This case illustrates retroperitoneal fibrosis manifesting with partial obstruction of the left renal tract. Retroperitoneal lymphoma is a consideration as a differential, but no discrete lymphadenopathy elsewhere and low metabolic activity makes it unlikely. 

A note is made that in between the presentation scan and second CT scan, pelvic surgery was performed for ignored reasons.

Biopsy was performed: 

Microscopy: The core biopsies should comprise dense fibrous connective tissue, admixed with moderate numbers of plasma cells and lymphocytes. The tissue extends into fatty tissues. There are no malignant cells.

Conclusion: Features in keeping with retroperitoneal fibrosis.