Primary dermal melanoma (PDM) is defined as a primary melanoma tumor confined to the dermis, subcutis, or both, without epidermal involvement. The significant overlap of histopathological features in PDM and cutaneous metastatic melanoma makes diagnostic accuracy of PDM challenging. We present a case of a 48-year-old man with a nontender 1.5 × 1.5 cm subcutaneous nodule on the left leg, which had been present for years. Biopsy revealed a dermal tumor with melanocytic differentiation noted to be positive for SOX-10. Additional pathology findings included a high Ki-67 proliferation index and a loss of p16 expression. Pathology reports were consistent with primary tumor stage 4a, and the patient was referred to surgical oncology where examination and workup demonstrated no evidence of the residual lesion representing a metastasis from a primary site. As PDM is histologically indistinguishable from melanoma metastasis to the skin, clues including a history of an evolving subepidermal nodule and exclusion of previous or concurrent melanomas can assist in its accurate diagnosis. Currently, a consensus on the criteria, staging, and management of PDM does not exist. Poorly defined diagnostic criteria and general lack of awareness of PDM result in high rates of incorrect and late-stage diagnoses. This case report highlights the importance of physician familiarity with PDM to ensure accurate recognition, evidence-based management, and improved patient outcomes.
