Myofibroblastoma of the breast: Cytologic, histologic, immunohistochemical, and ultrastructural findings in two cases with differing cellularity

We present the cytologic, histologic, immunohistochemical, and ultrastructural features of two cases of myofibroblastoma with different cellularity, one paucicellular the other hypercellular. Fine-needle aspiration suggested the diagnosis in the case of the hypercellular tumor of the breast and pointed out the benign nature of the paucicellular case, arising in the breast of a young woman. Immunohistochemistry, showed smooth muscle-specific actin, vimentin, and desmin positivity consistent with myofibroblastic differentiation, in both cases. The positivity of both tumors to CD34 suggests that they may be related to the so-called solitary fibrous tumor. In addition, there was strong progesterone- and weak estrogen-receptor positivity ion the hypercellular case, a novel finding in these tumors, with possible histogenetic implications. Ultrastructural findings in the first case were consistent with the myofibroblastic origin of the tumor