This single-center, open-label, randomized phase II trial (JAGC-1) will evaluate whether adjuvant chemotherapy can be safely omitted in patients with stage IB-III gastric cancer (cT2-4a and/or N+) who have achieved a pathological complete response (pCR) or TRG 4-5 on the Mandard scale, following neoadjuvant chemotherapy and surgery. The study aims to compare disease-free survival and quality of life in patients receiving or not receiving adjuvant chemotherapy after neoadjuvant treatment and surgery.

Population: patients with localized or locally advanced gastric cancer ( cT2-4a and/or N+ according to TNM, 8th revision, 2017) after having undergone the full extent of the planned neoadjuvant component of perioperative chemotherapy, radical surgical intervention, with a histological tumor regression grade classified as pCR or TRG 4-5 according to the Mandard scale

Study design Patients will be randomized in a ratio of approximately 1:1:

Patients who achieve pCR will be randomly assigned to one of two groups:

• Group A (no adjuvant therapy): Patients will not receive adjuvant chemotherapy
• Group B (adjuvant therapy): Patients will receive the adjuvant component of perioperative chemotherapy

Patients who achieve a TRG of 4-5 on the Mandard scale will be randomly assigned to one of two groups:

• Group C (adjuvant therapy): Patients will not receive adjuvant chemotherapy.
• Group D (no adjuvant therapy): Patients will receive the adjuvant component of perioperative chemotherapy.

Patients in the groups with adjuvant therapy will receive treatment until progression or maximal effect of therapy is detected (the longest duration of treatment is 3 months).

Inclusion Criteria:

• - Obtaining informed consent to participate in the study.
• Morphologically confirmed gastric adenocarcinoma.
• The presence of localized or locally advanced gastric cancer (cT2-4a and/or N+)
• The full volume of the planned neoadjuvant component of perioperative chemotherapy was performed
• Radical surgical intervention was performed for the primary tumor of gastric cancer
• The histological degree of tumor regression after surgical resection was classified TRG1 and without metastasis to regional lymph nodes N0 or TRG 4-5 based on the Mandard tumor Regression scale.
• ECOG score 0 - I.
• Absence of severe uncontrolled concomitant chronic and acute diseases.
• Adequate liver and bone marrow function
• Neutrophils > 1.5*109/l
• Hemoglobin >85 g/l
• Platelets >100*109/l
• Increased transaminases and/or bilirubin < 2 art.

Exclusion Criteria:

• Lack of a neoadjuvant component of perioperative chemotherapy or insufficient number of courses
• The time after surgical treatment is more than 12 weeks
• Stage IV of the disease
• Her2-positive and/or MSI\dMMR stomach cancer
• Severe uncontrolled concomitant chronic or acute diseases
• The presence of a second malignant tumor (with the exception of previously cured malignant neoplasms)
• Any conditions that, in the opinion of the doctor, interfere with the examination procedures
• With incomplete tumor response to treatment