Adenocarcinoma - GEJ

This phase II study evaluates dostarlimab followed by a watch-and-wait strategy in patients with localized dMMR/MSI-H gastric or gastroesophageal junction (GEJ) adenocarcinoma. The study aims to determine whether surgery can be safely avoided in patients achieving a complete response, defined by negative endoscopy and tumor-free biopsies after treatment.

The goal of the study is: * The collection of various tissue samples (blood, biopsies and "esophageal brushes") and their analysis. * To test a risk model based on genetic analyses (DNA-FISH and so-called single cell sequencing) on esophageal tissue samples. * Evaluating the quality of life of Barrett's Esophagus patients and the degree of fear of getting cancer. Patients with a Barrett's Esophagus can participate in the study if they are minimally 18 years old, are capable of giving informed consent (fully understanding what the study entails before giving consent to participate), have Barrett Esophagus and are referred to one of the participating centers due to suspicion of early esophageal cancer, for which the participant will be evaluated by endoscopic imaging and biopsy. Study procedures: An intake consultation will be planned, wherein the eligibility criteria will be assessed, and participant characteristics will be collected. A routine gastroscopy will be planned twice during which several minimally-invasive interventions will be performed: drawing a blood sample, brush cytology during the endoscopy (a brush is used to obtain cells from the surface of the esophagus) and obtaining biopsy samples (small pieces of tissue). Each participant will need to undergo all the interventions. Patients will have to complete questionnaires at several time points to assess their quality of life (EQ-5D-DL questionnaire) and fear of cancer recurrence (Cancer Worry Scale). This study is a randomized trial, meaning the study participants will be divided into two groups by the computer. One group will be informed of their risk profile, established based on the genetic analyses. The other group will not be informed of their risk profile. All patients will be followed-up in a more intensive surveillance schedule compared to the standard of care, for study purposes.