Pd1 Antibody Sintilimab ± Chemoradiotherapy for Locally Advanced Rectal Cancer

In this study, participants with locally advanced rectal cancer patients will be treated according to MMR/MSI status. There will be two cohorts in this study: Cohort A and Cohort B. For Cohort A, dMMR or MSI-H patients will receive 4 cycles of neoadjuvant Pd1 antibody Sintilimab,followed by one of the following treatments: (1) surgery and adjuvant treatment, (2）another 4 cycles of sintilimab, followed by radical surgery or observation (only for cCR) . For Cohort B, pMMR/MSS/MSI-L patients will be randomized to receive neoadjuvant chemoradiotherapy ± four cycles of Pd1 antibody Sintilimab，followed by one of the following treatments: (1) curative surgery and four cycles of adjuvant chemotherapy;(2)four cycles of chemotherapy then observation (only cCR after neoadjuvant therapy)