Retroperitoneal Ewing Sarcoma Masquerading as an Intraabdominal Abscess in a Teenager: A Rare Diagnostic Challenge.

Retroperitoneal Ewing sarcoma is a rare and aggressive extraskeletal variant of Ewing sarcoma, often presenting with nonspecific symptoms that can delay diagnosis. We discuss the case of a 19-year-old male who presented to the emergency room with progressive left abdominal and flank pain. Imaging revealed a retroperitoneal, necrotic mass and biopsy with immunohistochemistry confirmed retroperitoneal Ewing sarcoma with NKX2.2 and CD99 positivity. Fluorescence in situ hybridization (FISH) analysis showed that 98% of cells had an EWSR1 gene rearrangement. For treatment, the patient underwent a CT-guided biopsy and chemotherapy was initiated.  The sometimes-insidious progression of retroperitoneal Ewing sarcoma, coupled with the retroperitoneal space's ability to accommodate tumors without significant symptoms, often leads to late-stage diagnosis and poor prognosis. This report emphasizes the importance of early recognition and highlights the need for further research into optimal diagnostic protocols and treatment strategies.