Short-Course Radiotherapy Combined With Chemotherapy and Immunotherapy in Mid-Low Locally Advanced Rectal Cancer

The goal of this clinical trial is to test a new combination treatment for locally advanced rectal cancer (cancer in the lower or middle part of the rectum that has not spread to distant organs). The study aims to increase the chance of making the tumor disappear completely (called "complete response") and improve the quality of life by increasing the rate of anal sphincter preservation (avoiding permanent colostomy bags). The main questions it aims to answer are: Does the combination of short-course radiation therapy, two types of immunotherapy drugs (Qibeian and Aike), and chemotherapy (XELOX) increase the complete response rate to over 50%? Is this combination treatment safe, and what are the side effects? Can this treatment help more patients keep their anal function and avoid permanent stomas? This is a single-arm study, meaning all participants will receive the experimental treatment (there is no placebo or control group). Participants will: Receive short-course radiation therapy (25 Gy total, given once daily for 5 consecutive days). The radiation will target only the tumor and visible lymph nodes, intentionally avoiding unaffected lymph node areas to protect the immune system. Receive Qibei'an (a dual immunotherapy drug targeting both PD-1 and CTLA-4) once, 2 days after completing radiation. Receive Camrelizumab (a PD-1 immunotherapy drug) three times, combined with XELOX chemotherapy. Receive XELOX chemotherapy (Oxaliplatin ivgtt on Day 1, plus Capecitabine pills taken twice daily for 14 days, for each cycle) for up to 3 cycles. Undergo detailed assessments after treatment, including MRI scans, colonoscopy with biopsies, and blood tests (including ctDNA tests), to determine if the tumor has disappeared or if surgery is needed. Attend regular follow-up visits for up to 5 years after treatment (or surgery) to monitor for recurrence and assess quality of life. The study will enroll approximately 19 patients at Beijing Friendship Hospital, Capital Medical University. An independent safety monitoring board will regularly review the data to ensure participant safety.