Comparison of Ga-68-FAPI PET/CT and (18)FDG PET/CT Modalities in Gastrointestinal System Malignancies with Peritoneal Involvement

Purpose In this study, we aimed to investigate the utilization of Ga-68-FAPI PET/CT in comparison to (18)FDG PET/CT to evaluate the peritoneal involvement of the gastrointestinal malignancies alongside primary lesions and other metastatic foci. Procedures A total of 37 patients with various gastrointestinal malignancies with accompanying peritoneal involvement who underwent Ga-68-FAPI PET/CT and (18)FDG PET/CT imaging between September 2020 and June 2021 were included in this retrospective study. SUVmax values of Ga-68-FAPI and (18)FDG were compared according to lesion locations. Also, the lesion localization ability of both imaging was compared in patient basis. Results Of the 37 patients with peritoneal involvement (23 males and 14 females; median age, 62.8 +/- 12.7 years), 35.1% (n = 13) had colorectal cancer, 37.8% (n = 14) gastric cancer, and 27.0% (n = 10) pancreaticobiliary cancer. While 45.9% of them were operated, the remaining did not have surgery. The mean time interval between two studies was 3.2 days (range: 2-6 days). The mean SUVmax value of peritoneal metastases (p < 0.001) was significantly higher with Ga-68-FAPI PET/CT compared to that with (18)FDG PET/CT, as in primary lesions (p < 0.001), lymph node metastases (p = 0.006), liver metastases (p = 0.002), and bone metastases (p = 0.018). A total of 185 lesions was detected in the initial assessment with (18)FDG PET/CT. Of the total lesions detected with (18)FDG PET/CT, 5 of them were evaluated as benign lesions with Ga-68-FAPI PET/CT also in accordance with the reference standard. In addition to 180 lesions detected with (18)FDG PET/CT, a total of 37 additional malignant lesions, 12 of which were peritoneal metastases, were detected with Ga-68-FAPI PET/CT. Conclusion Ga-68-FAPI PET/CT was determined to be superior to (18)FDG PET/CT in terms of detection of peritoneal involvement with high image quality as well as primary tumor and other metastatic foci. Consequently, Ga-68-FAPI PET/CT can be used as a complementary imaging modality especially for inconclusive (18)FDG findings due to the lack of accuracy of (18)FDG PET/CT in some of the metastatic regions, especially in the liver.