Squamomelanocytic tumor: a new case of a unique biphenotypic neoplasm of uncertain biological potential

Squamomelanocytic tumor is an uncommon cutaneous neoplasm composed of an admixture of melanocytic and squamous cellular phenotypes. We describe a case of this tumor in a 94-year-old man who presented with a nodule on the back. Histologically, a well-demarcated expansive dermal nodule was composed of anastomosing epithelial strands with focal formation of squamous pearls and ductal structures commixed with elongated spindle cells with clear cytoplasm grouped in nests. The two cell types were diffusely admixed or clustered in groups within the nodule. Immunohistochemical studies showed that the spindle cells grouped in nests expressed S-100 and HMB-45 antigens, and the squamoid cells expressed cytokeratins and carcinoembrionic antigen (CEA) protein only in the inner layer of the ducts and the cystic space. Atypical features and high mitotic activity was observed in the melanocytic cells but slight atypia, mild dyskeratosis and mitotic figures were observed also in the squamoid component. This tumor represents a proliferation of two phenotypic cells that are distinctive for their intimate admixture and singular immunohistochemical profile. The authors discuss the histogenesis of this tumor, suggesting that it could represent an atypical solid-cystic hidradenoma colonized by a melanocytic malignant component. The biological behavior of this neoplasm remains currently uncertain.