Retroperitoneal extraosseous Ewing's sarcoma with renal involvement: US and MRI findings.

A case of retroperitoneal extraosseous Ewing's sarcoma (EES) with renal involvement, which simulated an exophytic renal mass, is reported. EES is a rare soft tissue tumor that can occur anywhere in the soft tissues, but is seen most commonly in the extremities. Although EES is histologically similar to osseous Ewing's sarcoma, it is usually found in older patients. EES should be included in the differential diagnosis of soft tissue tumors, especially in the second and third decade of life.