Retroperitoneal extrarenal Wilms tumor: A diagnostic challenge and kidney-sparing approach

This is a report of a 2-year-old female who presented with a large right retroperitoneal mass that was abutting the right kidney but appeared distinct with absence of the “claw sign.” Biopsy showed Wilms tumor. Following neoadjuvant chemotherapy, the tumor remained associated with the kidney, though without clear renal origin. Intraoperatively, an extrarenal Wilms tumor was confirmed, and tumor resection was performed without nephrectomy. Postoperatively, the patient underwent kidney-sparing proton beam flank radiation and completed chemotherapy. Imaging remains negative for recurrence, and there is no postradiation atrophy of the right kidney 18 months following completion of therapy. A high index of suspicion is needed for extrarenal, retroperitoneal Wilms tumor to allow for kidney-sparing treatment.