Retroperitoneal fat necrosis producing ureteral obstruction.

We present 3 cases of a rare syndrome of extrinsic ureteral obstruction produced by retroperitoneal fat necrosis and compare the medical records and histopathology to 19 cases of classic idiopathic retroperitoneal fibrosis. Clinical similarities include average age of onset in the fourth and fifth decades, male preponderance, abdominal or back pain as the presenting symptom, elevation of the blood urea concentration prior to diagnosis and tendency toward bilateral involvement. Histologically, the 2 conditions were different. Retroperitoneal fat necrosis is characterized by coalescence of fat cells into fat cysts bordered by foreign body giant cell granulomas, foam cells, light chronic inflammatory infiltrate and unimpressive fibrosis. The microscopic pathology of idiopathic retroperitoneal fibrosis is dominated by densely collagenized fibrous tissue of varying cellularity without evidence of fat necrosis. Of the 3 cases of retroperitoneal fat necrosis 2 featured prior ischiorectal abscesses and this is considered in a discussion of the etiology of this unusual cause of ureteral compression.