This is a single-arm, interventional study to explore the feasibility of circulating tumor DNA (ctDNA)-MRD testing to guide postoperative adjuvant treatment strategies in patients with stage II-III gastric cancer.

Inclusion Criteria:

1. Patients who have undergone D2 gastrectomy, achieved R0 resection, and are pathologically diagnosed with stage II-III gastric cancer.
2. No preoperative neoadjuvant or adjuvant therapy received.
3. Voluntary participation with signed informed consent, demonstrating good compliance and willingness to cooperate with follow-up procedures.
4. Age between 18-75 years, with no gender restrictions.
5. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.
6. Estimated survival of 6 months or more.
7. Baseline hematological and biochemical parameters within specified limits: a. Hemoglobin ≥ 80g/L. b. Absolute neutrophil count ≥ 1.5×10^9/L. c. Platelet count ≥ 100×10^9/L. d. Aspartate or alanine aminotransferase ≤ 2.5 times the upper limit of normal. e. Alkaline phosphatase ≤ 2.5 times the upper limit of normal. f. Thyroid-stimulating hormone (TSH) ≤ 1 times the upper limit of normal (if abnormal, T3 and T4 levels must be normal).

Exclusion Criteria:

1. Pregnant or lactating women, or women of childbearing potential with a positive pregnancy test at baseline.
2. Evidence of postoperative recurrence or metastasis.
3. Previous anti-tumor therapy, including chemotherapy, radiotherapy, or immunotherapy.
4. Positive resection margins identified in postoperative pathology.
5. History of other malignancies within the past five years, except certain skin cancers, superficial bladder cancer, in situ cervical or breast cancer.
6. Uncontrolled pleural effusion, pericardial effusion, or ascites.
7. Severe cardiovascular conditions such as symptomatic coronary artery disease, congestive heart failure (grade ≥ II), uncontrolled arrhythmias, or myocardial infarction within the last 12 months.
8. Uncontrolled infection, severe kidney disease, or other significant concurrent diseases.
9. Allergic reactions to study drugs.
10. Receipt of any anti-tumor therapy within 4 weeks prior to enrollment.
11. Administration of live attenuated vaccines within 4 weeks prior to the first study treatment dose or plans to receive such vaccines during the study period.
12. Positive HIV antibody, active hepatitis B or C (with specific viral load criteria).
13. Positive for COVID-19 nucleic acid or antigen test.
14. Other conditions deemed by the investigator to potentially affect patient safety or trial compliance, including serious illnesses requiring treatment, severe laboratory abnormalities, or other social or family reasons.