A collision tumor involving Basal cell carcinoma and lentigo maligna melanoma.

Basal cell carcinomas (BCC) are known to co-exist with other cutaneous lesions, but the collision of BCC with malignant melanoma is rare. We report on the case of an 82-year-old woman with a translucent papule set on a beige-brown plaque on the right side of the nose. Histologic examination showed lesions of lentigo maligna melanoma (LMM) in situ and invasive melanoma involving nests of BCC that invaded the dermis. Immunohistochemical studies with S100 protein, HMB-45, and Melan-A antibodies showed the melanocytic component in the epidermis and dense clusters of "atypical" melanocytes in the dermal nests of BCC. On examination of the biopsy specimen, melanoma was still in situ because it was limited to the nests of BCC and not detectable between dermal collagen bundles. However, the re-excision of the lesion showed residual BCC and invasive LMM, level II, measuring 0.2 mm in thickness. The diagnosis, pathogenesis, and prognosis of this collision tumor are discussed.