This study evaluates the safety and efficacy of endoscopic cryoballoon ablation treatment (ECAT) combined with a PD-1 inhibitor (sintilimab) as maintenance therapy in patients with advanced gastric cancer, and further explores the underlying immunoregulatory mechanisms.

This study is a single-center, randomized, controlled, open-label, prospective clinical trial. A total of 42 patients with advanced gastric cancer will be enrolled. Among them, 30 patients who achieve a partial response (PR) after receiving standard of care (SOC) treatment - such as FOLFOX, POLF, FLOT, SOX, CAPOX, or FOLFIRI - will be randomly assigned to two groups. Twenty patients will receive local gastric endoscopic cryoballoon ablation treatment (ECAT) combined with sintilimab, while the remaining ten patients will receive sintilimab monotherapy. Both groups will subsequently undergo standard maintenance therapy, followed by assessments of treatment efficacy, safety, and immunological evaluations of both peripheral blood and tumor tissues. Patients whose disease status is assessed as stable disease (SD) or progressive disease (PD) after SOC treatment will proceed to receive standard second-line therapies.

Inclusion Criteria:

1. Patients diagnosed with advanced gastric cancer (stage IV according to the AJCC 8th edition, including locally advanced unresectable gastric cancer [stage IVa] and gastric cancer with distant metastasis [stage IVb]; Borrmann classification types I, II, or III) who are deemed ineligible for surgical resection or unable to tolerate surgery, and meet the indications for first-line therapy with sintilimab combined with fluoropyrimidine- and platinum-based chemotherapy;
2. Age ≥18 years;
3. WHO pathological types: adenocarcinoma or neuroendocrine tumors;
4. Physician-assessed estimated life expectancy greater than 3 months, with distant metastases considered controllable;
5. Maximum diameter of the primary tumor ≤6 cm;
6. Adequate major organ function, defined as:① Liver function: ALT and AST ≤ 2.5 times the positive range, total bilirubin ≤ 2.0mg/dL; ② Renal function: creatinine clearance ≥ 40mL/min calculated using the EPI formula;
7. Hematological criteria: hemoglobin (Hgb) ≥70 g/L; absolute neutrophil count (ANC) ≥1.5×10⁹/L; platelet count (PLT) ≥80×10⁹/L;
8. Coagulation function: prothrombin time (PT) and activated partial thromboplastin time (APTT) both <2 times the normal value;
9. Negative pregnancy test for women of childbearing potential;
10. Eastern Cooperative Oncology Group (ECOG) performance status ≤2;
11. Signed informed consent form.

Exclusion Criteria:

1. Pregnant or breastfeeding women, or women planning to become pregnant within six months;
2. Patients with infectious diseases (such as HIV, syphilis, or active tuberculosis);
3. Patients with active hepatitis B or hepatitis C infection;
4. Patients with other concurrent primary malignancies;
5. Patients who have participated in another clinical trial within the past month;
6. Patients who have taken antiplatelet or anticoagulant medications within the past week;
7. Patients with gastric cancer complicated by active bleeding;
8. Patients with massive ascites (ascites volume ≥3000 mL);
9. Patients with cardia obstruction or pyloric obstruction;
10. Patients with active infections or autoimmune diseases;
11. Patients deemed unsuitable for treatment by the investigator for any other reason.