Total Neoadjuvant Therapy Followed by 'Watch and Wait' Approach or Organ Preservation for Low-risk Rectal Cancer

Inclusion Criteria:

* 1\. the age is more than 18 years old and less than 85 years old;
* 2\. ECOG score of physical condition is 0-1;
* 3\. adenocarcinoma of rectum confirmed by pathology; differentiated into Grade 1-3, i.e. high, medium and low differentiated adenocarcinoma
* 4\. the distance from the lower edge of the tumor to the anal edge ≤ 12cm (endoscopy) or to the anorectal ring (ARJ) ≤ 8cm;
* 5\. the initial MRI was T2 / T3a / T3B, with negative EMVI and negative CRM. There was no lymph node metastasis outside the ilium, general ilium, obturator and abdominal aorta
* 6\. the maximum diameter of tumor ≤ 4cm or the circumferentially invasive area is less than 1 / 3 of intestinal circumference
* 7\. no evidence of distant metastasis;
* 8\. no history of pelvic radiotherapy;
* 9\. no history of surgery or chemotherapy for rectal cancer;
* 10\. systemic infection without antibiotic treatment;
* 11\. blood routine test: neutrophil absolute value \> 1.5 × 10 9 / L, HGB \> 10.0 g / dl, PLT \> 100 × 10 9 / L;
* 12\. blood biochemistry: total bilirubin ≤ 1.5 x ULN, AST ≤ 3 x ULN, ALT ≤ 4 x ULN;
* 13\. the patient read and signed the informed consent of the study and agreed to participate in the study;

Exclusion Criteria:

* 1\. recurrent rectal cancer;
* 2\. initial local non resectable rectal cancer (no possibility of R0 whole block resection);
* 3\. the creatinine level is 1.5 times higher than the upper limit of normal value;
* 4\. have a history of pelvic radiotherapy;
* 5\. the patients could not tolerate the enhanced MRI;
* 6\. malignant tumors whose survival rate in the past 5 years is significantly lower than that of rectal cancer in our center (excluding well treated basal cell carcinoma, skin squamous carcinoma, small renal carcinoma, breast cancer and thyroid papillary carcinoma);
* 7\. in the past 6 months, the patients had arterial embolism diseases, such as angina, MI, TIA, CVA, etc;
* 8\. have received other types of anti-tumor or experimental treatment;
* 9\. the patients are pregnant or lactating women;
* 10\. patients with other diseases or mental disorders may affect their participation in this study;