Myofibroblastoma of the breast: a case report.

A tumor in the breast of a 67-year-old male was resected. It was grossly circumscribed and unencapsulated. Histologically, it was composed of plump and long bipolar, spindle cells arranged in fascicular clusters with intervening broad collagen bands. The cells lacked pleomorphism, and no mitotic figures were found. The margin was that of pushing border type. No epithelial component was seen in the tumor. Immunohistochemically, the tumor cells expressed variable reactivity to vimentin, desmin and smooth muscle actin and were negative for S-100 protein and keratin. Electron microscopy showed cell differentiation between fibroblast and smooth muscle cells.