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

Umut Elboga, Ertan Sahin, Tulay Kus, Yusuf Burak Cayirli, Gokmen Aktas, Merve Okuyan, Havva Yesil Cinkir, Fatih Teker, Ozlem Nuray Sever, Alper Aytekin, Latif Yılmaz, Aydın Aytekin, Ufuk Cimen, Vuslat Mumcu, Benan Kilbas, Kurtulus Eryilmaz, Davut Cakici, Yusuf Zeki Celen

Purpose: In this study, we aimed to investigate the utilization of ⁶⁸Ga-FAPI PET/CT in comparison to ¹⁸FDG 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 ⁶⁸Ga-FAPI PET/CT and ¹⁸FDG PET/CT imaging between September 2020 and June 2021 were included in this retrospective study. SUV_max values of ⁶⁸Ga-FAPI and ¹⁸FDG 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 SUV_max value of peritoneal metastases (p < 0.001) was significantly higher with ⁶⁸Ga-FAPI PET/CT compared to that with ¹⁸FDG 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 ¹⁸FDG PET/CT. Of the total lesions detected with ¹⁸FDG PET/CT, 5 of them were evaluated as benign lesions with ⁶⁸Ga-FAPI PET/CT also in accordance with the reference standard. In addition to 180 lesions detected with ¹⁸FDG PET/CT, a total of 37 additional malignant lesions, 12 of which were peritoneal metastases, were detected with ⁶⁸Ga-FAPI PET/CT.

Conclusion: ⁶⁸Ga-FAPI PET/CT was determined to be superior to ¹⁸FDG PET/CT in terms of detection of peritoneal involvement with high image quality as well as primary tumor and other metastatic foci. Consequently, ⁶⁸Ga-FAPI PET/CT can be used as a complementary imaging modality especially for inconclusive ¹⁸FDG findings due to the lack of accuracy of ¹⁸FDG PET/CT in some of the metastatic regions, especially in the liver.