Highlighting Welsh Cancer Research
The future of proton therapy in the UK
Prof Roger Taylor - Swansea University Medical School, Swansea, UK
Proton therapy is a means of delivering radiotherapy to patients. The majority of patients treated with radiotherapy have X-rays as the means of delivering their radiotherapy and the X-rays cause ionisations in biological molecules, particularly DNA, and that’s how radiotherapy works. But another means of delivering radiotherapy which is gaining increasing popularity and increasing interest at the moment is the use of protons to deliver radiotherapy and cause the ionisation in DNA. Now protons, unlike X-rays, because they’re particles have a finite track length within tissue and they deliver their energy up to a point which is known as the Bragg peak. So they deliver their energy up to that point and then beyond that there’s such a rapid fall-off that to all intents and purposes there’s no delivery beyond that Bragg peak.
So proton therapy is a means of delivering very targeted radiotherapy to tumours. Now, that can be used in various ways. Firstly, because of the physical characteristics of the dose delivery it can deliver radiotherapy to tumours which are very close to radiosensitive normal structures, for instance if you’re dealing with a tumour close to the spinal cord or the brainstem then it’s possible to deliver effective doses of radiotherapy very close to these structures without causing excessive damage to those radiosensitive structures.
The other important role of proton therapy is in the treatment of cancers in children. Because children have normal tissues which are very sensitive to quite low doses of radiation the targeted nature of proton therapy can be used to try to avoid irradiating normal tissues in children and try to minimise the impact of radiotherapy on their normal tissues. Now, children are susceptible to radiotherapy and, for instance, children who are quite young and haven’t fully grown and if the radiotherapy involves treating their bones then you get impaired bone growth. There’s a whole range of normal tissue toxicities in children and so an increasingly important use of proton therapy is to try to minimise the late effects of treating children and improve their long-term quality of life.
Can you tell us about local initiatives in this area?
In Swansea, where I’m based, I’m collaborating with a colleague, Richard Hugtenburg, who is a research physicist. Now, the work we’re doing in Swansea is based on a series of projects trying to better define the interaction of proton therapy with normal tissues. Unlike X-rays, protons are very susceptible to differences in density of the tissues they traverse like bone and air cavities and so on. So what we’re trying to do is to better define the way the proton beam is affected by transition through bone and other normal tissues of differing density. Now, the means of trying to do that is to do Monte Carlo simulations. Monte Carlo simulations involve a very complex computer algorithm using a series of codes which define the interaction of radiation beams with normal tissue and these codes are affected by parameters such as the machine characteristics and the normal tissues which are traversed by the beam. So the Monte Carlo simulations that we’re doing at the moment are trying define how proton dosimetry, proton dose distribution within the normal tissues, are affected by transition through bone. In other words, to what extent is the bone perturbing the dose distribution of proton therapy and that’s one area we’re looking at.
The other area we’re looking at at the moment is to try to better define, again using Monte Carlo simulations, the risk of radiation induced cancer with proton therapy and to what extent the risk of radiation induced cancer can be improved and reduced.
Can you tell us about national and international collaboration initiatives?
In order to better define the role of proton therapy this needs international collaboration. Proton therapy has been around a long time; proton therapy has been used for the treatment of cancer since the 1950s but in the last ten or twenty years there has been a lot of interest from machine manufacturers. So the means of delivering proton therapy has been enhanced by the interest from the industry but there are still a lot of unanswered questions. So in order to better define the role of proton radiotherapy in a range of clinical situations it’s necessary to collaborate.
In the UK we will be getting high energy proton facilities in Manchester and London in 2018 and 2019 and there will be a range of private facilities available as well, including our own local proton centre in Newport in 2017, we hope. So there is a plan to collaborate nationally in terms of organising collaborative research. Currently this is organised by CTRad which is an organisation to promote research into radiotherapy which is run the auspices of the National Cancer Research Institute, NCRI. So CTRad has convened a proton therapy research collaboration.
Internationally in Europe there’s an organisation called ENLIGHT which is the European Network for Light Ion Research where organisations involved in proton therapy research collaborate and share experiences and research outputs. There are other organisations which will collaborate in order to better understand the role of proton therapy. For instance, we recently held a workshop in Stockholm which looked critically at the role of proton therapy in children and we have recently produced a consensus statement which will be published soon.
What’s next for proton therapy?
Although proton therapy is gaining in popularity and usage across the world, particularly for children, there are a lot of unanswered questions. Proton therapy, when the beam is delivered there is uncertainty about the dose at the very end of the range, just at the point at the end of the Bragg peak. So further research needs to be done in order to define exactly what those uncertainties mean for the dose distribution of radiation at the very end of the Bragg peak with the proton beam. So further work needs to be done with Monte Carlo simulations in order to better define that and also when we’ve got better proton availability in the UK it will be possible to do some research with phantoms looking at the dosimetry of proton therapy at the very end of the Bragg peak.
The other work that we need to do is more dose comparative studies comparing intensity modulated radiotherapy with proton therapy and these planning studies using what is known about normal tissue late toxicity can better define the likely benefits of proton therapy compared with the best possible X-ray therapy which is, at the moment, IMRT. So we need to define further indications where proton therapy will be beneficial. At the moment most patients who are treated with proton therapy are either children or they have tumours close to radiosensitive structures such as the brain and spinal cord but there are probably other tumours or there may be some patients with common cancers where delivery of curative radiotherapy is particularly difficult who may well benefit from proton therapy. So we need to do further collaborative planning studies in order to define which patients are likely to benefit. When we have a greater availability of proton therapy in the UK it will be easier to do that sort of work.
