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Proton Beam Therapy

Proton beam therapy is a specialised form of radiotherapy that involves directing a beam of protons (energised sub-atomic particles) specifically at the tumour.

What is proton beam therapy?

Proton Beam Therapy (PBT) is a type of radiotherapy that uses beams of 'protons' (energised particles), instead of beams of X-rays ('photons'), that are used in conventional radiotherapy. It is more targeted than conventional radiotherapy so does less damage to the healthy tissue around the tumour and in the rest of the brain.

A machine (called a cyclotron or particle accelerator) speeds up the protons causing them to gain energy. Once a high level of energy is reached the protons are then beamed out of the machine and guided by magnets towards the tumour.

Protons accurately release most of their cell-killing energy at a particular depth in tissue. This depth can be varied by the medical team who programme the PBT machine. The beam of protons can be set up to stop once it has 'hit' the tumour cells (and travelled for a safe margin past the tumour to capture any tumour cells beyond the visible edge of the tumour).

This means that proton beam therapy destroys the tumour cells whilst not affecting so many of the surrounding cells. In particular, PBT delivers very little radiotherapy in a path through the rest of the brain on the other side of the tumour, unlike other high energy beams (X-rays) used in conventional radiotherapy.

What is the difference between Proton Beam Therapy and conventional radiotherapy?

However, the X-rays do also release some of their energy before they reach the tumour, and continue to release some after they have passed through the tumour and continue on through the brain to exit on the other side of the head.

This causes some damage to the normal, healthy cells in the path of the radiotherapy, but normal brain cells are more able to repair themselves than tumour cells.

This initial damage to the normal cells, however, can cause side-effects in the short-term. Some of the damage may be unrepairable, leading to long-term effects.

Similarly, in PBT the beams of protons are carefully planned to target most of their energy within the tumour.

However, by altering the beam energy, the beam can be designed so that the protons stop and release most of their energy accurately at a particular depth in the brain i.e. where the tumour is situated.

As proton beam therapy is highly targeted towards the tumour, it means it is often possible to treat areas closer to very sensitive structures such as the spinal cord or optic nerve.

The beam is usually set up to travel a little way past the tumour to make sure it gets any tumour cells beyond the edge of the tumour that can't be seen.

This means that fewer healthy cells nearby receive a dose of radiation. This is particularly important in children, whose brains are still developing.

The reduced damage to surrounding healthy tissue means the tumour can be given a higher dose of radiation.

Why isn't everyone given proton beam therapy?

Whether your child/you can have PBT depends on many factors, including:

Brain tumour type

PBT is not suitable for all types of brain tumour. It works best for smaller tumours and those where the edges are clearly defined.

It is also useful where the tumour is close to very important structures like the spinal cord or optic nerve, or when it is important to reduce the damage caused to surrounding normal tissue as much as possible e.g. brain tumours in young children whose brains are still developing.

There is a nationally approved list of tumour types that are suitable for referral.

With regards to brain tumours, these include:

Children, teenagers and young adults:

  • ependymoma
  • optic pathway and other selected low grade glioma
  • craniopharyngioma
  • pineal parenchymal tumours (not Pineoblastoma)
  • base of skull and spinal chordoma
  • base of skull chondrosarcoma


  • base of skull and spinal chordoma
  • base of skull chondrosarcoma

Though not on the approved list, PBT has sometimes been used to treat medulloblastomas in children. However, diagnosis alone is not enough. PBT will only be given if it is thought it will give a significant advantage over conventional radiotherapy.

Timing of when the radiotherapy needs to be given

Timing of when the radiotherapy needs to be given. (Referral abroad may delay the giving of radiotherapy for longer than the health professionals would advise) .

