Usefulness of 18F-Fluorodeoxyglucose Positron Emission Tomography in Diagnosing Polymyalgia Rheumatica and Large-Vessel Vasculitis: A Case-Control Study

Abstract

Background: Polymyalgia rheumatica (PMR) is a relatively common disease among the elderly. None of the most common imaging techniques provides diagnostic certainty of PMR. 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) may be a useful candidate as it can be used to visualize articular and periarticular FDG uptake at different locations, as well as associated large-vessel vasculitis (LVV). However, in the absence of large-scale case-control studies, its usefulness needs to be evaluated. The purpose of this study was to determine the usefulness of FDG-PET/CT in diagnosing PMR and LVV. Methods: We analyzed FDG-PET/CT scans performed between January 2015 and December 2019 on patients diagnosed with PMR. For comparisons, patients with PMR were matched 1:1 to controls according to age and sex. FDG-PET/CT scans had been performed on controls over the same period for diagnosis of cancer-associated stroke. FDG uptake was scored visually for 17 articular or periarticular sites and 13 vascular sites using a semi-quantitative scoring system (score 0-3). The case and control groups were compared using generalized linear mixed models (binomial distribution, logit function) for binary outcomes, and linear mixed models for continuous outcomes, with matched sets as a random effect. Receiver operating characteristics (ROC) curve analysis, based on the area under the ROC curve (AUC), was used to assess the ability of the number of sites with significant hyperfixation (score ≥ 2) and the FDG uptake score to differentiate the two groups. From the ROC curves, we determined optimal threshold values by maximizing the Youden index. Sensitivity and specificity for the optimal threshold were calculated. Results: 81 patients with a diagnosis of PMR and 81 controls were included (mean (SD) age 70.7 (9.8) years; 44.4% women). Significant differences between the PMR and control groups were found at all articular and periarticular sites for: 1) FDG uptake score (p<0.001 for all locations); 2) number of patients per site with significant FDG uptake (score ≥ 2); 3) global FDG articular uptake scores (score 0-51) (31 [IQR, 21 to 37] versus 6 [IQR, 3 to 10], p<0.001); and 4) number of sites with significant FDG uptake (score ≥ 2) (score 0-17) (11 [IQR, 7 to 13] versus 1 [IQR, 0 to 2], p<0.001). ROC curve analysis revealed that the presence of 6 or more sites with significant FDG uptake (≥ 2) was associated with the diagnosis of PMR, with a sensitivity of 84% and a specificity of 96% (AUC 0.90 [95% CI 0.8593-0.95]). No significant differences in global FDG vascular uptake scores (score 0-39) or in number of patients with at least 1 significant vascular uptake site (score ≥ 2) were found between the PMR and control groups (1 [IQR, 0 to 4] versus 4 [IQR, 0 to 6], p=0.06, and 8 (11.3%) versus 10 (14.1%), p=0.62 respectively). Conclusion: Our results demonstrate that the FDG uptake score and the number of sites with significant FDG uptake could be relevant criteria for the diagnosis of PMR. However, unlike other authors, we found no evidence suggesting that FDG-PET/CT may be useful in diagnosing silent underlying LVV in patients with isolated PMR. Funding: None to declare. Declaration of Interest: None to declare. Ethical Approval: Since this is a retrospective study, it did not require ethical committee approval in accordance with the French laws and regulations. Informed consent was obtained from all patients for the FDG-PET/CT scans.