The reproducibility of [68Ga]Ga-FAPI-04 PET uptake parameters at 15 min and 60 min post-injection

Gallium-68 labeled fibroblast activation protein inhibitor ([68Ga]Ga-FAPI-04) can be visualized just 15 min post-injection. However, the appearance of imaging at 15 and 60 min remains unclear. This study aimed to explore the relationship between quantitative values in [68Ga]Ga-FAPI-04 PET, specifically at 15 and 60 minutes post-injection, in patients with various tumor. We enrolled 30 patients with cancer who underwent [68Ga]Ga-FAPI-04 PET/CT scan between January 2021 and February 2025 at our institute. Image acquisition was performed using a PET/CT scanner at 15 min and 60 min after [68Ga]Ga-FAPI-04 injection. The maximum, mean and peak standardized uptake value (SUV_max, SUV_mean and SUV_peak), tumor-to-liver ratio (TLR), and uptake tumor volume (UTV) were measured in the region of interest of the target lesion, liver SUV_mean (SUV_liver) was also measured. Correlation coefficients of the between-image variables were evaluated by Spearman's rank correlation test. Agreement between the variables was evaluated by Bland-Altman plots with 95% limits of agreement. The SUV_max, SUV_mean, SUV_peak and UTV in tumors of all examinations were decreased from 15 min to 60 min. The SUV_max, SUV_mean, SUV_peak, TLR, and UTV at 15 min and 60 min were highly correlated (r = 0.945, 0.949, 0.959, 0.943, and 0.958; P < 0.001). The 95% limits of agreement ranged from -27.8 to 29.1 with a mean of 0.7 and -24.1 to 29.5 with a mean of 2.7 for SUV_max and SUV_mean, respectively. Other PET metrics demonstrated that all limits are above ± 30% between 15 min and 60 min. We observed a high correlation between the quantitative values at 15 min and 60 min. Meanwhile, 15 min and 60 min [68Ga]Ga-FAPI-04 PET SUV_max and SUV_mean have clinically acceptable reproducibility, and 15 min scan is feasible for all patients, but SUV_peak, TLR and UTV should not be used interchangeably.