Clinical Application of Fibroblast Activation Protein PET/MRI for Diagnosis and Staging

Positron emission tomography (PET) molecular imaging provides a valuable tool for the diagnosis and differential diagnosis, staging of various tumors. The most commonly used imaging agent is [F-18] fluorodeoxyglucose (18F-FDG), known as the "molecule of the century". However, in some low-grade gliomas, mucinous adenocarcinoma, bronchoalveolar carcinoma, primary hepatocellular carcinoma, renal clear cell carcinoma and some prostate cancers, factors such as the low expression level of tumor glucose transporter but high level of dephosphorylation, and the low number of tumor cells in tumor tissues can also be manifested as low absorption of 18F-FDG; in addition, 18F-FDG PET has limited ability to detect small lesions in some organs such as brain, liver, and kidneys that have physiological uptake or excretion of FDG with the relatively high background signal; moreover, the distribution of FDG in the body is easily affected by blood sugar. These factors limit the application value of 18F-FDG PET/CT in the differential diagnosis and staging of some malignant tumors.

A malignant tumor is composed of tumor cells and tumor stroma, which occupies the vast majority of the tumor. Cancer-associated fibroblasts (CAF) are an important part of the tumor stroma. Fibroblast activation protein (FAP) is over-expressed in CAF, which is closely related to tumor growth, invasion, metastasis, immunosuppression and prognosis; and the expression level of FAP in normal tissues and organs is very low, so it becomes an excellent target for cancer diagnosis and treatment. The use of radionuclide-labeled fibroblast activation protein inhibitors (FAPI) that specifically bind to FAP as a tracer for PET imaging can be applied for targeted diagnosis and treatment of cancer. Recently, some studies have found that gallium-68 (68Ga) -FAPI as a new novel positron tracer has shown to be with good application potential. The probe has very low background uptake in different types of cancer, so it can obtain high image contrast and clear tumor boundary. And it has good stability in serum and can be quickly removed from normal organs in vivo. In this project, we plan to apply the integrated PET / MR imaging of fibroblast activating protein (FAP) in the diagnosis and staging of malignant tumors, and compare it with 18F-FDG PET / CT imaging, so as to make up for the deficiency in FDG PET imaging in the diagnosis and staging of some tumors.