Malignant melanoma in situ colonizing basal cell carcinoma. A simulator of invasive melanoma.

Coexisting (collision) cutaneous neoplasms of various types and combinations are well documented but relatively uncommon. This report describes the unusual occurrence of a malignant melanoma in situ (MMIS) colonizing a basal cell carcinoma (BCC). A 69-year-old man was considered clinically to have a melanocytic neoplasm or a pigmented BCC of the right ear. Biopsy showed an MMIS, lentigo maligna type, juxtaposed to a typical BCC. The MMIS extended peripherally and into the BCC. Interspersed among the basaloid epithelial cell aggregates that extended 1.70 mm into the dermis were atypical melanocytes. Immunoperoxidase stains with high molecular weight cytokeratin (903) stained the basaloid keratinocytes but not the melanocytes. Conversely, HMB-45 and Mel-5 intensely stained the melanocytes throughout the BCC and in the epidermis but did not demonstrate any separate aggregates of invasive melanoma. These findings suggest that the position of the MMIS was a consequence of BCC colonization. Because the prognosis of malignant melanoma correlates most closely with the thickness of the lesion, this case poses a unique problem in predicting the biology of the lesion, which we believe should not be considered invasive melanoma.