Retroperitoneal vascular anatomy has multiple variants due to its complex embryological development. Between 6-8 weeks of embryonic life, the inferior vena cava (IVC) originates from the development and regression of the cardinal venous system, consisting of three pairs of venous axis, in order of appearance: posterior cardinal veins, subcardinal and supracardinal, which anastomose among themselves, with regression of segments to form the definitive venous system. The normal inferior vena cava is composed of different segments: prerenal segment, renal veins, postrenal segment and intrahepatic segment and ends in the right atrium. 
There are several venous anatomic variants described such as: left IVC, double IVC, azygos continuation of the IVC, circumaortic left renal vein, retroaortic left renal vein, double IVC with retroaortic right renal vein and hemiazygos continuation of the IVC, double IVC with retroaortic left renal vein and azygos continuation of the IVC, circumcaval ureter, absent infrarenal IVC with preservation of the suprarenal segment….[2, 3]
Vascular structures can usually be readily identified on contrast-enhanced CT. Identification of unusual venous arrangements may be difficult in those cases in which intravenous contrast material is contraindicated. In such patients, MR imaging may be used to distinguish aberrant vessels from masses by demonstrating flow voids or flow-related enhancement. 
In our case, a double IVC with a retroaortic left renal vein and azygos continuation of the IVC is found. It results from persistence of the left supracardinal vein and the dorsal limb of the renal collar with regression of the ventral limb. In similar cases, the subcardinal-hepatic anastomosis fails to form, but in our case it doesn't.
The study of retroperitoneal vascular distribution is important for increasing the safety of surgery, interventional radiology, diagnostic imaging of retroperitoneal masses and for the treatment of thromboembolic disease. [2, 3]