Non-operative Management

Primary Objectives： Evaluate the complete response rate (CR rate) and safety of short - course radiotherapy combined with ivonesimab (AK112) in patients with pMMR/MSS mid - low rectal cancer. Secondary Objectives： Evaluate treatment - related toxic reactions, the quality of life, long - term prognosis (local control \[LC\], disease - free survival \[DFS\] and overall survival \[OS\]). Patients will : Receive Radiotherapy: Pelvic IMRT or VMAT, DT 25Gy/5Fx. One week after radiotherapy, begin treatment with Ivorsimab (AK112) at a dose of 20mg/kg by intravenous drip on day 1. One cycle is 21 days, and a total of 6 cycles are to be carried out. Evaluate the curative effect after 3 cycles of treatment. Patients with progressive disease (PD) will withdraw from the study, and other treatment plans will be adjusted in a timely manner. Patients with CR/PR/SD will continue treatment for another 3 cycles. Conduct a comprehensive assessment after 6 cycles of treatment. Patients who achieve cCR can choose the watch - and - wait approach. For patients who do not achieve cCR, TME surgery is recommended. Decide whether to perform adjuvant chemotherapy based on the postoperative pathological findings.