Study of Comparing of With and Without Sequential Therapy of S-1

This trial is testing whether adding oral S-1 chemotherapy (a fluorouracil-based drug) as a follow-up treatment after surgery helps prevent stomach or gastroesophageal junction cancer from coming back in people with stage III disease. **You may be eligible if...** - You have been diagnosed with stomach cancer or gastroesophageal junction cancer (confirmed by tissue biopsy) - You had surgery with full removal of the tumor and complete lymph node dissection, with final pathology confirming stage III - Imaging after surgery shows no signs of recurrence or spread - You are between 18 and 75 years old and in reasonably good health (ECOG score 0–1) **You may NOT be eligible if...** - You have stage I, II, or IV disease - Your surgery did not achieve clear margins or adequate lymph node removal - You are in poor overall health or have significant organ dysfunction - Imaging shows signs of cancer recurrence after surgery Talk to your doctor to see if this trial is right for you.