Unveiling the Rarity: Melanotic Neuroectodermal Tumor of Infancy in the Forearm-A Diagnostic Challenge

Melanotic neuroectodermal tumor of infancy (MNTI) is a rare neural crest-derived neoplasm typically arising in the craniofacial region. Extracranial involvement, particularly in long bones, is exceedingly uncommon and may pose a diagnostic challenge. We report a 4.5-month-old male infant presenting with a rapidly enlarging forearm mass involving the radius with soft tissue extension. The initial biopsy suggested a malignant round cell tumor of neuroectodermal differentiation. A diagnosis of Ewing sarcoma was considered, based on clinicoradiological impression. The tumor showed poor response to chemotherapy, necessitating an above-elbow amputation. Histopathological examination of the resected specimen revealed a characteristic biphasic tumor composed of small neuroblastic cells and pigmented epithelial cells. Immunohistochemical staining confirmed the diagnosis of a MNTI. This extremely rare report highlights the diagnostic pitfalls on limited biopsy and underscores the importance of recognizing biphasic morphology with appropriate immunohistochemical correlation.