More importantly, HT-1080-FAP tumor uptake of [18F]21 was increased linearly up, while [68 Ga]Ga-FAPI-04 showed a constantly decreasing tumor uptake over the total scan time, resulting in 14.9-fold higher tumor uptake for [18F]21 than [68 Ga]Ga-FAPI-04 at 2 h pi (10.04%ID/g vs. 0.673%ID/g). Here, FAP is linked to neoplasm.