Abstract
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(s) = 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.