Pelvic Mass in a Pediatric Patient: A Rare Case of Extraosseous Ewing Sarcoma

Ewing sarcoma (ES) is a common malignant bone tumor in children, but primary extraosseous Ewing sarcoma (EES) in the pelvic region is particularly rare. We report a 21-month-old male with a 10-cm complex cystic mass adherent to the bladder. Core biopsy demonstrated a small round cell neoplasm positive for CD99, NKX2-2, and EWSR1:ERG fusion. The patient had a residual mass following induction chemotherapy, and underwent surgical resection with anterior detrusorrhaphy with no other pelvic organ involvement. Pathology showed <10% residual tumor with negative margins. Following completion of consolidation chemotherapy, imaging has shown no evidence of recurrent disease 13 months post-surgery.