Myofibroblastoma of the breast with diverse histological features

We report two cases of myofibroblastoma with unusual pathological features, in a 66-year-old woman and a 49-year-old man. Both tumours were unilateral, grossly nodular and well circumscribed, but not encapsulated. The lesions were made up of bipolar spindle cells arranged in fascicular clusters separated by bands of hialinized collagen; one included several islands of mature cartilage next to fat cells. The other contained atypical mononucleated and multinucleated giant cells. No mitotic figures were observed. Immunohistochemically, both tumours showed strong and diffuse cytoplasmic staining for vimentin and CD 34 and focal positivity for alpha-smooth muscle actin, and both were negative for cytokeratins, CD 68, Ham 5, 6, Mac 387, and S-100 protein. Desmin was positive in one case. Ultrastructural study revealed populations composed of fibroblastic cells without signs of myofibroblastic differentiation in one case; the second featured abundant undifferentiated mesenchymal cells with myofibroblastic differentiation. Both patients remain disease-free 38 and 36 months after lumpectomy.