This year in the United States, 6000 cancer patients—among them, those that suffer from pediatric, brain, and prostate cancer—will receive a new kind of treatment that irradiates tumors with a beam of protons rather than X-rays. New research suggests that those same protons could also provide more-accurate imaging and targeting of a patient’s tumor than the X-ray CT scans used today. Proton imaging would also drastically reduce a patient’s overall radiation exposure.
By using protons instead of photons, doctors can more easily kill cancer cells without also irradiating healthy cells around the tumor, says Sameer Keole, a radiation oncologist at the ProCure Proton Therapy Center, in Oklahoma City. He cites industry estimates that 240 000 cancer patients out of the more than 800 000 treated every year with X-rays would benefit from proton therapy. After his experience with proton therapy, he says he’s also ”bullish” about the prospects of new proton-imaging technologies.
Physicist Cinzia Talamonti, of the University of Florence, in Italy, says a prototype proton CT scanner should be ready for testing on small animals by next year. Human ”pCT” scanners, Talamonti projects, could be on the horizon in approximately five years.
Talamonti’s group’s work, published this month in the journal Nuclear Instruments and Methods in Physics Research Section A, points to the inherent 5 percent error rates when using X-ray CT scans as maps for proton therapy. Blurriness arises, Talamonti says, because X-rays and protons interact differently with matter. A CT scan of a patient’s torso, for example, reveals what the torso looks like to massless, electrically neutral X-rays. But a beam of massive, positively charged protons might scatter a little off a piece of bone or a dense mass of tissue, effectively reducing the usefulness of the CT scan with every proton’s bump and bounce.
Radiation oncologists use computer models to correct for the discrepancy between the CT scan and the proton beam’s scatterings. And some cancer treatments, Keole says, could benefit from this improved targeting. For instance, lower-malignancy brain tumors tend to stop within millimeters of where CT scans say they stop. Pointing a beam of protons at such a localized tumor today, Keole says, may sometimes also irradiate small pieces of healthy brain. Such misdirection, in part, blunts the great strength of proton therapy—its ability to irradiate the tumor and only the tumor.
”Protons release most of their energy at the end of their path,” says medical physicist Saverio Braccini, of the University of Bern, in Switzerland. ”From a physics point of view, this is a much more powerful basic tool [compared] to X-rays.”
A proton beam, in other words, acts like a barrage of subatomic depth charges. The proton beam’s intensity and direction determines the path length through the body, parameters that radiation oncologists fine-tune for each patient. The beam treads lightly until the protons slow down to a critical speed. Once slowed down enough, the protons deliver the largest fraction of their energy within a volume of just a few millimeters.
Braccini coauthored another recent paper suggesting a film-based proton imaging technology, which is cheaper but slower than Talamonti’s group’s silicon-detector-based imaging.
For his practice, Keole says the immediate results from a silicon pCT system like the one Talamonti’s paper proposes would be crucial. Film-based pCT scans—taking hours to develop at best—wouldn’t be acceptable, because proton therapy regimens often involve real-time imaging to align a patient’s body as precisely as possible along the proton beam path.
And with the promise of increased precision from pCT, Keole says the field of proton therapy is heading for a growth spurt. Spurring on this nascent industry, he says, are the technology’s decreasing costs, with the increased efficiency of proton accelerators projected to halve the US $100 million cost of building new proton-therapy centers in 10 to 15 years’ time. Many kinds of cancers—in the brain, prostate gland, GI tract, eye, spine, and lung, for instance—are becoming more treatable with proton therapy, he says.
”With protons, you have to build the [proton accelerator] from the ground up, and that costs tens of millions of dollars,” Keole says. ”If cost were not an issue and we could rapidly manufacture proton facilities like we do regular radiation facilities, we wouldn’t be having this discussion. Protons would be in every clinic across America.”
Seven proton-therapy clinics, Keole says, are in operation today in California, Florida, Indiana, Massachusetts, Oklahoma, Pennsylvania, and Texas. Two more are under construction in Chicago and Hampton, Va. In the United States alone, more than a dozen are on the drawing boards in cities such as Orlando and St. Louis. As more proton-therapy centers crop up and more radiation oncologists seek newer and better treatments, the proton is coming into its own technological niche alongside the workhorse electron. And now the particle that could be a lifesaver for some patients may be taking some nice pictures along the way, too.