Preoperative Prediction Model of Pathological Outcomes (Mesorectum Quality and Positive Circumferential Resection Margin) in Patients With Mid-low Rectal Cancer

Elaboration of a preoperative prediction model of the quality of the mesorectum and the involvement of the circumferential margin in patients with mid-low rectal cancer who undergo laparoscopic anterior rectal resection. In a second phase the investigators will study the utility of the prediction model in classifying patients with high risk of suboptimal quality of mesorectum and/or positive circumferential margin. Patients with high preoperative risk will undergo a transanal total mesorectal excision and patients with low risk a laparoscopic transabdominal mesorectal excision. The investigators finally will compare pathological outcomes ( quality of mesorectum and circumferential margin), survival and recurrence between the two groups.