Upper tract urothelial carcinoma arising in a horseshoe kidney with ureteropelvic junction obstruction: a case report.

Horseshoe kidney (HSK) is frequently associated with ureteropelvic junction obstruction (UPJO), but the occurrence of upper tract urothelial carcinoma (UTUC) in this setting is exceptionally rare. We report a 66-year-old man presenting with left flank pain and a giant hydronephrotic moiety of an HSK caused by longstanding UPJO. Computed tomography revealed massive hydronephrosis (50 × 46 × 28 cm) with enhancing urothelial wall thickening suspicious for UTUC. The patient underwent open heminephroureterectomy. Histopathology confirmed UTUC. This case highlights the need to suspect malignancy in chronically obstructed horseshoe kidneys and emphasizes the importance of meticulous preoperative imaging and surgical planning.