摘要Objective:This study aims to verify the feasibility and efficacy of laparoscopic lower mediastinal lymphadenectomy for Siewert type Ⅱ/Ⅲ adenocarcinoma of esophagogastric junction(AEG).Setting:An exploratory,observational,prospective,cohort study will be carried out under the Idea,Development,Exploration,Assessment and Long-term Follow-up(IDEAL)framework(stage 2b).Participants:The study will recruit 1,036 patients with cases of locally advanced AEG(Siewert type Ⅱ/Ⅲ,clinical stage cT2-4aN0-3M0),and 518 will be assigned to either the laparoscopy group or the open group.Interventions:Patients will receive lower mediastinal lymphadenectomy along with either total or proximal gastrectomy.Primary and secondary outcome measures:The primary endpoint is the number of lower mediastinal lymph nodes retrieved,and the secondary endpoints are the surgical safety and prognosis,including intraoperative and postoperative lower-mediastinal-lymphadenectomy-related morbidity and mortality,rate of rehospitalization,R0 resection rate,3-year local recurrence rate,and 3-year overall survival.Conclusions:The study will provide data for the guidance and development of surgical treatment strategies for AEG.Trial registration number:The study has been registered in ClinicalTrials.gov(No.NCT04443478).