Pigmented squamous cell carcinoma of the skin: morphologic and immunohistochemical study of five cases.

Invasive pigmented squamous cell carcinoma (PSCC) of the skin is reportedly rare. Herein, we evaluate an additional five cases and compare their relative frequency with non-pigmented squamous cell carcinoma (SCC). Of 46,791 archived cases of SCC, a total of five cases of PSCC were discovered for a relative frequency of approximately 0.01%. Grossly, each tumor presented as a rapidly growing crusted papule on actinic damaged skin of the face. Microscopically, all were composed of a mixture of keratininized squamous cells and melanin-producing dendritic melanocytes. The squamous cells stained for epithelial membrane antigen, and both low and high molecular keratins. The melanocytes stained for S-100 and HMB-45. A matched series of 31 SCCs were subjected to an identical immunohistochemical battery of stains to determine if a histologically subtle and unsuspected number of intratumoral melanocytes existed in SCC. Each of the cases failed to show intratumoral melanocytes. The differential diagnosis and possible histogenesis of PSCC is discussed and the importance of extensive pathologic examination to prevent misdiagnosis is emphasized. Despite the histologic dissimilarity, the long-term prognosis of the reported cases was similar to conventional SCC.