BCOR::CCNB3 Sarcoma of the Thigh: A Rare Case.

BCOR-rearranged sarcoma is a rare mesenchymal tumor and a recognized subtype of undifferentiated small round-cell sarcoma. It shares morphological similarities with other round-cell sarcomas but is distinguished by a unique molecular hallmark that differentiates it from Ewing sarcoma. These tumors primarily arise in bones and soft tissues. This case report details the case of an 11-year-old female who developed swelling on the medial aspect of her right distal thigh. There was no history of trauma, surgery, or fracture. Clinically, the presentation initially suggested osteosarcoma. However, histopathological evaluation confirmed a diagnosis of BCOR-rearranged sarcoma. This case highlights the importance of considering BCOR-rearranged sarcoma in the differential diagnosis of pediatric bone and soft tissue tumors, as its clinical and radiological features can mimic more common malignancies like osteosarcoma. Early recognition and accurate molecular diagnosis are crucial for guiding appropriate treatment.