Accelerating the uptake of proton beam therapy in the UK

Proton and ion beams exhibit better dose characteristics than x-rays for radiotherapy. Their superiority lies in the fact that the radiation dose can be confined largely to the tumour with much lower doses to the surrounding healthy tissue than can be achieved with x-rays. This allows for dose escalation in the tumour itself and therefore better control of the treatment outcome or far more sparing of dose to critical organs.

Proton and ion beam therapy is not new, having first been proposed in 1946, however in recent year’s improvements in the technology has meant it has become much cheaper and proton therapy can now be delivered at approaching twice the cost of x-ray therapy.

Emerging new accelerator technologies such as laser induced beams, dielectric wall accelerators and fixed field alternating gradient accelerators promise lowering the price tag even further.

The NHS has increased support for proton therapy by sending patients abroad as a transitory solution and by committing to support two large-scale proton therapy facilities. These are sited at the Christie Hospital in Manchester, which started treating patients at the end of 2018 and University College London Hospital which is due to come on line by 2020.

In order to make the most of this modality (optimal treatment outcome with minimal side effects) dosimetric accuracy similar to that currently achieved in x-ray therapy is required and improved primary standards and reference dosimetry is needed.

NPL's research efforts in proton and ion beam dosimetry focus on:

• the development of a primary standard for protons and ions based on a graphite calorimeter, similar to the one for photon and electron beams but smaller in size in order to be portable, with major emphasis on the conversion procedure from dose to graphite to dose to water in proton, alpha and carbon ion beams;
• in collaboration with the Institute of Physics in Medicine, the development of a dosimetry code of practice that will allow the proton and ion centres to calibrate their beams more accurately and consistently than the methods that are currently employed;
• improved measurements of the mean energy required to produce an ion pair in air for ionisation chamber dosimetry in proton and ion beams;
• improved measurement and simulation using Monte Carlo methods of ionisation chamber perturbation factors;
• characterisation of the energy dependent absorbed dose response of relative dosimeters such as alanine and radiochromic film;
• establishment of accurate dose conversion procedures from different materials to water;
• development of a micrometer-sized calorimeter based on SQUID technology to measure microdosimetric spectra in ion beams in order to underpin the definition of a new, more biologically relevant quantity for radiotherapy;
• development of water- and tissue-equivalent phantom materials specifically for proton and ion therapy beams;
• improve imaging modalities in order to reduce range uncertainties in proton and ion therapy treatments;
• development of reference and end-to-end dosimetry audits.

NPL has also established a physics research consortium with members from NHS centres, academia and other third party stakeholders and is running regular workshops aimed at promoting research collaborations within the UK. Learn more: Proton Physics Research Implementation Group (PPRIG).