RÉSUMÉ
Objective:To investigate the clinical utility of 68Ga-labeled fibroblast activation protein inhibitor (FAPI) PET/CT in the detection of primary and metastatic gastric signet-ring cell carcinoma (GSRCC) and compared the results with those of 18F-FDG PET/CT. Methods:A total of 21 patients (10 males, 11 females, average age 52 years) with primary and metastatic GSRCC who underwent 68Ga-FAPI and 18F-FDG PET/CT at the First Affiliated Hospital of Xiamen University from June 2020 to May 2022 were retrospectively analyzed. Pathological results of surgery and (or) biopsy were used as the " gold standard" for final diagnosis. In cases whose surgery or tissue biopsies were not available, clinical and radiographic follow-up results were used as the reference standards. Wilcoxon signed-rank test was used to compare the SUV max of 18F-FDG and 68Ga-FAPI. McNemar χ2 test was used to compare the detection rate between 18F-FDG and 68Ga-FAPI PET/CT. Results:68Ga-FAPI PET/CT showed higher SUV max than 18F-FDG in primary tumors (5.3(2.4, 15.7) vs 2.4(1.8, 2.5); z=2.31, P=0.021), local recurrences (7.8(6.0, 8.9) vs 2.4(1.9, 3.4); z=2.20, P=0.028), lymph nodes metastases (7.7(4.5, 12.2) vs 2.4(1.9, 3.6); z=6.01, P<0.001) and bone/visceral metastases (6.7(5.3, 11.1) vs 2.4(2.0, 3.4); z=11.36, P<0.001). Regarding diagnostic accuracy, 68Ga-FAPI PET/CT showed higher sensitivities than 18F-FDG for primary tumors (7/9 vs 2/9; χ2=3.20, P=0.063) and local recurrences (7/7 vs 2/7; χ2=3.20, P=0.063). It also demonstrated higher lesion detection rates than 18F-FDG for suspicious lymph node metastases (86%(65/76) vs 32%(24/76); χ2=31.37, P<0.001) and bone/visceral metastases (99%(184/185) vs 39%(73/185); χ2=107.08, P<0.001). Conclusions:68Ga-FAPI PET/CT showed higher tumor uptake and lesion detection rate than 18F-FDG in the primary and metastatic GSRCC. 68Ga-FAPI PET/CT demonstrates good diagnostic performance for tumor detection, staging, and restaging of GSRCC, which is helpful to further guide clinical treatment strategy.