Radical Surgery Versus Non-Surgery in Gastric Cancer Patients With Peritoneal Metastasis After Successful Conversion Therapy

Inclusion Criteria:

* Patients voluntarily participate in the study, sign the informed consent form, and demonstrate good compliance.
* Age ≥ 18 years and ≤ 70 years.
* Histologically and/or cytologically confirmed gastric cancer.
* No evidence of gastric outlet obstruction or intestinal obstruction.
* No distant metastasis on imaging examination. Initial laparoscopic exploration confirms CY0P1 or CY1P1, without distant organ metastasis, with a recorded peritoneal carcinomatosis index (PCI) ≤ 20, and with either positive or negative ascites. Patients meeting these criteria are eligible for pre-enrollment. Following successful conversion therapy, a second laparoscopic exploration must confirm conversion to CY0P0, with no distant organ metastases and no regional lymph node metastases beyond the second station. Only upon meeting these criteria are patients formally enrolled in the study.
* No prior chemotherapy, targeted therapy, or immunotherapy.
* ECOG performance status score of 0-2.
* Life expectancy ≥ 12 weeks.
* Laboratory requirements: Absolute neutrophil count (ANC) ≥ 1.5 × 10⁹/L, platelet count ≥ 75 × 10⁹/L, hemoglobin ≥ 8 g/dL; Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 × upper limit of normal (ULN); total bilirubin ≤ 1.5 × ULN. In the presence of liver metastases, AST and ALT ≤ 5 × ULN; Serum creatinine (Cr) ≤ 1.5 × ULN, or creatinine clearance (CCr) ≥ 60 mL/min; International normalized ratio (INR) ≤ 1.5 × ULN; prothrombin time (PT) and activated partial thromboplastin time (APTT) ≤ 1.5 × ULN.

Exclusion Criteria:

* Initial laparoscopic exploration reveals P0, or either of the two laparoscopic explorations reveals distant organ metastasis or distant lymph node metastasis.
* Prior anticancer therapy, including chemotherapy, targeted therapy, or immunotherapy.
* Hypertension that cannot be adequately controlled with antihypertensive medication (systolic blood pressure ≥ 140 mmHg or diastolic blood pressure ≥ 90 mmHg).
* Active bleeding or coagulation disorder within 2 months prior to enrollment, bleeding tendency, or ongoing thrombolytic therapy, deemed by the investigator as unsuitable for enrollment.
* Active cardiac disease within 6 months prior to treatment, including myocardial infarction, severe/unstable angina pectoris. Left ventricular ejection fraction (LVEF) \< 50% on echocardiography, or poorly controlled arrhythmia.
* Known allergy to immunotherapy agents, chemotherapy agents, or any of their excipients.
* Pregnancy (positive pregnancy test prior to treatment initiation) or breastfeeding.
* Active or uncontrolled severe infection: Severe infection within 4 weeks prior to enrollment (e.g., requiring intravenous antibiotics, antifungals, or antivirals); Known human immunodeficiency virus (HIV) infection; History of clinically significant liver disease, including viral hepatitis. For known hepatitis B virus (HBV) carriers, active HBV infection must be excluded, defined as HBV DNA positive (\> 1 × 10⁴ copies/mL or \> 2000 IU/mL); Known hepatitis C virus (HCV) infection with positive HCV RNA (\> 1 × 10³ copies/mL), or other hepatitis, cirrhosis.
* Urinalysis indicating proteinuria ≥ 2+, with 24-hour urine protein quantification \> 1.0 g.
* Any other condition deemed by the investigator as inappropriate for participation in this study.