Clear-cell smooth muscle tumor of the skin.

We report a case of a previously undescribed benign neoplasm characterized by a poorly circumscribed proliferation of clear cells, arranged both singly and in elongated fascicles oriented in haphazard fashion within the middle and lower reticular dermis. The fascicles consisted of spindle-shaped cells that were periodic acid-Schiff positive and diastase labile. These cells were characterized by oval, hyperchromatic, cigar-shaped nuclei and clear cytoplasm. Immunohistochemically, neoplastic cells were diffusely positive for smooth muscle actin (1A4), common muscle actin (HHF-35), and vimentin. Immunoreactivity for fibronectin was also focally noted. The neoplasm, which we designated clear-cell smooth muscle tumor of the skin, should be distinguished from smooth muscle hamartoma and other smooth muscle proliferations at cutaneous level. We highlight in this case report the histopathologic differential diagnosis among other tumors with myofibroblastic, melanocytic, and neural differentiation.