What is the evidence that proton beam therapy works?

Although the theory supporting the use of proton beam therapy is attractive and plausible, there is some controversy about whether there is hard evidence to show that it is just as good, or better, than radiotherapy that uses X-rays.

The most recent review, which looked at all the studies done to date, was published in 2012.
Aaron Allen and colleagues from the USA and Israel looked at the evidence for proton beam therapy on behalf of ASTRO – the American Society of Radiation Oncology.

What are the latest conclusions?

Proton beam therapy is an important development in the field of radiotherapy.

Evidence does exist that proton beam therapy is better than standard radiotherapy for key cancers.

Allen et al. said that there is evidence to show that proton beam therapy is the treatment of choice for the following cancers:

  • Central nervous system tumours that occur in children.
  • Adult eye cancers, particularly large eye melanomas.
  • Adult chordomas, tumours that arise from the spinal cord.

Some evidence shows that proton beam therapy is just as effective as radiotherapy for prostate cancers and primary liver cancers, but the data does not show that PBT is better.

Although proton beam therapy is emerging as a new therapy for other cancers, the evidence is not yet strong enough to show that it is as effective as or better than standard radiotherapy. This does not mean that PBT does not work in these cancers; it just means we don’t yet have the evidence to say either way.

Details of the evidence so far

The review by Aaron Allen and the other experts provided a summary of the evidence for the value of proton beam therapy at different body sites, in adults and in children.

Central nervous system cancers

Proton beam therapy seems to live up to its theoretical advantage when treating cancers of the spinal cord and at the base of the skull. Tumours at these locations are very close the spinal cord and the brain. Proton beam therapy allows a high dose of radiation to be accurately directed at the tumour but very little radiotherapy goes into the healthy nerve or brain tissue close by.

  • In one study, proton beam therapy was successful in treating 80% of chordomas, limiting local spread.
  • In another study, local control of meningiomas, a type of brain tumour, was achieved in 91.7 – 100% of patients treated.

Eye melanoma

Thousands of people with eye melanoma have been treated with proton beam therapy. Accumulated data from the USA and Europe shows that 95% of patients have their cancer controlled and 90% avoid having to lose their eye.

Childhood cancers

Proton beam therapy is accepted as a better treatment for childhood cancers because it has lesser effects on the healthy tissue around tumours.

  • Good results for PBT have been obtained in children with orbital rhabdosarcoma, a rare eye cancer and ependymomas, a tumour of the brain and spinal cord.
  • Several studies show that proton beam therapy is the best treatment for childhood brain tumours.

Adult head and neck cancers

Results suggest that proton beam therapy may have the advantage over intensity modulated radiotherapy for tumours in the nose/sinuses as the less damage to surrounding tissues spares brain damage.

There is little evidence at the moment to show that PBT is better than standard radiotherapy for head and neck cancers in general.

Prostate cancer

Published studies have reported around 2000 patients with prostate cancer being treated with proton beam therapy. This is more than any other cancer type.

Although the findings are positive, there is no clear winner between methods of standard radiotherapy, which are very good in the case of prostate cancer, and PBT. More trials, probably with a direct comparison between the two treatments would be required to find out.

Lung cancer

Proton beam therapy has been used in non-small cell lung cancer but evidence does not show it is more effective than standard radiotherapy. It is more difficult to program PBT to hit a moving target. As the lungs move during breathing, this means additional challenges when planning and focusing the proton beam therapy.

GI cancers

Very few studies have been done in this area. Some studies suggest that proton beam therapy could have fewer side effects in pancreatic cancers or oesophageal cancers, as these may be close to vital tissues that would be more damaged by standard radiotherapy. Primary liver cancers have been treated successfully with PBT but not enough data has been collected to show it is better than radiotherapy.

Does the lack of evidence matter?

Obviously, it is not acceptable to press ahead with a treatment when there is no evidence that it does any good. However, when a treatment seems to have an effect on tumour tissue that is very similar to standard radiotherapy but has fewer side effects, then further research to discover the longer term benefits is sensible.

As proton beam therapy is only just starting to be used in patients with cancers outside the central nervous system and the eye, we need to carry on using PBT in clinical trials to gain more information. Only then can doctors in the future make their decisions based on clearer information.

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