Tislelizumab Combined With Neoadjuvant Chemotherapy Used in the Perioperative Treatment.

Inclusion Criteria:

* The patient voluntarily joined this study and signed an informed consent form;
* Age ≥ 18 years old, ≤ 75 years old
* Pathological diagnosis of gastric adenocarcinoma or adenocarcinoma of the gastroesophageal junction
* Patients must be able to provide fresh slices of tumor tissue (FFPE tissue blocks or approximately 15 slides), unstained FFPE slides, and if clinically feasible, fresh biopsy samples will be preferred. If archived samples cannot be obtained, fresh tumor biopsy specimens must be collected during the baseline period, with the same requirements for glass slides as archived tumor tissue
* Clinical staging determined by CT and laparoscopy for curative resection is cT3\~4aN+M0 or cT4bNanyM0 for gastric and gastroesophageal junction adenocarcinoma patients (according to AJCC 8th edition staging)
* Have not received anti-tumor treatment (such as surgery, radiotherapy, chemotherapy, targeted therapy, immunotherapy, etc.)
* Plan to undergo surgical treatment after the completion of neoadjuvant therapy
* Able to swallow pills normally
* ECOG score 0-1 points
* Expected survival time ≥ 12 months
* Normal function of major organs, i.e. meeting the following criteria:The standard for blood routine examination must comply with: (No blood transfusion or blood products within 14 days, no correction using G-CSF or other hematopoietic stimulating factors)，Neutrophil absolute count ≥ 1.5 × 109/L; Platelets ≥ 80 × 109/L; Hemoglobin ≥ 80g/L
* Biochemical examination must meet the following standards: Total bilirubin\<1.5

  * ULN; ALT and AST ≤ 2.5 × ULN; Serum Cr ≤ 1.5 x ULN or endogenous creatinine clearance rate\>50ml/min (male: endogenous creatinine clearance rate=(140 age) x body weight)/(72 x serum Cr); Female: Endogenous creatinine clearance rate=(140 age) x body weight/(72 x serum Cr) x 0.85; Weight unit: kg; Serum Cr unit: mg/mL
* Female subjects with pregnancy ability must undergo a serum pregnancy test within 7 days before the first medication, and the result is negative. They are willing to use efficient contraception methods during the trial period and 120 days after the last dose. For male subjects whose partners are women of childbearing age, surgical sterilization or agreement to use efficient methods of contraception during the trial period and 120 days after the last dose should be considered

Exclusion Criteria:

* There are non resectable factors, including tumor reasons that cannot be resected or surgical contraindications that cannot be resected or those who refuse surgery
* Previously or currently suffering from other malignant tumors
* Suffering from any chronic or major illness that is considered intolerable to treatment (such as severe heart disease, uncontrolled hypertension, certain degrees of liver and kidney dysfunction, etc.)
* Those who have experienced gastrointestinal perforation, abdominal abscess in the past, or recent (within 3 months) intestinal obstruction or imaging and clinical symptoms indicating accompanying intestinal obstruction
* If there are significant clinically significant bleeding symptoms or clear bleeding tendencies, such as gastrointestinal bleeding, hemorrhagic gastric ulcer, or vasculitis, within the first 3 months prior to the first use of the study drug, and if fecal occult blood is positive during the baseline period, a follow-up examination can be conducted. If the results are still positive after the follow- up examination, a gastroscopy examination is required (excluding those who have undergone gastroscopy examination within 3 months prior to enrollment to exclude such situations)
* If the patient is in the active infection stage and needs treatment (such as using antibiotics, antiviral drugs, or antifungal drugs)
* Active hepatitis (hepatitis B reference: HBsAg positive and HBV DNA ≥ 500 IU/ml; hepatitis C reference: HCV antibody positive and HCV copy number\>upper limit of normal value)
* Patients with congenital or acquired immune dysfunction (such as HIV infected individuals)
* Patients with any active autoimmune diseases or a history of autoimmune diseases with the possibility of recurrence
* Planned or previously received organ or allogeneic bone marrow transplantation
* Subjects who currently have interstitial pneumonia or interstitial lung disease, or have a history of requiring hormone therapy for interstitial pneumonia or interstitial lung disease, or those whose screening period CT shows active pneumonia or severe lung function impairment; Active pulmonary tuberculosis
* Current or recent use of immunosuppressive drugs or systemic corticosteroids for the purpose of achieving immunosuppression
* Patients who have received attenuated live vaccines within 28 days prior to the first use of the study drug, or who need to receive such vaccines during treatment or within 60 days after the last dose
* Known to be allergic to any investigational drug or excipient
* Lactating women
* Any factors determined by the researcher that may affect patient safety and result in the forced termination of this study midway