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What is external radiotherapy?

External radiotherapy uses a machine outside the body to direct radiation beams at cancer to destroy it.

This is different to internal radiotherapy which means giving radiotherapy to the cancer from inside the body.

How radiotherapy works

Radiotherapy treatment works by damaging the DNA within the cancer cells. DNA is the genetic code that controls how the body's cells behave. 

Before starting your treatment

Before starting your treatment your doctor will discuss with you the possible short and long term side effects. Most side effects are temporary and can usually be well controlled with medicines.

Your treatment team uses images from a range of tests to plan and monitor your treatment. The tests might include:

  • x-rays
  • CT scans
  • MRI scans
  • PET scans

Your treatment team carefully plans the radiotherapy to give a high dose to the cancer. 

How you have external radiotherapy

You have treatment in the radiotherapy department. Radiotherapy machines are very big and can vary in how they look and work.

Photo of a linear accelerator

The most common type of machine is called a linear accelerator (LINAC), which uses electricity to create the radiotherapy beams.

The machine doesn't touch you and you don’t feel anything during treatment. But you may get some discomfort or pain later on from the side effects. 

For the radiotherapy to work as well as possible, it is important that the radiotherapy field covers the whole cancer, plus a border around it. Doctors try to give as low a dose as possible to the surrounding healthy tissue to reduce the risk of side effects.

Radiotherapy fractions

The full dose of radiation is usually divided into a number of smaller doses called fractions. This allows healthy cells to recover between treatments. You have the fractions as a series of treatment sessions that make up your radiotherapy course. You might have a course of treatment over several weeks, but this varies.

Radiotherapy to relieve cancer symptoms, such as pain, is called palliative radiotherapy. You often have this in fewer fractions and sometimes it is just one treatment. The radiotherapy dose with each fraction is bigger but you have fewer fractions so the overall radiotherapy dose is lower.

Palliative radiotherapy has fewer side effects than radiotherapy that aims to cure the cancer.

Types of external radiotherapy

Most types of radiotherapy treatments use photons, but electrons and protons can also be used.

Photons and protons can treat cancers that are deep inside the body or those on or close to the skin (superficial cancers). Electrons are only used for superficial cancers. 

Treatments using photons, protons and electrons will vary slightly but the experience of having radiotherapy will be similar.

Your doctor chooses the type of radiotherapy and the machine for your treatment according to the type of cancer you have and where it is in your body. There are different types of external radiotherapy treatment, which include:

  • conformal radiotherapy
  • intensity modulated radiotherapy (IMRT)
  • image guided radiotherapy (IGRT)
  • 4-dimensional radiotherapy (4D-RT)
  • stereotactic radiotherapy (SABR) and radiosurgery (SRS)
  • adaptive radiotherapy

Side effects of radiotherapy

External radiotherapy doesn't make you radioactive. It is safe to be with other people, including children, throughout your treatment course.

People react to radiotherapy in different ways:

  • some carry on working and just take time off for their treatment
  • others feel tired and want to stay at home
  • some people stay in hospital for all or part of the treatment

Your doctors, radiographers and nurses will advise you about how best to manage your course of treatment. If you have family members to look after, you may need extra help. You can ask for help from your employer, family or friends, or the staff in the radiotherapy department. 

As your treatment goes on you'll have more of an idea of how it makes you feel. You can then make any necessary changes to your daily life to help you cope with the rest of your treatment course.

Last reviewed: 
21 Dec 2018
  • External Beam Therapy (2nd edition) 
    Peter Hoskin
    Oxford University Press, 2012

  • Devita, Hellman and Rosenberg's Cancer Principles and Practice of Oncology (10th edition)
    VT Devita, TS Lawrence and SA Rosenberg
    Wolters Kluwer Health, 2015. 

  • Radiotherapy Services in England 2012
    Department of Health, November 2012

  • Walter and Miller's Textbook of Radiotherapy: Radiation Physics, Therapy and Oncology (7th edition)
    R Symonds and others
    Elsevier LTD, 2012

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