This study aims to evaluate the efficacy and safety of HIPEC combined with NIPS and tislelizumab conversion therapy for gastric/gastroesophageal junction cancer with positive cytology alone (CY1P0) or a Peritoneal Carcinomatosis Index (PCI) ≤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.