A Randomized, Controlled, Open-Label, Multicenter Clinical Study Comparing the Efficacy of Close Follow-Up versus Adjuvant Chemotherapy in Patients with Stage IB-IIA Gastric or Gastroesophageal Junction Adenocarcinoma Following Radical Surgery

INTERVENTION: High‐Risk Close Follow‐Up Group:Only standard postoperative follow‐up is accepted.;Low‐risk control group:None;High‐Risk Adjuvant Chemotherapy Group:Take Tegafur capsules orally. Dosage: Body surface area < 1.25 m2: 40 mg twice daily; 1.25 m2 <= body surface area < 1.5 m2: 50 mg twice daily; body surface area = 1.5 m2: 60 mg twice daily. Administer continuously for 14 days, followed by a 7‐day break; this constitutes one treatment cycle. A total of 8 cycles are required.; CONDITION: Stage IB‐IIA adenocarcinoma of the stomach or gastroesophageal junction PRIMARY OUTCOME: Differences in DFS Between the Close Follow‐Up Group and the Adjuvant Chemotherapy Group in a High‐Risk Randomized Cohort; SECONDARY OUTCOME: Overall Survival (OS) in the Close Follow‐Up Group vs. the Adjuvant Chemotherapy Group in a High‐Risk Randomized Cohort;Safety endpoints in the adjuvant chemotherapy group of the high‐risk randomized cohort, including AEs, treatment‐related AEs, Grade >=3 AEs, SAEs, AEs leading to dose modification or discontinuation, and treatment‐related deaths;Comparison of DFS and OS Between the Low‐Risk Registration Follow‐up Group and the High‐Risk Adjuvant Chemotherapy Group;Comparison of DFS and OS Between the Low‐Risk Registration Follow‐up Group and the High‐Risk Close Follow‐up Group;Recurrence Patterns in High‐Risk Randomized Cohorts (Local Recurrence, Regional Recurrence, Peritoneal Recurrence, Distant Metastasis);Treatment completion rate, relative dose intensity, and proportion of dose delays/reductions in the high‐risk adjuvant chemotherapy group;Description of DFS and OS in the low‐risk registration and follow‐up cohort, and evaluation of the prognostic discriminatory ability of the risk stratification model; INCLUSION CRITERIA: 1. Age >= 18 years, any gender; 2. Diagnosed with gastric adenocarcinoma or gastroesophageal junction adenocarcinoma confirmed by pathology, with ECOG score <= 2; 3. TNM stage of gastric adenocarcinoma or gastroesophageal junction adenocarcinoma is IB‐IIA; 4. Liver resection has been completed; 5. Participants must meet the following lab criteria: (1) Absolute neutrophil count (ANC) >= 1.5×10^9/L without using granulocyte colony‐stimulating factor in the past 14 days; (2) Platelets >= 100×10^9/L without a blood transfusion in the past 14 days; (3) Hemoglobin > 9g/dL without transfusion or erythropoietin use in the past 14 days; (4) Total bilirubin <= 1.5× upper limit of normal (ULN); (5) Aspartate transaminase (AST) and alanine transaminase (ALT) <= 2.5×ULN; (6) Serum creatinine <= 1.5×ULN and creatinine clearance (calculated by Cockcroft‐Gault formula) >= 60 mL/min; (7) Good coagulation function, defined as international norm