Intestinal Microbiome Modulation With Antibiotics in the Neoadjuvant Treatment of Locally Advanced Rectal Cancer

Inclusion Criteria: * Patients over 18 years old with middle and lower rectal adenocarcinoma, classified as locally advanced or where the tumor´s location necessitates rectal amputation surgery; * Tumors located below the peritoneal reflection, as determined by MRI, will be defined as middle and lower rectum; * Patients eligible for rectal amputation will be those with tumor whose distal margin is less than 2,0cm from the pectineal line, as determined by rectoscopy, or less than 2,0cm from the anorectal ring by palpation or MRI. * Patients with middle rectum cancer candidates for radiotherapy as part of neoadjuvant treatment will be included according to the following criteria: mesorectal fascia compromised by direct extension of the primary tumor or by an affected lymph node; cT4 tumors. Exclusion Criteria: * Patients without performance status for total neoadjuvant treatment; * Histologies other than adenocarcinoma; * Adenocarcinomas with a microsatellite instability phenotype, as determined by immunohistochemistry; * Patients with middle rectal tumors who are not indicated for radiotherapy in the neoadjuvant treatment regimen, according to the institutional protocol (tumors cT3N0, with free mesorectal fascia; cT1-3 N+, with free mesorectal fascia).