Abstract

Tumors with components of both melanoma and basal cell carcinoma are generally referred to as basomelanocytic neoplasms. A classification scheme was recently proposed to better characterize these rare tumors depending on the growth pattern and the boundaries of both elements and to help better unify the use of terms such as collision and colonization. Nevertheless, issues related to diagnosis, treatment, and prognosis remain controversial. Our case features malignant melanoma closely associated with basal cell carcinoma in a 91-year-old man with a past medical history of non-melanoma skin cancers and atypical melanocytic proliferations. He presented with a growing 9-mm pigmented papule on postauricular skin. Biopsy revealed nodular basal cell carcinoma nests, many of which are heavily populated and near-completely replaced by melanoma cells except for thin peripheral rims of epithelium. Only rare invasive melanoma nests were noted in the mid dermis and confirmed by immunohistochemical testing with SOX10, Melan A, and Ber-EP4 stains. Flanking skin showed limited melanoma in situ. Breslow thickness measurement was challenging as the invasive nests were juxtaposed to islands of basal cell carcinoma. This raised the possibility of invasion through basal cell carcinoma (0.4 mm) rather than surface epidermis (1.3 mm) which could change the pathologic staging to pT1a instead of pT2a. Whether the melanoma component arose from passenger melanocytes within the basal cell carcinoma could not be determined. We hope to elucidate some of the continued challenges associated with nomenclature and staging in these rare neoplasms. Treatment decisions related to Mohs micrographic surgery and sentinel lymph node biopsy are dependent on a better understanding of the pathogenesis of these tumors.

Financial Disclosure:
No current or relevant financial relationships exist.