Melanoma Colonizing Basal Cell Carcinoma: A Rarely Described Entity

Collision tumors combining basal cell carcinoma (BCC) and melanoma are exceptionally rare, and their distinction from melanoma colonization remains controversial.

A 68-year-old man presented with a persistent ulcer on the lower lip, initially believed to represent BCC. Wedge resection revealed a 2.2 cm ulcerated lesion composed of BCC with malignant melanocyte colonization. Immunohistochemical analysis confirmed the biphasic nature: the melanocytic component was positive for S100, Melan-A, SOX10, and PRAME (with p16 loss), and the epithelial component was positive for p40 and Bcl-2. The melanoma measured 7.5 mm in Breslow thickness (pT4b) with ulceration and perineural invasion, but no lymphovascular involvement. Margins were negative. A sentinel lymph node biopsy was recommended but was declined by the patient.

This case illustrates the diagnostic complexity and clinical effect of BCC-melanoma collision/colonization tumors. Prognosis is dictated by the melanoma component, underscoring the need for meticulous histopathologic evaluation and immunohistochemistry in guiding patient management.