Why doesn’t the UK have proton therapy? Actually, we do.
Back in 1984, when proton therapy was still a research topic in physics labs, the UK opened the world’s first hospital-based therapy facility. The Clatterbridge centre for oncology was the first place in the world where patients could have proton therapy in a hospital, rather than a physics lab. The facility continues to treat patients today, but can only treat eye tumours due to the low energy of the beam.
There hasn’t been another proton therapy centre built in the UK since. We haven’t entirely ignored proton therapy, but we have been somewhat left behind.
In my first post on how physics is used to treat cancer I explained how proton therapy works. In this post, I’ll share some of the new research and initiatives that could bring the UK back to the forefront of proton therapy treatment.
Since 2008, the NHS has had a program to send limited numbers of UK patients abroad for proton therapy treatment. But it’s not cheap. In 2012, the Department of Health finally announced the decision to provide a proton beam therapy service in the UK. Two sites were selected for the service, which should treat its first patients in 2017 and be fully operational in 2018.
From the viewpoint of a hospital, having a particle accelerator in the basement still represents both considerable complexity and investment. Current accelerator technology is doing the job, but it could be better. In the words of Marco Schippers from Paul Scherrer Institute (PSI) in Switzerland “Don’t treat tomorrow’s patients with yesterday’s proton therapy technology.”
The UK will have a choice to make when it comes to accelerator technology. Compared to the Large Hadron Collider, where protons are accelerated to energies of 7 TeV, the 250 MeV needed for proton therapy (roughly 30,000 times less energetic) seems pretty measly.
Today, the UK has two large accelerator complexes, ISIS and Diamond, and a multitude of cutting-edge smaller facilities. In the last decade the UK has been steadily building further expertise in accelerators with the Accelerator Science and Technology Centre (ASTeC) and not one but two institutes, the John Adams Institute for Accelerator Science and the Cockcroft Institute.
So perhaps it’s not surprising that with all this expertise the UK is fairly well placed to start considering how to improve the ‘beast in the basement’.
There are currently two major players for proton and ion therapy: cyclotrons and synchrotrons.
Cyclotrons are compact machines, where the beam starts in the centre and, as it gains energy, follows a spiral path outwards (see diagram above from Ernest Lawrence’s 1934 patent for the cyclotron). Once the particles reach maximum energy the beam is extracted and used. Unfortunately, the maximum energy is limited by the size of the spiral, and that in turn is limited by the fact that once particles get close to the speed of light they get out of sync with the radio frequency accelerating field. This means cyclotrons can be used for proton therapy, but generally not for therapy using heavier ions.
Cyclotrons can only provide beams at a single energy, while proton therapy treatment requires beams with variable energy to control how far they go into the body. To achieve this, cyclotron facilities introduce ‘range shifters’ which intercept the beam and control the energy of the particles. However, these can degrade the beam quality and in some cases can mean throwing away up to 99% of the original beam. It might work, but it’s not ideal.
The other option is synchrotrons. Synchrotrons are relatively large machines where the strength of the magnets and the frequency of the accelerating field are ‘synchronised’ with the particles so that they stay on a neat orbit, rather than spiraling outwards. They have the advantage of being able to accelerate both protons and other ions. But their size and cost are a concern, as is the time it takes to vary the output energy. Each magnet ramping cycle takes about a second, which, in some cases, may limit how a treatment plan is delivered.
What we really need is a compact, reliable, flexible accelerator that is also cheap, easy to maintain and doesn’t require a team of PhDs to run. It might seem a far-off dream, but there are groups going back to the drawing board to try and make just that.
HOW COULD WE DO IT BETTER?
One idea is a new type of accelerator, first invented in the 1950s that mixes the advantages of both the cyclotron and the synchrotron: the so-called ‘Fixed Field Alternating Gradient’ (FFAG) accelerator. During my PhD I was involved in a design study for a combined proton and carbon ion accelerator system, which showed that, in principle, this new type of machine could be a good solution.
It has a fixed magnetic field so the particles spiral out, but is cleverly arranged so they only spiral out a little way. This means it could reach high energies with a relatively compact size, and could vary the energy of the beam a thousand times faster than most synchrotrons. That solves the energy limitations of the cyclotron and the slow-cycling nature of the synchrotron.
But the accelerator is only one part of the story. Unless you can translate the benefits of having a super-whizzy accelerator all the way through to treating the patient, there is little point in having it. The rest of the system also needs to be ‘tomorrow’s technology’.
To bring the beam to the patient at any angle requires a magnetic gantry system, a chain of focusing and bending magnets that bend the beam into the correct position, focus and shape it while being able to rotate all the way around the patient. The current gantries for protons are pretty hefty devices, weighing in at 100 tons or more. The main issue comes down to basic physics. It takes a strong magnet to bend a beam of 250 MeV protons and an even stronger one to control heavier ions. You can’t escape that.
This is a key area that needs development, particularly for ions. There is only one existing gantry for heavy ions, and it is a behemoth weighing 670 tonnes located at Heidelberg Ion-Beam Therapy Center in Germany. Words don’t quite cover the scale of this device, so thankfully there is a video which should help put things in perspective.
It turns out the same concept behind the FFAG accelerator design can also be applied to gantries to help solve this problem. While it’s early days, there is some exciting research happening that could help shrink down the size and cost of both proton and carbon ion gantries.
REDUCING THE COST OF THE SYSTEM
The initial cost of a proton or ion therapy system is always going to be higher than a comparable X-ray radiotherapy system as it usually requires new buildings, significant shielding and multiple treatment rooms. For a small or rural hospital, the initial purchase cost may seem prohibitive. This is why a number of companies are starting to offer single-room systems that include the accelerator and treatment system together, saving space.
However, there is more to consider than just the initial investment. Only a small fraction of the time it takes to treat a patient is actual ‘beam-on’ time. Most of the time is taken up positioning, imaging and preparing the patient. Single room systems might be cheaper, but if you’re not using the accelerator 90% of the time, is it a sensible choice? Optimising the system to treat as many patients as possible while getting efficient use of the accelerator is not an easy task, but it is one that hospitals must consider.
LOOKING INTO THE FUTURE
What lies over the horizon could be even more exciting.
One concept might make it possible to have a single room system using linear acceleration instead of large magnets.
Further afield, the development of a whole new way of accelerating particles, called laser plasma acceleration or plasma wakefield acceleration, could shrink down the size of the accelerator to something the size of a tabletop. In the best case, the accelerator might even be something that could fit in the palm of your hand.
While this exciting new technique is difficult to control in the lab at the moment, if successful it certainly has the potential to be a huge step towards a wider uptake of proton and ion therapy. It’s also an area where the UK has a world-leading research base.
The UK might be playing catch-up at the moment, but there is certainly a lot of room for innovation in this burgeoning field. If we are truly are a ‘knowledge economy’ perhaps we ought to be developing some of these ideas to build up a small but expert industry right here in the UK?
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