Cutaneous carcinosarcoma with myoepithelial differentiation: immunohistochemical and cytogenetic analysis of a case presenting in an unusual location.

Primary cutaneous carcinosarcoma is an uncommon tumor that is characterized by an admixture of malignant epithelial and mesenchymal elements. To our knowledge, only approximately forty cases have been reported to date in the literature. The majority of these neoplasms occurred on the extremities and the head and neck region of older individuals. We report an additional case, which presented in an 87-year-old man in an unusual location, the buttock. Histologically, the lesion was characterized by a biphasic proliferation of malignant epithelial areas admixed with an infiltrative, poorly differentiated spindle cell component. The epithelial areas consisted of nodular aggregates of basaloid to squamoid cells forming focal ductal structures with central lumens containing proteinaceous debris. The sarcomatoid component consisted of malignant spindle to epithelioid-appearing cells admixed with bizarre-appearing tumor giant cells within a myxochondroid stroma. The epithelial cells showed positive staining with pan-cytokeratin, BerEP4, p63, and keratin 5/6 and were focally positive for S-100, calponin, and actin, consistent with myoepithelial differentiation. An epithelial membrane antigen stain highlighted the scattered ductal structures. The high-grade spindle cell areas were negative for S-100, actin, desmin, calponin, p63, epithelial membrane antigen, and keratin 5/6.