This study aims to investigate the efficacy and safety of neoadjuvant cadonilimab in combination with perioperative SOX chemotherapy, compared to perioperative SOX chemotherapy alone, in patients with diffuse or mixed-type locally advanced gastric or gastroesophageal junction adenocarcinoma. The main questions it seeks to answer are: 1. Is neoadjuvant cadonilimab plus SOX chemotherapy superior to neoadjuvant placebo plus SOX chemotherapy in terms of the pathological complete response (pCR) rate at the time of surgery? 2. To evaluate and compare the 3-year OS rate in patients receiving neoadjuvant cadonilimab plus SOX chemotherapy versus patients receiving placebo plus neoadjuvant SOX chemotherapy regimen. Participants will be divided into two groups: 1. Experimental group: Participants will receive intravenous cadonilimab (10 mg/kg) in combination with the SOX regimen (oxaliplatin 130 mg/m² and S-1, with the initial dose determined based on body surface area). 2. Control group: Participants will receive a placebo in combination with the SOX regimen. After completing 3-4 cycles of treatment, patients in both the experimental and control groups will undergo radical surgery with D2 or D2+ lymphadenectomy. Following surgery, patients will receive 4 cycles of adjuvant SOX chemotherapy at 70% of the standard dosage, administered every 21 days, starting within 3-6 weeks post-surgery.
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Pathological complete response (pCR) rate
Timeframe: From enrollment to surgical treatment, it is expected to require 4 months.
3-year survival rate (3-year OS rate)
Timeframe: 3 years