The patient was a 55-year-old woman with a 4-month history of pain in left flank that irradiated to the lumbar region. Computed tomography revealed a cystic tumor in the retroperitoneal region that compresses the ureter and left renal pelvis in contact with pancreatic body and tail. Surgery was performed and the definitive pathological report diagnosed nodular retroperitoneal fibrosis associated with IgG4; Ki67-positive in germinal centers (5%) and IgG4-positive (40 plasma cells in 3 fields of 40x) by immunohistochemistry. Retroperitoneal fibrosis is a rare disease, that develops gradually and has an excellent response to steroid management. Surgical treatment is reserved for cases that compromise adjacent structures. Thus, identifying it when studying a retroperitoneal tumor leads to better prognosis and survival.
