Persistent FDG/PET CT uptake in idiopathic retroperitoneal fibrosis helps identifying patients at a higher risk for relapse.

Background: The aim of this study was to evaluate the prognostic value of persistent retroperitoneal fibrosis FDG uptake using FDG/PET CT in patients with idiopathic retroperitoneal fibrosis (IRF).

Methods: In this monocentric retrospective cohort study, all patients admitted for IRF from January 2009 to December 2017 underwent a FDG/PET CT at diagnosis and during follow up. Metabolic activity of IRF was assessed by retroperitoneal fibrosis FDG uptake measured as maximal standardized uptake value (SUVmax). The primary outcome was IRF relapse rate during follow-up.

Results: 23 consecutive patients (54.7 [36.9-89] years, 73.9% of men) diagnosed with IRF had FDG/PET CT imaging performed at diagnosis, 3.1 [1-8.7] months (i.e 1st evaluation) and 10.4 [4.9-17.5] months (i.e 2nd evaluation) after diagnosis. High FDG retroperitoneal fibrosis uptake was present in all patients at diagnosis (SUVmax 6.5 [3.8-11.9]) and persisted in 16 (69.6%; SUVmax 3.65 [2.1-5.4]) and 12 (52.2%; SUVmax 3.75 [2.7-7.8]) patients, at 1st and 2nd evaluation respectively. All but one patient had received steroids at IRF diagnosis and 21 (91.3%) were in complete remission at both 1st and 2nd evaluation. During a median follow-up period of 38.7 [3-106.9] months, 6 (26.1%) patients suffered IRF relapse that occurred 15.7 [9.2-42.8] months after diagnosis. Multivariate analysis showed that only persistent retroperitoneal fibrosis FDG uptake at 2nd evaluation was associated with IRF relapse (p = .046).

Conclusions: In IRF, persistent retroperitoneal fibrosis FDG uptake during follow up is associated with clinical outcome. FDG/PET CT may help to better stratify the risk of relapse and target therapy in IRF.