This is a single-center, open-label, Phase Ib/II study aiming to assess the perioperative safety and postoperative outcomes of a novel surgical technique in treating primary adenocarcinoma located in the upper 1/3 of the stomach or gastroesophageal junction (Siewert II or III).

The study will enroll 30 patients who will undergo totally laparoscopic proximal gastrectomy with esophagogastrostomy by fissure technique. Clinical data will be collected to evaluate perioperative safety. Patients will be followed for at least 3 months, during which endoscopy will be performed to analyze occurrences and reasons for anastomotic-related complications. Additionally, the quality of life after surgery will be evaluated by QLQ-C30 and QLQ-STO22.

Inclusion Criteria:

1. Age between 18 and 75 years old;
2. Pathologically confirmed as adenocarcinoma;
3. Primary tumor located in the upper 1/3 of the stomach or the gastroesophageal junction (Siewert II or III);
4. If it is adenocarcinoma of the upper 1/3 of the stomach, cT1N0M0 should be met.
5. For gastroesophageal junction adenocarcinoma, cT1-2N0M0 should be met, and clinical judgment should indicate no distant lymph node metastasis around the stomach.
6. Bilateral resection margins should be greater than 2 cm, and more than half of the residual stomach should be preserved.
7. No history of upper abdominal surgery (excluding laparoscopic cholecystectomy).
8. No preoperative comprehensive treatments such as chemotherapy, radiotherapy, targeted therapy, immunotherapy, etc.
9. Preoperative ECOG (Eastern Cooperative Oncology Group) score of 0/1.
10. Preoperative ASA (American Society of Anesthesiologists) score I-III.
11. Good function of important organs.
12. Signed informed consent.

Exclusion Criteria:

1. Preoperative assessment indicating cT4b or Bulky lymph nodes enlargement or distant lymph nodes metastasis;
2. Pregnant or lactating women;
3. Patients with severe mental illness;
4. Preoperative temperature ≥38°C or infectious diseases requiring systemic treatment;
5. Severe respiratory diseases, with FEV1 < 50% of predicted value;
6. History of other malignant tumors in the past 5 years;
7. Severe liver or kidney dysfunction;
8. Unstable angina or myocardial infarction within the last 6 months;
9. History of stroke or cerebral hemorrhage within the last 6 months (excluding old infarcts);
10. Systemic use of glucocorticoids within the last 1 month;
11. Emergency surgery required due to complications of gastric cancer (bleeding, perforation, obstruction);
12. Patient has participated in or is currently participating in other clinical trials (within the last 6 months).