Abstract:

Melanoma with rhabdomyosarcomatous differentiation is an exceedingly rare disease whereby malignant melanoma dedifferentiates and undergoes rhabdomyoblastic metamorphosis. Only 11 cases have been confirmed in the literature, making management of this aggressive subtype a challenge. We present a case of a 74-year-old male with a history of primary myelofibrosis and BCC who presented for evaluation of a new concerning lesion on the left posterior scalp. Physical examination revealed an ulcerated crusted pink papule on the left superior occipital scalp with no lymph node involvement. Shave biopsy of the lesion was performed, and histopathology revealed well circumscribed, non-encapsulated, highly cellular, and plump spindled cells in poorly arranged fascicles. Florid pleomorphism with atypical mitosis were observed, and the lesion was read-out as atypical fibroxanthoma. Mohs micrographic surgery was performed, and the defect allowed to heal by secondary intention. Histopathology of the excised specimen demonstrated a central nodular area strongly positive for desmin and MyoD1, focally positive for myogenin, and negative for SOX10 and Melan-A. At the periphery, cells stained strongly and diffusely positive for SOX10 and Melan-A, and negative for desmin, MyoD1, and myogenin. Immunostains for p40, CK903, and ERG were negative in both components and no perineural or lymphovascular invasion was identified. De/trans-differentiated melanoma are a rarely described phenomenon, but likely under-recognized and under-reported. Tumors are large, ulcerated, and often widely distributed, with a predilection for sun-damaged skin on the head and neck. As in our case, tumors are histologically biphasic with a pre-existing conventional melanoma, and a dedifferentiated component closely resembling atypical fibroxanthoma which can result in misdiagnosis. Treatment options are poorly described but include surgery with adjuvant radiation, chemotherapy, and most recently immunotherapy.

Financial Disclosure:
No current or relevant financial relationships exist.

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Alfredo Siller, MD