Abstract
Background. Positron emission tomography (PET)-computed tomography (CT) scans are generally used in idiopathic inflammatory myositis (IIM) to exclude malignancy due to its high association in specific IIM subtypes. PET-CT scans are often not protocoled to look at the muscle. Magnetic resonance imaging (MRI) remains the most sensitive test to detect muscle inflammation. Our study aims to begin preliminary validation of PET-CT scan in IIM against commonly used clinical and imaging biomarkers.
Methods. Patients underwent assessment by clinical examination (manual muscle testing 26 scores [MMT 26]), laboratory testing (creatinine kinase and myositis serology), self-reported health assessment questionnaires (HAQ), and imaging (whole-body PET-CT scan and myositis-protocoled lower limb MRI). The SUVmax for four predetermined muscles (deltoid, biceps, gluteus, and quadriceps) and the two most FDG-avid muscles in the upper limbs and lower limbs were tabulated bilaterally and compared against a physiological baseline (the musculus longissimus thoraces [MLT]). PET findings were compared with patient, laboratory, and MRI findings.
Results. Ten patients with myositis were included in the study, 8 (80%) were males, and 2 (20%) were females. The median age was 67 (IQR: 61-72), CK was 972 (129-3538), MMT 26 was 239 (217-255), and HAQ was 1(0-2). Seven par¬ticipants had necrotising autoimmune myopathy, one with dermatomyositis, one with polymyositis and one with granulomatous myositis. Half of the patients were seronegative. Fifty-four muscles (54/80, 67.5%) with FDG-avid uptake on PET also had muscle oedema on MRI. The median SUVmax of FDG-avid muscles was higher at 1.27 (CI: 1.28-1.63) when muscle oedema was present on MRI, com¬pared to 0.96 (CI:0.83-1.08) when muscle oedema was absent on MRI (p<0.001). The mean SUVmax of 16 muscles showed no significant correlation with CK (r=0.467, p=0.174), MMT 26 (r=-0.192, p=0.620) and HAQ (r=-0.478, p=0.162).
Conclusion. PET-CT scan appears comparable to MRI when analysing muscle in IIM in most cases. This is a promising alternative when MRI scans are con¬traindicated or not available. Future studies could focus on comparative analysis with muscle biopsies, differentiating IIM phenotypes, and exploring visceral in¬volvement.