Speaking in a symposium entitled All you need to know about hadron therapy, Marco Schippers from the Paul Scherrer Institute in Switzerland took a look at some new developments in particle accelerator technologies. In particular, he cited one key area as the continuing quest for smaller, lower-cost accelerators that could one day be implemented as stand-alone systems serving a single treatment room.
There are various approaches to this challenge under development, both by accelerator laboratories and within industry. First up, there's the compact proton-therapy system being developed by Still River Systems, a start-up company based in Littleton, MA. Still River's technology is based on an 8-10 T synchrocyclotron that could ultimately be mounted on a gantry to rotate around the patient.
Such a system could work with both scattering and spot-scanning beam delivery. However, Shippers says that it may not be suitable for fast pencil-beam scanning - an approach that's being investigated to help deal with the problems caused by moving targets. Elsewhere, Varian Medical Systems (Palo Alto, CA) is also developing a compact synchrocyclotron, which will offer an energy-selection system on the gantry.
Another option in the pipeline is the dielectric wall accelerator (DWA), a technology that was described in more detail at ESTRO's presidential symposium. George Caporaso of Lawrence Livermore National Laboratory (Livermore, CA) presented details of a compact DWA system designed to deliver intensity-modulated proton therapy with fast scanning.
The DWA works by using uses fast-switched, high-voltage transmission lines to generate pulsed electric fields on the inside of a high-gradient insulating acceleration tube. This high-gradient insulator can handle electric fields large enough to speed protons passing through to energies of more than 200 MeV. In principle, the system should be able to achieve energies comparable to those of the most powerful conventional proton accelerators, while being only a couple of metres in length.
Such a system will produce individual pulses that can be varied in intensity, energy and spot-width. "One exciting feature of this is that you can control many parameters on a pulse-to-pulse basis," Caporaso explained. "Varying the energy on a pulse-to-pulse basis, for example, leads to an intensity-modulated proton beam."
The Lawrence Livermore team is working with TomoTherapy (Madison, WI) and the Compact Particle Acceleration Corporation (a newly formed company focused on development of the DWA) to bring this to the clinic. The researchers have built a small prototype and plan to add the proton injector to the set-up by the end of this year.
The ultimate goal is a 200 MeV system with a tomotherapy-style gantry that rotates 200° around the patient, Caporaso explained. "The various components that are necessary to put this together have field stresses in a range that can lead to a system that will fit in a small room."
Even further into the future lies the possibility of employing laser-driven accelerators, in which laser-induced plasmas are used to accelerate protons (Laser Physics 16 639). Such a system would be capable of delivering energy-and intensity-modulated proton therapy, though Schippers notes that in its current form, the technology generates a lot of neutrons so is not feasible for clinical use.
Finally, Schippers cited the "science fiction" option: a plasma wakefield accelerator (Nature 445 741), which can produce accelerating fields orders of magnitude larger than those used in conventional colliders. He notes, however, that this is "very far ahead".
"Currently, the choice is between synchrotron and cyclotron, but people are looking for new accelerator technologies," Schippers concluded. He finished on a note of caution: "These new accelerators are all good ideas, but do they provide at least the same quality treatments as we have now? And if they do, what are the advantages?"