Caporaso and his colleagues take an innovative approach to energizing protons. They use a tube made from a special insulator material–layers of metal such as stainless steel alternating with plastic–that can sustain extremely high electric fields of 100 megavolts per meter without getting short-circuited. That means a tube that is about 2.5 meters long could create 250-million-electron-volt protons for zapping tumors.
Another advantage of the design is that the researchers can control how much energy they give to the proton beam. Conventional accelerators that use magnets always produce the maximum energy, says Thomas Mackie, cofounder of TomoTherapy and a professor of medical physics at the University of Wisconsin. Physicians then have to slow the beam down so that it can be given to the patient. This process creates neutrons, so current proton-therapy centers need concrete walls to shield the neutrons. That adds to the therapy center’s size and cost. “We’re not making a high energy and having to slow it down to lower energy,” Mackie says. “We’re just creating energy you absolutely need for the patients.”
So far, the researchers have shown that a three-millimeter-long tube can carry a 100-megavolts-per-meter electric field. The success of the technology banks on the 20-centimeter-long small-scale prototype that the researchers are now building. They need to show that the proof-of-concept prototype can sustain high electric fields. Once that works, they will have to make a full-scale clinical prototype just as safe and effective for treating cancers as current machines.
Leonard Arzt, executive director of the National Association of Proton Therapy, believes that it is too early to say whether the technology will work. And even if it does, he cautions that it would take many years for it to be available in hospitals. “The machine’s clinical trials are at least five years away; then it will have to get FDA approval,” Arzt says.
DeVere White, on the other hand, is cautious but optimistic. “This machine must deliver the same characteristics as the present ones,” he says. “We really expect that this is not only going to do what the present machines do; it’s going to do more.”