A Trial of Robotic-assisted Versus Laparoscopic Abdominoperineal Resection for Treating Low Rectal Cancer

Inclusion Criteria: * Histologically proven rectal adenocarcinoma * Inferior edge of the tumor located within 5 cm from the anal verge as determined by colonoscopy withdrawing and digital rectal examination * No evidence of distant metastases (including pelvis, peritoneum, liver, lung, brain, bone, distant lymph node, etc) according to ultrasound, CT, PET-CT, etc * Tumor assessed as cT1-T3 or ycT1-T3 after preoperative neoadjuvant chemoradiotherapy by pelvic MRI * No other malignancies in medical history except adequately treated basocellular carcinoma of the skin or in situ carcinoma of the cervix uteri * Suitable for both robot-assisted and laparoscopic surgery * American Society of Anesthesiologists (ASA) class I - III * No other preoperative treatment except neoadjuvant chemoradiotherapy * Informed consent Exclusion Criteria: * Tumors assessed as cT1N0 and suitable for local excision * Signs of acute intestinal obstruction, bleeding or perforation needing emergency surgery * More than one colorectal tumor * Familial Adenomatosis Polyposis, Lynch Syndrome, acute inflammatory bowel disease * Schedules need for other synchronous colon surgery * Absolute contraindications to general anesthesia or prolonged pneumoperitoneum (ASA class \> III) * Pregnancy or lactation * Patients and/or family members can not understand and accept this study * Patients received chemoradiotherapy or other anti-tumor therapy before surgery