Benign myoepithelioma of the breast: origin and development.

A case of benign myoepithelioma of the breast in a 55-year-old woman is described. The tumor was a well-circumscribed solid mass, measuring 13 x 12 mm. Histopathology indicated that the tumor was composed of entirely myoepithelial cells, which was confirmed by immunoreactivity to calponin and S-100. There was no ductal differentiation in the tumor, and staining for pan-cytokeratin and epithelial membrane antigen was weak and negative, respectively. Although the biological behavior of the tumor remains to be ascertained, the tumor was considered to be myoepithelioma with benign features due to mild nuclear pleomorphism, sparse mitotic figures, low Ki-67 labeling index and low S-phase fraction. Diagnostic confusion between benign myoepithelioma and other myoepithelial-rich cell tumors is possible. Considering the classification of myoepithelial tumor in the salivary glands, benign myoepithelioma of the breast may possess a different development process from adenomyoepithelioma.