Parasitism of basal cell carcinoma by lentigo maligna melanoma.

The coexistence of 2 types of malignant neoplasms is relatively uncommon. We report a pigmented melanocytic lesion occurring adjacent to a previously treated basal cell carcinoma. Histology showed melanoma (lentigo maligna type) colonizing basal cell carcinoma demonstrated by hematoxylin-eosin and immunocytochemistry stains. It is unclear whether this lesion has the metastatic potential of an invasive melanoma of similar thickness, or simply reflects melanoma in situ extending into an epidermal-derived tumor analogous to appendageal extension of lentigo maligna. We review the literature on melanoma colliding with basal cell carcinoma. Therapeutic and prognostic problems are posed by this unique situation.