Watch and Wait in PD-1 Monoclonal Antibody Treated dMMR/MSI-H Distal Rectal Cancer

Inclusion Criteria: Preliminary inclusion criteria： * Histological identified rectal adenocarcinoma, * Tumor biopsy immunohistochemical (IHC) identified dMMR, including one or more deficient of the MSH1,MSH2,MSH6 and PMS2 protein expression and diagnosed as deficient mismatch repair(dMMR), or next generation sequencing identified (MSI-H)； MRI identified tumor inferior margin lower than peritoneal reflection, * Clinical staging TxNxM0, with or without positive MRF, with or without positive EMVI, * Staging method：all patients undergo rectal palpation, high resolution MRI ± transrectal Ultrasound examination，positive perienteric lymph node(LN): short diameter ≥10mm LN or LN with typical metastatic shape and MRI character, clinical data should be re-evaluated and judged by center evaluation group when there are contradictory stagings，distant metastasis were excluded by chest and abdominal enhanced CT and pelvic enhanced MRI, * No intestinal obstruction symptom，or obstruction relieved after proximal colostomy, * No rectal surgery history, * No chemotherapy or radiotherapy history, * No biopharmaceutical treatment history(such as monoclonal antibody), immunotherapy(such as anti PD-1antibody, anti PD-L1 antibody, anti PD-L2 antibody or anti CTLA-4), or other research drug treatment, * Endocrinotherapy history restriction:No * Informed consent assigned, Final inclusion criteria： * Clinical complete response (cCR)(Chest,abdominal and pelvic enhanced CT or pelvic enhanced MRI or transrectal ultrasound proved) * Transrectal ultrasound biopsy or endoscopic biopsy proved pathologically complete response (pCR) Exclusion Criteria: * Arrhythmia need anti-arrhythmia treatment(except β-blocking agent or Digoxin)，symptomatic coronary heart disease or myocardial ischemia(myocardial infarction within 6 months) or congestive heart-failure (CHF) \> NYHA grade II, * Severe hypertension not well controlled by drugs, * HIV infection history or active phase of chronic Hepatitis B or C(high copies of virus DNA), * Active tuberculosis(TB)，accepting anti-TB treatment or anti-TB treatment within 1 year before trial screen, * Other active clinical severe infection(NCI-CTC V5.0), * Outside pelvic distant metastasis evidences, * Dyscrasia, organ dysfunction, * Pelvic or abdominal radiotherapy history, * Multiple CRC or Multi-primary tumors； * Epilepsy need treatments(Steroid or anti-epilepsy therapy), * Other malignant tumor history within 5 years, * Over abuse of drugs, medical and psychological or social conditions that might interfere patients or evaluation of the study results, * Any active autoimmune disease or autoimmune disease history (including but not restricted：interstitial pneumonia, uveitis,enteritis, hepatitis,hypophysitis, nephritis, hyperthyroidism, hypothyroidism, asthma need bronchodilators), * Any anti-infection vaccine injection 4 weeks before inclusion , * Long-term exposure to immune-suppressor, combination of systemic or topical use of corticosteroids (dose\>10mg/day prednisolone or equivalent hormone)； * Known or suspicious allergy to any study related drugs, * Any unstable state might cause damage to the safety and compliance of patients, * Pregnant or breast feeding women who has ability to have children while without contraception, * Refuse to sign informed consent