Single centre double-blinded three-arm randomised controlled trial of extended margin surgery + IOERT at standard dose (10 Gy) versus extended margin surgery + IOERT at higher dose (15 Gy) versus extended margin surgery alone in a 1:1:1 ratio in patients with Locally Advanced Rectal Cancer (LARC) or Locally Recurrent Rectal Cancer (LRRC).

Rectal cancer is a cancer that occurs in the pelvis from the rectum. Locally advanced rectal cancer outgrows the rectum and attaches to other body parts in the pelvis and locally recurrent rectal cancer is a rectal cancer that comes back after surgery, and usually attaches to many different pelvic structures. They are both difficult to manage. The standard of care treatment involves chemotherapy and radiotherapy, followed by what is known as an extended margin operation to remove all cancer affected organs and not leave any cancer cells behind. If cancer cells reach the edge of the removed tissue, there is a high chance of leaving cancer cells behind. This is a key predictor of negative outcome in patients. Intraoperative electron beam radiotherapy (IOERT) was developed to help improve patient outcomes. Once the cancer has been removed, the surgeon and a cancer radiotherapy specialist examine the patient's scans, the cancer specimen and the area the cancer was in, and if there is concern about small numbers of cancer cells being left behind they treat the area with radiotherapy to destroy these cells. Patients that are due to receive treatment for these subsets of rectal cancer will be approached to take part. If eligible on the day of surgery the patient will be randomised to one of three arms: Arm A - standard of care (No IOERT), Arm B - extended margin surgery plus IOERT (10 Gy), or Arm C - extended margin surgery plus higher dose IOERT (15 Gy). The surgeon, cancer specialist team and patient will be blinded to study treatment. Patients will be followed up at 30 days, 3 months and for a minimum of 12 months post surgery as part of the trial and they will be followed up for 5 years as part of standard care.

Inclusion Criteria:

• Age ≥ 16
• Non-metastatic/oligo-metastatic (up to 3 lesions from 2 sites predicted to be radically treatable) - locally advanced or locally recurrent disease involving the posterior or lateral components of the pelvis and predicted to be resectable but with close margins from imaging as determined by a specialist MDT (sMDT)
• Colorectal sMDT review with experience in pelvic exenteration, which has proposed IntrOperative Electron Radiotherapy (IOERT) as an option for treatment
• Patient suitable for IOERT as component of treatment in the view of the responsible Clinical Oncologist
• Performance status ≤1 as defined by the Eastern Cooperative Oncology Group (ECOG)
• Deemed medically fit for surgery
• Written informed consent

Exclusion Criteria:

• Unresectable disease/likelihood of R2 resection
• sMDT determined excess prior radiotherapy within IOERT target zone
• Women who are pregnant or breastfeeding
• Participation within an interventional clinical trial within 3 months of the point of registration within ELECTRA

This is a double-blinded trial, which means both the participant and clinicians will not know the treatment allocation. The surgeon, oncologist and patient will remain blinded throughout the study, while the radiotherapy delivery team will know the outcome of the randomisation.

Patient allocation will be provided to the unblinded radiotherapy staff via an Interactive Web Response System (IWRS). The randomisation code will be retained by the system. Deaths and serious adverse events (SAE) will be reviewed in a blinded manner.

Unblinding will be required in the event that any participants further treatment may benefit from further radiotherapy, and if potential IOERT treatment and its dose may impact this.