Radiotherapy is a treatment that involves the use of high-energy radiation. It is commonly used to treat cancer.

Almost half of all people with cancer have radiotherapy as part of their treatment plan.

Radiotherapy is also sometimes used to treat benign (non-cancerous) tumours and other conditions – such as thyroid disease and some blood disorders.

This information focuses mainly on the use of radiotherapy as a treatment for cancer.

Why it is used

Radiotherapy can be used, alone or in combination with chemotherapy (chemoradiotherapy), to try to cure cancers.

For people with incurable cancers, radiotherapy is a very effective method of controlling symptoms.

Radiotherapy can also be used before surgery to shrink a tumour so it's easier to remove (known as neoadjuvant treatment), or after surgery to destroy small amounts of tumour that may be left (known as adjuvant treatment).

How it is carried out

Radiotherapy can be given in two different ways – from outside the body (external radiotherapy) or inside the body (internal radiotherapy).

External radiotherapy usually involves using a machine called a linear accelerator, which focuses high-energy radiation beams onto the area requiring treatment. External beam radiotherapy is completely painless.

External beam radiotherapy usually involves a series of daily treatments over a number of days or weeks.

Internal radiotherapy can involve placing a small piece of radioactive material temporarily inside the body near the cancerous cells (known as brachytherapy), or the use of a radioactive liquid that's swallowed or injected. The radiation emitted by internal radiotherapy is painless, though the procedure to insert the source can sometimes cause mild discomfort

The type of radiotherapy you have – and the length of the course of treatment – will depend on the size and type of cancer and where it is in the body.

Read more about how radiotherapy is performed.

How it works

The high-energy radiation used during radiotherapy permanently damages the DNA of cancer cells, causing them to die.

Nearby healthy tissues also suffer temporary cell damage from radiation but these cells are usually able to repair the DNA damage and continue growing normally.

Side effects

The temporary DNA damage to normal tissues causes side effects, most of which are short-lived.

Common side effects include sore skin, tiredness and hair loss. These tend to get better within a few days or weeks of treatment finishing.

In rare cases, radiotherapy can cause significant long-term effects. For example, treatment to the genitals or pelvic region can cause permanent infertility.

Read more about the side effects of radiotherapy.


Despite the side effects, radiotherapy can be a highly effective treatment for cancer. Four out of every 10 cancer cures include radiotherapy as part of the treatment plan.

However, radiotherapy does not cause cancerous tumours to shrink immediately and it can take some time for the beneficial effects to become apparent. These effects will depend on the type of cancer you have, other treatments that may be given alongside (such as chemotherapy or surgery) and how advanced the tumour is when treatment begins.

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