18F-fluorodeoxyglucose PET/CT uptake characteristics in immune checkpoint inhibitor-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-centre 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 five of the six 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.