Colliding, Colonizing, or Combining?: Four Cases Illustrating the Unique Challenges of Melanoma arising in conjunction with Basal Cell Carcinoma

Biphasic lesions comprised of melanocytic and epithelial components are rare entities believed to arise either as a collision of two histologically distinct lesions in the same anatomic location or as a singular progenitor tumor differentiating along two differing lineages. Regardless of mechanism of origin, these tumors present unique challenges in pathologic interpretation and in determining appropriate measurements, which assigns subsequent prognosis to the patient.

We present 4 tumors of melanoma co-existing with basal cell carcinoma and discuss relevant literature regarding these biphasic entities. Patients consisted of 3 males and 1 female, ranging in age from 62–93, with lesions located on the shoulder, frontal scalp, forearm, and nose. Three of four lesions demonstrated melanoma cells limited to basal cell carcinoma tumor lobules, without evidence of direct dermal invasion by melanoma cells, raising question of whether or not these tumors should be classified as in situ or invasive melanoma.

These cases highlight the complexity that such lesions pose to dermatopathologists, in terms of their uncertain origin and variable microscopic appearance. In the absence of data regarding outcomes for these tumors (given their rarity), it is important to utilize a case-by-case approach with careful clinical correlation and appropriate use of ancillary techniques.