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Proton Radiation for All

Proton-beam cancer therapy could become more available, thanks to a new device.
July 24, 2007

A new machine that produces highly energized protons could make proton-radiation therapy more easily accessible to cancer patients. The machine, being developed by researchers at Lawrence Livermore National Laboratory, will be a fifth of the size and cost of the proton-therapy machines that are currently found at six specialized medical centers in the United States.

The smaller, lighter, and cheaper machine should be easier for smaller hospitals to buy and install. That will make proton therapy available to more people. When the device is ready for the market, it will “probably be the world’s most sophisticated radiation-therapy machine available at a very affordable price,” says Ralph deVere White, director of the UC Davis Cancer Center, which supported early research on the device. “It will change the bar for what is standard therapy.”

Radiating tumors with protons has proved to be better than conventional x-ray radiation for treating certain types of cancers, such as head and neck, lung, and prostate. Proton beams can be controlled very precisely, so they spare healthy tissue around tumors and cause fewer side effects in patients. But current proton-therapy machines, owing to the large magnets that create the energetic particles and the concrete walls that are needed to shield the radiation, take up a room the size of a basketball court. The machines also come with a hefty price tag–between $150 and $200 million.

The new machine, called a dielectric wall accelerator, should fit inside conventional radiation-treatment rooms. The goal is for hospitals to replace x-ray machines with the new proton-radiation machine, says George Caporaso, a physicist who is leading the research at Lawrence Livermore.

Caporaso and his colleagues expect to have a small version of the device ready by the end of this year, and a full-scale prototype within the next three years. The machine will be clinically tested at the University of California Davis Cancer Center. If tests succeed, TomoTherapy, based in Madison, WI, will market the machines.

To kill tumors, protons need to have energies of around 250 million electron volts. That requires speeding them up, which is done using a high electric field in machines called accelerators. Accelerators can be made of metal tubes that are tens of meters long, through which particles travel to gain energy. Proton-therapy machines use a type of accelerator called a circular accelerator, which bends the particle beam so that the particles go on a spiral path while gaining energy. Bending the proton beam requires large magnets that can weigh hundreds of tons.

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.”

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