局所進行性胃または胃食道接合部腺癌に対する低線量放射線療法およびチセリズマブを用いた周術期化学療法

Inclusion Criteria:

1.Voluntary participation with written informed consent obtained; 2. Histologically confirmed gastric or gastroesophageal junction adenocarcinoma (Siewert type II/III) by endoscopic biopsy (pathology from outside institutions must be reviewed by the study center); 3. Staged as cT1-2N1-3M0 or cT3-4aN0-3M0 according to the AJCC 8th edition, based on endoscopy and imaging (CT, MRI, or PET-CT); 4. Age ≥20 and ≤80 years, male or female; ECOG performance status of 0-1; 5. Presence of measurable and/or non-measurable disease per RECIST v1.1; 6.No prior systemic antitumor therapy, including chemotherapy, radiotherapy, targeted therapy, immunotherapy, biologic therapy, local therapy, or investigational agents; Adequate organ function (no blood products or growth factors within 2 weeks prior to screening); 7. Women of childbearing potential must have a negative pregnancy test within 72 hours prior to first dosing and agree to use effective contraception during the study and for 5 months after the last dose; men with partners of childbearing potential must use effective contraception during the study and for 7 months after the last dose.

Exclusion Criteria:

1. History of surgery for gastric or gastroesophageal junction tumors;
2. Prior history of fistula formation caused by invasion of the primary tumor; 3.High risk of gastrointestinal bleeding or perforation;

4.Poor nutritional status, defined as BMI <18.5 kg/m² or PG-SGA score ≥9; 5. Major surgery or severe trauma within 4 weeks prior to first study drug administration; Uncontrolled pleural effusion, pericardial effusion, or ascites requiring repeated drainage; 6. Prior or ongoing treatment with anti-PD-1/PD-L1 antibodies, chemotherapy, radiotherapy, or targeted therapy; 7. Use of any investigational drug within 4 weeks prior to first study drug administration; Requirement for systemic corticosteroids; 8. Prior receipt of antitumor vaccines or live vaccines within 4 weeks before first study drug administration; 9. Active autoimmune disease or history of autoimmune disease; 10.History of immunodeficiency, including HIV infection, other acquired or congenital immunodeficiency, solid organ transplantation, or allogeneic bone marrow transplantation; 11.Any condition requiring systemic corticosteroid or immunosuppressive therapy within 14 days prior to treatment, except for minimal systemic absorption routes or short-term (≤7 days) prophylactic use; 12.Uncontrolled clinically significant cardiac disease, including NYHA class II or higher heart failure, unstable angina, myocardial infarction within 1 year, or clinically significant arrhythmias requiring intervention; 13.Severe infection (CTCAE grade >2) within 4 weeks prior to treatment, including infections requiring hospitalization; evidence of active pulmonary infection at baseline imaging or infections requiring antibiotics within 2 weeks prior to enrollment (prophylactic antibiotics excluded); 14.History of interstitial lung disease, noninfectious pneumonitis, pulmonary fibrosis, or other uncontrolled acute pulmonary disease; 15.Active tuberculosis, history of active tuberculosis within 1 year prior to enrollment, or inadequately treated tuberculosis more than 1 year prior; 16.Active hepatitis B (HBV DNA ≥2,000 IU/mL) or hepatitis C infection (HCV antibody positive with detectable HCV RNA); 17.Any condition requiring systemic corticosteroid or immunosuppressive therapy within 14 days prior to treatment, except for minimal systemic absorption routes or short-term (≤7 days) prophylactic use; 18.Uncontrolled clinically significant cardiac disease, including NYHA class II or higher heart failure, unstable angina, myocardial infarction within 1 year, or clinically significant arrhythmias requiring intervention; 19.Severe infection (CTCAE grade >2) within 4 weeks prior to treatment; History of interstitial lung disease, noninfectious pneumonitis, pulmonary fibrosis, or other uncontrolled acute pulmonary disease; 20.Active tuberculosis, history of active tuberculosis within 1 year prior to enrollment, or inadequately treated tuberculosis more than 1 year prior; 21.Active hepatitis B (HBV DNA ≥2,000 IU/mL) or hepatitis C infection (HCV antibody positive with detectable HCV RNA); 22.Grade >1 abnormalities in serum sodium, potassium, or calcium within 2 weeks prior to enrollment that cannot be corrected with treatment.