Brain radiotherapy side effects

You might have radiotherapy for a cancer that started in the brain. This is a primary brain tumour. Or you might have radiotherapy for a secondary brain cancer. This means a cancer that has spread to the brain from another part of the body.

The main side effects of radiotherapy to the brain include:

  • tiredness
  • sickness
  • worsening of symptoms

You may also experience more general side effects of radiotherapy such as hair loss and sore skin.

Like all side effects of radiotherapy, you might not feel these straight away. Side effects tend to come on gradually as you go through your treatment. And for a few weeks afterwards. Everyone reacts differently to radiotherapy, so how you feel varies from person to person. 


Radiotherapy can cause tiredness. There are different reasons for this. The body uses up energy reserves to repair healthy cells damaged by the radiotherapy. If you have taken steroids as part of your treatment Open a glossary item, you might also find that you feel extremely tired when you stop taking them. Travelling to the hospital each day for treatment can also make you tired.

You might feel weak and lack energy as well as being tired. Various things can help you to reduce tiredness and cope with it. This includes exercise. This can give you more energy but you need to balance it with resting.

Extreme tiredness (somnolence)

In a few people, the tiredness can become very severe a few weeks after treatment has finished. You may also feel drowsy and irritable. This is a rare side effect and is called hypersomnia or somnolence syndrome.

It is extreme tiredness that can make you feel very drowsy and want to sleep a lot. You might also have:

  • headaches
  • a high temperature
  • loss of appetite
  • feeling and being sick (nausea and vomiting)

If you are sleeping a lot or have these symptoms and are worried, contact the team looking after you. It usually gets better on its own over a few weeks.


Radiotherapy to the brain can make you feel or be sick. This is called nausea and vomiting. The sickness might last for a few weeks after the treatment has finished. Medicines, diet and sometimes complementary therapies can help to control sickness.

Sickness can usually be well controlled with medicines. Your radiotherapy team can give you anti sickness tablets. Some people find that it helps to take an anti sickness tablet about 20 to 60 minutes before having radiotherapy.

Other people find they manage better by taking anti sickness tablets regularly throughout the day during their course of treatment. You can discuss which would be best for you with your radiotherapy team.

Your doctor might give you steroids to take while having brain radiotherapy. Radiotherapy to the brain can cause swelling which can lead to nausea. Steroids can help to relieve the pressure. 

Worsening symptoms

Radiotherapy to the brain causes short term swelling in the treatment area. This raises the pressure in the brain.

Doctors call this oedema. It can make your symptoms worse for a time. This can be frightening because you might think the radiotherapy isn't working. Raised pressure in the brain might cause:

  • headaches
  • feeling sick
  • fits (seizures)

The swelling from radiotherapy goes down after treatment. While you have the treatment your doctor gives you medicines to take. 


Steroids can reduce the swelling in the brain. Your doctor will tell you how long to take the steroids for. Usually, you gradually lower the dose of steroids after the treatment ends. But if the swelling hasn't gone down enough, your symptoms might come back.

It is important to talk to your radiotherapy team if you think your symptoms are getting worse. They will be able to reassure you and might increase your dose of steroid tablets until your symptoms gets better.

Bevacizumab (Avastin)

You might have a targeted cancer drug called bevacizumab (Avastin) if you can't take steroids for any reason. Bevacizumab can lower raised pressure in the brain. It does this by changing the growth of blood vessels around the tumour.

This video shows people affected by cancer talking about the side effects of radiotherapy for brain cancer. The video is around 5 and a half minutes long.

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Last reviewed: 
05 Feb 2024
Next review due: 
05 Feb 2024

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