SOX Combined With Sintilimab and Trastuzumab Versus SOX Regimen in the Perioperative Treatment of HER2-positive Locally Advanced Gastric Adenocarcinoma

Inclusion Criteria:

* Sign the informed consent form.
* Locally advanced adenocarcinoma of the stomach or gastroesophageal junction (Siewert type II/III) confirmed by pathology or cytology.
* The definition of a positive HER2 test result is as follows: IHC detects HER2 3+ or IHC detects HER2 2+ and FISH is positive.
* Clinically, based on chest, abdomen and pelvic CT, gastroscopy, endoscopic ultrasonography, gastrointestinal contrast, ordinary ultrasound, or laparoscopy if possible, it is judged as T3-4a N+ or T4bN any gastric cancer or gastroesophageal junction cancer (refer to AJCC Article Version 8 in stages).
* Patients have not received chemotherapy and/or immunotherapy and/or trastuzumab treatment and/or radiotherapy in the past.
* Age 18-75 years old.
* The Eastern Cooperative Oncology Group (ECOG) performance status score was 0 or 1, and there was no deterioration within 2 weeks before the first administration of the study drug.
* Good organ function:

Blood routine: hemoglobin ≥90g/L, white blood cell ≥3.0×109/L, neutrophil ≥1.5×109/L, platelet ≥100×109/L; Renal function: creatinine≤1.5×upper limit of normal (UNL) or creatinine clearance ≥60ml/min; Liver function: total bilirubin (TBIL)≤1.5×upper limit of normal (UNL); ALT≤2.5×UNL, AST≤2.5×UNL.

Exclusion Criteria:

* The pathology is other types besides adenocarcinoma, such as squamous cell carcinoma, adenosquamous carcinoma, neuroendocrine carcinoma and so on.
* Have received chemotherapy and/or radiotherapy in the past.
* Have received any anti-PD-1, anti-PD-L1/L2 antibodies, anti-CTLA-4 antibodies and other immunotherapy in the past.
* Have received any anti-HER2 therapy in the past.
* Intra-abdominal dissemination or distant metastasis (M1).
* Clinically significant ascites.
* Known to have allergic reactions to oxaliplatin and any ingredients or excipients of Tiggio.
* Known to have allergic reactions to any ingredients or excipients of Sintilimab and Trastuzumab.
* Inability to swallow, intestinal obstruction, or other factors that affect the administration and absorption of the drug.