MedicalPhysicsWeb
Aug 7, 2007
Proton therapy offers a number of advantages over conventional X-ray radiotherapy. Most importantly, protons deliver all of the energy at the end of their path - unlike X-rays. This means they can be "tuned" to dump all 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.
Trouble is, conventional proton accelerators require a space the size of a basketball court and can cost well in excess of $100 million, once all the necessary infrastructure is factored in. That alone provides strong motivation for physicists to develop smaller, cheaper accelerators that will make proton therapy accessible to many more clinics - and many more patients.
By the sound of things, they have been rising to the challenge. During a symposium entitled "Novel particle acceleration techniques" at last month's annual meeting of the American Association of Physicists in Medicine (AAPM) in Minneapolis, MN, a number of physicists from around the world presented their latest work on compact particle accelerators.
Chang-Ming Charlie Ma from Fox Chase Cancer Center (Philadelphia, PA) kicked off the session with a reminder about why all of this effort is necessary. "Proton/ion therapy has great potential for improving local control and normal-tissue sparing because of its superior dose distributions," he explained.
One option for shrinking the cost of proton therapy is simply to build a smaller, lighter accelerator using conventional technology. This is the approach being taken by Still River Systems, a Littleton, MA-based start-up that's developing a compact proton-therapy system powered by a synchrocyclotron (a cyclotron in which the frequency of the driving RF electric field is varied to compensate for the mass gain of the accelerated particles). "When you scale a 2 T cyclotron to 10 T, it goes from weighing 450 tonnes to less than 20 tonnes," said Kenneth Gall, co-founder and CEO of Still River Systems.
Essentially this is what Gall and his team have done, using superconductors as the basis for the 10 T electromagnet. The result will be an accelerator, just a couple of metres across, which can be mounted on a gantry in a similar fashion to the familiar linac set-up. So far, Gall reported, the ion source is operating successfully at 10 T, the RF system has been tested, and the vacuum system is complete. A prototype system is set to be installed at Washington University (St Louis, MO) next year.
Laser focus
Elsewhere, researchers are trying a more radical approach. In recent years, an increasing number of scientists have become interested in the idea of laser-energized proton acceleration. Focusing a high-power laser pulse onto a thin target causes massive ionization in the target and expels a large number of relativistic electrons. This leaves the target with a strong positive charge and so creates a transient electric field. Any protons present will then be accelerated to high energies by this field.
However, there are several challenges to overcome before laser-generated proton beams become a ready-to-go clinical technology. For starters, therapists need a beam of at least 60 MeV; that beam also has to be monoenergetic. Neither of these specifications is currently easy to achieve with laser acceleration. Nevertheless, Toshi Tajima of the Japan Atomic Energy Agency (Kizugawa, Japan) is confident that this is the route to affordable proton therapy. "My goal is to 'compactify' the laser accelerators into the size of a hospital photon machine," he told AAPM delegates.
His team believes it can solve both the MeV and "monoenergy" challenges by combining a technique known as adiabatic acceleration with a specially designed graded target. Add in a PET scanner for confirming the irradiation and you have the basis of a clinical proton-therapy system, Tajima claims. The project has just been given funding equivalent to $100 million by the Japanese government. It remains to be seen whether the researchers can make their ideas work in practice, though.
Electrons, not protons
Laser-based accelerators could also facilitate the clinical uptake of very-high-energy electron therapy (VHEET). Compared to photons, electrons are little-used therapeutically. When they are, it is usually at the low energies a conventional linac is capable of producing, which in turn restricts their usefulness to superficial tumours. By and large, electron-beam therapy is most commonly employed in conjunction with photon radiotherapy to boost the dose.
VHEET, on the other hand, requires a dedicated accelerator, though current technology is bulky and expensive. Too bulky and too expensive for the clinical mainstream, despite the fact that that high-energy electrons offer dosimetric advantages over photons. According to Victor Malka of the Laboratoire d'Optique Appliquée (Palaiseau, France), electrons have a narrower penumbra compared to photons, a characteristic which improves the sparing of sensitive structures by as much as 20%.
"The lack of compact and cost-efficient electron accelerators could be overcome by laser-plasma systems," Malka claimed. In such an accelerator, a high-intensity pulsed laser is fired into a jet of dense, ionized gas. The subsequent interaction creates plasma waves, and electrons caught in these waves are pushed to high speeds and emitted as a high-energy electron beam. Using this technique, electrons can be accelerated to therapeutic energies in a few millimetres, rather than hundreds of metres.
The question is: will any of these technologies actually deliver what that their proponents claim? Some members of the audience expressed scepticism during the ensuing debate. For their part, most of the speakers were more than a little vague about what their systems might cost if they ever make it to market. Clearly, this remains a watching brief for the time being.
About the author
Michelle Jeandron is science and technology reporter on medicalphysicsweb.
