Idiopathic Retroperitoneal Fibrosis Mimicking Malignancy and Presenting With Obstructive Uropathy and Acute Kidney Injury: A Case Report.

Retroperitoneal fibrosis is a rare fibroinflammatory disorder that can lead to progressive encasement of retroperitoneal structures, including the aorta, inferior vena cava, ureters, and iliac vessels, most commonly resulting in ureteral obstruction. It may be idiopathic or secondary to medications, malignancy, radiotherapy, or other causes. Its radiological appearance often mimics malignant disease, which may complicate the diagnostic approach, particularly in patients with a previous oncologic history. We report the case of a 70-year-old man with a past medical history significant for chronic lymphocytic leukemia under surveillance and intermediate-risk prostate adenocarcinoma previously treated with hormone therapy and currently without evidence of active disease, who presented to the emergency department with persistent left-sided flank pain. Contrast-enhanced abdominal computed tomography revealed an irregular left retroperitoneal soft-tissue density, measuring approximately 8 cm in longitudinal extent, located anteroinferior to the aortic bifurcation, encasing the left ureter and causing mild to moderate ipsilateral hydronephrosis. Given concern for malignancy, a biopsy of the retroperitoneal lesion and an adjacent lymph node was performed, demonstrating fibro-adipose tissue with a mononuclear inflammatory infiltrate and no evidence of neoplasia. Serum immunoglobulin G4 levels were within normal limits. After exclusion of alternative diagnoses, corticosteroid therapy was initiated, resulting in rapid clinical improvement and complete radiological resolution on follow-up imaging. Retroperitoneal fibrosis should be considered in patients presenting with unexplained obstructive uropathy, and an etiological workup is essential to exclude secondary causes and guide appropriate management.