Comparison of the Pathological Effect Between 2 and 4 Cycles Neoadjuvant CAPOX for Low/Intermediate Risk II/III Rectal Cancer

Inclusion Criteria: 1. Age: 18-75 years old; No gender limitation; 2. Patients diagnosed with low/intermediate risk stage II/III rectal cancer under MRI and transanal ultrasound,defined as: low:T3a-bN0-1M0, EMVI (±), MRF (-) (≥2mm); Middle-high rectal cancer: T3a-cN0-1M0, EMVI (±), MRF (-) (≥2mm); No more than 3 lymph nodes with short diameter over 8mm or highly suspected metastases; Patients with very low rectal cancer who met the above criteria and could achieve negative circumferential resection margin under ELAPE surgery could be included in the group 3. tumor located \<=12cm from anal verge by colonoscopy or anal examination 4. no distant metastasis confirmed by CT examination; 5. rectal adenocarcinoma confirmed by pathology, 6. ECOG score: 0-1; 7. Patients with primary rectal cancer who did not receive surgery (except palliative stomy), radiotherapy, systemic chemotherapy or other anti-tumor therapy before enrollment; 8. Main organs function normally, that is, meet the following characteristics: ① Blood routine examination criteria should meet: Hb ≥9g/dL, WBC ≥ 3.5/4.0×109/L, neutrophils ≥ 1.5×109/L, PLT≥ 100×109/L. ② Biochemical tests should meet the following criteria: CREA and BIL ≤ 1.0 times upper limit of normal (ULN), ALT and AST≤ 2.5 times upper limit of normal (ULN), alkaline phosphatase (ALP) ≤2.5×UNL, total bilirubin (TBIL) ≤1.5×UNL. 9. No history of allergy to platinum drugs when no 5-FU drugs are allergic; 10. Women of childbearing age must have had a pregnancy test (serum or urine) 7 days prior to enrolment, be negative, and be willing to use an appropriate method of contraception during the trial and 8 weeks after the last dosing. For men, surgical sterilization or consent to use an appropriate method of contraception during the trial or for 8 weeks after the last dosing; 11. Subjects volunteered to participate in this study, signed the informed consent, and showed good compliance and followed up. Exclusion Criteria: 1. patients suspect to Lynch syndrome; 2. Patients showed distant metastasis during treatment; 3. Previously or coexisting malignancies (including concurrent colon cancer), except for cured basal cell carcinoma of the skin and carcinoma in situ of the cervix; 4. pregnant or breastfeeding women; 5. Patients with severe cardiovascular diseases and diabetes that is not easily controlled; 6. People with mental disorders; 7. Severe infection; 8. sever renal disfunction; 9. History of gastrointestinal fistula, perforation, bleeding, or severe ulcer; 10. Allergic to 5-FU or platinum; 11. The presence of serious gastrointestinal diseases that affect the absorption of oral chemotherapeutic drugs; (12) Participants in additional clinical trials within 4 weeks prior to the start of treatment.