Utility of positron emission tomography as a new tool for muscle involvement in patients with idiopathic inflammatory myositis: a controlled study

OBJECTIVES: Idiopathic inflammatory myositis (IIM) represents a rare group of disease that can affect multiple organs in addition to the muscles. 18F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) is an emerging scanning method that is widely used in diagnosing, staging and response to treatment in patients with cancer. In this study, we aimed to evaluate the muscle involvement in PET/CT which was performed for malignancy screening and its correlation with myositis-specific antibodies (MSA) and/or myositis-associated antibodies (MAA) in patients with IIM. METHODS: IIM patients who fulfilled 2017 EULAR/ACR classification criteria and had PET-CT scans during the active phase of myositis (within two weeks of starting steroids) were included into the study. Age and sex matched participants with history of malignancy (non-IIM patients) were defined as control group. RESULTS: Data of 160 IIM patients were evaluated and 34 patients (of 64.7% female) whose PET/CT results were available, included into the study. Fourteen patients with diagnosis of malignancy without IIM (non-IIM patients) defined as the control group. Sensitivity and specificity of a positive FDG muscle uptake were 37.1% and 100%, 65.7% and 92.9%, 91.4% and 7.1% compared to liver, mediastinum and LTM uptakes, respectively. In multivariate analysis, higher baseline CRP (p=0.017, confidence interval [CI] 95%: 1.03-1.36, OR:1.18) and LDH (p=0.029, CI 95%:1.001-1.017, OR:1.01) levels were associated with muscle PET/CT positivity. CONCLUSIONS: In patients with active IIM, median muscle FDG uptake with PET/CT was higher compared to non-IIM. PET/CT may be used for the evaluation of extent and activity in patients with IIM.