Abstract
241046
Introduction: Fibroblast activation protein inhibitor (FAPI) is an attractive new PET tracer for the diagnosis of malignant tumors. Recently, [18F]-labeled FAPI has been expected to replace [68Ga]-labeled FAPI due to its easy handling and cost advantages. Although [68Ga]-labeled FAPIs are known to be capable of PET/CT imaging as early as 10 minutes after administration, an optimal imaging time for [18F]FAPI-74 has not yet been established due to limited clinical case reports. The purpose of this study is to estimate the appropriate imaging time by examining changes of [18F]FAPI-74 uptake after administration in normal organs and tumors.
Methods: Of the subjects enrolled in the clinical trial (UMIN000051687) who underwent [18F]FAPI-74 PET/CT, 10 subjects (5 females, 5 males) who also agreed to undergo 3-phase imaging were included in the study. All eligible patients had a history of malignancy: 5 had colorectal cancer, 2 had gastric cancer, 1 had esophagogastric junction cancer, and 1 had mycosis fungoides. PET/CT images were acquired three times at 10, 30, and 60 minutes after administration of [18F]FAPI-74. We measured SUVmax of gallbladder, bile duct, and nipple, and SUVmean of left ventricle (blood), liver, spleen, pancreas, kidney, muscle, bone (L5), lung, parotid gland, submandibular gland, thyroid, and brain. SUVmax was also measured in 22 tumor lesions, and tumor-blood ratio (TBR) was calculated by dividing SUVmax of tumor by SUVmean of left ventricle (blood). These changes were statistically analyzed using the Wilcoxon signed rank test, and p < 0.05 was considered significant.
Results: Tumor uptake did not change significantly due to large variations (median SUV of 10- and 60-minute scans were 6.8 and 5.1, respectively, p = 0.18). For organs, uptake increased significantly over time in the gallbladder (2.8 and 16.7, respectively, p < 0.01), bile duct (6.5 and 9.8, respectively, p < 0.05), and nipple (1.6 and 2.5, respectively, p < 0.01). Brain uptake did not change significantly (0.09 and 0.06, respectively, p = 0.17). Uptake in the other organs decreased from the 10-min to the 60-min scan (p < 0.01 for each). TBR was gradually increased (2.6 and 3.2, respectively, p < 0.01).
Conclusions: Many normal organs showed a decrease in uptake from 10 to 60 minutes after [18F]FAPI-74 administration, with the exception of the gallbladder, bile duct, and nipple, which showed an increase. In particular, the gallbladder and bile duct showed extremely high uptake in the 60-min images, which could be considered as a possible influence in the assessment of lesions in the surrounding area. Although 60-min scan may generally be the best due to the highest TBR, the appropriate imaging time may need to be determined by the surrounding organs of the tumor being evaluated. More cases should be analyzed to evaluate this hypothesis as the number of cases and subjects was limited.
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