MYOFIBROBLASTOMA OF THE BREAST WITH DIVERSE DIFFERENTIATIONS

Background.-The histogenesis of myofibroblastoma of the breast remains unknown. Design.-Two cases of myofibroblastoma of the female breast w ere analyzed using light microscopy and immunohistochemistry. Results. -The tumors were well circumscribed and predominantly composed of blan d bipolar spindle cells and interspersed bands of hyalinized collagen. Additionally, one tumor contained a cartilaginous island, and the oth er contained a well-defined small nodule that consisted of fascicular arrangements of mitotically inactive atypical cells, simulating atypic al leiomyoma. Both lesions contained a fatty element. Immunohistochemi cally, spindle tumor cells expressed desmin, alpha-smooth muscle actin , muscle actin, and CD34. The atypical cells were strongly and diffuse ly positive for all these markers. Both tumors were DNA diploid and ha d a moderate S-phase fraction. The patients have no evidence of diseas e 4 and 18 months after simple excision. Conclusion.-Myofibroblastoma of the breast may be a benign mesenchymal neoplasm capable of diverse lines of differentiation.