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

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.