The purpose of this prospective, single-center, phase II study is to evaluate the clinical efficacy and safety of combining first-line systemic therapy plus immunotherapy with personalized ultrafractionated stereotactic adaptive radiation therapy (PULSAR) in patients with unresectable locally advanced gastric cancer.

Standard first-line treatment for unresectable locally advanced gastric cancer is systemic chemotherapy combined with immunotherapy, but the overall prognosis remains poor, and local complications like bleeding or obstruction frequently occur.

ULSAR delivers a high dose of radiation per fraction using a 3-to-4 week interval. This extended fraction interval allows normal tissue recovery and provides an opportunity for adaptive replanning before each fraction to minimize gastrointestinal toxicities while maximizing tumor dose delivery. Furthermore, this unique scheduling allows seamless integration with systemic immunotherapy without interrupting the regular chemotherapy cycle. This study specifically aims to evaluate if PULSAR can improve local control, conversion to surgery, and progression-free survival in HER-2 negative, EBV negative, and MSS gastric cancer patients whose tumors have not progressed after at least 3 cycles of first-line immunotherapy-containing systemic regimens.

Inclusion Criteria:

Age 19 or more years. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2. Histologically confirmed gastric adenocarcinoma. Tumor biomarker status: HER-2 negative, EBV negative, and Microsatellite Stable (MSS).

Unresectable, locally advanced extent at initial staging (Para-aortic lymph node [PALN] and Supraclavicular lymph node [SCN] metastases are allowed).

Has completed 3 or more cycles of first-line systemic therapy combined with immunotherapy without evidence of disease progression.

Presence of at least one evaluable lesion according to RECIST v1.1 that is deemed safely irradiable by the investigator.

Voluntary written informed consent provided by the subject.

Exclusion Criteria:

Pregnant or lactating women. Presence of brain metastases or leptomeningeal involvement. Prior history of radiation therapy to the intended target site. Severe uncontrolled comorbidities that, in the investigator's opinion, limit study participation or treatment compliance (e.g., uncontrolled infection, heart failure, arrhythmia, psychiatric illness).

Inability or unwillingness to comply with the study protocol procedures. Any condition deemed inappropriate for study participation by the principal investigator or attending physician.