Abstract: Objectives: Clinical improvement following tocilizumab (TCZ) therapy in patients with large-vessel (LVV) giant cell arteritis (GCA) is well established. However, information on TCZ effect on imaging vascular activity is limited. We aimed to determine if clinical improvement correlated with reduction of vascular 18F-fluorodeoxyglucose (18F-FDG) uptake in positron emission tomography (PET/CT) scans. Methods: Observational study of patients with refractory LVV-GCA treated with TCZ who had a baseline and a follow-up 18F-FDG-PET/CT scan. For the visual analysis of 18F-FDG vascular uptake, a total vascular score (TVS) was defined, ranging from 0 to 15. Besides, a semiquantitative analysis was performed as a target to background ratio (TBR)= SUVmax thoracic aorta wall/SUVmax aortic vascular pool. The baseline and follow-up TVS and TBR were compared. Clinical and lab¬oratory outcomes were also assessed. Results: We included 30 patients (24 women/6 men); mean age± standard deviation 65.7± 9.8 years. Baseline PET/CT scans were performed due to active disease at a median [interquartile range-IQR] of 1.5 [0.0-4.0] months before TCZ onset. Following TCZ therapy, 25 (83.33%) patients achieved clinical remission and reduction of 18F-FDG vascular uptake was also observed after a mean ± standard deviation of 10.8±3.7 months. TBR decreased from 1.70 ± 0.52 to 1.48 ± 0.25 (p=0.005) and TVS from 4.97±2.62 to 3.13±1.89 (p< 0.001). However, only 9 (30.0%) patients showed complete normalisation of TBR and only 3 (10%) normalisation of TVS. TBR and TVS showed a good correlation (r=0.576). Conclusions: Although most of LVV-GCA patients achieve clinical remission after TCZ therapy, less than one-third show normalisation of 18F-FDG vascular uptake.

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