Myofibroblastoma of the breast with diverse differentiations.

Abstract

The histogenesis of myofibroblastoma of the breast remains unknown. Two cases of myofibroblastoma of the female breast were analyzed using light microscopy and immunohistochemistry. The tumors were well circumscribed and predominantly composed of bland bipolar spindle cells and interspersed bands of hyalinized collagen. Additionally, one tumor contained a cartilaginous island, and the other contained a well-defined small nodule that consisted of fascicular arrangements of mitotically inactive atypical cells, simulating atypical leiomyoma. Both lesions contained a fatty element. Immunohistochemically, spindle tumor cells expressed desmin, alpha-smooth muscle actin, muscle actin, and CD34. The atypical cells were strongly and diffusely positive for all these markers. Both tumors were DNA diploid and had a moderate S-phase fraction. The patients have no evidence of disease 4 and 18 months after simple excision. Myofibroblastoma of the breast may be a benign mesenchymal neoplasm capable of diverse lines of differentiation.