Long-course Chemoradiotherapy or Short-course Radiotherapy Combined With CAPOX, PD-1antibody, and COX-2 Inhibitor for MSS Locally Advanced Rectal Cancer (SERRAC)

SERRAC is a prospective, multicentre, randomized phase II trial. 138 LARC (T3-4/N+M0, distance from anal verge ≤10cm) patients will be treated with neoadjuvant therapy and assigned to Group A and Group B (1:1). Group 1 receives LCRT (50Gy/25Fx) followed by 3 cycles of CAPOX.Group 2 receives SCRT (25Gy/5Fx) followed by 4 cycles of capecitabine plus oxaliplatin (CAPOX) chemotherapy and PD-1 antibody. The COX2 inhibitor celecoxib 200 mg was started orally twice a day during chemotherapy until the end of neoadjuvant treatment.TME surgery is scheduled after TNT while a watch and wait (W\&W) option can be applied to patients achieving clinical complete response (cCR). The primary endpoint is complete response (CR, pathological complete response \[pCR\] plus cCR) rate. The secondary endpoints include the grade 3-4 acute adverse effects (AE) rate, anal preservation rate, 3-year DFS rate, etc.