Impact of 68Ga-FAPi PET/CT on Staging or Restaging Digestive System Tumors in Patients with Negative or Equivocal 18F-FDG PET/CT Findings

Objectives: This study aimed to evaluate the potential efficacy of 68 Ga-fibroblast activation protein inhibitor (FAPi) positron emission tomography/ computed tomography (PET/CT) for detecting, staging, and restaging digestive system malignancies that are 18 F-fluorodeoxyglucose ( 18 F-FDG) negative or show equivocal 18 F-FDG uptake.Methods: We conducted a prospective analysis of 30 patients with pathologically confirmed primary tumors or metastases of the digestive system.Participants underwent 68 Ga-FAPi PET/CT and 18 F-FDG PET/CT imaging for staging or restaging purposes within the same week.The efficacy of 68 Ga-FAPi PET/CT was assessed by comparing its ability to detect lesions and influence disease staging with that of 18 F-FDG PET/CT.Results: 68 Ga-FAPi PET/CT imaging was performed in 30 patients with 18 F-FDG-negative or indeterminate lesions.Of the 30 patients, 23 had gastric cancer and 7 had colorectal cancer.Among all patients, histopathological diagnosis of signet ring cell carcinoma was present in 15 (50%) patients.Primary tumor or local recurrence was detected in 19 (63%) patients, lymph node metastasis in 8 (27%) patients, visceral metastasis in 4 (13%) patients, peritoneal metastasis in 14 (47%) patients, and bone metastasis in 3 (10%) patients on 68 Ga-FAPi PET/CT images.All patients underwent histopathological confirmation on 68 Ga-FAPi PET/CT images.The disease stage was upgraded in 20 patients (67%) after 68 Ga-FAPi PET/CT imaging.Of the 20 patients, 12 had no evidence of recurrence or metastasis on 18 F-FDG PET/CT.Conclusion: Based on our study, 68 Ga-FAPi PET/CT alters the disease stage in the majority of gastrointestinal malignancies with negative or equivocal 18 F-FDG PET/CT findings. 68Ga-FAPi PET/CT appears to be effective in both staging and restaging of gastrointestinal malignancies, such as signet-ring cell carcinomas of the stomach that frequently show low 18 F-FDG -avidity.