Short Course Radiotherapy With Sequential Disitamab Vedotin Combined With S-1 and Sintilimab as Whole Course Neoadjuvant Therapy for HER2 Expressed Locally Progressive Gastric Cancer

This trial is testing a combination of four treatments given before surgery — short-course radiotherapy, disitamab vedotin (an antibody-drug conjugate targeting HER2), the chemotherapy drug S-1, and sintilimab (an immunotherapy) — in people with locally advanced HER2-expressing stomach cancer. **You may be eligible if...** - You are between 18 and 75 years old - You have locally advanced gastric or gastroesophageal junction cancer confirmed by tissue testing, with stage cT3-4N+M0 (cancer has not spread distantly) - Your tumor tests positive for HER2 expression (IHC 1+, 2+, or 3+) - You have not received any prior treatment for this cancer - You have a good performance status (ECOG 0-1) and adequate organ function - Your tumor is considered resectable (surgeons believe it can be removed) **You may NOT be eligible if...** - Your cancer has spread to distant organs - You have already received any cancer treatment (chemotherapy, radiation, immunotherapy) - You are pregnant or breastfeeding - You have significant organ dysfunction Talk to your doctor to see if this trial is right for you.