18F-fluorodeoxyglucose PET/CT uptake characteristics in ICI-induced polymyalgia rheumatica compared to idiopathic polymyalgia rheumatica

Objectives Polymyalgia rheumatica (PMR)-like manifestations are among the most common rheumatological immune checkpoint inhibitor (ICI)-related adverse events. 18F-fluorodeoxyglucose (FDG) PET/CT is useful for the follow-up of various solid organ cancers and for PMR assessment. The aim of this study was to describe PET/CT aspects of ICI-related PMR (ICI-PMR) in comparison with idiopathic PMR. Methods This was an observational, comparative, retrospective, single-center study. Inclusion criteria were a diagnosis of ICI-PMR or idiopathic PMR and availability of an 18F-FDG PET/CT within 3 months before diagnosis. 18F-FDG uptake in 20 musculoskeletal regions of interest (ROIs) on PET/CT was assessed by two independent readers. PET/CT features of ICI-PMR and idiopathic PMR were compared. Results Twenty patients with ICI-PMR were compared with 23 patients with idiopathic PMR. Patients with ICI-PMR were mostly treated for metastatic melanoma. Compared with patients with idiopathic PMR, patients with ICI-PMR had similar C-reactive protein levels but a higher frequency of peripheral arthritis. On PET/CT, patients with ICI-PMR had significantly lower 18F-FDG uptake at ischial tuberosities, greater trochanters, anterior pubic symphysis, iliopectineal bursae, sternoclavicular joints and interspinous bursae than patients with idiopathic PMR. These differences remained significant for 5 of the 6 ROIs after stratification according to peripheral arthritis. Leuven and Besançon scores were significantly lower in ICI-PMR, regardless of the presence of peripheral arthritis. Conclusion The distribution of musculoskeletal inflammation on 18F-FDG PET/CT differed between ICI-PMR and idiopathic PMR. ICI-PMR was associated with less intense musculoskeletal uptake, especially in extra-articular regions, even after stratification based on peripheral arthritis.