Background: Polymyalgia rheumatica (PMR) and giant cell arteritis (GCA) are chronic inflammatory disorders that predominantly affect older adults. ¹⁸ F-FDG-PET/CT has become valuable for evaluating disease activity in PMR and GCA. PET imaging with FAP inhibitors (FAPI) has shown potential for producing high-contrast images in various oncological and inflammatory diseases. However, the role of PET-FAPI in assessment of PMR and GCA is unknown.

Objectives: To perform a head-to-head comparison of the novel FAPI-PET/CT with the established FDG-PET/CT for evaluating disease activity and localization in patients with PMR and GCA.

Methods: In this prospective study, 78 patients (27 FAPI-PET, 51 FDG-PET) with PMR and/or GCA according to international criteria were enrolled. All patients underwent clinical evaluation, laboratory testing, and PET/CT imaging. The FAPI-PET Visual Index was calculated by summing visual scores (0-3) across involved sites. Imaging findings, their correlation with the Polymyalgia Rheumatica Activity Score (PMR-AS), and clinical outcomes were analyzed.

Results: The cohort consisted predominantly of patients with PMR (83.3%). In these patients, FAPI-PET demonstrated a distinct imaging profile, characterized by superior sensitivity in the shoulder girdle (91.3% vs 66.7%, P =0.023) and significantly higher uptake intensity (SUVmax 9.5 vs 5.6, P <0.01) compared to FDG-PET. FDG-PET showed higher detection rates at other sites including the ischial tuberosities. Most notably, we found a strong positive correlation between the FAPI-PET Visual Index and clinical disease activity (PMR-AS: R=0.751, P =0.0002), which was significantly stronger than the non-significant correlation observed with FDG-PET (R=0.312, P =0.0603). In the smaller GCA subgroup, trends were observed, with FDG-PET showing more frequent vascular involvement.

Conclusions: FAPI-PET demonstrates superior performance for the assessment of PMR, closely reflecting clinical disease activity and outperforming FDG-PET in evaluating characteristic girdle involvement. FAPI-PET emerges as a highly specific imaging biomarker for PMR, holding significant promise for improving diagnosis, phenotypic stratification, and objective monitoring of treatment response.

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Acknowledgments: NIL.

Disclosure of Interests: None declared.