Myofibroblastoma of breast: Evidence favoring smooth-muscle rather than myofibroblastic differentiation

A histopathological study of two cases of the tumor known in the literature as myofibroblastoma of the breast is presented. The tumors occurred in Cau casian males aged 57 and 62 years. Histologically, these were moderately ce llular, lobulated spindle-cell lesions, each with a reasonably well-delinea ted edge with surrounding fatty connective tissue. No breast ducts or lobul es were present. Tumor cell nuclei were bland, with small nucleoli and some nuclear grooving. Nuclear atypia and mitoses were absent. Immunostaining r evealed positivity for alpha-smooth-muscle actin, desmin, and CD34. Tumor c ells contained rough endoplasmic reticulum, bundles of myofilaments with fo cal densities, intermediate filaments, attachment plaques alternating with plasmalemmal caveolae, and focal lamina. Ultrastructural findings pointed t o true smooth-muscle differentiation, and the cell-surface in particular la cked surface features of myofibroblasts (fibronectin fibrils [microtendons] and fibronexus junctions). These and published data suggest that at least some of the lesions referred to in the literature as myofibroblastoma may n ot be myofibroblastic and may be better designated as myogenic stromal tumo rs or as variants of leiomyoma.