Prospectively Investigate the effectiveness and safety of neoadjuvant Bevacizumab + chemotherapy (mFOLFOX6) combined with short-course radiotherapy (25Gy/5Fx) for RAS mutant-type locally advanced rectal cancer

Inclusion Criteria:

1. A biopsy proven histological diagnosis of rectal adenocarcinoma；

2. An ARMS-PCR proven KRAS, NRAS mutation, excluding BRAF mutation or microsatellite instablility-High;

3. No intestinal obstruction, distant metastasis was excluded by CT, MRI or PET / CT;

4. MRI proven T3c-T4, CRM positive, N2, extramural vascular invasion positive or lateral lymph node positive of locally advanced rectal cancer;

5. Age between 18-75 years;

6. ECOG (Eastern US Cooperative Oncology Group) score: 0-1;

7. Has sufficient organ function:

   • Hemopoietic function: hemoglobin ≥ 90 g / L, platelet ≥ 80 g / L × 109 / L,
   • neutrophils ≥ 1.5 × 109/L
   • Liver function: ALT and AST < 2.5 × ULN;
   • Renal function: serum creatinine < 1.5 ULN;

8. Willing to participate and informed consent signed；

Exclusion Criteria:

1. An ARMS-PCR proven RAS wild type, BRAF mutation and microsatellite instablility-High;
2. Patients having undergone chemotherapy, radiotherapy or surgery for colorectal cancer;
3. Patients with other uncontrolled malignant tumors (except early-stage basal cell carcinoma or cervical carcinoma in situ) ;
4. Female patients who are pregnant or breastfeeding;
5. Patients with severe heart, liver, or kidney, or neurological or psychiatric disease;
6. Patients with active infection;
7. Poor overall health status, ECOG ≥ 2;
8. Patients with concomitant diseases that seriously endanger the safety of patients or affect the completion of the study in the judgment of the investigator;
9. Known hypersensitivity reactions to any investigational drugs;