Proton therapy offers a number of advantages over conventional X-ray radiotherapy. Those advantages stem from some basic physics - chiefly the fact that the rate at which protons transfer their energy to the medium is at a pronounced peak right at the end of their range. This means that they can be "tuned" to dump a high fraction of their killing power in the tumour, which considerably reduces the damage incurred by surrounding healthy tissue as well as offering the possibility of upping the dose.
Right now, only a small fraction of cancer patients can benefit from proton therapy. Conventional proton-therapy systems rely on synchrotrons or cyclotrons to accelerate the protons. These are basketball-court-sized machines costing upwards of $100 million each. Because of the high barrier to entry, there are still only 26 proton-therapy centres worldwide.
The task, according to Mackie, is to dismantle the cost and logistics bottlenecks associated with proton therapy, and to do that sooner rather than later. In a talk entitled "Compact intensity-modulated proton therapy (IMPT)", he explained how TomoTherapy, in partnership with the Lawrence Livermore National Laboratory (Livermore, CA) and others, is working to do just that.
So why compact IMPT? Using rapidly scanned beams (steered by magnets), the technique produces better dose distributions (versus conventional proton delivery) and can treat deeper-lying targets. IMPT can also create a larger field while generating fewer neutrons, which cause unwanted dose outside the treatment volume and may thus compromise the advantages of proton therapy.
Mackie summed it up nicely: "The Bragg peak is a sharp knife for avoiding sensitive structures - we shouldn't dull it or be afraid of it. Compact IMPT is the best solution to maintain high dose conformality and to reduce integral dose."
Think small, win big
The core enabling technology for compact IMPT is a next-generation particle accelerator that was initially conceived as part of a system for monitoring nuclear weapons. Developed by scientists at Livermore, the so-called dielectric-wall accelerator (DWA) works by using 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.
Significantly, such a system will be able to generate individual pulses that can be varied in intensity, energy and spot-width. "The DWA is the only accelerator technology for which energy intensity and beam spot size can be varied pulse to pulse on-the-fly," Mackie added.
Alan Nahum, professor of physics at the Clatterbridge Centre for Oncology, UK, and lead organizer of the Future Perspectives workshop, told medicalphysicsweb that compact IMPT "could be the big breakthrough we have been waiting for in charged–particle therapy: an affordable one-room solution using state-of-the-art beam scanning with the huge advantage of variable beam energy and consequently producing the best dose distributions 'that money can buy'."
He continued: "It [compact IMPT] is the most exciting thing happening in radiation therapy at present. I believe that besides the 'must treat with protons' base-of-skull and childhood cancers, we will also treat (medium-size) lung and (left-sided) breast tumours with this new machine."
Make it real
There's still a long way to go to, however. The Livermore team is currently working with TomoTherapy and the Compact Particle Acceleration Corporation (CPAC), a newly formed company that aims to commercialize various applications of DWA technology, to bring compact IMPT systems to market.
According to Mackie, the big technical challenge is proving the optical switch technology required to enable fast switching of capacitor-like devices to create a moving electric field that supplies the acceleration. "The commercial case is made when a reasonable proton energy has been achieved," he told medicalphysicsweb. "We have accelerated protons to low energy, but now we need to demonstrate a beam that achieves minimum clinical performance (e.g. 70 MeV) before doubts about this technology are retired."
Overall investment in the project is expected to be of the order of $40-50 million, though it's worth noting that CPAC is also looking to push non-medical uses of the DWA in sectors such as security and defence. Mackie says that the first compact IMPT system release to customers is planned for late 2011, with regulatory approval and commercial release set for 2012.