To evaluate the potential usefulness of 18F-AlF-FAPI positron emission tomography/computed tomography (PET/CT) for the diagnosis of primary and metastatic lesions in gastrointestinal tumors, and compared with 18F-FDG PET/CT.

Subjects with gastrointestinal tumors underwent contemporaneous 18F-AlF-FAPI and 18F-FDG PET/CT either for an initial assessment or for recurrence detection. Tumor uptake was quantified by the maximum standard uptake value (SUVmax). The numbers of positive tumor lesions of 18F-FDG and 18F-AlF-FAPI PET/CT were recorded by visual interpretation. The sensitivity, specificity, and accuracy of 18F-AlF-FAPI, 18F-FDG PET/CT were calculated and compared to evaluate the diagnostic accuracy.

Inclusion Criteria:

• adult patients (aged 18 years or order);
• patients with suspected or newly diagnosed or previously treated gastrointestinal tumors (supporting evidence may include MRI, CT, tumor markers and pathology report);
• patients who had scheduled both 18F-FDG and 18F-AlF-FAPI PET/CT scans;
• patients who were able to provide informed consent (signed by participant, parent or legal representative) and assent according to the guidelines of the Clinical Research Ethics Committee.

Exclusion Criteria:

• patients with non-malignant lesions;
• patients with pregnancy;
• the inability or unwillingness of the research participant, parent or legal representative to provide written informed consent.