Comparison of ⁶⁸Ga-FAPI and ¹⁸F-FDG uptake in gastric, duodenal, and colorectal cancers

Background: Accurate clinical staging is crucial to managing gastrointestinal cancer. But ¹⁸F-fluorodoxyglucose (FDG) PET/CT has limitations. Targeting fibroblast-activation-protein (FAP) is a newer diagnostic approach for the visualization of tumor stroma, and ⁶⁸Ga-labeled FAP-inhibitors (⁶⁸Ga-FAPI) present a promising alternative to ¹⁸F-FDG.

Purpose: To evaluate the diagnostic efficacy of ⁶⁸Ga-FAPI PET/CT in the primary and metastatic lesions of gastrointestinal malignancies, compared with ¹⁸F-FDG PET/CT.Materials and Methods: Participants with gastric, duodenal and colorectal cancers, who received contemporaneous ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT between October 2019 through June 2020, were retrospectively analyzed. ¹⁸F-FDG and ⁶⁸Ga-FAPI uptakes were compared using the Wilcoxon signed-rank test. McNemar’s test was used to compare the diagnostic performance between the two scans.

Results: 35 participants (median, 64 years; interquartile range (IQR), 53-68 years; 18 men) were evaluated. In treatment-naïve participants (n = 19), ⁶⁸Ga-FAPI PET/CT upstaged the clinical TNM (tumor-node-metastasis) staging in 4 (21%) participants, compared with ¹⁸F-FDG PET/CT. ⁶⁸Ga-FAPI PET/CT showed a higher tracer uptake in primary lesions (gastric cancer, 12.7 vs. 3.7, P = 0.003; colorectal cancer, 15.9 vs. 7.9, P = 0.03), involved lymph nodes (6.7 vs. 2.4, P < 0.001), bone and visceral metastases (liver-mets: 9.7 vs. 5.2, P < 0.001; peritoneal-mets: 8.4 vs. 3.6, P < 0.001; bone-mets: 4.3 vs. 2.2, P < 0.001; lung-mets: 4.4 vs. 1.9, P = 0.01) than ¹⁸F-FDG PET/CT, with higher sensitivities in the detection of primary tumors (100% [19/19] vs. 53% [10/19], P = 0.004), lymph nodes (79% [22/28] vs. 54% [15/28], P < 0.001), bone and visceral metastases (89% [31/35] vs. 57% [20/35], P < 0.001).

Conclusions: ⁶⁸Ga-FAPI PET/CT was superior to ¹⁸F-FDG PET/CT in the detection of primary and metastatic lesions in gastric, duodenal, and colorectal cancers, with higher tracer uptakes of most primary and metastatic lesions.