Comparation of Endoscopic Biopsy and Postoperative Specimen for the Molecular Diagnosis of Gastric Cancer

This study is comparing tumor tissue samples taken by endoscopy before surgery versus samples taken from the surgical specimen after stomach cancer surgery, to see whether both samples give the same molecular (genetic) information — which is important for selecting targeted therapies. **You may be eligible if...** - You are between 18 and 75 years old - You have been diagnosed with primary stomach adenocarcinoma (confirmed by biopsy within the past 12 months) - You are scheduled for stomach cancer surgery with intent to remove all cancer (R0 gastrectomy) **You may NOT be eligible if...** - You are pregnant or breastfeeding - You have previously received chemotherapy or radiation for this cancer - You have other serious conditions that prevent safe surgery Talk to your doctor to see if this trial is right for you.