• Age: ≥18 years

• Histologically confirmed gastric or gastro-oesophageal junctional (GOJ) adenocarcinoma (inclusive of Siewert-stein classification type I-III (62))

• MMRd/MSI-H. There are three different methods validated for detection (63) :

‣ Immunohistochemistry (IHC) staining for expression of MMR proteins (MLH1, MSH2, PMS2 and MSH6), MMRd defined as loss of function or one or more of these proteins.

⁃ Polymerase chain reaction (PCR) amplification of microsatellite sequences

⁃ Next-generation sequencing (NGS) for detection of MSI

⁃ Stage II-IIIB: TNM T2-T4, N0-N3, M0

• Absence of distant metastatic disease on CT scan + PET CT + staging laparoscopy prior to study entry.

• MDT determined suitable for surgery and MDT believes an R0 resection is achievable after neo-adjuvant therapy (resectable disease)

• No prior anti-cancer therapy for gastric / GOJ adenocarcinoma

• ECOG performance status 0-2

∙ Laboratory parameters

∙ • Adequate haematologic and end-organ function defined by the following laboratory test results: Haematology: Absolute neutrophil count \> 1.5 x 109/L Platelets \> 100 x 109/L Haemoglobin \> 90 x 109/L (can be post-transfusion) Biochemistry: Serum Creatinine Clearance \>50ml/min (calculated using Cockcroft-Gault formula Appendix X)

∙ Liver function: Bilirubin within normal limits ALT/AST ≤2.5x ULN

∙ Coagulation profile (for patients not receiving therapeutic anticoagulation):

∙ International Normalised Ratio (INR) \< 1.5 Activated Prothrombin Time (APTT) \< 1.5xULN

• Before patient registration/randomisation, written informed consent must be given according to ICH/GCP, and national/local regulations

• Patient is fit to undergo all protocol investigations and receive all protocol treatment based on the assessment in the surgical / oncology clinic

• Signed and dated informed consent document indicating that the patient (or legally acceptable representative) has been informed of all the pertinent aspects of the trial prior to enrolment

• Willingness and ability to comply with the protocol for the duration of the study including scheduled visits, examinations, investigations and treatment plans