Reconstruction Techniques and Vascular Approach After Laparoscopic Left Hemicolectomy and Rectum Low Anterior Resection

This is an interventional, controlled and randomized single-center clinical trial with patients diagnosed with colorectal cancer that will undergo laparoscopic left hemicolectomy or laparoscopic low anterior resection for rectal cancer. All patients included in the study (study 1) are randomized into two groups (group 1A and group 1B): in the group 1A the surgical technique initially dissects the inferior mesenteric vein (IMV) and in the group 1B the surgical technique initially dissects the inferior mesenteric artery. The patients who will undergo a colorectal anastomosis will be included in the study 2 and will be randomized to group 2A and group 2B: in group 2A the patients will have a latero-terminal colorectal anastomosis and in group 2B a termino-terminal colorectal anastomosis. Study hypothesis: Study 1: the surgical technique that initially dissects the IMV presents a greater number of total resected lymph nodes compared to the technique that initially dissects the IMA. Study 2: latero-terminal anorectal anastomosis has a better anorectal functional outcome than termino-terminal anorectal anastomosis. Primary outcome measures: * Study 1:To compare the total number of resected lymph nodes and the distance to the surgical margins between the two arms (1A and 1B). * Study 2: to compare the anorectal functional outcome at 12 months after surgery or ileostomy closure between the two arms (2A and 2B). The anorectal function will be assessed by the scores of the validated Colorectal Functional Outcome Questionnaire (COREFO) and the validated scale LARS (Syndrome anterior resection of the rectum). Secondary outcome measures: * Study 1: to compare the duration of surgery, bleeding and need for surgical conversion between the two arms (1A and 1B). Another objectives are to compare the incidence of postoperative complications (measured by the Clavien-Dindo classification) and survival. * Study 2: to compare the quality of life and anorectal functional outcomes