A Prospective, Biomarker-Stratified Phase 2 Study: β-Hydroxybutyrate as a Predictive Biomarker for Short-Course Radiotherapy Followed by Chemotherapy Plus PD-1 Inhibitor Versus Chemotherapy Alone in Locally Advanced Rectal Cancer

This is a prospective, single-arm, biomarker-stratified, multicenter phase 2 clinical trial. All enrolled patients first receive short-course radiotherapy (SCRT), followed by measurement of serum β-hydroxybutyrate (β-HB) levels to calculate the post-radiotherapy elevation rate. Patients are stratified into an β-HB-positive stratum (elevation significantly) and an β-HB-negative stratum (no elevation), then randomized 1:1 within each stratum to receive either perioperative chemotherapy plus PD-1 inhibitor or perioperative chemotherapy alone. The primary endpoint is the complete response (CR) rate, defined as the composite of pathological complete response (pCR) and clinical complete response (cCR). The study aims to evaluate the efficacy and safety of adding PD-1 inhibitor to perioperative chemotherapy in LARC, and to validate post-radiotherapy β-HB elevation as a predictive biomarker for immunotherapy benefit.