S-1/Oxaliplatin for High Risk Patients Who Underwent Gastrectomy

Inclusion Criteria: * Patients who voluntarily provide written informed consent prior to entering into this study * Newly definitely diagnosed with primary gastric or gastroesophageal junction adenocarcinoma histologically * Patients who underwent radical resection with wide lymph node dissection * TNM stage (6th AJCC) of IIIB or IV on post-operative staging * Patients who can be randomized within 6 weeks after surgery Exclusion Criteria: * Aged \< 20 years or ≥ 76 years * Eastern Cooperative Oncology Group (ECOG) performance status \>2 * Patients who underwent surgery for neoplasm in stomach in the past * History of malignant disease The following cases can be included in this study - adequately treated basal cell or squamous cell skin cancer, cervical carcinoma in situ * Gastric or gastroesophageal junction adenocarcinoma with distant metastasis (M1) including distant lymph node (behind the pancreas, along the aorta, portal vein, behind the peritoneum, mesenteric lymph node) * Residual cancer on post-operative staging (R1 and R2 resection) * Patients who received adjuvant chemotherapy, or neoadjuvant chemotherapy and/or radiotherapy and/or immunotherapy in the past for treatment of gastric cancer * Any of the following within 6 months prior to the study recruitment: Myocardial infarction, severe/unstable angina, coronary/peripheral artery bypass, NYHA class III or IV congestive heart failure, stroke or transient ischemic attack, serious cardiac arrhythmia requiring treatment * Patients of childbearing potential who do not agree to use generally accepted effective method of birth control during the study treatment period and for at least 6 months after the end of study treatment * Pregnant women, breastfeeding women, or women of childbearing potential whose pregnancy test result is positive• * History of hypersensitivity to the investigational products (S1 and Oxaliplatin)