ELECTRA is a feasibility trial to determine whether it is acceptable and feasible to recruit, randomise and deliver Intraoperative Electron Radiotherapy (IOERT) in a randomised controlled trial setting. Patients with locally advanced rectal cancer or locally recurrent rectal cancer involving the pelvic sidewall or posterior bony pelvis will be randomised to receive either extended margin surgery along, extended margin surgery with IOERT at standard dose (10 Gy), or extended margin surgery with IOERT at higher dose (15 Gy). Patients will be randomised to the three treatment arms in a 1:1:1 ratio. Patients will be followed up for a minimum of 12 months and up to 24 months following their surgery.