Comparison of 68Ga-FAPI PET/CT and 18F-FDG PET/CT in Gastric Cancer: Impact on Metastatic Detection and Stage Migration

OBJECTIVES: To compare the diagnostic performance of ¹⁸F-FDG PET/CT and ⁶⁸Ga-FAPI PET/CT in patients with gastric cancer, with particular emphasis on metastatic detection and stage migration. METHODS: Fifty-two patients with histopathologically confirmed gastric cancer who underwent both ¹⁸F-FDG PET/CT and ⁶⁸Ga-FAPI PET/CT were retrospectively analyzed. Primary tumor uptake, metastatic involvement, and tumor staging were evaluated for each modality. Detection rates for lymph node, peritoneal, liver, and bone metastases were compared. Diagnostic performance and stage migration between modalities were assessed. Quantitative parameters including SUVmax and tumor-to-background ratio were analyzed. RESULTS: ⁶⁸Ga-FAPI PET/CT demonstrated significantly higher uptake in primary tumors compared with ¹⁸F-FDG PET/CT (p < 0.001). Peritoneal metastases were detected in 30.8% (16/52) of patients on ⁶⁸Ga-FAPI PET/CT compared with 13.5% (7/52) on ¹⁸F-FDG PET/CT (p = 0.004). Nodal metastases were also identified more frequently with ⁶⁸Ga-FAPI PET/CT. Both modalities showed high diagnostic performance for liver metastases, while ⁶⁸Ga-FAPI PET/CT demonstrated slightly higher sensitivity for bone lesions and ¹⁸F-FDG PET/CT showed slightly higher specificity. Overall stage migration between the two modalities occurred in 25.0% (13/52) of patients, predominantly due to upstaging associated with newly detected metastatic disease. In subgroup analysis, ¹⁸F-FDG uptake tended to be lower in signet-ring cell carcinoma, whereas FAPI uptake appeared less influenced by histological subtype. CONCLUSION: ⁶⁸Ga-FAPI PET/CT improves detection of metastatic disease, particularly peritoneal involvement, compared with ¹⁸F-FDG PET/CT in patients with gastric cancer and may contribute to more accurate disease staging. ADVANCES IN KNOWLEDGE: This study highlights the added value of 68Ga-FAPI PET/CT in identifying metastatic disease and altering clinical staging, particularly through improved detection of peritoneal involvement, which may impact treatment decision-making.