Comparison of [¹⁸F]FAPI-42 and [¹⁸F]FDG PET/CT in the evaluation of systemic vasculitis

The role of fibroblast activation protein (FAP)-targeted imaging in systemic vasculitis is currently unclear. We aimed to evaluate the clinical value of fluorine-18-labeled FAP inhibitor 42 ([¹⁸F]FAPI-42) in patients with systemic vasculitis and to compare with [¹⁸F]fluorodeoxyglucose (FDG) imaging.

Patients with systemic vasculitis who underwent dual-tracer PET/CT([¹⁸F]FDG and [¹⁸F]FAPI) imaging from September 2020 to March 2022 were retrospectively analyzed. Positive lesions are defined as vascular/extravascular lesions with increased tracer uptake above surrounding background, which cannot be attributed to the physiologic biodistribution of the radiotracer. The vascular/extravascular lesion detection rate and semiquantitative values (SUVmax, TBR_blood and TBR_liver) of [¹⁸F]FAPI and [¹⁸F]FDG were compared, and the correlation between the extent and range of tracer uptake and levels of inflammatory markers was investigated.

Thirty patients (13 males and 17 females; mean age, 52.5 ± 17.2 years) with systemic vasculitis were included (17 large vessel vasculitis, 10 anti-neutrophil cytoplasmic antibody-associated vasculitis, 2 Behcet's disease and 1 polyarteritis nodosa). [¹⁸F]FDG PET/CT had positive findings in 93.3% (28/30) of patients, while [¹⁸F]FAPI PET/CT had positive findings in all patients (100%, P = 0.500). Compared with [¹⁸F]FDG PET/CT, [¹⁸F]FAPI PET/CT detected more lesions (161/168 vs. 145/168, P = 0.005), and more extensive vascular involvement in 60% (18/30) of patients. Although SUVmax did not differ significantly between [¹⁸F]FAPI and [¹⁸F]FDG (median, 5.94 vs. 5.46, P = 0.517), [¹⁸F]FAPI had higher TBR_liver (median, 9.59 vs. 3.15, P < 0.001) and TBR_blood (median, 5.45 vs. 4.20, P = 0.006). The total number of positive lesions in FAPI PET/CT show a moderate correlation with erythrocyte sedimentation rate (r_s =0.478, P = 0.008) and C-reactive protein (r_s =0.486, P = 0.006). After treatment, follow-up FAPI PET/CT of 6 patients showed decreased SUVmax, TBR and number of detected lesions, paralleling the clinical remission.

[¹⁸F]FAPI PET/CT imaging is a promising imaging modality for the diagnosis and therapeutic monitoring of systemic vasculitis.