Giant Clear Cell Renal Cell Carcinoma With Continuous Tumor Thrombus Extension Into the Left Renal, Gonadal, Ascending Lumbar, and Hemiazygos Veins: A Case Report.

Venous tumor thrombus is a well-recognized feature of clear cell renal cell carcinoma (ccRCC), most commonly involving the renal vein and, in advanced cases, the inferior vena cava. However, simultaneous extension into multiple collateral venous pathways, particularly the gonadal and hemiazygos veins, is exceedingly uncommon. We report the case of a patient with a giant left-sided ccRCC measuring 20 cm, presenting with extensive venous tumor thrombus involving the left renal vein, the left gonadal vein, the left ascending lumbar vein, and the hemiazygos vein without direct inferior vena cava invasion. Contrast-enhanced computed tomography demonstrated complete distortion of the renal architecture, marked local mass effect on adjacent abdominal structures, encasement of the left renal artery, and extensive perirenal fat infiltration. The tumor thrombus reached the ostium of the left renal vein at the inferior vena cava while remaining confined to the renal venous system and collateral veins. Additional findings included delayed bilateral renal excretion, mild-to-moderate ascites, and indeterminate small hepatic lesions without significant retroperitoneal lymphadenopathy. Histopathological examination confirmed ccRCC. This case illustrates an exceptionally rare pattern of multivenous tumor thrombus involving the renal, gonadal, ascending lumbar, and hemiazygos veins. Comprehensive preoperative CT evaluation is essential for accurately delineating the extent of venous involvement, guiding therapeutic planning, and raising awareness of atypical collateral venous extension in advanced RCC.