The purpose of this single center prospective study is to assess the value of FAPI PET imaging in predicting early postoperative recurrence of malignant tumor of digestive system and to evaluate the diagnostic performance of FAPI PET/CT in the detection of primary tumor and metastatic lesions.

Malignant tumor of digestive system is one of the most common cancer types worldwide with high cancer-related mortalities. FDG PET/CT is an essential imaging modality in the tumor treatment and management. However, FDG PET/CT has certain limitations. For example, the detection sensitivity and tracer uptake in signet ring cell carcinoma (SRCC) is low. Furthermore, the physiological uptake of the gastrointestinal tract and acute gastroenteritis may mask the detection of lesion in the abdomen and pelvis. Therefore, a novel molecular imaging tracer is needed for the accurate evaluation of malignant tumor of digestive system.

Cancer-associated fibroblasts (CAFs) specifically express fibroblast activation protein (FAP), which is highly expressed by CAFs in more than 90% of human epithelial cancers. FAP-specific small molecule inhibitors (FAPIs) show much advantages over FDG, including high tumor accumulation, low background tissue uptake, and rapid clearance in vivo.

The purpose of this single center prospective study is to assess the value of FAPI PET/CT imaging in predicting early postoperative recurrence of malignant tumor of digestive system and to evaluate the diagnostic performance of FAPI PET/CT in the detection of primary tumor and metastatic lesions.

In this study, qualitative and quantitative analyses will be used. The diagnostic criteria of FAPI PET/CT: if the FAPI uptake of lesion is higher than that of the surrounding background tissue, it is determined as a positive lesion; if the FAPI uptake of the lesion is the same as or lower than that of the surrounding background tissue, it is determined as a negative lesion. Pathological result is used as the gold standard for diagnosis. The diagnostic performance of FAPI PET/CT including sensitivity, specificity, positive predictive value, negative predictive value, and accuracy will be calculated.

The quantitative indicators include the following metabolic parameters: maximum standard uptake value (SUVmax), mean standard uptake value (SUVmean), tumor metabolic volume (MTV), total tumor glycolysis (TLG), tumor-to-liver ratio, and tumor-to-background tissue ratio.

Inclusion Criteria:

1. Voluntarily participate and sign the informed consent.
2. Patients with suspected or new diagnosed gastrointestinal cancer or other malignant tumor of digestive system.
3. Complete clinical information, biochemical examination results, CT and/or MRI imaging examination, pathological information, and clinical/imaging follow-up data.

Exclusion Criteria:

1. Female patient who plan to become pregnant within 6 months, or pregnant or breast-feeding women.
2. Allergic constitution.
3. Those who are in poor general condition and cannot tolerate PET/CT examination.
4. The researchers believe that he or she is not suitable to participate in this study.

Patients with suspected or new diagnosed gastrointestinal cancer or other malignant tumor of digestive system.

The individual participant data will not be shared with other researchers. Our research results will eventually be shared with other researchers in the form of paper.