Breast tumors with myofibroblastic differentiation: Clinico-pathological observations in myofibroblastoma and myofibrosarcoma

This report describes the clinico-pathological features of myofibroblastic tumors of the breast in six patients. Four women and one man presented with a benign myofibroblastoma. The sixth patient was a woman with myofibrosarc oma. All myofibroblastomas were composed of a fascicular arrangement of spi ndle cells embedded in dense bundles of collagen. Tumors differed with resp ect to their proportion of neoplastic cells and collagenous stroma as well as cellular pleomorphism. Based on this variation, the tumors could be subc lassified as classic, collagenized, epithelioid and cellular myofibroblasto ma. Immunohistological staining confirmed myofibroblastic differentiation b y strong expression of either desmin or smooth muscle actin with coexpressi on of vimentin. In addition, numerous cells re acted with antibodies to CD6 8. Proliferative activity was rather low in the myofibroblastoma with an av erage of 0-2 mitotic figures per 10 HPF. DNA cytometric analysis was perfor med in two cases and showed diploid stem lines with minor S-phase fractions (1% and 3%). In the myofibrosarcoma, cells contained pleomorphic nuclei wi th some giant cells and numerous mitotic figures (6-7/10 HPF) and had infil trating margins that were apparent even grossly. Immunohistochemically, tum or cells strongly expressed vimentin, smooth muscle actin and fibronectin. Ultrastructurally, neoplastic cells met the criteria of myofibroblasts, i.e . contained abundant intermediate filaments and myofilament bundles with fo cal densities as well as fibronexus junctions. DNA cytometric analysis exhi bited again a diploid stemline but marked proliferative activity was presen t as indicated by an S-phase fraction of 20%. In conclusion, in benign myof ibroblastoma there may be some cellular pleomorphism but mitotic activity i s always low. The malignant counterpart, myofibrosarcoma, is characterized by marked cellular pleomorphism, infiltrating margins and high mitotic rate .