Intra-patient comparison of tracer uptake and biodistribution of [68Ga]Ga-FAPI-46 and [18F]AlF-FAPI-74.

PURPOSE: To compare the tracer uptake and biodistribution of [68Ga]Ga-FAPI-46 and [18F]AlF-FAPI-74 in an intra-patient study design. METHODS: This substudy of a large prospective trial included five patients with fibrotic lung disease who underwent both [68Ga]Ga-FAPI-46 and [18F]AlF-FAPI-74 PET/CT. Imaging was performed 60 min post-injection using EARL2-compliant reconstructions. Spherical volumes of interest were placed in normal organs, background tissues (blood pool, liver, and skeletal muscle) and reactive tissue uptake foci. SUVmean and SUVmax were calculated and the means were compared using paired t-tests. RESULTS: Visual analysis showed similar overall biodistribution for both tracers, with higher apparent vascular and intestinal activity on [18F]AlF-FAPI-74 PET/CT compared with [68Ga]Ga-FAPI-46 PET/CT. Quantitatively, [18F]AlF-FAPI-74 demonstrated significantly higher SUVmean and SUVmax in the blood pool region (p = 0.01 and p = 0.03, respectively). SUVmean was also significantly higher in the spleen (p = 0.01) and jejunum (p = 0.02) for [18F]AlF-FAPI-74 compared with [68Ga]Ga-FAPI-46. No significant differences between both radiopharmaceuticals were observed in other normal organs or reactive tissue foci. CONCLUSION: At 1 h post-injection, [18F]AlF-FAPI-74 shows higher blood pool, splenic, and small bowel activity but otherwise similar normal-organ and reactive tissue uptake compared with [68Ga]Ga-FAPI-46. Although both tracers exhibit low physiological background uptake, these biodistribution differences should be considered when interpreting FAPI PET images and when comparing quantitative metrics across studies using different FAPI tracers.