Abstract:

We present a case of a 61 year old female with a 15.1mm deep malignant melanoma arising in the setting of a giant congenital melanocytic nevus of the posterior scalp. On re-excision, multifocal melanocytomas were identified in the deep dermis and subcutaneous tissue. The melanocytomas were nodular and composed of epithelioid melanocytes with oblong nuclei, fine chromatin, prominent cherry red nucleoli, and focal areas of intracytoplasmic pigment deposition. The mitotic rate in these areas was low with a benign immunohistochemistry staining pattern (HMB weak to negative, low proliferative index on Ki-67/Mart-1, p16 staining retained). Intradermal lymph nodes within the nevus were found to have collections of subcapsular and parenchymal bland appearing melanocytes with a reassuring immunohistochemistry staining pattern (positive Sox-10 and Mart-1; negative HMB-45). Given the three separate melanocytic proliferations in this case with propensity for lymph node spread, it was difficult to definitively assign a lineage to the melanocytes, although histology and immunohistochemistry suggested a benign origin. Melanocytomas are uncommon low-grade neoplasms that may occur sporadically or in association with Carney complex. Sporadic melanocytomas that occur in association with a nevus are termed combined melanocytomas. Classic histologic findings include epithelioid and spindled melanocytes with prominent nucleoli and abundant cytoplasm with pigmentation present in melanocytes and melanophages. Mitotic figures in these lesions are rare. Melanocytomas pose a diagnostic challenge due to overlapping histologic features with malignant melanoma and benign melanocytic nevi. In difficult cases, molecular studies may be of use, as melanocytomas are reported to have a NTRK3-SCAPER gene fusion. Increased awareness of the histologic and molecular features of melanocytomas is critical to avoid a potentially devastating misinterpretation.

Financial Disclosure:
No current or relevant financial relationships exist.

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Hannah Rose, DO