ctDNA-MRD Monitoring After Neoadjuvant Therapy in Gastric Cancer With pCR or MPR

This prospective observational study will enroll adults with gastric or gastroesophageal junction adenocarcinoma who achieve pathological complete response or major pathological response after neoadjuvant therapy and R0 gastrectomy. The study will evaluate whether postoperative circulating tumor DNA-based molecular residual disease status can identify participants at increased risk of recurrence despite a favorable pathological response. All postoperative treatment decisions will be made by the treating clinicians according to routine care and will not be assigned by the study. Tumor-informed ctDNA-MRD testing will be performed longitudinally, and results will not be used to alter treatment. Participants will be followed for disease-free survival, overall survival, recurrence detection, and adverse events.

Inclusion Criteria: * Age 18 to 75 years. * Histologically confirmed gastric or gastroesophageal junction adenocarcinoma. * Completion of preoperative neoadjuvant therapy followed by R0 gastrectomy. * Postoperative pathological complete response, defined as Becker tumor regression grade 1a or Ryan tumor regression grade 0, or major pathological response, defined as Becker tumor regression grade 1b or Ryan tumor regression grade 1 with 10 percent or less residual viable tumor. * Willingness to undergo serial blood collection and follow-up and provision of written informed consent. * Eastern Cooperative Oncology Group performance status of 0 or 1 with adequate major organ function. * Adequate diagnostic biopsy or preoperative tumor tissue for 1021-gene panel testing. Exclusion Criteria: * Postoperative pathological assessment showing neither pathological complete response nor major pathological response, including more than 10 percent residual viable tumor or Becker tumor regression grade 2 to 3. * Distant metastatic disease identified intraoperatively or postoperatively. * Another malignant tumor within the previous 5 years. * Severe postoperative complications preventing any postoperative antitumor therapy. * Contraindication to immunotherapy, including active autoimmune disease.