A Prospective, Phase II Clinical Study of HIPEC Combined With NIPS and Tislelizumab Conversion Therapy for Gastric Cancer With Peritoneal Metastasis With Positive Cytology Alone or PCI Score ≤10

Inclusion Criteria: 1. Treatment-naïve patients who have not received chemotherapy, radiotherapy, or any other antitumor therapy prior to the start of the clinical trial; 2. Age between 18 and 75 years; 3. Male or non-pregnant, non-lactating female; 4. Gastric or gastroesophageal junction adenocarcinoma confirmed by gastroscopy and pathological diagnosis; 5. HER-2 negative by immunohistochemistry (IHC), and PD-L1 CPS ≥1; 6. Laparoscopic exploration confirming either positive cytology alone (P0CY1) or peritoneal metastasis (PCI score ≤10); 7. No other distant metastases; 8. Hematological criteria: white blood cell count ≥3.5×10⁹/L, neutrophils ≥1.5×10⁹/L, platelet count ≥100×10⁹/L, hemoglobin ≥90 g/L; 9. Biochemical criteria: ALT ≤2.5×ULN, AST ≤2.5×ULN, total bilirubin ≤1.5×ULN, serum creatinine ≤1.5×ULN; 10. Left ventricular ejection fraction ≥50%; 11. ECOG performance status 0-1; 12. Ability to comply with the study protocol and voluntarily provide signed informed consent. Exclusion Criteria: 1. Inability to comply with the study protocol or procedures; 2. Known HER2-positive status; 3. Known diagnosis of squamous cell carcinoma, undifferentiated carcinoma, or other histological types of gastric cancer, or adenocarcinoma mixed with other histological types; 4. Current conditions or diseases affecting drug absorption; 5. Patients preoperatively confirmed as unsuitable for conversion therapy; 6. Severe cardiovascular diseases, such as uncontrolled heart failure, coronary artery disease, arrhythmia, or uncontrolled hypertension; 7. Symptomatic active central nervous system metastases (e.g., clinical symptoms, cerebral edema, spinal cord compression, carcinomatous meningitis, leptomeningeal disease, and/or progressive growth); 8. Known allergy to the investigational drug(s); 9. Prior treatment with anti-PD-1/PD-L1 antibodies, anti-PD-L2 antibodies, anti-CD137 antibodies, anti-CTLA-4 antibodies, or other drugs/antibodies targeting T-cell co-stimulation or checkpoint pathways; 10. Clinically uncontrolled active infections, such as acute pneumonia, active hepatitis B or C (HBV DNA ≥1×10⁴ copies/mL or \>2000 IU/mL despite prior antiviral therapy); Known primary immunodeficiency or active tuberculosis; History of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation; Known history of human immunodeficiency virus (HIV) infection (HIV antibody positive); 11. Significant malnutrition (weight loss ≥5% within 1 month or \>15% within 3 months prior to informed consent, or food intake reduced by ≥50% within 1 week), unless corrected for ≥4 weeks before the first dose of investigational drug; 12. History of other primary malignancies, except: * Malignancies in complete remission for at least 2 years prior to enrollment with no required treatment during the study; * Adequately treated non-melanoma skin cancer or malignant lentigo with no evidence of recurrence; * Adequately treated carcinoma in situ with no evidence of recurrence; 13. Female patients who are pregnant or breastfeeding; 14. Any concomitant illness that, in the investigator's judgment, seriously endangers patient safety or affects study completion; 15. Patients deemed ineligible for the study by the investigator.