©
Aug 7, 2007
Proton therapy offers a number of advantages over conventional X-ray radiotherapy. Most importantly, protons deliver all of the energy at the end of their path - unlike X-rays. This means they can be "tuned" to dump all 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.
Trouble is, conventional proton accelerators require a space the size of a basketball court and can cost well in excess of $100 million, once all the necessary infrastructure is factored in. That alone provides strong motivation for physicists to develop smaller, cheaper accelerators that will make proton therapy accessible to many more clinics - and many more patients.
By the sound of things, they have been rising to the challenge. During a symposium entitled "Novel particle acceleration techniques" at last month's annual meeting of the American Association of Physicists in Medicine (AAPM) in Minneapolis, MN, a number of physicists from around the world presented their latest work on compact particle accelerators.
Chang-Ming Charlie Ma from Fox Chase Cancer Center (Philadelphia, PA) kicked off the session with a reminder about why all of this effort is necessary. "Proton/ion therapy has great potential for improving local control and normal-tissue sparing because of its superior dose distributions," he explained.
One option for shrinking the cost of proton therapy is simply to build a smaller, lighter accelerator using conventional technology. This is the approach being taken by Still River Systems, a Littleton, MA-based start-up that's developing a compact proton-therapy system powered by a synchrocyclotron (a cyclotron in which the frequency of the driving RF electric field is varied to compensate for the mass gain of the accelerated particles). "When you scale a 2 T cyclotron to 10 T, it goes from weighing 450 tonnes to less than 20 tonnes," said Kenneth Gall, co-founder and CEO of Still River Systems.
Essentially this is what Gall and his team have done, using superconductors as the basis for the 10 T electromagnet. The result will be an accelerator, just a couple of metres across, which can be mounted on a gantry in a similar fashion to the familiar linac set-up. So far, Gall reported, the ion source is operating successfully at 10 T, the RF system has been tested, and the vacuum system is complete. A prototype system is set to be installed at Washington University (St Louis, MO) next year.
Laser focus
Elsewhere, researchers are trying a more radical approach. In recent years, an increasing number of scientists have become interested in the idea of laser-energized proton acceleration. Focusing a high-power laser pulse onto a thin target causes massive ionization in the target and expels a large number of relativistic electrons. This leaves the target with a strong positive charge and so creates a transient electric field. Any protons present will then be accelerated to high energies by this field.
However, there are several challenges to overcome before laser-generated proton beams become a ready-to-go clinical technology. For starters, therapists need a beam of at least 60 MeV; that beam also has to be monoenergetic. Neither of these specifications is currently easy to achieve with laser acceleration. Nevertheless, Toshi Tajima of the Japan Atomic Energy Agency (Kizugawa, Japan) is confident that this is the route to affordable proton therapy. "My goal is to 'compactify' the laser accelerators into the size of a hospital photon machine," he told AAPM delegates.
His team believes it can solve both the MeV and "monoenergy" challenges by combining a technique known as adiabatic acceleration with a specially designed graded target. Add in a PET scanner for confirming the irradiation and you have the basis of a clinical proton-therapy system, Tajima claims. The project has just been given funding equivalent to $100 million by the Japanese government. It remains to be seen whether the researchers can make their ideas work in practice, though.
Electrons, not protons
Laser-based accelerators could also facilitate the clinical uptake of very-high-energy electron therapy (VHEET). Compared to photons, electrons are little-used therapeutically. When they are, it is usually at the low energies a conventional linac is capable of producing, which in turn restricts their usefulness to superficial tumours. By and large, electron-beam therapy is most commonly employed in conjunction with photon radiotherapy to boost the dose.
VHEET, on the other hand, requires a dedicated accelerator, though current technology is bulky and expensive. Too bulky and too expensive for the clinical mainstream, despite the fact that that high-energy electrons offer dosimetric advantages over photons. According to Victor Malka of the Laboratoire d'Optique Appliquée (Palaiseau, France), electrons have a narrower penumbra compared to photons, a characteristic which improves the sparing of sensitive structures by as much as 20%.
"The lack of compact and cost-efficient electron accelerators could be overcome by laser-plasma systems," Malka claimed. In such an accelerator, a high-intensity pulsed laser is fired into a jet of dense, ionized gas. The subsequent interaction creates plasma waves, and electrons caught in these waves are pushed to high speeds and emitted as a high-energy electron beam. Using this technique, electrons can be accelerated to therapeutic energies in a few millimetres, rather than hundreds of metres.
The question is: will any of these technologies actually deliver what that their proponents claim? Some members of the audience expressed scepticism during the ensuing debate. For their part, most of the speakers were more than a little vague about what their systems might cost if they ever make it to market. Clearly, this remains a watching brief for the time being.
About the author
Michelle Jeandron is science and technology reporter on medicalphysicsweb.
©
