Safety and Efficacy of PD-1 ± mFOLFOX6 Neoadjuvant Therapy in Local Advanced sMPCC

Inclusion Criteria: 1. Histological confirmation of simultaneous multiple primary colorectal cancer (sMPCC); 2. Tumor biopsy immunohistochemistry of at least one tumor lesion identified dMMR, including the expression loss of one or more of the four proteins MSH1, MSH2, MSH6 and PMS2; or at least one tumor lesion identified MSI-H by polymerase chain reaction or next-generation sequencing technique; 3. Clinical staging T3-4NxM0, with or without positive MRF, with or without positive EMVI; 4. Staging method: all patients undergo chest,abdominal and pelvic enhanced CT, rectal palpation, high resolution MRI 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; 5. No intestinal obstruction symptom，or obstruction relieved after proximal colostomy; 6. No colorectal surgery history; 7. No chemotherapy or radiotherapy history; 8. 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; 9. No endocrinotherapy history restriction; 10. informed consent assigned. Exclusion Criteria: 1. 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; 2. Severe hypertension not well controlled by drugs; 3. HIV infection history or active phase of chronic Hepatitis B or C(high copies of virus DNA); 4. Active tuberculosis(TB), accepting anti-TB treatment or anti-TB treatment within 1 year before trial screen; 5. Other active clinical severe infection(NCI-CTC AE V5.0); 6. Outside pelvic distant metastasis evidences; 7. Dyscrasia, organ dysfunction; 8. Pelvic or abdominal radiotherapy history; 9. Epilepsy need treatments(Steroid or anti-epilepsy therapy); 10. Other malignant tumor history within 5 years; 11. Over abuse of drugs, medical and psychological or social conditions that might interfere patients or evaluation of the study results; 12. Any active autoimmune disease or autoimmune disease history (including but not restricted: interstitial pneumonia, uveitis, enteritis, hepatitis, hypophysitis, nephritis, hyperthyroidism, hypothyroidism, asthma need bronchodilators); 13. Any anti-infection vaccine injection 4 weeks before inclusion; 14. Long-term exposure to immune-suppressor, combination of systemic or topical use of corticosteroids (dose\>10mg/day prednisolone or equivalent hormone); 15. Known or suspicious allergy to any study related drugs; 16. Any unstable state might cause damage to the safety and compliance of patients; 17. Pregnant or breast feeding women who has ability to have children while without contraception; 18. Refuse to sign informed consent.