Leiomyoma of the breast.

Leiomyoma of the breast parenchyma proper is a rare neoplasm. This tumor occurs predominantly in late middle-aged women and can be clinically difficult to discern from carcinoma. Mammographically, the tumor lacks typical features of carcinoma, but this possibility cannot be absolutely excluded. Immunohistochemical, ultrastructural, and DNA characteristics are reported in a new case. Mammographic findings are reported for the first time. Immunoperoxidase findings with antibodies to muscle-specific actin, desmin, vimentin, S-100 protein, and cytokeratins, as well as the ultrastructural features, suggest a pure smooth muscle cell origin and not a myoepithelial cell origin.