Neoadjuvant Nab-Paclitaxel Plus Oxaliplatin, S-1, and Sintilimab in Early-Onset Resectable Gastric Cancer

Inclusion Criteria: * Participants must meet all of the following inclusion criteria: * Male or female, aged ≥16 and ≤45 years; * Karnofsky performance score ≥70% or ECOG performance status 0-1; * Histologically confirmed adenocarcinoma of the stomach or gastroesophageal junction (GEJ). * For gastric body cancer: clinical stage cT3-T4a N+ M0; * For GEJ cancer: clinical stage cT2-T4a N+ M0; * For cT4b Nany M0 cases: location may be gastric body or GEJ. * Staging is based on contrast-enhanced CT, MRI (if needed), and endoscopic ultrasonography (EUS) (if needed); * Assessed as resectable after multidisciplinary team (MDT) discussion; * Surgical evaluation confirms that D2 radical gastrectomy is feasible; * Sufficient physical condition and organ function to tolerate major abdominal surgery; * Baseline laboratory tests meet the following criteria: * Hemoglobin ≥90 g/L * Absolute neutrophil count (ANC) ≥1.5×10⁹/L * Platelets ≥100×10⁹/L * ALT and AST ≤2.5× upper limit of normal (ULN) * Alkaline phosphatase (ALP) ≤2.5× ULN * Total bilirubin \<1.5× ULN * Serum creatinine \<1× ULN * Serum albumin ≥30 g/L * No severe comorbidities that may limit life expectancy to \<3 years; * Participant is willing and able to comply with the study protocol during the study period; * Written informed consent must be signed before screening. Participants must understand their right to withdraw at any time without any loss of benefits; * Willing to provide blood and tissue samples. Exclusion Criteria: * Participants meeting any of the following criteria will be excluded: * Patients with HER-2 positive gastric cancer, as determined by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH); * Patients with dMMR (deficient mismatch repair) or MSI-H (microsatellite instability-high) tumors, as determined by IHC; * Pregnant or breastfeeding women; * Prior treatment for gastric cancer with cytotoxic chemotherapy, radiotherapy, or immunotherapy; * History of other malignancies within the past 5 years; * Active autoimmune diseases, including but not limited to systemic lupus erythematosus, rheumatoid arthritis, myasthenia gravis; * Active inflammatory bowel diseases, such as Crohn's disease or ulcerative colitis; * Currently receiving systemic corticosteroids or other immunosuppressive therapy; * Prior organ transplantation or use of anti-rejection medications; * Active tuberculosis, HIV infection, or severe active hepatitis B or C; * History of uncontrolled epilepsy, central nervous system disorders, or psychiatric illnesses that, in the investigator's judgment, may interfere with informed consent or compliance with oral medication; * Clinically significant (active) cardiac diseases, including symptomatic coronary artery disease, NYHA class II or above congestive heart failure , severe arrhythmias requiring medical intervention, or myocardial infarction within the past 12 months; * Presence of upper gastrointestinal obstruction that may impair the oral intake or absorption of S-1; * Severe uncontrolled recurrent infections or other uncontrolled serious comorbidities; * Use of any investigational drugs within 4 weeks prior to study enrollment.