Cancer associated fibroblasts (CAFs) can promote tumor cell proliferation, migration, invasion, and angiogenesis through immunosuppressive effects and the production of mediators, thereby promoting tumor growth and progression. The characteristic of CAFs is high expression of fibroblast activation protein (FAP). In approximately 90% of epithelial derived tumors, FAP is highly overexpressed on the membrane of CAFs. Contrary to CAFs, FAP expression is lower or absent in normal tissues. Therefore, FAP inhibitors (FAPI) targeting FAP can overcome the limitations of 18F-2-fluoro.2-deoxy-D-glucose fluorodeoxyglucose(18F-FDG) PET imaging. But like 18F-FDG, wound healing, fibrosis, and inflammation can also uptake FAPI.. Therefore, a comparison of the performance of 18F-FDG and 18F-FAPI PET imaging in diagnosing primary and metastatic lesions of various types of cancer is conducted to evaluate the potential value of these new radiopharmaceuticals as effective alternatives to 18F-FDG, highlighting their advantages and limitations.

This study is a prospective, single study and has been approved by the ethics committee. The subjects of this studywere from January 1,2024 to December 31, 2029.The detailed description is as follows:

1. Patients: The subjects we selected are adults who are not restricted by gender. For details, please refer to the "Eligibility Criteria" column.
2. Clinical data collection: Record the course of disease, laboratory tests (tumor markers, inflammatory markers), PET examinations, and follow-up related imaging (ultrasound, CT, or magnetic resonance imaging（MRI）) information of all patients.
3. CT or MRI image analysis: Record the location and number of lesions, measure the long diameter of the primary and metastatic lesions, and the short diameter of lymph nodes on CT or MRI images.
4. PET image analysis: Record and evaluate the following indicators: the maximum, mean and peak standardized uptakevalue (SUVmax, SUVmean and SUVpeak), Metabolic lesion volume (MLV), total lesion glycolysis (TLG).
5. Statistical analysis: Use descriptive statistical methods to compare the age of patients and the standardized uptake values of FDG and FAPI. Normal distribution data is represented as mean ± standard deviation, while non normal distribution data is represented as median with IQR. Compare the normal distribution data between two groups using paired two sample t-test, and compare the non normal distribution data between two groups using McNemar test. Using a four grid table McNemar χ Compare the diagnostic efficacy of 18F-FDG PET and 18F-FAPI PET through 2 tests, calculate and compare the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of 18F-FDG and 18F-FAPI PET. Due to the potential impact of tumor type on diagnosis, subgroup analysis was conducted on the diagnostic efficacy of 18F-FDG PET and 18F-FAPI PET. Due to the possibility of multiple metastatic lesions in a given participant, the diagnostic results may be correlated within the participant. Therefore, sensitivity analysis of metastatic lesions is also performed based on a generalized linear mixed effects model by combining this correlation between different lesions within the same participant. Double tailed P<0.05 indicates a statistically significant difference.

Inclusion Criteria:

1. Adult patients (≥ 18 years old);
2. Suspected or newly diagnosed malignant tumor patients;
3. Patients who underwent 18F-FDG and 18F-FAPI PET scans within one week;
4. Patients who are able to provide informed consent forms (signed by participants, parents, or legal representatives) and consent forms in accordance with the guidelines of the Clinical Research Ethics Committee.

Exclusion Criteria:

1. Pregnant women;
2. Scanned images with poor image quality due to factors such as body movement;
3. Patients with mental illness or other unsuitable conditions for examination, poor compliance, etc.

The subjects we selected are adults who are not restricted by gender. For details, please refer to the "EligibilityCriteria" colymn.