"We like to treat as many pediatric patients as we can because that is where proton therapy is going to have the most benefit," said Andrew K. Lee, M.D., M.P.H., Director of the Proton Therapy Center at University of Texas M.D. Anderson Cancer Center, one of the nation's five operational proton centers. (See sidebar at bottom)
"Kids tend to be more sensitive to late effects of even low doses of radiation and the effects can be pretty profound. If you treat [children's] brains, not only do they have issues regarding neuro-cognition but any bone you treat to a certain dose is probably not going to grow at the same rate as non-exposed bone. So anything you can do to avoid total radiation exposure to a child but still get at the tumor is just a good thing," Dr. Lee said.
Technological Enhancements
Compared to conventional X-ray radiation therapy, protons are particles that are accelerated and aimed precisely to target only tumors, sparing nearby healthy tissue.
"Once the protons reach a certain velocity and depth of tissue, they will deposit all their energy over a very finite range. It maximizes the radiation dose deposition right where you want it and perhaps more importantly, there is no dose after that point," Dr. Lee said. "That is the difference between X-rays and protons. We can stop the protons over a very short distance."
Gantry room installation at the
University of Florida Proton
Therapy Institute in Jacksonville
(Image courtesy of (IBA)
Ion Beam Applications)
As advanced as proton therapy is, the approach continues to be refined by medical physicists and doctors to maximize clinical benefit. For example, M.D. Anderson was the first to employ a pencil beam technology in conjunction with its Hitachi synchrotron. This spot scanning approach uses magnets to guide fine proton beams toward a tumor and away from critical structures. Applications include complex of tumors of the prostate, brain, base of the skull and eye.
To explain the technology, Dr. Lee drew the analogy of painting. Protons are the paint, but many techniques can apply it-a sprayer, roller, or fine airbrush.
"Conventional protons use a little bit larger spray and block the overspray with an aperture typically made out of brass. That has an opening in it that conforms to the shape of the tumor target. Protons are allowed to pass through the opening and overspray is blocked," he said. "With the pencil beam scanning, imagine taking away that block and instead of a spray you use an airbrush of protons. You start at the deepest layer and spray one spot, then another until you cover that entire layer; then move up to the next layer."
