Proton Therapy Congress 2016
Potential of laser-driven methods for proton beam delivery
Prof Marco Borghesi- Queen’s University Belfast, Belfast, UK
I have been presenting some of the work that we do as part of a large UK-wide project which I lead. It’s called A-SAIL which stands for Advanced Strategies for Accelerating Ions with Lasers. Effectively the work we do is in trying to develop a new acceleration technique for protons but also different types of ions which is based on high powered lasers. It’s radically different from other acceleration techniques which have been discussed at this meeting or that are used conventionally in proton therapy. It is a technique which is still being developed so certainly we are at a low TRL level at the moment but it presents some possible aspects that might be of interest for future options, let’s say, for different therapies. It’s radically different so it might lead to a very different way of doing therapy with ions.
Could you tell us about the advantages of laser acceleration?
Our work is in trying to develop techniques which might make the treatment cheaper in the future. Of course it’s a long-term proposition, it’s not going to happen soon, and also we are exploring opportunities that might make it different, might make it more interesting, let’s say, for implementation on a broader scale. Particularly using lasers for example we have recent results that show that we can efficiently accelerate not only protons but also other ion species, carbon for example. As you will see by, for example, looking at the stands here there are several solutions on the market for protons but not much is available in terms of carbon which is even more expensive to implement. Therefore that could be a niche that for example we could target with a different type of acceleration if we can develop it to the required standard.
How do the laser accelerators work?
To accelerate particles with a laser, particularly ions which is the focus here, you need very high powered lasers, very powerful lasers, and there are different techniques through which the ions can be accelerated, different mechanisms which are pursued by researchers worldwide. The most established is called target normal sheath acceleration. Effectively an accelerator, the concept of an accelerator using lasers it’s very simple, you just irradiate with a high powered laser, thin foil, or any material really but particularly metallic and of course we are talking about pulse lasers so these are electromagnetic radiation which travels in short pulses, let’s say of a duration of a picosecond or even less, femtosecond, and you focus it onto a target with some specialised optics clearly into spots of say a micrometre radius or so. Therefore you have an enormous concentration of energy in space and time at that point, you immediately ionise the target so you can rip out electrons and ions from the target and with that process through which the energy of the laser is transferred very efficiently to the electrons in that case. So the electrons then are energetic enough to stream through a foil, they reach the rear of the foil, ionise the surface, establish a very strong electric field there, and this electric field accelerates almost instantaneously the ions from the target, so all the processes take place in a picosecond or so and the electric field that you generate during this process are of many orders of magnitude higher than what you can sustain in a conventional accelerator.
The main reason is that conventional accelerators accelerate particles using radiofrequency fields, so an electromagnetic wave, but this is done in a vacuum inside some metallic vessels and if you raise the amplitude of this field too much you start breaking down the container somehow. In a plasma you can raise your field as much as you want because the plasma is already broken down somehow, it’s already matter reduced to its elemental components. There are other mechanisms which are currently being developed, there’s a range of them, one which is very interesting is called radiation pressure acceleration, in that case you irradiate similarly with a very powerful laser a very thin foil, in this case on the scale on tenths of nanometre. Under the right conditions, so if you control the parameters of the laser appropriately you can actually just propel the irradiated portion of a target forward by the radiation pressure of the laser pulse. This is very promising because you can accelerate not only ions present on the surface but also the main component of the target. So for example if you irradiate carbon foils, you can radiate very efficiently carbon, and so there is very much interest in this development for future therapeutic opportunities.
How do you see laser accelerators being used to advance the field beyond proton therapy?
To introduce other species into therapy, like I was talking about carbon but there is interest also in intermediate species like helium or lithium and so on, and one of the features of laser ion acceleration is that it’s very flexible in the sense that with the same set-up you can accelerate different particles just by simply changing the target that you irradiate. Not only that, irradiating a target you can produce x-rays, electrons so there are a range of different options which could also be combined because with a laser it’s relatively easy to have several pulses arriving in the same interaction environment from different angles, for example. So there are options also in terms of mixed field irradiation or new diagnostic opportunities, for example. Of course it’s all hypothetical at this stage I should say. It will make sense to do it only if the cost is reasonable of course but there are some opportunities for reducing the cost even over a standard installation because one of the nice features of a hypothetical laser centre for therapy would be that you could propagate. Imagine you have a multi-centre for proton therapy, at the moment the way it works is that you produce the ions from an accelerator and then these are transported to the several interaction treatment rooms. In this case, basically what you have to transport is a very rigid beam of high energy ions which requires large magnets for steering and requires radiation protection because these particles when they are bent radiate, of course. If you could do that with a laser, in principle with the same laser system, you could service several treatment rooms by transporting the laser radiation to close to patient and producing the ions in the room itself. That would certainly bring some savings because of course this building requires thick concrete to shield radiation. Also at the end of a system at the moment, let’s say the larger centre use gantries which are a weak magnetic system used to steer the beam around the patient, with an optical driver you could probably reduce significantly the size of these gantries because, as you know, lasers can be carried around by mirrors so part of these gantries could be optical again. I mean it’s still too early to think about this in too much detail but there are still already design studies for gantries based on this extrapolation of what the laser driven ions could provide which are interesting and show already a reduced size that is better than what is done now.
What’s next?
From here one message that I get, even beyond the biology, it seems like that there is need of some hard evidence from clinical treatment and there is still some controversy about that so it is not completely clear that everybody agrees that protons are better for all the cases that are being put forward. I don’t know now about that; on the basis of the physics the position should be better so if it is not like that it is interesting and needs to be explored.
