Compact proton therapy

For killing malignant tumors, protons are potentially more effective than x rays because protons deposit most of their cell-killing energy at the end of their trajectory—in the tumor—and very little in the intervening healthy tissue. To be therapeutic, the protons must be accelerated to 70-250 MeV depending on the nature of the tumor and its location in the human body; such energies require a large facility and an expensive accelerator. (For more, see “Treating Cancer with Protons,” Physics Today September 2002, page 45 .) At the July meeting of the American Association of Physicists in Medicine (AAPM), Thomas Mackie, a professor at the University of Wisconsin and cofounder of TomoTherapy Inc, presented a new proton-therapy design developed by George Caporaso and colleagues at Lawrence Livermore National Laboratory and licensed by Mackie’s company. Using laminated high-gradient insulators that can withstand enormous electric fields without breaking or breaking down, the new “dielectric wall accelerator” can energize protons to 100 MeV in just 1 meter. Its compact size means that the accelerator could be mounted on a gantry and rotated around a patient to precisely aim the proton beam. In addition, a DWA-based system could vary both proton energy and proton-beam intensity, allowing further control over where the particles dump their energy in the patient. Mackie cautions that a prototype is only now being built at Livermore and clinical trials of the system are many years away. (AAPM paper TH-C-AUD-9.)