Background: Positron emission tomography (PET/CT) has gained importance in the diagnosis and assessment of large vessel vasculitis (LVV) recently. Objectives: We aimed to investigate the diagnostic importance and clinical significance of PET/CT findings in giant cell arteritis (GCA). Methods: Data of the patients who underwent PET/CT to investigate large vessel involvement and who had at least 6 months of follow-up with a clinical diagnosis of GCA were retrospectively evaluated. PET/CT images were assessed by an experienced nuclear medicine specialist, regions of interest were drawn for major vascular territories and standardized maximum uptake values (SUVmax) of these areas were recorded. Results: Twenty-nine consecutive patients (median age 68 (50-83), mean follow-up time 37.1 ± 48.8 (6-242)) were included into the study. All patients were over 50 years old and had erythrocyte sedimentation rate (ESR) over 50 mm/h at the time of imaging. Twenty patients (68.9%) met the ACR 1990 Classification criteria (ACR (+) group). The number of patients who had hypermetabolism in the aorta and its major branches in favour of LVV in PET/CT was 23 (79.3%) (PET-CT (+) group). Thoracic and abdominal aorta involvement were detected in 22 (75.8%) and 16 (55.2%) patients, respectively. There was positive correlation between SUVmax in thoracic and abdominal aorta on PET/CT and ESR at diagnosis (r = 0.63 p = 0.002 and r = 0.77 p <0.001, respectively) and SUVmax in thoracic aorta and CRP (r=0.50 p=0.026). PET/CT (-) patients had more frequent disease flares during the follow-up (4/6 vs. 5/23 p = 0.035 OR = 7.2 (1.01- 51)). Three distinct subgroups were defined by implementing both ACR criteria and PET/CT positivity. Among ACR (+) patients (n=20); comparison of PET/CT (+) (n=14) and PET/CT (-) (n=6) patients did not show any difference in age of diagnosis, presence of polymyalgia rheumatica (PMR), flare rate and damage scores. Among PET/CT (+) patients (n=23), the mean age at diagnosis was higher, PMR and bilateral axillary artery involvement was more frequent in ACR (+) group (n=14) (Table 1). Conclusion: PET/CT is increasingly used in the diagnosis and assessment of GCA in our center. The level of FDG uptake of the vessel wall in PET/CT correlates with the acute phase response. Flare was rarely observed in PET/CT (+) patients at diagnosis. Axillary artery involvement detected on PET/CT may be associated with the classical GCA clinic in ACR(+) patients (1). PET/CT (+) patients who does not met ACR criteria seems to have a diverse clinic features like young age and rare presence of PMR. PET/CT findings may be helpful in recognizing subgroups and predicting prognosis of GCA although prospective studies with follow-up scans are warranted. References: 1Grayson PC, Maksimowicz-McKinnon K, Clark TM, Tomasson G, Cuthbertson D, Carette S, et al. Distribution of arterial lesions in Takayasu’s arteritis and giant cell arteritis. Annals of the rheumatic diseases. 2012;71(8):1329-34. Table 1. Comparison of patients who fulfilled and not fulfilled ACR 1990 classification criteria among PET/CT (+) patients. ACR (+) PET/CT (+) (n=14) ACR (-) PET/CT (+) (n=9) p OR (%95 CI) Age at diagnosis 68,8±4,5 63.3±9,2 0.004 PMR 10 2 0.021 2.5 (1 – 6.1 ) History of flare 4 1 NS CRP at diagnosis 75,1±30,6 130,8±93,4 0.024 ESR at diagnosis 93,9±28,1 112,5±21,2 NS Brachiocephalic artery 9 6 NS Right subclavian 8 5 NS Lef subclavian 9 5 NS Right carotid 8 5 NS Left carotid 9 6 NS Right axillary 7 0 0.011 2 (1.18 – 3.3 ) Bilateral axillary 6 0 0.022 1.75 (1.1-2.7 ) Thoracic aorta SUVmax (mean) 3,9±1,1 4,6±1,3 NS Abdominal aorta SUVmax (mean) 4,5±1,2 5,3±1,8 NS Disclosure of Interests: None declared