Adenocarcinoma/therapy; Fluorouracil/therapeutic use; Leucovorin/therapeutic use; Motor Neurons; Peripheral Nervous System Diseases/etiology; Rectal Neoplasms/therapy; Rectum/innervation; Adenocarcinoma/drug therapy; Adenocarcinoma/pathology; Adenocarcinoma/radiotherapy; Adenocarcinoma/surgery; Adult; Aged; Antimetabolites, Antineoplastic/adverse effects; Antimetabolites, Antineoplastic/therapeutic use; Biopsy; Female; Fluorouracil/adverse effects; Follow-Up Studies; Humans; Leucovorin/adverse effects; Male; Manometry; Middle Aged; Motor Neurons/drug effects; Motor Neurons/radiation effects; Peripheral Nervous System Diseases/physiopathology; Preoperative Care; Pressure; Prospective Studies; Radiotherapy, Adjuvant/adverse effects; Rectal Neoplasms/drug therapy; Rectal Neoplasms/pathology; Rectal Neoplasms/radiotherapy; Rectal Neoplasms/surgery; Rectum/physiopathology; Risk Factors; Vitamin B Complex/adverse effects; Vitamin B Complex/therapeutic use