Efficacy and Safety of Anti-PD-1, Thymalfasin, and SOX in Neoadjuvant Treatment of cStage III Gastric/Gastroesophageal Junction Adenocarcinoma

This trial tests whether adding immunotherapy (an anti-PD-1 drug) and thymalfasin (an immune-boosting agent) to standard SOX chemotherapy before surgery improves outcomes for people with Stage III stomach or gastroesophageal junction cancer. **You may be eligible if...** - You are between 18 and 75 years old - You have been diagnosed with Stage III gastric (stomach) or gastroesophageal junction (GEJ) adenocarcinoma confirmed by endoscopy and biopsy - Your cancer is HER2-negative and is considered surgically removable with curative intent - Your cancer has been assessed by CT scan and meets Stage III criteria - You are in good general health and can tolerate surgery **You may NOT be eligible if...** - You have had another cancer in the past 5 years (with some exceptions for early-stage skin or in-situ cancers) - You have had a blood transfusion within a certain time period before starting - You have had or are planning an organ or bone marrow transplant - You have uncontrolled infections or bleeding issues Talk to your doctor to see if this trial is right for you.