Retroperitoneal fibrosis.

Retroperitoneal fibrosis is the formation of a fibrotic plaque in the prelumbar or presacral area which becomes clinically significant when it produces ureteral obstruction. It can occur as a result of taking certain drugs, retroperitoneal fluid collections, inflammatory disease of various intraabdominal organs, previous surgery or radiation therapy, and various metastatic neoplasms. The majority of the causes of retroperitoneal fibrosis are considered idiopathic.