Heavier particles like carbon ions have a much higher linear energy transfer than photons or protons, making them more effective at treating tumours with an anoxic core. To date, only 4450 patients have received carbon-ion therapy. But this number's not that surprising considering that there are so few carbon-beam facilities in the world, just two in Japan: the Heavy Ion Medical Accelerator in Chiba (HIMAC) and the Hyogo Ion Beam Medical Center (HIBMC); and two in Germany: GSI, the heavy-ion research centre in Darmstadt and the Heidelberg Ion-beam Therapy Center.

HIMAC was the site of the world's first carbon-ion treatment in 1994 and the facility has now treated 3795 patients. One recent clinical study has shown some pretty impressive early results for carbon-ion therapy of renal-cell carcinoma - a notoriously radioresistant tumour for which there have been few reports of curative radiotherapy. The HIMAC team studied 10 patients, including three with stage IV disease and tumour sizes of up to 120 mm. Carbon-ion treatment resulted in a 100% five-year local control rate, as well as progression-free survival and cause-specific survival rates of 100% - a similar outcome to that of radical surgery (Int. J. Radiat. Oncol. Biol. Phys. doi: 10.1016/j.ijrobp.2008.01.043).

The study's lead researcher Takuma Nomiya, from the National Institute of Radiological Sciences in Chiba, certainly seems impressed by the results. "If my parents got cancer, I would recommend carbon-beam therapy, even if it was operable," he told medicalphysicsweb." Having said that, Nomiya points out that a follow-up trial is not in the pipeline - mainly due to the increasing amount of treatments at HIMAC creating a situation where the patient number exceeds the institution's capacity.

"With public attention to carbon-ion radiotherapy increasing, the supply is not sufficient to meet demand," Nomiya explained. "Building new carbon-beam institutions seems to be a problem that needs to be solved." While there are a few facilities currently under construction (in Japan and Italy, plus Siemens' projects at the Particle Therapy Center Marburg and the North European Radiooncological Center Kiel, both in Germany), the set-up costs are a big obstacle. Establishing a carbon-beam facility requires an investment of up to a hundred million dollars (although prices are now decreasing slightly) - almost twice that of a proton-therapy system.

This cost issue is being addressed in part by companies such as IBA in Belgium, which is developing a carbon-ion source based on a superconducting cyclotron (See Forward thinking on heavy ions). According to IBA's founder and chief research officer Yves Jongen, the new accelerator will be "five times smaller, less expensive and simpler" than a synchrotron-based source.

Beyond protons and heavy-ions, what other options could one day emerge in the particle-therapy armoury? Researchers at CERN are looking to antiprotons to provide the ultimate "magic bullet" (see Antiproton therapy: worth the investment?). Initial data from CERN's antiproton cell experiment (ACE) showed antiprotons to be four times as effective as protons at targeted cell damage. Although, given the costs involved in setting up an antiproton facility - somewhere between $0.7 and 1.4 billion - it may be some time before this particular technology hits the market.

For further information about the current status of particle therapy, and an insight into the future of this exciting technology, head on over to Jacksonville, Florida, next week for PTCOG 47 - the group's annual scientific meeting. I'll be attending the meeting, so be sure to check out medicalphysicsweb over the next couple of weeks for exclusive reports and updates from the event.