Neoadjuvant chemotherapy or chemoradiotherapy has become the standard neoadjuvant regimen for locally advanced G/GEJ cancer and has been recommended by a series of treatment guidelines. Although with clinical benefits of neoadjuvant chemotherapy or chemoradiotherapy, the pCR and long-term survival rates are still unsatisfactory and perioperative treatment mode for locally advanced G/GEJ cancer still needs further optimization. In this study, we will explore the efficacy and safety of chemotherapy combined with tislelizumab and LDRT in the neoadjuvant treatment for locally advanced G/GEJ cancer.

This is a prospective, multicenter, single-arm, phase Ib/II trial. In the phase Ib, 5 cases will be enrolled in each treatment group. In the phase II study, a total of 44 patients will be enrolled. Eligible patients will be registered and receive three cycles of SOX plus tislelizumab regimen. Simultaneously, LDRT will be planned and administered. Radical D2 gastric cancer resection will be performed 4-6 weeks after the last administration of chemotherapy plus tislelizumab. The primary endpoint of Phase Ib is to determine the optimal radiation dose for phase II study. The primary endpoint of phase II is the pathological complete response (pCR) rate. Secondary endpoints include R0 resection rate, major pathological response (MPR), objective response rate (ORR), 2-year disease-free survival (DFS) rate, 2-year overall survival (OS) rate and safety profile of the neoadjuvant regimen.

Inclusion Criteria:

1. Age 18-75 years.
2. Histologically or cytologically confirmed diagnosis of locally advanced G/GEJ adenocarcinoma (cT3-T4a, N+, M0) as assessed by exploratory laparoscopic surgery, ultrasonography and/or CT/MRI.
3. Resectable G/GEJ cancer, as judged by experienced surgeons.
4. Eastern Cooperative Oncology Group performance score (ECOG PS) ≤1.
5. Agree to provide blood, feces, and tissue specimens.
6. The expected survival is longer than 3 months.
7. There was no previous antitumor treatment (including chemotherapy, radiotherapy, targeted therapy, immunotherapy, interventional therapy, and other treatments with antitumor effects).
8. Adequate organ and hematological function.
9. Strict contraception.
10. Patients must be able to understand and be willing to sign the written informed consent form. A signed informed consent form must be appropriately obtained prior to the conduct of any trial-specific procedure.

Exclusion Criteria:

1. Undergoing other drug clinical trials or having participated in any drug clinical trials one month before enrollment.
2. Other tumors that have not been treated or exist at the same time, except carcinoma in situ of the cervix, treated basal cell carcinoma or superficial bladder tumor. If the tumor was cured and no evidence of disease was found for more than 3 years, the patient can be enrolled. All other tumors must be treated at least 3 years before enrollment.
3. Allergic to oxaliplatin, S-1, tislelizumab and similar drugs, or suspected allergies.
4. With serious gastrointestinal diseases that affect the absorption of oral chemotherapy drugs, such as chronic inflammatory bowel disease.
5. History of gastric perforation within 6 months.
6. Clinical manifestations include significant arrhythmia, myocardial ischemia, severe atrioventricular block, heart failure, and severe heart valve disease; Or those with severe lung function impairment; Or those with impaired blood system, liver and kidney function who cannot tolerate radiotherapy and chemotherapy; Or severe bone marrow failure; Or those with uncontrollable infections.
7. With uncontrollable mental illness.
8. Pregnant or lactating women.
9. Unable to comply with the research program or procedures.