dMMR/MSI-H is a key molecular subtype of gastric cancer, found in 8-22% of cases. It is typically associated with older age, female sex, distal tumor location, and intestinal histology (Lauren classification). While this subtype predicts better survival in locally advanced disease, its prognostic role in metastatic settings is less clear.

Notably, dMMR/MSI-H tumors are often resistant to conventional chemotherapy. Conversely, they demonstrate exceptional sensitivity to immunotherapy. This has led to effective strategies using immune checkpoint inhibitors, either alone or combined with chemotherapy, in both neoadjuvant and advanced disease settings.

However, key challenges remain. Prospective data are largely from Western populations, leaving the efficacy in Asian patients-who bear a high disease burden-less defined. Furthermore, about half of dMMR/MSI-H patients exhibit primary or acquired resistance to immunotherapy. A deeper understanding of the tumor-immune dynamics during treatment is crucial to uncover resistance mechanisms and improve patient outcomes.

Inclusion Criteria:

• Male or female, aged 18 to 85 years.
• Histologically confirmed gastric cancer or adenocarcinoma of the esophagogastric junction (only Siewert types II and III are included).
• dMMR status confirmed by immunohistochemistry (IHC) or MSI-H status confirmed by PCR/NGS.
• Tumor clinical staging meeting the following criteria:

cT≥2, any N, M0, assessed by the investigator as potentially resectable and planned for preoperative treatment followed by surgery.

• Willing to receive treatment with immune checkpoint inhibitors (including, but not limited to, various PD-1 inhibitors, PD-L1 inhibitors, CTLA-4 inhibitors, PD-1/CTLA-4 bispecific antibodies, etc.), which may be combined with or without standard chemotherapy regimens for gastric cancer.

Exclusion Criteria:

• Tumor histology other than adenocarcinoma, such as squamous cell carcinoma, neuroendocrine carcinoma, etc.
• Presence of central nervous system metastases and/or leptomeningeal carcinomatosis.
• Prior antitumor therapy directed at the current gastric cancer (excluding palliative gastrointestinal bypass surgery performed to relieve obstructive symptoms).

Informed consent forms did not include provisions for public data sharing