Background: There is insufficient knowledge about the usefulness and methodology of using a whole body positron emission tomography (PET) tracer such as F18-fluordeoxyglucose ([ ¹⁸ F]-FDG) for assessment of muscle pathology in rheumatic musculoskeletal diseases (RMDs). In RMDs, characterised by affected joints, a common symptom such as muscle weakness is shown to be associated with altered muscle glucose metabolism ^[1] . [ ¹⁸ F]-FDG PET can be used to visualise the altered glucose metabolism in muscles which allows for a non-invasive investigation of changes in muscle metabolism of the whole body and at the cellular level ^[2] . Consequently, leading to more personalised interventions of weakened muscles.

Objectives: The objectives were to determine the most appropriate methodology to assess [ ¹⁸ F]-FDG muscular uptake in RMDs and to compare muscular uptake between patients with rheumatoid arthritis (RA), osteoarthritis (OA), inflammatory idiopathic myopathy (IIM), and controls.

Methods: Whole body [ ¹⁸ F]-FDG PET scans were retrospectively analyzed for muscular tracer uptake. The deltoid; biceps brachii; triceps brachii; psoas; quadriceps and; hamstrings were evaluated in 31 RMDs (11 RA, 10 OA, 10 IIM) and eight control persons. For the qualitative assessment, regions with positive uptake and the uptake pattern were recorded. Two methods were compared to quantitatively assess [ ¹⁸ F]-FDG uptake in the muscles: fixed volume of interest (VOI) (fixed height position on the bone) and hotspot VOI (visually highest uptake determined qualitatively), whereafter standardized uptake values (SUVs) were compared for different muscle groups between the RMDs and controls.

Results: [ ¹⁸ F]-FDG PET enables qualitative and quantitative assessment of muscle glucose uptake in RA, OA and IIM patients. Differences in [ ¹⁸ F]-FDG muscle uptake between the RMDs were observed in this study. Qualitative assessment: [ ¹⁸ F]-FDG uptake was observed in all muscle groups of OA, RA and IIM patients. A heterogenous uptake pattern of [ ¹⁸ F]-FDG was mostly seen in the quadriceps and the hamstring muscles whereas, in the other muscle groups, the uptake pattern varied between heterogenous and homogenous. Quantitative assessment: SUVs of FDG uptake either assessed by fixed or hospot VOI method were highly associated with each other (R ² =0.9, p<0.0001). However, the hotspot VOI method was preferred due to its higher sensitivity to detect differential [ ¹⁸ F]-FDG muscle uptake as depicted by the Bland-Altman plot (mean=0.147, 95% CI [-0.411, 0.117]). In all muscle groups, the IIM patients had the highest uptake followed by the OA and RA patients, respectively. The muscles that were the most affected in the RMDs were the biceps brachii, deltoids and the quadriceps. Compared to the controls, the RA, OA and IIM patients had a 1.43, 1.46 and 2.43 uptake in the biceps brachii, respectively; a 1.14, 1.30 and 1.99 higher uptake in the deltoids, respectively and; a 1.35, 1.63 and 2.09 higher uptake in quadriceps, respectively.

Conclusion: [ ¹⁸ F]-FDG PET enables qualitative and quantitative assessment, and differentiation of muscle glucose uptake in RA, OA and IIM patients both at the muscle level and at the patient group level. The hotspot method and the SUV _peak measurement are recommended to quantitatively identify differential uptake between RA, OA, IIM patients and controls. Remarkably, FDG uptake was increased in several muscle groups in all studied RMDs as compared to controls, pointing at altered glucose metaolism in muscles of these RMDsThis study provides the evidence for future exploration of using [ ¹⁸ F]-FDG PET to study muscle pathology in RMDs.

REFERENCES: [1] Vaamonde-García C, López-Armada MJ. Role of mitochondrial dysfunction on rheumatic diseases. Biochem Pharmacol. 2019;165:181-195. DOI:10.1016/j.bcp.2019.03.008.

[2] Vaidyanathan S, Patel CN, Scarsbrook AF, Chowdhury FU. FDG PET/CT in infection and inflammation--current and emerging clinical applications. Clin Radiol. 2015;70(7):787-800. DOI:10.1016/j.crad.2015.03.010.

Acknowledgements: NIL.

Disclosure of Interests: None declared.