Timing of when the radiotherapy needs to be given in relation to other treatments

Timing of when the radiotherapy needs to be given in relation to other treatments, such as chemotherapy. (It is most important that PBT seamlessly links with surgery and chemotherapy. Disruption in the pathway, and the resulting delays, can lead to less intensive therapy and a reduction in tumour control. This could counteract any longer term benefits. This is particularly important in the treatment of high grade, 'malignant' brain tumours, such as medulloblastoma)

It is also important to realise that:

  • PBT is a relatively new therapy that is usually used for rare types of tumours. This means there is little evidence about its effectiveness compared to conventional radiotherapy, particularly about long-term side-effects
  • the way it works when it reaches the tumour is much the same as conventional radiotherapy
  • the quicker 'stopping' of the radiation in PBT means getting the target right is very important
  • it requires extra training and skill to work out the required depth and dosage of PBT
  • it is harder to check what is actually being treated during PBT treatment - conventional radiotherapy can use imaging to check the accuracy of the treatment area, which is important when near to critical structures, e.g. spinal cord or optic nerve
  • people who travel to receive PBT often respond well, but they are 'optimally chosen' who are more likely to do well

What happens if Proton Beam Therapy is suitable?

If the Clinical Oncologist feels that PBT may be suitable for you/ your child, they will, with your agreement, refer your/your child's case to the Proton Clinical Reference Panel (PCRP) of the UK Proton Overseas Programme for consideration. This is the same for patients from England, Wales, Scotland and Northern Ireland.

There is a strict set of criteria that a patient must fulfil for a referral for PBT to be made. Funding for treatment will not be approved outside these strict criteria. It is also unlikely that the treatment will be successful if these criteria are not filled.

How is Proton Beam Therapy given?

The procedure for giving PBT is very similar to conventional radiotherapy.

In preparation:

There will be several planning appointments at the treatment centre before you/your child starts PBT. These include:

  • scans to plan the treatment and programme the machine accurately
  • a radiotherapy mask will be custom-made to keep your child's/ your head still and in position during the PBT sessions. The mask will be custom-made from a mouldable sheet, which is soaked in warm water and laid on your face. It will feel a bit like having a warm flannel across your face. You will wear the mask during PBT sessions to help hold you in position and keep you still. Some centres use tiny beads implanted under the skull (under anaesthesia) to act as pinpoints to ensure the head is located in the same place instead of a mask.
  • the chance to discuss the treatment, what to expect and any possible side-effects.

After these appointments there is usually a two week gap whilst the treatment plan is calculated and prepared specifically for you/your child.

The Proton Beam Therapy procedure:

  • Your child (or you) will lie on a treatment couch, and the mask will be placed over their/your face and attached to the couch. You will be positioned correctly. Young children may need to have a general anaesthetic each day of treatment to make sure they stay still.
  • Before the PBT begins, the radiotherapy team will leave the room, but they can still see and hear your child or you. You will not be allowed to stay in the room with your child.
  • A machine called a cyclotron accelerates the protons to high energies. The protons then exit the machine, guided by magnets, towards the tumour.
  • Once at the site of the tumour, the protons deposit their energy, destroying the tumour cells.
  • After treatment, the team will re-enter the room and help take off the mask.

You will then usually be able to go straight home.

The treatment generally only lasts one minute, but with the positioning and adjustments, the session is likely to be about 20 minutes.

It is usually given daily (Monday to Friday) for up to 5-7 weeks. This depends on your child's/your tumour type and grade.

You will need to be away from home for at least 8 – 10 weeks.

Will I need to stay in hospital when having PBT treatment?

No, PBT is usually given as an outpatient treatment, which means you do not need to stay in hospital.

Occasionally, you /your child may need to stay in hospital. For example, if you/your child have side-effects that need inpatient care, or if you/your child need inpatient chemotherapy alongside the PBT.

It is worth bearing in mind, however, that brain tumour patients are usually referred to Switzerland or the USA for PBT, which can mean substantial time away from home.

Will proton beam therapy hurt?

The PBT itself shouldn't cause any discomfort, but you may experience some temporary unwanted side-effects following the treatment.

What are the side-effects?

These are likely to be fewer than with conventional radiotherapy, but how your child/you feel will depend on the size of the dose of PBT you had and whether you are having any other treatment, such as chemotherapy. Find out more about the side-effects of proton beam therapy.

If you have further questions, need to clarify any of the information on this page, or want to find out more about research and clinical trials, please contact our team:

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