The Efficacy of Watch and Wait Strategy or Surgery After Neoadjuvant Immunotherapy for Locally Advanced Colorectal Cancer With dMMR/MSI-H Guided by MRD Dynamic Monitoring: A Single-center, Open-label, Prospective, Phase II Clinical Trial.

Inclusion Criteria:

* ( 1 ) Disease characteristics 1. Histologic confirmation for colorectal adenocarcinoma; 2．Immunohistochemical identified as dMMR and/or pCR and/or NGS detection for MSI-H; 3．According to UICC/AJCC TNM staging system (in 2017 version 8) assessment for locally advanced colorectal cancer (II - III, namely cT3-4 and/or N + ）； 4.\* clinical staging methods: colon CT staging, approved by pelvic magnetic resonance imaging (MRI) and the rectum rectum cavity ultrasound combined with staging, 5．No signs of intestinal obstruction;Or intestinal obstruction after proximal colon obstruction has been relieved after colostomy surgery; 6．No distant metastasis was confirmed by comprehensive examination (distant organs or (and) the distant lymph node metastasis); ( 2 ) Previous treatment 1．No colorectal cancer surgery before; 2. No previous chemotherapy or radiotherapy ; 3. had not received biological treatment ; 4. Previous endocrine therapy : no restriction.

Exclusion Criteria:

* 1.Immunohistochemistry of tumor biopsy specimens suggested pMMR or microsatellite instability detection suggested MSS ; 2. Chronic hepatitis B or C ( high-copy viral DNA ) with a history of HIV infection or active phase ; 3.Autoimmune diseases ; 4.Other active clinical severe infections ( \> NCI-CTC3.0 version ) ; 5. patients with clinical stage I ; 6.Preoperative evidence of distant metastasis ; 7. cachexia, organ dysfunction ; 8. Have a history of pelvic or abdominal radiotherapy ; 9. Patients whose seizures require treatment ( e.g. steroids or antiepileptic treatment ) ; 10.5 years have a history of other malignant tumors, except for cured cervical carcinoma in situ or skin basal cell carcinoma ; 11. Chronic inflammatory bowel disease, intestinal obstruction ; 12. Drug abuse and medical, psychological or social conditions may interfere with patient participation in the study or have an impact on the evaluation of the results of the study ; 13.Known or suspected allergies to the study drug or any drug related to this test ; any unstable condition or condition that may endanger patient safety and compliance ;