Clinical value of a [18F]-FDG PET-CT muscle-to-muscle SUV ratio for the diagnosis of active dermatomyositis

Objective To study a muscle-to-muscle standardised uptake value (SUV) ratio with FDG-PET/CT (FDG-PET) as a marker for the detection of disease activity in dermatomyositis (DM). Methods Patients with DM ( n = 24) who met the European Neuro-Muscular Centre diagnostic criteria were retrospectively identified over a 3-year period through a national survey. Muscle biopsy was performed in all patients. Maximum SUV was measured in proximal muscles (SUV PROX ) that had the highest radiotracer uptake on visual grading as well as in the musculus longissimus thoracis (SUV MLT ), whereas mean SUV was measured for the liver (SUV LIV ). Muscle-to-liver SUV ratios for either muscle group were compared and a SUV PROX /SUV MLT ratio was calculated. SUV PROX /SUV MLT of DM patients were compared with age- and sex-matched control subjects ( n = 24) with melanoma who had received FDG-PET scans. Results DM patients presented with proximal and symmetrical muscle uptake. Differences in SUV PROX /SUV LIV and SUV MLT /SUV LIV ratios in DM subjects were significant ( p < 0.001). SUV PROX /SUV MLT ratios in DM and their controls also differed significantly ( p = 0.0012). The SUV PROX /SUV MLT ratio threshold between DM subjects and controls was 1.73 with a sensitivity of 50% (CI95%, 29.1 to 70.9%) and specificity at 83.3% (CI95%, 62.6 to 95.3%). When amyopathic DM patients were removed from the analysis, specificity was increased to 95% (CI95%, 75.1 to 99.9%) with a likelihood ratio of 10 and an AUC of 83.4% (CI95%, 71.4 to 95.4%). Conclusion A muscle-to-muscle SUV PROX /SUV MLT ratio with a cut-off value of 1.73 in FDG-PET imaging might serve as a non-invasive marker to determine disease activity in dermatomyositis. Key Points • [18F]-FDG PET-scanner standardised uptake value (SUV) could reflect disease activity in dermatomyositis (DM). • A ratio of SUV in proximal muscles (SUV PROX ) to SUV in musculus longissimus thoracis (SUV MLT ) could be used to determine active DM. • Active disease is suspected for SUV PROX /SUV MLT ratios greater than 1.73.