The purpose of this study is to evaluate the efficacy of Tislelizumab in the neoadjuvant (prior to surgery) or adjuvant (after surgery) treatment of previously untreated adults with gastric and gastroesophageal junction (GEJ) adenocarcinoma.
The primary study hypotheses are that:
Neoadjuvant and adjuvant Tislelizumab plus chemoradiotherapy, followed by adjuvant Tislelizumab and chemotherapy is superior to neoadjuvant chemoradiotherapy or chemotherapy, followed by adjuvant chemotherapy in terms of rate of Pathological Complete Response (pathCR) at the time of surgery.
Has previously untreated localized gastric or GEJ adenocarcinoma as defined by T3~4aN+M0 or T4bNanyM0 （AJCC Version 8）
Has an Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 1
Has adequate organ function.
Male participants of childbearing potential must agree to use an adequate method of contraception for the course of the study through 180 days after the last dose of chemotherapy.
Has a known additional malignancy that is progressing or has required active treatment within the past 5 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ that have undergone potentially curative therapy are not excluded.
Has received prior systemic anti-cancer therapy including investigational agents for the current malignancy.
Has an active infection requiring systemic therapy.
Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis.