Organ Preservation Strategy of Total Neoadjuvant Chemoradiotherapy for Low Rectal Carcinoma

Inclusion Criteria: * Age≥18 years * cT1-3N0M0 assessed by CT, MRI and ultrasound colonoscopy * Rectal adenocarcinoma confirmed by biopsy pathology and the lower edge of the lesion was within 5cm from the anal edge * Patients who have a strong desire to preserve the anus and are willing to accept neoadjuvant treatment, meanwhile is suitable for the implementation of organ preservation strategy after discussion within the multidisciplinary team of the hospital * No previous treatment (including endoscopic resection, transanal local resection, radiotherapy, chemotherapy, etc.) before enrollment * No contraindications to chemoradiotherapy * No other colorectal organic diseases * Voluntarily sign the informed consent Exclusion Criteria: * Have received previous treatment (including endoscopic resection, transanal local resection, radiotherapy, chemotherapy, etc.) before enrollment * Patients with concurrent colorectal organic diseases * Patients with familial polyposis * Patients with a previous history of colorectal surgery or pelvic radiotherapy that may affect the outcome of this treatment * Pregnant or lactating women * The patient or family members could not understand the conditions and objectives of this study * With a history of uncontrolled epilepsy, central nervous system diseases, or mental disorders, their clinical severity may hinder the signing of an informed consent form or affect their compliance with oral medication * It is difficult to achieve complete remission base on existing evidence , such as: maximum diameter of cT2/T3 tumor\>4cm; suspicious Lateral lymph nodes (maximum diameter) =5mm; baseline CEA≥100; biopsy pathology contains signet ring cell carcinoma mucinous adenocarcinoma; evaluation team determines whether circumferential constrictive tumor should be included or not when necessary Severe heart disease, such as symptomatic coronary heart disease, New York Cardiology Association (NYHA) grade II or above severe congestive heart failure, or severe arrhythmia requiring pharmacological intervention, or a history of myocardial infarction within the last 12 months * Organ transplantation requires immunosuppressive therapy * Serious uncontrolled recurrent infections, or other serious uncontrolled concomitant diseases * The baseline blood routine and biochemical indexes of the subject do not meet the following criteria: hemoglobin ≥90g / L; absolute neutrophil count (ANC) ≥1.5\*10\^9 / L; platelet ≥100\*10\^9 / L; ALT and AST≤ 2.5 times normal upper limit; ALP ≤2.5 times normal upper limit; serum total bilirubin \<1.5 times normal upper limit; serum normal creatinine"\<1 time upper limit; and serum albumin ≥30g / L