Whole-body and site specific [¹⁸F]FDG uptake patterns on PET/CT have limited value in differentiating between polymyalgia rheumatica and other inflammatory diseases: two cohorts of treatment-naïve suspected polymyalgia rheumatica

Background: It has been hypothesized that 2-[¹⁸F]fluoro-2-deoxy-D-glucose ([¹⁸F]FDG) positron emission tomography (PET) computed tomography (CT) can distinguish polymyalgia rheumatica (PMR) from non-PMR patients based on the [¹⁸F]FDG-uptake patterns. Nevertheless, a comprehensive assessment of whole-body [¹⁸F]FDG-patterns across all uptaking musculoskeletal sites, as well as site-specific [¹⁸F]FDG-uptake patterns, has not been conducted. Therefore, this study aimed to investigate both the overall whole-body [¹⁸F]FDG-uptake patterns and the specific uptake patterns at individual sites in patients suspected of having PMR. Methods: Two distinct cohorts of patients with suspected PMR from Denmark and the Netherlands were prospectively included, encompassing 66/27 and 36/21 PMR/non-PMR patients, respectively. The cohorts consisted of treatment-naïve patients, who underwent pre-treatment [¹⁸F]FDG-PET/CT scans. The [¹⁸F]FDG-uptake was then assessed across 34 different anatomical sites. Furthermore, the site-specific [¹⁸F]FDG-uptake pattern within each anatomical site was categorized according to its shape. Results: Patients with PMR were more likely than non-PMR patients to have bilateral [¹⁸F]FDG-uptake equal to or above liver compared at the ischial tuberosities (91%/41%), shoulder joints (86%/45%), hip joints (83%/52%), and along the lumbar spinal processes (70%/30%). However, a subgroup analysis comparing non-PMR patients with other inflammatory conditions to patients with PMR revealed that several non-PMR patients exhibited a similar whole-body [¹⁸F]FDG-uptake pattern. Furthermore, site-specific [¹⁸F]FDG-uptake patterns were similar in patients with PMR and non-PMR. Conclusion: Assessing whole-body or site-specific [¹⁸F]FDG-uptake patterns does not improve the diagnostic accuracy in distinguishing PMR from other inflammatory diseases. Consequently, [¹⁸F]FDG-PET/CT should mainly be used to rule out a clinical diagnosis of PMR. Trial registration: ClinicalTrials.gov (NCT04519580). Registered 17th of August 2020.