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

Prior treatment

• Pelvic or abdominal radiotherapy history,
• 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)；

Other illness

• 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),
• 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),
• Known or suspicious allergy to any study related drugs,

Other

• Outside pelvic distant metastasis evidences,
• Dyscrasia, organ dysfunction,
• Multiple CRC or Multi-primary tumors；
• 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

• 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)