Making proton therapy practical

Proton therapy is a better fit than conventional radiation therapy for some, if not most, cancer patients. The subatomic particles of its beam—accelerated protons cut loose from atomic nuclei and marshaled into a tightly focused beam—have greater mass than other forms of radiation, such as x-rays. As a result, if properly aimed, they deposit more energy in cancerous tissue with less collateral damage to surrounding healthy tissue.

This makes proton therapy especially attractive when treating pediatric cancers, in whose patients collateral damage from x-rays can do the most long-term harm. But despite their promise, proton facilities are rare. Only about 25 facilities offer proton therapy worldwide and only another 25 are in the planning stages, which can drag on for years.

The problem is size—and money. Whereas the linear accelerators that create high-energy x-ray beams are discrete machines that can be sited within a cancer center, proton accelerators typically are the centers, speeding and concentrating protons that are then channeled into any of several treatment rooms. A single center may cost as much as $125 million to build.

One of the newest such facilities, the ProCure Proton Therapy Center in Oklahoma City, opened its doors just two months ago. Patients with head and neck, brain, central nervous system, prostate, and some pediatric cancers, among others, will be treated at the new 60,000-square-foot center, the first in a network of ProCure centers to open across the country.

"Our pledge as a company is to build proton centers until every patient with cancer who could benefit has access to this advanced treatment," said Hadley Ford, CEO of ProCure.

That will be tough. ProCure is constructing a second center near Chicago, scheduled to open in early 2011. Others are planned for Detroit and sites in New Jersey and South Florida. But the need for proton therapy is enormous.

An estimated 250,000 patients in the U.S. could benefit from proton therapy annually, according to the Oklahoma City center, which itself can treat only about 1500 per year. Total capacity for proton therapy delivered in the U.S. currently is only about 6000.

To meet this demand, the oncology community will have to think smaller and cheaper. Two scientists are doing just that.

Dale Litzenberg , PhD, is leading an effort at the University of Michigan to accelerate protons by bombarding a thin foil with light from a 300-terawatt laser. The electric fields within the short laser pulses cause a "coulomb explosion" in the foil that liberates protons. Litzenberg, a research assistant professor in the UM radiation oncology department, is working on a way to herd them into a beam for use in proton therapy.

Meanwhile, George Caporaso, PhD, and colleagues at Lawrence Livermore National Laboratory in Northern California are developing a compact and linear proton accelerator using high-gradient vacuum insulators and advanced dielectric materials and switches. The goal of this "dielectric wall," according to Caporaso, beam research project leader at the laboratory, is to produce a proton accelerator as small as a standard linear accelerator, one that can deliver intensity-modulated proton rather than x-ray therapy.

There's reason to believe such a development may not be too far away. A vendor of conventional radiation therapy, TomoTherapy in Madison, WI, has already licensed the new accelerator technology from Lawrence Livermore. TomoTherapy is working with the laboratory and its partner Compact Particle Acceleration to develop a prototype accelerator for cancer therapy.