Nivolumab Combined With SOX Used in the Perioperative Treatment

This study is testing whether adding nivolumab (an immunotherapy drug) to a standard chemotherapy regimen (SOX: S-1 plus oxaliplatin) given before and after surgery can improve outcomes for people with stomach or gastroesophageal junction cancer that is surgically removable. **You may be eligible if...** - You are between 18 and 75 years old - You have been diagnosed with stomach cancer or cancer at the junction of the stomach and esophagus, confirmed by biopsy - Your cancer is at a locally advanced stage (T3–T4 or involves lymph nodes) but has not spread to distant organs - You have not received any prior cancer treatment - Your general health is good (ECOG score 0–1) - You are able to swallow tablets - Your blood, liver, and kidney functions are within acceptable ranges **You may NOT be eligible if...** - Your cancer has spread to other parts of the body - You have had previous surgery, radiation, or chemotherapy for this cancer - You have serious autoimmune conditions or active infections - Your organ function does not meet required thresholds Talk to your doctor to see if this trial is right for you.