This study is a single-center, prospective, phase II clinical trial aimed at assessing the impact of HIPEC combined with systemic chemotherapy and immune checkpoint inhibitors on the R0 resection rate in patients with peritoneal metastasis from gastric cancer. Furthermore, it seeks to analyze the effects of this treatment strategy on overall survival (OS), progression-free survival (PFS), PCI and adverse reaction rates.

Gastric cancer with peritoneal metastasis represents an advanced stage of the disease, with a very poor prognosis for patients. It is crucial to develop more effective treatment options to improve their prognosis. CRS with HIPEC has shown promise in extending survival for these patients. Currently, a PCI value ≥12 is widely recognized as significantly impacting the prognosis of gastric cancer peritoneal metastasis.

Recent clinical trials have resulted in the global endorsement of immune checkpoint inhibitors as a third-line therapy for gastric cancer. In March 2020, the National Medical Products Administration (NMPA) granted approval for nivolumab to be used in advanced or recurrent patients with gastric or esophagogastric junction adenocarcinoma who have received two or more systemic treatments.

However, there remains a lack of widely accepted effective treatment regimens for patients with peritoneal metastasis of gastric cancer. The CSCO recommends that such patients be referred for standard advanced gastric cancer treatments or consider participation in clinical trials. Consequently, our center has developed a comprehensive treatment protocol involving systemic chemotherapy (SOX), immune checkpoint inhibitors (Tislelizumab) and HIPEC, aiming to investigate the safety of this regimen and further enhance the prognosis of patients with peritoneal metastasis from gastric cancer.

Inclusion Criteria:

1. 18-75 years
2. Men, or non-pregnant and non-lactating women
3. Pathological diagnosis as gastric malignant tumor
4. diagnosis of peritoneal metastasis [peritoneal cancer index (PCI) ≤ 12 points] by laparoscopic exploration
5. Normal major organ function:

1) HB ≥ 90 g/L； ANC ≥ 1.5×109/L; plt ≥125×109/L; 2) TBIL<1.5ULN; ALT, AST <2.5ULN; Cr≤1.25ULN; ALB ≥ 30 g/L 6. Not receive radiation therapy, chemotherapy, targeted therapy, or immunotherapy.

7. Eastern Cooperative Oncology Group Performance Status (ECOG): 0-1 8. Patients are volunteers, have signed informed consent, and agree to cooperate with investigators in data collection.

Exclusion Criteria:

1. Patients have other malignant tumors within the past 5 years.
2. Patients use immunosuppressant within 30 days prior to the initial use of tirelizumab, excluding nasal sprays and physiological doses of inhaled corticosteroids or systemic steroids from consideration.
3. Patients have a documented history of ongoing autoimmune conditions or prior autoimmune diseases.
4. Patients are allergic to oxaliplatin, S-1, other related chemotherapy drugs or immune checkpoint inhibitors.
5. Patients with epilepsy or other mental disorders.
6. Patients who are unable to undergo surgery due to severe heart, lung and vascular diseases.
7. Pregnant or lactating women.
8. Patients with other distant metastasis