POS0399 DIAGNOSTIC PERFORMANCE OF HEAD AND NECK FDG-PET/MR IN NEW-ONSET GCA
Authors: P. H. Nienhuis, A. H. Dias, C. M. Sørensen, K. Keller, I. Hansen, E. Naeser, M. Nyhuus Bendix Rasch, A. P. A. Appelman, E. Brouwer, R. H. J. A. Slart, E. M. Hauge, L. C. Gormsen, B. Dalsgaard Nielsen
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> Accurate and timely diagnosis is crucial in patients with suspected giant cell arteritis (GCA) as prompt treatment with high-dose glucocorticoids may prevent severe ischemic complications, such as sight loss. However, the use of high-dose glucocorticoids in patients without verified GCA should also be done with caution due to potential side effects. Currently, multiple diagnostic imaging may be required to establish an accurate diagnosis. In addition to ultrasonography, 18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography (FDG-PET/CT) and Magnetic Resonance Imaging (MRI) are used in clinical practice but carry a significant risk of false negative results, especially after initiated (high-dose) glucocorticoid treatment. Since FDG-PET imaging measures metabolic activity, while MRI demonstrates morphological changes, combining these modalities may result in a composite hybrid diagnostic test with higher diagnostic accuracy. <h3>Objectives:</h3> To evaluate the diagnostic performance of hybrid FDG-PET/MR in new-onset GCA and to estimate the effect of 10 days glucocorticoid treatment on FDG PET/MR sensitivity. <h3>Methods:</h3> Patients with suspected new-onset GCA were included in this prospective study. Patients who had received treatment with glucocorticoids for more than 3 days prior to imaging were not included. All patients underwent a standard diagnostic workup that included ultrasonography of the temporal and axillary arteries and whole-body FDG-PET/CT. All patients underwent head and neck FDG-PET/MRI after FDG-PET/CT using the same FDG administration. Those with a confirmed clinical diagnosis of GCA underwent additional FDG-PET/CT and FDG-PET/MRI after 10 days of treatment. The final clinical diagnosis based on clinical presentation and standard diagnostic workup was used as the reference diagnosis. The cranial arteries were assessed on FDG-PET, T1-weighted contrast-enhanced (T1WCE) MRI, and time-of-flight (TOF) MRI. The scoring system is described in Table 1. FDG-PET/CT and FDG-PET/MRI scans were independently assessed by two experienced nuclear medicine physicians (NMPs) who were blinded for final clinical diagnosis and imaging timepoint (baseline or day 10). NMPs were also asked to give a final overall impression of the presence or absence of GCA. The diagnostic performance was calculated as sensitivity and specificity of the diagnostic test and inter-rater agreement was calculated using Fleiss' Kappa. <h3>Results:</h3> In total, 30 patients with suspected GCA were included and completed FDG-PET/CT and FDG-PET/MRI imaging at time of diagnosis. Baseline characteristics are displayed in Table 2. Twenty patients received a final clinical diagnosis of GCA. In ten patients a diagnosis of GCA was ruled out. Of the 20 patients with GCA, 14 patients completed a second FDG-PET/CT and FDG-PET/MRI scanning in a mean of 10 days (range 8 to 13) after the initiation of treatment. FDG-PET/MRI had a sensitivity of 95% and a specificity of 90% for GCA. Two patients had no FDG-uptake but showed clear signs of vessel wall thickening or stenosis on MR imaging. FDG-PET/CT had a sensitivity of 85% and specificity of 100%. Three of the 14 GCA patients who were scanned a second time on day 10 did not show any signs of GCA on FDG-PET/CT or FDG-PET/MRI despite showing those signs on the scans at time of diagnosis, thus resulting in a sensitivity of 79%. NMPs showed full agreement (100%) regarding the final diagnosis (GCA vs no GCA) on imaging. <h3>Conclusion:</h3> This is the first study to evaluate the diagnostic performance of fully integrated hybrid FDG-PET/MRI in GCA with assessment of the cranial arteries. FDG-PET/MR showed high diagnostic sensitivity and specificity was more sensitive than FDG-PET/CT in the diagnosis of GCA. The sensitivity of FDG-PET/CT and FDG-PET/MRI decreased after 10 days of glucocorticoid treatment. <b>Table 1.</b> Description of the scoring methods for the imaged sequences. <b>Table 2.</b> Baseline characteristics for patients with the final clinical diagnosis of GCA and patients without (No GCA). CRP, C-reactive protein; GC, glucocorticoid. <h3>REFERENCES:</h3> <b>NIL.</b> <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>