Optimizing [18F]FAPI-74 PET/CT imaging: insights from Early-phase tumor uptake and tissue contrast

Fibroblast activation protein inhibitor (FAPI) is a novel PET tracer that targets various malignant tumors. Although some studies have demonstrated that 68Ga-labeled FAPI PET provides diagnostic images shortly after injection, reports on early-phase imaging using [18F]FAPI-74 are limited. This study evaluated the diagnostic potential of early-phase [18F]FAPI-74 PET/CT imaging. The cohort comprised 32 patients who underwent whole-body [18F]FAPI-74 PET/CT at three time points: 10, 30, and 60 min after administration. The uptake of [18F]FAPI-74 in normal organs and tumors at each time point was evaluated and compared using standardized uptake value (SUV). The contrast between tumor and background uptake was assessed using tumor-to-blood ratios (TBRs), calculated as the ratio of tumor SUVmax to blood pool SUVmean. The uptake of [18F]FAPI-74 in most organs gradually decreased, except in the mammary gland and nipple. Moreover, uptake due to the excretion increased over time in the gallbladder, and bile duct. Among the 72 lesions identified across patients, the median SUVmax of the tumors remained relatively stable across the time points (5.00, 5.16, and 5.11 at 10, 30, and 60 min, respectively), while the median TBRs gradually increased (1.57, 2.24, and 2.94, respectively). Subgroup analysis revealed a similar pattern of increasing median TBRs in primary tumors (2.76, 3.72, and 4.04, respectively), lymph node metastases and peritoneal dissemination (1.67, 2.47, and 3.58, respectively), and lung metastases (0.99, 1.35, and 1.48, respectively). TBRs were highest at 60 min after administration. However, early-phase imaging was considered useful because of its sufficient diagnostic contrast and low physiological uptake in the biliary system. UMIN Clinical Trials Registry, UMIN000051687. Registered 22 July 2023, https://center6.umin.ac.jp/cgi-open-bin/ctr_e/ctr_his_list.cgi?recptno=R000058594.