Primary mucinous adenocarcinoma of the renal pelvis associated with obstructive urolithiasis and giant hydronephrosis.

Primary mucinous adenocarcinoma of the renal pelvis is an exceptionally rare malignancy that often mimics chronic stone disease or infected hydronephrosis. We report a 55-year-old man presenting with bilateral flank pain, bilateral nephrolithiasis, and giant right hydronephrosis. Ultrasound and contrast-enhanced computed tomography suggested obstructive infected hydronephrosis caused by a large ureteropelvic junction stone. During surgery, abundant gelatinous mucin and thickened friable renal pelvic mucosa raised suspicion for malignancy. Biopsy and final histopathology confirmed moderately differentiated mucinous adenocarcinoma. This case highlights the diagnostic difficulty of this tumor and the importance of intraoperative suspicion and histopathological assessment in longstanding obstructive stone disease.