell types were noted with accompanying myxoid and/or hyalinized extracellular matrix. Ten tumors were high grade cytologically and 15 were low grade. The mitotic rate ranged from three to 51 mitoses per 10 high-power fields. Necrosis was present in 15 tumors and perineural and vascular invasion were identified in 11 and four neoplasms respectively. Immunoreactivities included CAM5.2 (89%), AE1:AE3 (100%), 34 beta E12 (92%), cytokeratin 7 (21%), cytokeratin 14 (53%), vimentin (100%), S-100 protein(100%), smooth muscle actin (50%), calponin (75%), muscle-specific actin (31%), glial fibrillary acidic protein (31%), carcinoembryonic antigen (0%), and epithelial membrane antigen (21%). Ultrastructural examination of three tumors showed myoepithelial features. Ten patients developed recurrences, mostly multiple. Follow up of 17 patients showed that eight patients (47%) developed metastases (six high grade, two low grade) and five patients (29%) died of disease (four high grade, one low grade) after a mean of 32 months. Two patients were alive with disease (19 and 49 mos). Ten patients (59%) were without any evidence of disease after a mean of 42.2 months. Myoepithelial carcinomas exhibit a wide spectrum of cytomorphologic features and diverse clinical outcomes. As a result of their morphologic heterogeneity, they can be confused easily with many tumors. Myoepithelial carcinomas have been underrecognized in the past, primarily by being lumped under a broader category of "malignant mixed tumor." Awareness of their unique cytoarchitectural patterns and immunohistochemical profile is crucial for accurate identification.