Primary objective:~Evaluate the CCR rate of low rectal cancer using adaptive and optimized chemotherapy and radiotherapy strategies (all population and dMMR/MSI-H subgroup)~Secondary objectives:~2.1 Evaluate the 2-year anal preservation rate, recurrence rate, quality of life, DFS and OS 2.2 Explore the subgroup of patients suitable for observation.~Outline:~Patients after long-course chemoradiation are grouped based on their MSI-H/dMMR status. For patients with MSI-H/dMMR, consolidation immunotherapy of Tislelizumab (BGB-A317) will be assigned. For patients with MSS/pMMR, consolidation chemotherapy will be given according to their tumor response. After completion of consolidation therapy, those who reach clinical complete response will receive organ preservation (watch and wait) strategy in place of radical surgery. During treatment, once local regrowth occurs or poor tumor response, total mesorectal excision (TME) surgery will be performed.
The study is designed to test the hypothesis that the clinical complete response (CCR) rate of patients with locally advanced rectal cancer (LARC) treated with neoadjuvant chemoradiotherapy will increase after an adaptive-design paradigm, as well as the rate of 2-year organ preservation, recurrence, quality of life, DFS and OS.