Mucinous adenocarcinoma of the renal pelvis masquerading as xanthogranulomatous pyelonephritis.

Primary adenocarcinoma of the renal pelvis accounts for less than 1% of renal pelvis tumors. We encountered a patient who presented with long-term nephrolithiasis and hydronephrosis. The initial impression was xanthogranulomatous pyelonephritis. The postoperative histopathologic report showed mucinous adenocarcinoma of the renal pelvis with signet ring cell formation. Signet ring cell phenomenon is extremely rare in this kind of tumor. We describe the patient's clinical presentation, imaging findings, histopathologic examination, and immunohistochemistry results.