Early Discontinuation of Steroid Treatment in Negative FDG-PET/CT Patients With Idiopathic Retroperitoneal Fibrosis

Inclusion Criteria: * Patient over 18 years old * New onset or untreated relapsing of active idiopathic retroperitoneal fibrosis (IRF) defined by the association of: * Related-disease symptoms (Appendix 17.2) or elevated CRP level (\>20 mg/l) AND * Retroperitoneal peri-aortic mass that surrounds the abdominal vessels on CT-scan Exclusion Criteria: * Secondary retroperitoneal fibrosis including drug-related retroperitoneal fibrosis, active infections (such as tuberculosis) or malignancies, systemic vasculitis (such as Anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis), Erdheim-Chester disease (Appendix 17.3),patients with IgG4 disease may be enrolled * Contraindication to perform FDG-PET/CT, * Contraindication to perform CT scan with injection of contrast agent, * Contraindication to treatment by prednisone * Active infection * Acute or chronic liver disease that is deemed sufficiently severe to impair their ability to participate in the trial, * Active or history of malignancy in last 5 years. Individuals with squamous cell or basal cell skin carcinomas and individuals with cervical carcinoma in situ may be enrolled if they have received curative surgical treatment, * Serum creatinine level greater than 400 µmol/L that cannot be attributed to underlying IRF, * Live vaccination received from 4 weeks before inclusion, * Inhaled glucocorticoids (except for patients with documented asthma), * Any previous treatment with rituximab, methotrexate, alemtuzumab, cyclophosphamide, azathioprine, mycophenolate mofetil, infliximab, adalimumab, etanercept within the past 3 months, * Pregnancy or breastfeeding, * Non-affiliation to a social security regime, * Subject deprived of freedom, subject under a legal protective measure * Refusal to participate