PD1 antibody is now recommended for dMMR/MSI-H metastatic colorectal cancer patients as second line. Chemoradiotherapy is standand treatment for locally advanced rectal cancer and is also recommended as an alternative choice for unresectable locally advanced colon cancer. Thus, this study will investigate the efficacy and toxicity of combination strategy using PD1 antibody and chemoradiotherapy for dMMR/MSI-H locally advnaced colorectal cancer patients.

PD1 antibody is recommended for dMMR/MSI-H metastatic colorectal cancer patients as second line. Chemoradiotherapy is standand treatment for locally advanced rectal cancer and is also recommended as an alternative choice for unresectable locally advanced colon cancer. Thus, this phase II, single arm study will investigate the efficacy and toxicity of combination strategy using PD1 antibody and chemoradiotherapy for dMMR/MSI-H locally advnaced colorectal cancer patients, which is expected to yield higher tumor regressiona with tolerable toxicity.

Inclusion Criteria:

1. Histologically proven diagnosis of colorectal adenocarcinoma;
2. Biopsy tissues with IHC indicates deficient mismatch repair(dMMR),that is,the loss of at least one of the four proteins ,MSH1,MSH2,MSH6,PMS2;or gene detection implies MSI-H;
3. Clinical stage for rectal cancer patients is cT3-4N0M0 or cTxN+M0;clinical stage of colon cancer should meet the following criteria (any one is sufficient): a)Tumor penetrates the whole wall and adherents to other organs or structures around(T4b).Tumor cannot reach R0 resection by imaging assessment; b)The intestinal lymph nodes involved are closely adjacent to large abdominal vessels. Lymph nodes dissection is not feasible by imaging assessment; c)Surgeons assess it is hard to achieve R0 resection after surgical exploration; d)Surgeons assess tumor is extensive multiviseral resection is needed, which is expected to damage the organs and seriously affect the quality of life after operation;
4. Preoperative staging methods: all patients need to accept digital rectal examination(DRE).Patients with rectal cancer undergo high-resolution MRI±ultrasound colonoscopy/transrectal ultrasound for preoperative staging. Perienteric lymph nodes with short diameter ≥10mm or the shape of lymph nodes and its MRI characteristics are consistent with typical lymph node metastasis. If endoscopic ultrasonography is used in combination, and there is a contradiction between staging methods, the data should be submitted to the evaluation team of our center for the accurate staging;
5. No symptoms of ileus; or ileus is alleviated after proximal colostomy.

Previous treatment:

1. No surgery except preventative stoma;
2. No chemotherapy or radiotherapy;
3. No biotherapy (e.g.monoclonal antibodies), immunotherapy (e.g.anti-PD-1 antibody,anti-PD-L1 antibody,anti-PD-L2 antibody or CTLA-4 antibody),or other clinical trials agents;
4. No limit to previous endocrine therapy.

Patient characteristics:

1. Age between 18 and 72 years;
2. ECOG performance status of 0 or 1;
3. Life expectancy: more than 2 years;
4. Hematopoietic: WBC>3×109/L;PLT>80×109/L; Hb>90g/L;
5. Hepatic: ALT and AST<2 times upper limit of normal (ULN); bilirubin<1.5 times ULN;
6. Renal: creatinine <1.5 times ULN or creatinine clearance ≥ 60 mL/min.

Exclusion Criteria:

All patients enrolled cannot have any of the following situation:

1. Arrhythmias require antiarrhythmic therapy (with the exception of β-blockers or digoxin), symptomatic coronary artery disease or local myocardial ischemia (myocardial infarction within the past 6 months) or congestive heart failure exceeding NYHA II;
2. Severe hypertension with poor drug control;
3. A known history of testing positive for HIV or chronic hepatitis B or C (high copy virus DNA) at active stage;
4. Patients with active tuberculosis (TB) are receiving anti-tuberculosis treatment or have received anti-tuberculosis treatment within 1 year before screening;
5. Other active severe clinical infections (NCI-CTC5.0);
6. Apparent distant metastasis away from the pelvic before surgery;
7. Cachexia, organ function decompensation;
8. Previous pelvic or abdominal radiotherapy;
9. Multiple primary colorectal cancers;
10. Epilepsy require medical treatment (such as steroid or antiepileptic therapy);
11. Other malignancy within the past 5 years with the exception of effectively treated carcinoma in situ of the cervix or basal cell carcinoma of the skin;
12. Drug abuse and medical, psychological or social factors that may interfere with patients' participation in the study or affect the evaluation of the study;
13. Patients have any active autoimmune diseases or a history of autoimmune diseases(including but not limited to: interstitial pneumonia, uveitis, enteritis, hepatitis, hypophysitis, nephritis, hyperthyroidism and decreased thyroid function; patients with vitiligo or with complete remission of asthma in childhood and without any intervention in adulthood may be included; patients with asthma requiring bronchodilators intervention are not included.
14. Received any anti-infection vaccine (e.g. influenza vaccine, chickenpox vaccine, etc.) within 4 weeks before enrollment;
15. Complications require long-term treatment with immunosuppressive drugs, or requiring systemic or local use of immunosuppressive corticosteroids(>10mg/day prednisone or other therapeutic hormones);
16. Known or suspected allergy to the study drugs or to any drugs related to this trial;
17. Any unstable condition or which endangers the patients' safety and compliance;
18. Pregnant or breast-feeding women who are fertile without effective contraception;
19. Refuse to sign the informed consent.

Exit Criteria:

1. Patients withdraw the informed consent and ask for quit;
2. Poor compliance;
3. Disease progression during treatment;
4. Serious adverse events or serious adverse reactions (SAE) occurred during the study;
5. Any delay of treatment for more than two weeks (including two weeks) (referring to the delay of all drugs in the medication plan) shall be discussed by the researchers whether to quit.

Cessation Criteria:

Study suspension refers to the cessation of the whole study before the end of the program. The main purpose of this action is to protect the rights and interests of the subjects, ensure the quality of the study, and avoid unnecessary economic losses. The whole study will be stopped for the following reasons:

1. Researchers find serious safety issues;
2. Efficacy is poor that there is no need to continue the study;
3. Severe mistakes in the scheme design or important deviations in the implementation process;
4. Funds or management problems;
5. The administrative department decide to cancel the study. A complete suspension of research is either temporary or permanent. When discontinued, all study records shall be retained for future reference.