The goal of this clinical trial is to find out if adding a PD-1 antibody (serplulimab) to FOLFOXIRI chemotherapy and radiotherapy works better than CAPOX chemotherapy with radiotherapy as total neoadjuvant therapy for adults with pMMR locally advanced low rectal cancer. It will also look at the safety of these treatments and how they affect long-term outcomes such as organ preservation and survival. The main questions it aims to answer are: Does PD-1 antibody plus FOLFOXIRI with radiotherapy improve 3-year event-free survival compared with CAPOX with radiotherapy? Does this treatment increase the chance of clinical complete response and avoiding a permanent stoma (sphincter-preserving or non-surgical "watch-and-wait" management)? What side effects and medical problems occur during and after these treatments? Researchers will compare: Group A (experimental group): PD-1 antibody (serplulimab) plus FOLFOXIRI chemotherapy combined with long-course radiotherapy as total neoadjuvant therapy. Group B (control group): CAPOX chemotherapy combined with long-course radiotherapy as total neoadjuvant therapy. Participants will: Sign an informed consent form and have screening tests (physical exam, blood tests, ECG, imaging such as MRI/CT, endoscopy) to confirm they can join the trial. Be randomly assigned (like drawing lots) to Group A or Group B. Receive several cycles of chemotherapy together with a 5-week course of pelvic radiotherapy before surgery; the experimental group will also receive PD-1 antibody during part of the chemotherapy and radiotherapy period. Have regular clinic visits for checkups, blood tests, and assessment of side effects during treatment. After neoadjuvant therapy, have MRI/CT and endoscopy to assess tumor response. Depending on the response, they may: Receive surgery to remove the rectal tumor, or If a clinical complete response is achieved and both doctor and patient agree, enter a "watch-and-wait" program instead of immediate surgery. Provide blood samples and allow tumor tissue to be collected (for example, from biopsy and surgery) for future research (such as building PDX models and testing blood markers). Be followed regularly for at least 5 years with clinic visits, blood tests (including CEA), imaging, and colonoscopy to check for tumor recurrence, side effects, and quality of life.
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3-year Event-Free Survival (EFS)
Timeframe: Up to 3 years after randomization