Collision tumor of cutaneous malignant melanoma and basal cell carcinoma.

An extremely rare case of a collision tumor of malignant melanoma (MM) and basal cell carcinoma (BCC) located in the temporal scalp of a 78-year-old male is reported. Microscopic examination revealed a collision tumor consisting of the nodular type, Clark's level IV MM, and a pigmented BCC. The tumor was studied by immunohistochemistry and was analyzed for point mutations of codons 12 and 61 of H-ras, K-ras and N-ras oncogenes. Tumor cells of the MM were positive for S-100 protein, HMB-45 and vimentin, but negative in the BCC. The malignant melanoma possessed an activating A to T transversion in codon 61 of N-ras gene, which is the code for leusine instead of glutamine. This is the first report of ras oncogene mutation in such a collision tumor. MM and BCC are clearly different in terms of histological, immunohistochemical and ras gene mutation analyses. These findings support the view that the association of MM and BCC in this collision tumor is a chance occurrence.