The Therapeutic Efficacy of 18F-FDG Combined With 18F-FAPI PET/MR in Neoadjuvant Therapy for Gastric Cancer

Evaluation efficacy of tumor regression level

Gastric cancer is the fifth most common cancer worldwide and the third leading cause of
 cancer-related deaths. Although surgical treatment can benefit the survival of the vast
 majority of patients, currently only early gastric cancer patients can be cured directly
 through endoscopic resection or surgery alone. Neoadjuvant therapy reduces tumor volume and
 improves tumor response rate through preoperative radiotherapy and chemotherapy, thereby
 increasing R0 resection rate and improving overall survival, without increasing postoperative
 complications and mortality. Timely imaging re staging during neoadjuvant therapy can allow
 patients to enter the surgical stage earlier, thereby reducing their preoperative burden.
 According to the different stages of neoadjuvant therapy, clinical staging can be divided
 into baseline stage (cBSstage) and clinical rest stage (cReStage) after neoadjuvant therapy.
 
 At present, the conventional imaging methods for diagnosing cBStage in gastric cancer include
 CT, endoscopic ultrasonography (EUS), and MRI. The NCCN guidelines recommend CT for cBStage,
 with a diagnostic accuracy of 77.1% to 88.9%. Similarly, EUS and MRI were also used for
 cBStage, with accuracy rates of 65.0% to 92.1% and 71.4% to 82.6%, respectively. The
 application of diffusion-weighted imaging (DWI) has improved the accuracy of MRI diagnosis of
 cBStage to 93%.
 
 However, due to the destruction of the gastric wall structure by neoadjuvant therapy,
 accurate imaging re staging is difficult. Currently, accurate tumor regression grading can
 only be obtained through surgical resection of pathological specimens. For cReT after
 neoadjuvant therapy, the diagnostic accuracy of EUS is only 63% (T2: 44%, T3: 68%, T4: 90%).
 Due to the presence of chronic inflammatory reactions, such as tumor cell apoptosis,
 necrosis, fibrosis, etc., in both the tumor and the critical normal gastric wall after
 neoadjuvant therapy, imaging cannot accurately identify the level of gastric wall, leading to
 the current low value of CT for cReT. Meanwhile, due to the fact that the pathological
 reactions of lymph nodes after neoadjuvant therapy are mainly subacute inflammatory reactions
 accompanied by scar tissue formation, and not all lymph node volumes that experience these
 pathological reactions will rapidly decrease, the accuracy of CT diagnosis of cReN is only
 44%, while the sensitivity and specificity of EUS diagnosis of cReN are 50% and 56%,
 respectively.
 
 In addition, positron emission tomography (PET) can reflect the abnormal metabolism, protein
 synthesis, DNA repair, and cell proliferation of tumors at the molecular level, providing
 important information in tumor grading diagnosis, prognosis evaluation, treatment
 decision-making, and efficacy monitoring. The conventional positron tracer 18F-FDG can
 reflect the glucose metabolism ability of different tissues, while most types of malignant
 tumors exhibit high metabolism. Therefore, 18F-FDG can be used for the diagnosis, staging,
 and treatment monitoring of cancer. However, in gastric cancer patients, 18F-FDG has certain
 limitations, including 1) interference with physiological or inflammatory uptake of the
 gastric wall; 2) Low uptake of 18F-FDG is present in signet ring cell carcinoma, mucinous
 adenocarcinoma, or other poorly differentiated cancers with high mucus content; 3) There are
 cases of false positive FDG after immunotherapy. In the study of SUV changes in the tumor
 area before and after treatment, it was found that patients with postoperative pathological
 regression grades 1-5 Δ SUVs are between 0-70%.
 
 Tumor associated fibroblasts are closely related to tumor growth, invasion, and distant
 metastasis, and their activation requires the involvement of fibroblast activation protein
 (FAP). Therefore, radiolabeled fibroblast activation protein inhibitor (FAPI) can achieve in
 vivo FAP targeted tracing and quantification by specifically binding to FAP. Currently, a
 large number of studies have shown that 18F-FAPI is superior to 18F-FDG in the staging and re
 staging of gastric cancer. Furthermore, prospective studies have shown a certain relationship
 between tumor regression grade (TRG) and 18F-FAPI rate of change parameters (SUVmax, SUVavg,
 SUVR).
 
 Therefore, in the early stage of this study, 18F-FAPI combined with 18F-FDG PET/MRI imaging
 was used to evaluate the efficacy of neoadjuvant therapy for gastric cancer, preoperative
 assessment of tumor regression grade after treatment, and re staging to guide the development
 of further clinical treatment plans.