The goal of this study is to analyze the tumor microenvironment (TME) in gastric cancer patients treated with combined immunotherapy and chemotherapy. The main questions it aims to answer are:

• Provide profiles of TME between pre-treatment and post-treatment to gain insights into the mechanisms of immunotherapy combined with chemotherapy in advanced gastric cancer
• Investigate the crucial factors affecting treatment efficacy by comparing gastric cancer patients with varying treatment responses

Inclusion Criteria:

• Non-bedridden, aged 18 to 70 years old;
• Eastern Cooperative Oncology Group (ECOG) score is 0 to 1;
• Histologically confirmed gastric adenocarcinoma with evaluable lesions based on RECIST 1.1;
• Advanced gastric cancer;
• Ready for receiving S-1 plus oxaliplatin chemotherapy and either nivolumab or sintilimab

Exclusion Criteria:

• History of other malignant diseases in the last 5 years;
• Prior chemotherapy, radiotherapy or immunotherapy;
• Not receiving S-1 plus oxaliplatin chemotherapy and either nivolumab or sintilimab.

Advanced gastric cancer patients receiving S-1 plus oxaliplatin chemotherapy and either nivolumab or sintilimab