Polymyalgia rheumatica (PMR) is an inflammatory disease often misdiagnosed as certain cancers, among other diseases. Researchers have recently made progress to better diagnose PMR using a standard uptake value index based on fluorodeoxyglucose with positron-emission tomography/computed tomography.
Measuring standard uptake value (SUV) in various sites around the body may be an effective method for diagnosing polymyalgia rheumatica (PMR), according to a study published in Clinical Radiology. The use of fluorodeoxyglucose (FDG) with positron-emission tomography/computed tomography (PET/CT) to measure SUV proved useful for distinguishing PMR from other types of disease.
PMR is a rheumatic disease—mostly characterized by pain and stiffness in the shoulder girdle, pelvic girdle, and neck—affecting individuals over the age of 50, and affected patients are more often women.
Diagnosing PMR has some obstacles. Scarcity in serological indicators and specific clinical manifestations lead the presence of PMR to be easily confused with rheumatoid arthritis (RA), forms of cancer, spondyloarthropathy (SpA), and more.
FDG is a modified form of glucose typically utilized by oncologists as a radiotracer in PET/CT scans. FDG accumulates in tumors and inflammatory tissues alike because both sites are high in glucose demand. Although this is a known marker for many cancers, the feasibility of SUV as a metabolic parameter for PMR diagnosis is unknown.
In the study, researchers derived an SUV index from the ratio of maximum levels of SUV (SUVmax) to liver mean SUV (SUVmean) as determined by FDG PET/CT and assessed the feasibility of utilizing the SUV index in diagnosing patients with PMR.
Data on patients admitted to the Department of Immuno-rheumatology of The Third Affiliated Hospital of Soochow University, Changzhou, China who underwent PET/CT between November 2014 through December 2022 were retrospectively analyzed. All patients were 50 years or older, exhibited pelvic girdle and/or bilateral shoulder pain, were unanimously diagnosed by members of the immune-rheumatology treatment team, and met the 1984 Healey criteria for PMR. Any patients who did not have disease activity during the time of their PET/CT, had severe impairments in liver function, or whose fasting blood glucose levels were greater than 11.1 mmol/l were excluded.
Seventy-eight patients made up the PMR group, and the control group was comprised of 121 patients with differing inflammatory diseases and infections ranging from RA and SpA to idiopathic inflammatory myopathies, undifferentiated connective tissue diseases, among other infections.
Among the 18 sites where analyses of FDG uptake were conducted, the characteristic sites for PMR were identified as the ischial tuberosity, periarticular hip, symphysis pubis enthesis, and interspinous bursa. The mean SUV index of the characteristic sites was found to be independently associated with PMR. The authors determined the best cutoff value for SUV index was 1.685, where the specificity registered at 92.6% and sensitivity at 84.6%.
The probability of PMR increased in a nonlinear manner alongside the characteristic site SUV index, and whenever the mean SUV index for characteristic sites was at or above 2.56, the likelihood of PMR incrementally reached the threshold effect—which was 90% or higher in some cases.
Several limitations were listed by the authors, including the small cohort, the study’s retrospective nature, and the consistent need to externally validate their findings. Additionally, the authors were unable to include FDG intake for the wrist, knuckle, elbow, ankle, or knee because of PET/CT limitations and the fact that these sites are more likely to be diagnosed as an inflammatory disease other than PMR if they exhibit high uptake.
The authors concluded that “PMR should be highly suspected when the characteristic site SUV index is 1.685”; however, before these findings can be clinically translated, further studies, validation, and consideration are necessary.
Sun S, Shao X, Lu X, et al. Assessing the feasibility of SUVindex (a metric derived from FDG/PET/CT) for the diagnosis of polymyalgia rheumatica. Clinical Radiol. Published online June 28, 2023. doi: 10.1016/j.crad.2023.06.007