Primary Visceral Ewing Sarcoma (Gastric and Bladder) with KIT Expression: A Report of Two Patients and a Differential Diagnostic Approach.

Extra-skeletal Ewing sarcoma is a rare tumor, most commonly occurring in the paravertebral region, extremities, retroperitoneum, and very rarely in visceral locations. We present two primary visceral Ewing sarcomas confirmed at the molecular level: one located in the stomach and the other in the bladder. Both tumors demonstrated strong and diffuse membranous CD99 expression, accompanied by variable KIT immunoreactivity. Additionally, we provide a review of the literature on primary gastric and bladder ES, including an analysis of differential diagnoses for gastric and bladder tumors exhibiting CD99 and KIT expression.