Proton radiotherapy: hype, fiction and a few facts
The tragic case of the missing boy with a brain tumour sparked headlines around the world. When five-year-old Ashya King’s parents were arrested for removing him from a UK hospital to seek alternative medical treatment abroad, few people had heard of proton beam therapy. However, all that has changed now that Ashya has been transferred to a clinic in the Czech Republic to undergo the treatment.
Proton beam therapy (or our preferred title of proton radiotherapy), is a form of radiotherapy delivered with beams of high energy protons rather than the more usual beams of high energy x-rays (or less frequently beams of electrons). It is part of the general field of particle radiotherapy that includes use of particles that are heavier than protons. Beams of carbon ions are the main topic for clinical research around the world, although there is also interest in other particles such as helium ions.
Radiotherapy involves the delivery of very large radiation doses to tumours with the aim of ensuring that none of the cancer cells in the tumour are able to go on reproducing after the treatment.
The main potential advantage of proton radiotherapy comes from the basic physics of proton interactions and the generation of what is termed a ‘Bragg peak’. Energy is deposited at a relatively low rate in the entrance path of the beam, and rises to a maximum at depth. The position of this maximum can be varied to match the depth of a tumour and beyond this depth, no dose is delivered. This is quite different from the physics of x-ray interactions which follow a more continuous deposition of dose, being higher in the entrance region and reducing gradually with depth into the patient. Actual treatments often involve beams delivered from many angles all converging on the identified tumour.
This suggests that proton therapy represents a dramatic and substantial improvement on x-ray radiotherapy for many patients. This is not always correct, however, as modern x-ray techniques such as intensity modulated radiotherapy (IMRT) can do an excellent job of shaping dose distributions and so detailed treatment planning studies and clinical evaluations are required to understand what benefit proton therapy might deliver. In addition, certain technical aspects, such as image guidance to target treatment, may not be as advanced in some proton centres when compared to modern IMRT departments. The area of least debate is in the treatment of some children with cancer.
Access to proton radiotherapy for patients in England is organised though the NHS Specialised Services team. A list of agreed indications was developed based upon tumour sites with most clinical experience and potential for benefit. The list is dominated by childhood cancers as the use of proton radiotherapy over conventional treatment should help to minimise any detrimental effects of radiotherapy on developing tissues and late side effects such as radiation induced cancer. Treatment is delivered in highly selected international centres with expertise for specific indications. Funding covers all treatment-related travel and accommodation costs. Although proton radiotherapy has the potential to benefit a wider range of tumours, these remain to be clinically confirmed.
It is most important that proton radiotherapy seamlessly integrates with surgery and chemotherapy in a timely fashion. Disruption in the pathway can lead to a reduction in tumour control off-setting longer term benefits. Patients with medulloblastoma often require such a co-ordinated approach. Furthermore, due to the cancer and treatment, medulloblastoma patients are often acutely unwell making integration of proton beam radiotherapy into routine care particularly challenging while it is not available in the UK.
When the two NHS proton radiotherapy centres open in London and Manchester in 2018, the number of funded indications will expand. The national and local NHS infrastructure will be used to ensure that all aspects of the required care will be delivered in an optimum way, even though many patients will still be far from home. Childhood cancers like medulloblastoma will almost certainly be included in the agreed indications for treatment in the London and Manchester centres.
A consortium led by University Hospital Birmingham and involving the University, Birmingham Children’s Hospital and the Royal Orthopaedic Hospital was successful in being designated as the third NHS centre for proton radiotherapy. The timescale on which this centre will be developed has not yet been agreed.
Honorary Professor, School of Physics and Astronomy and Director of Medical Physics, University Hospital Birmingham
Clinical Service Lead for Radiotherapy, University Hospital Birmingham