Dose Escalated Radiotherapy for Rectal Cancers Using MR-guided Radiotherapy

Inclusion Criteria: * All patients aged ≥18 years * Histological confirmation of locally advanced rectal adenocarcinoma requiring neoadjuvant chemoradiotherapy as per RMH clinical guidelines. * Patients suitable for concomitant chemotherapy * Patients with disease included within treatment field size of MR-Linac. * WHO Performance status 0-2 * Ability of the participant understand and the willingness to sign a written informed consent form. * Ability/willingness to comply with the patient reported outcome questionnaires schedule throughout the study. Exclusion Criteria: * Contraindications to MRI (e.g. pacemaker, potentially mobile metal implant, claustrophobia) * Disease outside maximum radiotherapy treatment field length. * Comorbidities which predispose to significant toxicity (e.g. inflammatory bowel disease) or preclude long term follow up * Unilateral or bilateral total hip replacement, or other pelvic metalwork which causes artefact on diffusion-weighted imaging * Previous pelvic radiotherapy * Patients needing induction chemotherapy prior to chemoradiotherapy * Previous invasive malignancy within the last 2 years excluding basal or squamous cell carcinomas of the skin, low risk non-muscle invasive bladder cancer (assuming cystoscopic follow up now negative) or small renal masses on surveillance.