Retroperitoneal fibrosis   

Retroperitoneal soft tissue mass measuring 5.9 x 3.8 cm encasing aorta, iliac arteries, bilateral ureters, and with diffuse lymphadenopathy requiring ...

Renal perfusion scan shows 14 % perfusion on affected side. 

What imaging do you use to monitor disease activity? Imaging only shows chronic, fibrotic lesions making it hard to assess disease activity.