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Case Reports

Abstract

Giant congenital melanocytic nevi (CMN), defined by a size greater than twenty centimeters, carry a 6-20% risk of malignant transformation. Melanoma with rhabdomyosarcomatous transdifferentiation arising in a CMN is exceedingly rare. Here, we report a case of malignant melanoma with rhabdomyosarcomatous transdifferentiation arising within a CMN in an infant. A 2-month-old boy presented with subcutaneous nodules within a giant CMN of the hand and forearm. On initial biopsy, there was an unusual minute population of PRAME-positive small cells in the deep soft tissue. Its significance was indeterminate, without sufficient evidence of malignant melanoma. On repeat biopsy for rapid growth approximately 3 months later, the subcutaneous nodules were composed of atypical small round blue cells with mitoses and infiltrative growth that were strongly positive for MyoD1 and PRAME, and negative for melanocytic markers (SOX10, MelanA, S100). H3K27me3 expression was significantly reduced. The findings fit with melanoma with rhabdomyosarcomatous transdifferentiation. The patient underwent multidisciplinary surgical resection with narrowly negative margins and a negative axillary sentinel lymph node biopsy. However, 4 months after resection a focal metastasis in an antecubital fossa lymph node was found on imaging and excised. We aim to add to the limited literature on this rare entity and highlight the histopathological features and associated diagnostic challenges.