<h3>Background:</h3> Giant cell arteritis (GCA) represents the most common form of aortitis in individuals &gt;50 years old. Aortic dilation has been reported both in the early and late course of GCA, and patients with GCA who develop aortic dilation have an increased mortality. Patients with early aortic FDG uptake at PET/CT seem at higher risk of late aortic complications, but consistent predictors of aortic dilation are lacking. <h3>Objectives:</h3> To evaluate aortic involvement (inflammation/dilation) and predictors of aortic dilatation in a longitudinally followed cohort of patients with newly diagnosed GCA. <h3>Methods:</h3> This was a retrospective study on prospectively collected data of consecutive patients diagnosed with GCA in two European centers. Patients older than 50 years, with a new diagnosis of GCA confirmed by temporal artery biopsy or imaging, who underwent thoracic aortic imaging at diagnosis and after at least 6 months of follow-up were included. All imaging studies were reviewed by radiologists and nuclear medicine physicians at the two study centers. Circumferential wall thickening with or without contrast enhancement on CT and MRI was considered evidence of inflammation (aortitis). Increased FDG uptake ≥ 2 on PET/CT in the aorta was deemed consistent with inflammation (aortitis). CT and MRI of the aorta were reconstructed in a plane perpendicular to the flow of blood to obtain measurements of aortic caliber. Aortic dilation was defined by a diameter &gt;40 mm in the ascending aorta, ≥40 mm in the thoracic descending aorta and ≥30 mm in the abdominal aorta. <h3>Results:</h3> A total of 157 patients were included. Baseline features are reported in Table 1. At diagnosis, 43 (27.4%) patients underwent aortic MRI, and 114 (72.6%) PET/CT. Aortitis was present in 92 (58.6%) patients, and aortic dilation in 31 (19.7%). Aortitis was present in 70.3% of PET/CT and 32.6% of MRI, p&lt;0.0001, and aortic dilation in 22.7% and 14% respectively, p=0.420. Compared with those without, patients with aortitis had higher baseline aortic diameters [mean (SD) ascending aorta 3.74 (0.45) vs 3.62 (0.39) cm, p=0.090; mean (SD) aortic arch 3.13 (0.38) vs 2.95 (0.37) cm, p=0.005; mean (SD) descending aorta 2.99 (0.39) vs 2.81 (0.31) cm, p=0.008; mean (SD) suprarenal aorta 2.40 (0.33) vs 2.23 (0.28) cm, p=0.001]. At univariate logistic regression analysis, significant predictors of baseline aortic dilation were male sex [OR 5.37 (95% CI 2.33 to 12.40)], fever [OR 2.02 (95% CI 0.90 to 4.55)], hypercholesterolemia [OR 0.35 (95% CI 0.13 to 0.90)], and aortitis [OR 2.56 (95% CI 1.02 to 6.42)]. At multivariate analysis, predictors of aortic dilation were aortitis [OR 3.65 (95% CI 1.25 to 10.71)] and male sex [OR 8.33 (95% CI 3.13 to 22.18)]. Median (Q1, Q3) time between baseline and follow-up aortic imaging was 30 (17, 69.5) months. At last imaging 53 (33.8%) patients underwent aortic MRI, 74 (47.1%) PET/CT and 30 (19.1%) CT angiography. There was a significant increase in aortic diameters between the first and last imaging [mean change (95% CI) 1.1 (0.6 to 1.6) mm at the ascending aorta, p&lt;0.0001; 0.9 (0.6 to 1.2) mm at the aortic arch, p&lt;0.0001; 0.9 (0.4 to 1.4) mm at the descending aorta, p&lt;0.0001; 0.2 (-0,003 to 0.4) mm at the suprarenal aorta, p=0.099]. The mean (SD) annual growth rate was 0.31 (1.73) mm/year in the ascending aorta, 0.26 (0.95) mm/year in the aortic arch, 0.29 (1.95) mm/year in the descending aorta and 0.09 (1.39) mm/year in the suprarenal aorta, without differences between patients with and without baseline aortitis. At follow-up imaging, 12 (9.8%) patients developed a new aortic aneurysm. Cox proportional hazards regression analysis did not show a significantly higher risk for incident aortic aneurysms in patients with baseline aortitis (sex- and age-adjusted HR 1.65 (95% CI 0.59 to 5.62). <h3>Conclusion:</h3> Aortitis and aortic dilation were common findings in patients with newly diagnosed GCA, and baseline aortitis was associated with higher aortic diameters. Significant increase in aortic diameters was observed during the study follow-up. These data confirm that patients with GCA need to be screened for aortic dilation. However, the best screening modality and timing remain unclear.REFERENCES: NIL. <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> Chiara Marvisi: None declared, Konstanze V. Guggenberger: None declared, Caterina Ricordi: None declared, Rudolf A. Werner: None declared, Giulia Besutti: None declared, Sebastian E. Serfling: None declared, Roberto Farì: None declared, Matthias Fröhlich: None declared, Rexhep Durmo: None declared, Michael Gernert: None declared, Lucia Spaggiari: None declared, Annibale Versari: None declared, Pierpaolo Pattacini: None declared, Carlo Salvarani: None declared, Marc Schmalzing BMS, Novartis, AbbVie, AstraZeneca, Chugai/Roche, Janssen-Cilag, Gilead, Boehringer/Ingelheim, Mylan, onkowissen.de, Galapagos, EUSA-Pharma, Novartis, AbbVie, AstraZeneca, Chugai/Roche, Janssen-Cilag, Gilead, Boehringer/Ingelheim, Mylan, onkowissen.de, Galapagos, EUSA-Pharma, Hexal/Sandoz, Amgen, medac, Lilly, UCB, Chugai, Novartis, Francesco Muratore: None declared, Torsten Bley BioTel Research, Bracco, Chugai, Guerbet, Roche, Novartis, Siemens Healthineers, BioTel Research, Bracco, Chugai, Guerbet, Roche, Novartis, Siemens Healthineers, Deutsche Forschungsgemeinschaft (DFG), Siemens Healthineers.

POS0932 AORTIC INVOLVEMENT IN GCA: A MULTICENTER RETROSPECTIVE STUDY