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Abstract

Basomelanocytic tumors (BMTs) are rare cutaneous neoplasms characterized by an intimate admixture of atypical basaloid epithelial and melanocytic cells. While basal cell carcinoma (BCC) and melanoma are commonly encountered individually, only a limited number of BMTs have been documented. A classification scheme of four BMT subtypes has been proposed by Satter et al. (PMID:18727668): (1) collision: two tumors close together yet with defined boundaries, (2) colonization: atypical melanocytes confined within BCC nests, (3) combined: two intertwined tumor cell populations, and (4) biphenotypic: tumor cells with overlapping immunohistochemical and molecular profiles. We report the clinical and histologic features of four BMTs: 76-year-old woman, 80-year-old man, 89-year-old woman, and 91-year-old-man, presented with lesions on the right back, left upper lip, left ear, and left forehead, respectively.  Three cases were colonization tumors, with histologic sections showing tumors comprised of two different neoplastic populations. One component exhibited a nodular proliferation of basaloid nests with peripheral palisading and stained positive for keratin 5/6, keratin 17, and p63. Scattered within these basaloid nests were melanocytes highlighted by Sox10, Melan-A, MiTF, and HMB-45. The remaining case is a combined tumor of p63+ BCC and Sox10+ melanoma. Some authors have proposed that colonizing and colliding tumors may be best classified as melanoma in situ. Reports of the indolent biologic behavior of BMTs appear to support this view. Of the 27 BMTs in our literature review, metastatic disease was noted in only one collision and one combined BMT (PMID:24765446, PMID:26209219), and in none of the colonized tumors.