Idiopathic inflammatory myositis

Objectives: Idiopathic inflammatory myositis (IIM) is a group of connective tissue diseases characterized by proximal muscle weakness. IIM is frequently accompanied with interstitial lung disease (ILD) and malignancy. ¹⁸F-FDG PET/CT, a standard tool for diagnosing malignancies, can also be used to detect inflammatory lesions. However, about its usefulness in clinical course from diagnosis to treatment evaluation of IIM has not been well established. The aim of this study is to investigate the significance of PET/CT in the detection of malignancy and evaluation of muscular and pulmonary involvement in patients with IIM.

Methods: Thirty-eight consecutive patients with clinical diagnosed IIM who underwent ¹⁸F-FDG PET/CT were retrospectively studied, including 15 males and 23 females, with an average age of 56.0±12.5 years (range 24 to 83 years). Clinical complaint, muscle strength level and serum creatine kinase level was collected in all patients. ¹⁸F-FDG PET/CT scan was underwent in routine standard condition, and pulmonary high-resolution computed tomography (HRCT) was simultaneously obtained in each patient. Image observations for each patient include the following aspects: (1)Identify and record abnormal FDG uptake lesions suspected malignancy. (2) Evaluate FDG uptake in proximal limb muscles with visually observation and measured its SUV_max. (3)Evaluate FDG uptake in the area of ILD. According to clinical classification criteria, polymyositis (PM) and dermatomyositis (DM) were defined, and DM was further divided into classic DM (cDM) and clinical amyopathic dermatomyositis (CADM). ILD was identified by pulmonary HRCT. If the interval between onset of respiratory symptom and respiratory failure were less than 3 months, a rapidly progressive interstitial lung disease (RP-ILD) was diagnosed.

Results: Final clinical classification for the group of patient was 3 PM, 17 cDM and 18 CADM. ¹⁸F-FDG PET/CT diagnosed malignant tumor in 7 patients and was confirmed by histopathologic findings. Increased uptake of FDG in proximal limb was observed in 21/38 patients (55.3%), which were seen in 3 PM, 13 cDM and 5 CADM. And Spearman’s rank correlation analysis showed that SUV_max in proximal limb muscle in patient with IIM was correlated with muscle strength level (r=-0.605, P<0.001) and serum creatine kinase level (r=0.332, P=0.042). On PET/CT, HRCT detected ILD in 30/38 (78.9%) cases, including 1 PM, 12 cDM and 17 CADM. Twenty-six of 30 (92.9%) ILD cases showed increased uptake in lung lesions, with a average SUV_max of 2.3±0.7 and ranged from 1.3 to 4.7. Seven of the 30 ILD cases were diagnosed as RP-ILD. In these 7 cases, FDG uptake of lung lesion was higher than other 23 cases with chronic ILD (SUV_max:3.0±0.8 versus 1.8±0.6, t=4.268,P<0.001). If use a cut-off value of SUV_max蠅2.5 to predict RP-ILD, it yielded a diagnostic sensitivity, specificity and accuracy of 100% (7/7), 87.0% (20/23) and 90.0% (27/30), respectively.

Conclusion: In patients with IIM, ¹⁸F-FDG PET/CT is useful in detecting malignancies and evaluate severity of inflammation in muscle and lung. Research Support: None