Bilateral multifocal nephrogenic adenomas arising in the renal pelves: a case report and review of the literature.

Background: Nephrogenic adenoma (NA) is a benign epithelial lesion of the genitourinary tract resulting from the reimplantation and proliferation of exfoliated renal tubular cells at sites of urothelial injury. It most commonly arises in the bladder, while involvement of the renal pelvis is exceedingly rare. Bilateral multifocal presentation with atypical histologic features has not been previously reported. Case presentation: A 50-year-old male with a history of bladder stone lithotripsy presented with painless gross hematuria and lumbodorsal aching of nine months' duration. Preoperative workup included three urine cytology examinations, which showed atypical cells suspicious for malignancy, and an outside hospital biopsy of the left renal pelvis. Review of the biopsy at our institution revealed no definitive malignant components (the specimen contained renal parenchyma, fibromuscular tissue, and scant urothelium with chronic inflammation), rendering the diagnosis indeterminate. Ultrasonography revealed bilateral multiple renal calculi and multiple solid masses in the bilateral renal collecting systems, suggestive of bilateral renal pelvic carcinoma. Given the bilateral disease and diagnostic uncertainty, the patient underwent staged percutaneous nephroscopic tumor resections. The right-sided lesion showed NA with focal atypia (CK7+, PAX8+, P504S+, CK20-, GATA-3-, Ki-67 ~10%). Two months later, the patient was readmitted for left-sided lesions. Non-contrast computed tomography (CT) demonstrated multiple nodular and strip-like lesions at the left renal pelvis and ureteropelvic junction, with the largest measuring 14 × 8 mm, accompanied by mild dilatation of the left pelvicalyceal system. Left percutaneous nephroscopic tumor resection was performed, and pathology confirmed atypical NA (PAX8+, EMA+, TFE3+, focal CK7/CK20/P504S/CAIX+, Ki-67 ~10%, p53 patchy ~80%). Specimens were fragmented, precluding margin assessment, though no macroscopic residual tumor was observed intraoperatively. Follow-up at 10 months showed no recurrence or metastasis. Conclusion: This is the first report of bilateral multifocal NA arising in the renal pelvis with atypical histologic features and an elevated Ki-67 proliferation index. This case expands the known morphological spectrum of NA and highlights important diagnostic challenges. Urologists and pathologists should be aware of this rare entity to avoid misdiagnosis as bilateral renal pelvic carcinoma and subsequent overtreatment.