This is a typical case of retroperitoneal fibrosis. The main differential would be retroperitoneal lymphoma, which however would typically demonstrate mass effect and displace the aorta medially and the ureters laterally, not pulling them in medially like retroperitoneal fibrosis frequently does.

Biopsy and subsequent histopathology demonstrated "lymphoplasmatic and hystiocytic infiltration consistent with idiopathic retroperitoneal fibrosis (Ormond disease), not IgG4 positive, with numerous CD68 histiocytes; MDM2 negative ruling out liposarcoma, AE1/AE3 and CK7 negative ruling out other carcinoma".

The patient received an ureteral stent and got systemic steroid therapy for 5 months, after which it was switched to mycophenolate mofetil.