Radiotherapy is the use of controlled, high-energy radiation, usually X-rays, to destroy cancerous cells. The treatment doesn’t make you radioactive and it’s perfectly safe to be with others during the course of your treatment. It is only used in rectal or anal cancer. Some specialised techniques may be used in advanced colon or rectal cancer.

If you have been diagnosed with cancer in your rectum, radiotherapy may be offered to you as a treatment option. It is usually given at the same time as chemotherapy, as this makes the cancer cells more sensitive to the radiation. Chemotherapy and radiotherapy together is known as ‘chemo-radiation’. Radiotherapy is not usually used to treat cancer in the colon.


Chemo-radiotherapy is an effective alternative to surgery for anal cancer. Please see our Anal Cancer Factsheet for further information.

Radiotherapy machine

How is radiotherapy given?

Radiotherapy can be given in two ways:


External radiotherapy is delivered from outside of the body by a machine and only takes a few minutes. For this type of radiotherapy you will often need a CT scan 7 – 10 days before treatment to help plan how to give it.


Internal radiotherapy involves positioning radioactive sources inside or near to the tumour. The main advantage of this treatment is that it can deliver a high dose of radiation directly to the cancer while limiting damage to surrounding tissues and organs. This treatment, also known as brachytherapy or contact radiotherapy, is not currently available at all cancer centres, and you may have to travel to a specialist centre.


The Papillon technique is an example of contact radiotherapy for rectal cancer. It is performed at Clatterbridge in Liverpool and Castle Hill in Hull, Nottingham City Hospital and St Luke’s in Guildford. It may be used where appropriate as an alternative to surgery on tumours which are fairly low in the rectum, less than 3 cm diameter and where the cancer is at an early stage (T1 or T2). Patients who are not medically fit for other treatments may benefit from this technique, although not all rectal cancers respond. The conventional treatment for rectal cancer is with external radiotherapy followed by surgery.


Radiotherapy can be used at several different stages of treatment:

Pre-operative radiotherapy

Radiotherapy can be given before surgery (also known as neo-adjuvant radiotherapy) to shrink the tumour in order to make it easier to remove and reduce the risk of the cancer coming back after your surgery.

If you are prescribed external radiotherapy, the treatment could either be a short course (eg five treatments over a period of five days) or a long course (eg five treatments a week, for five or six weeks). The length of the treatment will depend on the size of your tumour.


If prescribed internal radiotherapy, this treatment usually takes one or two sessions.


Whether you have external or internal radiotherapy, there will be a period of time between the radiotherapy treatment ending and your planned date for surgery. This delay is intentional, as the radiotherapy continues to shrink the tumour for several weeks after the end of treatment.

Post-operative radiotherapy

If you have not had radiotherapy before your surgery, you may be advised to have external radiotherapy  with chemotherapy after surgery (also known as ‘adjuvant radiotherapy’) to destroy any cancer cells that may be left behind. this might be because the tumour was difficult to remove or has grown through the wall of the rectum or spread to nearby lymph nodes.

The course of treatment may vary from five to seven weeks, five days a week.

Palliative radiotherapy

Lower doses of radiotherapy, over a shorter period of time, can also be given if surgery is not an option to relieve symptoms and slow the spread of the cancer.

If your clinical oncologist believes you will benefit from radiotherapy, they will discuss the proposed treatment plan with you. Ask your doctor about what they are recommending for you, how the radiotherapy will be given and the choices you have.

Specialist radiotherapy techniques

Radiotherapy and other similar technologies can also be used in novel ways to treat metastases that may have developed in other parts of the body eg the liver, brain or lung. These very special techniques can only be carried out in a few specialist centres around the UK, but they may offer individual patients a viable alternative to surgery, if the tumours cannot be removed safely by surgery. There are a number of ways these new techniques are used, and you may hear them referred to as IMRT (Cyberknife) and RFA (Radiofrequency Ablation).