This trial is a prospective, monocentric, non-therapeutic, interventional cohort study aiming to decipher the immune TME through standard neoadjuvant CT in resectable G/GEJ adenocarcinomas. This study will also longitudinally monitor MRD during neoadjuvant and adjuvant therapy.

Inclusion Criteria:

• Male or female patient > 18 years of age on day of signing informed consent.
• Histologically proven non-metastatic resectable gastric and gastroesophageal junction adenocarcinoma, Stage IB to III to be treated with standard neoadjuvant treatment.
• Surgery of primary tumor to be done at Centre Léon Bérard.
• Availability of archival FFPE tumor block from initial diagnosis with at least 20% of tumor cells.
• Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) 0, 1 or 2.
• Patient should understand, sign, and date the written voluntary informed consent form at the screening visit prior to any protocol-specific procedures performed. Patient should be able and willing to comply with study visits and procedures as per protocol.
• Patients must be covered by a medical insurance.

Exclusion Criteria:

• Any condition contraindicated with blood sampling procedures required by the protocol.
• Known additional malignancy that is progressing or requires active treatment. Exceptions include adequately treated basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or in situ cervical cancer
• Any psychological, familial, geographic or social situation, according to the judgment of investigator, potentially preventing the provision of informed consent or compliance to study procedure.
• Pregnant or breast-feeding woman.

Adult patients with localised non-metastatic operable gastric and oeasogatric junction adenocarcinoma, stage IB à III, treated with standard neoadjuvant therapy and by surgery.