Exploratory Study on the Application of Molecular Residual Disease (MRD) in Postoperative Treatment of Gastric Cancer

Inclusion Criteria: * Histologically confirmed adenocarcinoma, mucinous adenocarcinoma, or signet ring cell carcinoma of the stomach or gastroesophageal junction (GEJ). * Clinical stage cT4NanyM0, deemed suitable for radical resection. * Age between 18 and 75 years. * ECOG performance status of 0 or 1. * No prior neoadjuvant therapy and strong patient/family preference for primary surgery. * Adequate organ function defined as: Absolute neutrophil count (ANC) \> 1.5 x 10⁹/L (or \> 1500/µL)；Hemoglobin (Hb) \> 90 g/L；Platelet count (PLT) \> 100 x 10⁹/L (or \> 100,000/µL)；Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) \< 2.5 x Upper Limit of Normal (ULN)；Total bilirubin (TBIL) \< 1.5 x ULN；Serum creatinine (Cr) \< 1.0 x ULN * Good compliance and willingness to undergo long-term follow-up. * Provision of signed written informed consent. Exclusion Criteria: * Pathological specimens containing \>10% of other histological components (e.g., squamous cell carcinoma, neuroendocrine carcinoma, etc.). * Uncontrolled gastrointestinal obstruction or recurrent bleeding. * Inability to swallow oral medications. * Active infection requiring systemic therapy. * Moderate to severe cirrhosis due to any cause. * Cardiac function classified as NYHA class \> I. * History of myocardial infarction, unstable angina, stroke, or uncontrolled arrhythmia. * Anticipated inability to tolerate postoperative adjuvant chemotherapy. * Concurrent medical conditions that contraindicate chemotherapy. * Pregnancy or lactation; history of psychiatric illness; poor compliance.