Primary Ewing sarcoma of the kidney mimicking hydatid cyst: A case report with literature review.

Primary renal Ewing sarcoma (EWS) is a rare and aggressive disease with a poor prognosis. Due to its nonspecific presentation and radiological findings, it can be misdiagnosed. This study reports a case of primary renal EWS mimicking a hydatid cyst. The case (31-year-old female) presented with right loin pain for a one-month duration. Abdominal magnetic resonance imaging demonstrated a well-defined, thick-walled cystic lesion occupying the upper and middle poles of the kidney, compressing the adjacent calyces and renal vessels, and showing thin peripheral septa. The features were suggestive of a hydatid cyst. The latex agglutination test for hydatid disease was negative. An excisional biopsy followed by a partial nephrectomy was performed, and histopathology confirmed the cyst as primary renal EWS. Extraosseous cases are rare, accounting for approximately 6% of all EWS cases. Primary renal EWS comprises only 1% of renal tumors, with fewer than 150 cases reported. Early detection and management of renal EWS may be challenging due to the nonspecificity of imaging features, as the disease can mimic other differential diagnoses, including hydatid cysts.