Melanotic neuroectodermal tumor of infancy presenting in the subcutaneous soft tissue of the thigh.

Melanotic neuroectodermal tumor of infancy (MNTI) is a rare and diagnostically challenging neoplasm typically presenting in the bones of the maxilla, skull, or mandible. Only 6 of approximately 357 reported cases have involved the subcutis. We describe a case of MNTI presenting as a palpable, subcutaneous, thigh mass in a 5-month-old girl. By ultrasound, the mass was round with well-defined borders, minimal vascularity, and heterogeneous echogenicity. Microscopically, the tumor consisted of nested foci of primitive-appearing small round blue cells with an increased nuclear to cytoplasmic ratio, stippled chromatin, and occasional mitotic figures. A larger and more epithelioid second cell population exhibited eosinophilic cytoplasm and sparse pigmented granules. The background stroma was fibrous and densely sclerotic. The differential diagnosis of soft tissue MNTI can include melanoma, neuroblastoma, rhabdomyosarcoma, desmoplastic small round cell tumor, and other pediatric "small round cell" neoplasms. The tumor had the characteristic immunophenotype of MNTI: cytokeratin+, HMB-45+, neuron-specific enolase+, and synaptophysin+. MNTI should be considered in the differential diagnosis of pigmented soft tissue lesions in children. Our patient remains disease-free 40 months after excision, although these tumors can locally recur (10%-20%) and rarely metastasize.