DEWI: Watch-and-Wait With Dostarlimab in Localized dMMR/MSI-H Gastric/GEJ Adenocarcinoma Phase II Study

Inclusion and exclusion criteria are identical in both cohorts, except for inclusion criteria 3 and 4. Inclusion criteria The patient is eligible to be included in the study only if all the following criteria apply: 1. Is capable of giving signed and dated informed consent, 2. Has an ECOG PS of 0-1, 3. Age ≥18 years old: * A patient over 70 years of age is eligible to participate in the main initial cohort if the patient respects all the criteria and has a score G8 of ≥14. * A patient over 70 years with a score G8 \<14 should have a consultation with an onco-geriatrician to determine the best treatment or therapeutic strategy based on age and co-morbidity. After this consultation, the patient is eligible for main initial cohort if "fits for surgery" with no contraindications to repeated UGI endoscopy with biopsies (no change to the initial protocol) and can follow the study plan and procedures outlined in the study. * If a patient is over 70-year-old, has a G8 score \<14, and is unfit for surgery (as determined by onco-geriatric advice and multidisciplinary team \[MDT\] conclusion), the patient cannot be included in the phase II study (main initial cohort). This patient can be included in the unfit supplemental cohort, with the available Geriatric Core Dataset (G-CODE) test result being mandatory for inclusion (No switching between cohorts is allowed). 4. Has histologically proven non-metastatic gastric or OGJ adenocarcinoma cT2 to T4, Nx, M0 after computed tomography thorax-abdomen-pelvis (TAP-CT) and echo-endoscopy (EUS), performed within 6 weeks before inclusion, according to the 7th Edition of the International Union Against Cancer; NB1: Echo-endoscopy will be performed only if the tumor is not obstructive at UGI endoscopy ± a new UGI endoscopy with 10 biopsies, photos (if not done at the first UGI endoscopy done for diagnosis). If obstructive, the tumor will be classified as cT3 or cT4 (in the situation when the tumor was obstructive and prevented EUS, it was classified T3N+, if it did not invade the adjacent organs on CT scan, because obstructing tumors represented locally advanced disease in the vast majority of cases in previous studies). In this case a new UGI endoscopy must be done with 10 biopsies, photos (if not done at the first GGI endoscopy done for diagnosis). NB2: Echo endoscopy is not mandatory/performed for patient included in additional unfit cohort. TNM stage will be determine by CT scanner. 5. Has no peritoneal carcinomatosis (optional coelioscopy; recommended in case of doubt/ suspicious on CT/ imaging), 6. Has not received prior therapy (chemotherapy, radiotherapy, or immunotherapy) for localized gastric or OGJ adenocarcinoma, 7. Tumor status confirmed to be dMMR/MSI-H as follows: \- MMR protein expression status will be evaluated by immunohistochemistry (IHC) with four antibodies (anti-hMLH1, anti-hMSH2, anti-hMSH6, anti-hPMS2) according to the local procedures. dMMR will be defined as loss of MLSH1 and PMS2, loss of MSH2 and MSH6, or loss of only one protein with presence of MSI-H. MSI analysis will be performed by polymerase chain reaction \[PCR\] using a pentaplex panel (BAT-25, BAT-26, NR-21, NR-24, and NR-27; PROMEGA). MSI-H is defined as instability in two or more of the five studied markers. For this study, samples with 2 unstable markers will also undergo MMR analysis by IHC. Agreement of Sponsor (GERCOR) on a dMMR/MSI status is mandatory to include the patient (the patient's file \[an anonymized mail\] must be sent to Sponsor). Approval/refusal email for inclusion of the patient will be sent by the Sponsor within 24 hours of receipt of the Investigator email. In case of discrepancy between IHC and PCR, the final decision about the dMMR/MSI status will be taken by GERCOR or coordinating investigator, 8. Has hematological status: absolute neutrophil count (ANC) ≥1.5 x 109/L; platelets ≥100 x 109/L; hemoglobin ≥9 g/dL, 9. Has adequate renal function: serum creatinine level ≤150 µM and clearance ≥30 ml/min (Modification of the Diet in Renal Disease \[MDRD\] or Cockcroft and Gault), 10. Has adequate liver function: ≤1.5 x upper limit of normal (ULN) of direct bilirubin ≤ ULN for participants with total bilirubin levels \>1.5 x ULN (inclusion possible if known Gilbert syndrome), alkaline phosphatase \<5 x ULN, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) ≤2.5 x ULN, 11. Has international normalized ratio (INR), prothrombin time (PT), and activated partial thromboplastin time (aPTT) ≤1.5 x ULN, except for the patient on anticoagulant therapy who must have PT-INR-aPTT within therapeutic range is deemed appropriate by the Investigator, 12. Has radiological tumor assessment at screening performed within 6 weeks before inclusion according to RECIST version 1.1 by chest, abdomen, and pelvis CT, showing the absence of metastatic or non-surgical disease, 13. A female participant is eligible to participate if she is not pregnant or breastfeeding, and one of the following conditions applies: * Is a woman of non-childbearing potential as defined: i/ ≥ 45 years of age and has not had menses for \>1 year, ii/ Amenorrheic for \<2 years without a hysterectomy and oophorectomy and have a follicle stimulating hormone (FSH) value in the postmenopausal range upon pre-study (screening) evaluation, iii/ post-hysterectomy, post-bilateral oophorectomy, or post-tubal ligation documented hysterectomy or oophorectomy must be confirmed with medical records of the actual procedure or confirmed by an ultrasound, magnetic resonance imaging (MRI), or CT scan. Tubal ligation must be confirmed with medical records of the actual procedure, otherwise the patient must fulfill the criteria in Inclusion criteria 15. Information must be captured appropriately within the site's source documents, * Has negative pregnancy blood test within 72 hours before the first dose of dostarlimab, AND * If woman of childbearing potential (WOCBP), femal