FDG PET/CT for the Diagnosis of Cranial and Extra-Cranial Giant Cell Arteritis

1633 Objectives: During the COVID-19 lockdown, temporal artery biopsy (TAB) procedures were not available at our institution for the diagnosis of giant cell arteritis (GCA). For the evaluation of large vessel vasculitis, 18F-fluorodeoxyglucose PET/CT (FDG PET/CT) has good diagnostic performance (pooled sensitivity of 90% and specificity of 98%). Recently, limited data has been reported that cranial artery inflammation can be detected on newer generation non-digital PET/CT scanner. With such advancements, it has been suggested that FDG PET/CT may be able to replace TAB. We hypothesized that digital FDG PET/CT can be used for the diagnosis of GCA through the integrated assessment of cranial and extracranial artery inflammation. Methods: We report data from the use of our GE Discovery MI digital PET/CT for the diagnosis of GCA as an alternative to TAB during the COVID-19 lockdown. Subjects were included if they were referred for FDG PET/CT for clinically suspected GCA. Exclusion criteria included corticosteroid therapy > 3 days prior to PET/CT or history of known vasculitis. 185-370 MBq of FDG was injected intravenously and imaging acquired 60-90 minutes later. Images were interpreted by two expert readers and a consensus was obtained for every case. Results: Fifteen (9 women, 5 men) subjects were included in the analysis. The mean age was 72 years, mean CRP 67.9 mg/L (normal 0-10 mg/L), and mean ESR 56.9 mm/h (normal 2-39 mm/h). Seven subjects were scanned within 3 days of corticosteroid initiation and the other 8 had not received corticosteroids. Four of 15 subjects (27%) were diagnosed with GCA by PET with abnormal cranial or large vessel artery uptake (2 subjects with cranial uptake alone and 2 with both cranial and extracranial uptake), and 3 had evidence of PMR on PET without vasculitis. In those diagnosed with GCA by PET, vascular inflammation was visualized in the temporal arteries in all 4, maxillary and occipital arteries in 3 subjects (Fig.1). Alternate diagnoses explaining the clinical presentation were made in 5 of 8 remaining subjects (63%) including subacute thyroiditis, active sinusitis, metastatic malignancy and masticator space malignancy. During a follow-up period averaging 58 days (range 22 to 94 days), good response to prednisone was seen in the patients diagnosed with GCA by PET/CT and no evidence of GCA in the remaining non-GCA patients. Conclusions: In conclusion, digital FDG PET/CT appears promising for the diagnosis of GCA through its enhanced sensitivity and resolution to detect small cranial artery inflammation. Further prospective studies comparing digital FDG PET/CT with current GCA diagnostic modalities is warranted.