Abstract:

Background: Collision tumors are characterized by two distinct malignant or benign neoplasms at one anatomic site. The most commonly reported collision tumors consist of a combination of a basal cell carcinoma (BCC) with another neoplasm such as a seborrheic keratosis (SK) or nevus; however, many other combinations have been reported in the literature including combinations with squamous cell carcinoma and melanoma. Here we present a case of a melanoma arising over a dermatofibroma. Case description: A 37-year-old previously healthy male with a family history of metastatic melanoma presented to dermatology clinic for mole mapping. The patient noted a growing lesion on his left upper arm; exam revealed a 11x12mm lesion consisting of a brown macule with irregular borders overlying an ill-defined pink papule. Biopsy was performed and demonstrated a subtle atypical proliferation of melanocytes overlying a dermatofibroma. Positive staining for SOX10 was demonstrated in melanocytes, nests and the superficial dermis, but not in the subjacent fibrohistiocytic tumor. PRAME staining confirmed crisp nuclear expression within junctional melanocytes. Overall, these features suggested melanoma-in-situ developing in a nevus, arising over a dermatofibroma. The patient received excision of the lesion with clear margins, and to our knowledge, the lesion has not recurred. Conclusion: There are few cases of collision tumors containing a dermatofibroma in the literature. Other reports have described dermatofibromas in combination with BCCs, dysplastic nevi and melanoma, as in the case presented above. Dermatofibroma collision tumors may be difficult to diagnose; dermoscopy can prove extremely valuable in identifying various lesion structures and components in order to promptly diagnose and treat life-threatening malignant cutaneous neoplasms that appear in combination with other neoplasms.

Financial Disclosure:
No current or relevant financial relationships exist.

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Andrew Choi