Evaluation of Efficacy and Safety De-escalation Versus Standard Adjuvant Chemotherapy in Patients With Low Risk Localized Gastroesophageal Adenocarcinoma

The ATTENUATION study targets patients with gastric adenocarcinoma (GA), esophageal adenocarcinoma (EAC) or gastro-esophageal junction (GEJ) who have received 4 cycles of FLOT chemotherapy before the surgery. Standard post-operative management consists of chemotherapy with 4 cycles of FLOT. However, the nature and duration of postoperative treatment are standardized and are not adapted to the specific tumor response of each patient. All patients are therefore referred to the same treatment regimen. As a result, good responders (defined in particular by wide resection of the tumor and a good response to preoperative chemotherapy on the tumor removed during surgery) may be over-treated and exposed to unnecessary adverse events. Only 50-60% of patients can start chemotherapy post-operatively, due to the potential residual adverse effects associated with surgery in particular. Thus, it would appear that preoperative chemotherapy is the most important factor in the overall efficacy of the treatment sequence. Moreover, numerous retrospective studies have reported a favorable outcome in patients with a major response to pre-operative treatment but who were unable to receive post-operative chemotherapy. The hypothesis of this study is that surveillance after surgery in patients with gastric or gastroesophageal junction tumors, with a good response to preoperative chemotherapy could provide significant clinical benefit and favorable disease progression. Participants will: * be distributed in one of the two arms * will be followed up every 3 months for 2 years, then every 6 months (clinical examination, imaging, quality-of-life questionnaire) subsequent years until 3 years after the randomization of the last patient. * followed up until their death or their progression whether local, regional or metastatic.