Superior correlation with clinical activity and unique detection patterns: a prospective study of FAPI versus FDG-PET in polymyalgia rheumatica and giant cell arteritis

To perform a prospective comparative study of the novel FAPI-PET/CT with the established FDG-PET/CT for identifying characteristic inflammatory patterns in patients with polymyalgia rheumatica (PMR) and giant cell arteritis (GCA). In this prospective study, 78 patients (27 FAPI-PET, 51 FDG-PET) with PMR/GCA were enrolled using the 2012 EULAR/ACR criteria for PMR and the 2022 ACR/EULAR criteria for GCA, with diagnosis confirmed after a median 9.2-month follow-up. All underwent clinical and laboratory evaluation plus PET/CT imaging before any immunosuppressive treatment. The FAPI-PET Visual Index was derived by summing visual scores (0–3) across involved sites. Imaging findings were correlated with the PMR-AS and clinical outcomes. Pure PMR accounted for 83.3% of the cohort. In PMR, FAPI-PET showed superior sensitivity in the shoulder girdle (91.3% vs. 66.7%, P = 0.02) and higher uptake intensity (SUVmax 9.5 vs. 5.6, P < 0.01) versus FDG-PET, which showed higher detection at sites like the ischial tuberosities. Crucially, the FAPI-PET Visual Index correlated strongly with PMR-AS (R = 0.751, P = 0.0002), significantly outperforming FDG-PET (R = 0.312, P = 0.0603). In the smaller GCA subgroup, FDG-PET appeared more sensitive for vascular involvement. FAPI-PET demonstrates a distinct imaging profile and shows a superior correlation with clinical disease activity in PMR compared to FDG-PET in this comparative study. In the small subgroup of patients with GCA, FDG-PET appeared more sensitive for vascular involvement, but these findings require confirmation in larger studies. FAPI-PET shows promise as an imaging biomarker of active PMR, potentially improving diagnosis and phenotypic stratification.