[F]FDG uptake in proximal muscles assessed by PET/CT reflects both global and local muscular inflammation and provides useful information in the management of patients with polymyositis/ dermatomyositis

Objective. This study aimed to determine whether [F]fluorodeoxyglucose-PET/CT ([F]FDG-PET/CT) discriminates PM/DM from non-muscular diseases and also whether FDG uptake in proximal muscles reflects the activity and severity of muscular inflammation in PM/DM. Methods. Twenty treatment-naı̈ve PM/DM patients who underwent [F]FDG-PET/CT were retrospectively identified by reviewing medical records. The same number of ageand sex-matched control patients with non-muscular diseases were also identified. Standardized uptake value (SUV) was calculated for each of the seven proximal muscles. For patient-based assessment, mean proximal muscle SUV was calculated by averaging the SUVs for these proximal muscles bilaterally. Results. Mean proximal muscle SUVs were significantly greater in PM/DM patients than in control patients (median 1.05 vs 0.69, P<0.001). Mean proximal muscle SUVs significantly correlated with mean proximal manual muscle test scores ( = 0.49, P = 0.028) and serum levels of creatine kinase ( = 0.54, P = 0.015) and aldolase ( = 0.64, P = 0.002). Furthermore, SUVs in proximal muscles from which biopsy specimens were obtained significantly correlated with histological grade for inflammatory cell infiltration ( = 0.66, P = 0.002). Conclusion. Our results suggest that [F]FDG-PET/CT is useful in the diagnosis of PM/DM when inflammation in proximal muscles is globally assessed with quantitative measurements. Our results also indicate that local FDG uptake in a proximal muscle reflects the activity of inflammation in the same muscle and provides useful information in determining the region for muscle biopsy.