Peri-renal collateral circles.

The figures illustrate the computed tomographic findings in 2 patients with idiopathic retroperitoneal fibrosis (IRF) who developed obstruction of the renal veins and peri-renal collateral circles late in the course of their disease. The involvement ab extrinseco of the renal veins is an overlooked feature of IRF. It often progresses slowly and only rarely leads to clinically overt thrombosis; collateral circles thus develop and, as in our patients, may be a late manifestation of the disease.