Value of ¹⁸F-FDG PET/CT in assessing muscle disease activity in patients with inflammatory myopathy without malignant tumors

Objective To investigate the value of ¹⁸F-fluorodeoxyglucose (FDG) PET/CT in assessing muscle disease activity in patients with inflammatory myopathy (IM) without malignant tumors.

Methods  In this retrospective case-control study, the clinical and ¹⁸F-FDG PET/CT imaging data of 66 patients with IM without malignant tumors who underwent ¹⁸F-FDG PET/CT at Nanjing Drum Tower Hospital, the Affiliated Hospital of Nanjing University Medical School, between October 2015 and August 2024, were analyzed. The cohort comprised 18 males and 48 females, with an average age of (56.9±15.4) years. The maximum standardized uptake value (SUV_max) was measured bilaterally in a total of 16 muscles (deltoid, biceps brachii, triceps brachii, pectoralis major, psoas major, gluteal muscles, adductor group, and quadriceps femoris) for all enrolled patients. The average SUV_max for all measured muscles was calculated as the muscle standardized uptake value (SUV_M), which was then normalized using the liver SUV (SUV_LIV) to determine SUV_M/SUV_LIV. Muscle disease activity was evaluated using the myositis intention to treat activities index (MITAX), which classified patients into active and inactive groups. The diagnostic ability of SUV_M/SUV_LIV in assessing muscle disease activity in patients with IM was examined via receiver operating characteristic (ROC) curve analysis. Additionally, Spearman′s rank correlation analysis was conducted to explore the relationships between SUV_M/SUV_LIV and patients′ general characteristics and laboratory parameters. The Mann-Whitney U test was used for intergroup comparisons of continuous variables with non-normal distributions.

Results Among the 66 patients, 49 were classified into the active group, whereas 17 were categorized into the inactive group. SUV_M/SUV_LIV in the active group was 0.66 (0.54, 0.95), which was significantly higher than that in the inactive group (0.48 (0.34, 0.52); Z=3.952, P<0.001). ROC curve analysis demonstrated that the optimal cut-off value of SUV_M/SUV_LIV for distinguishing active from inactive patients was 0.57, yielding a sensitivity of 65.3% and a specificity of 94.1%, with an area under the curve of 0.824 (95%CI: 0.710–0.906). Correlation analysis revealed that SUV_M/SUV_LIV was significantly correlated with MITAX (r=0.595, P<0.001), creatine kinase (r=0.590, P<0.001), creatine kinase-myocardial band (r=0.420, P<0.001), ferritin (r=0.558, P<0.001), and C-reactive protein (r=0.355, P=0.005), but showed no significant correlation with disease subtype (r=0.073, P=0.559).

Conclusion The semiquantitative metric SUV_M/SUV_LIV derived from ¹⁸F-FDG PET/CT is effective in evaluating muscle disease activity in patients with IM without malignant tumors.