Renal Ewing sarcoma with extensive metastasis diagnosed via pulmonary thrombus.

Primary Ewing sarcoma of the kidney (ESK) is a rare and aggressive malignancy often misdiagnosed as renal cell carcinoma (RCC) due to non-specific radiological features. Accurate diagnosis requires histology, immunohistochemistry (IHC) and molecular studies. We report the case of a female in her 20s presenting with abdominal pain, haematuria and weight loss, initially diagnosed with RCC. Imaging revealed a large renal mass and pulmonary embolus. Mechanical thrombectomy (MT) was performed, and biopsy of the pulmonary thrombus confirmed metastatic ESK through IHC and next-generation sequencing, identifying the EWSR1-FLI1 gene fusion. The patient was treated with neoadjuvant chemotherapy (NAC) and interventional procedures. This case highlights the importance of considering ESK in differential diagnoses of renal masses and the utility of MT in obtaining diagnostic tissue, which allowed for the timely initiation of NAC and improved clinical management.