Tumor Microenvironment in Gastric Cancer Patients Treated With Combined Immunotherapy and Chemotherapy

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.