Adult patients with a diagnosis of idiopathic retroperitoneal fibrosis Prospective multicentric cohort study Intervention : administration of prednisone during 9 to 21 months at 1mg/kg/day at inclusion.

At baseline visit: Eligible patients will be screen during a standard visit care (consultation or hospitalization). A clinical examination, an abdominal CT scan, blood and urine biological tests will be performed.

At inclusion visit: After verification of inclusion and non inclusion criteria, if the patient meets the eligibility criteria, the investigator, will provide the patient with information and details regarding the trial. The consent is obtained and signed after a reflection period of 30 minutes.

The following procedure will be scheduled within 7 days:

• 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET/CT) (pregnancy test if mandatory)
• Specimens for the biocollection Patients with positive FDG PET/CT (hypermetabolism grade II or III) at M0 will receive oral steroids (prednisone) at 1mg/kg/day during 1 month and then the dose will be tapered to obtain <10mg/day at 6 months and <7,5mg/day at 9 months.

Patients with a negative FDG-PET/CT (hypermetabolism grade 0 or I) at M0 will be excluded of the study.

Follow-up visits : M6, M9,M12,M15,M21, relapse At M6, M12, and M15: During these visits clinical examination (blood pressure measurement, body temperature, heart rate, weight and clinical signs or symptoms related to IRF) will be performed. An abdominal CT scan may be performed as part of the care depending on the clinician's judgment. Glucocorticoid compliance and tapering, concomitant medications and adverse events (including serious cardiovascular adverse events) will be assessed and recorded. A nurse will collect blood and urine.

At M9, M21 or relapse : During these visits clinical examination, an abdominal CT scan, a FDG-PET/CT blood and urine biological tests will be performed.

At M9: The patients who failed to reach remission at M9 are considered as treatment failure and will be treated on best medical judgment by the investigator and excluded to the study. The patients who had a dose of prednisone ≥7,5mg / day at M9 will also be excluded to the study.

Patients who achieved remission at M9 and have a retroperitoneal fibrosis visual score grade 0 or I under a dose of prednisone <7,5mg / day will discontinue steroids treatment.

Patients who achieved remission at M9 and have a retroperitoneal fibrosis visual score grade II or III under a dose of prednisone <7,5mg / day will continue steroids treatment at the actual dose (medical judgment).

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