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Vital Signs

Getting radiation where it should go . . . and not where it shouldn't

It's a little-known fact that most cancer patients who receive radiation therapy get a tattoo. But instead of the elaborate body art favored by rock musicians and athletes, they get a line of tiny blue-black dots. The dots help doctors to aim the radiation beam. Fast and precise computers at Dartmouth's Norris Cotton Cancer Center can direct a beam with accuracy as close as a single millimeter—the width of just 16 pages of this magazine.

Yet for years, doctors have tried to achieve still greater accuracy. A tumor attacks a living human whose lungs, diaphragm, and heart are constantly in motion. Radiation technicians can't tell patients not to move their hearts any more than photographers can make squirming toddlers sit still.

Beams: This is not a trivial concern, explains Eugen Hug, M.D., Norris Cotton's director of radiation oncology. Radiation beams must be widened to compensate for these body movements, as well as to catch tumors that are too microscopic to show up on an x-ray. But because radiation damages any tissue in its path, treatments can harm the lungs or the heart while it's attacking the cancer.

"Almost all of the side effects come from radiation given where it doesn't belong," says Hug.

To reduce this collateral damage, the Cancer Center has adopted a new technique called radiation pulsing, to deliver the beam with more accuracy than ever before. Physicists, among them DHMC's David Gladstone, Sc.D., call it radiation gating—a term derived from the electronic description for a signal that's switched on and off. Norris Cotton, Hug says, is one of just a handful of cancer centers nationwide with the equipment and technical skill to use radiation pulsing effectively.

Pulsing: In one of DHMC's radiation suites, Gladstone retrieves from a high cabinet a clear plastic box slightly larger than a pack of cigarettes. The comparison to a cigarette pack is apt, since radiation pulsing is particularly effective against lung cancer. One side of the box contains two reflectors similar to the "hot dots" on a child's backpack. When the box is placed on a patient's chest, a device that looks like a shower head—except it sprays infrared light, not water—uses the reflectors to calibrate the precise rhythm of the patient's breathing.

This information is then correlated with a CT scan, a threedimensional picture of the patient's insides. It is this image that determines the placement of the tattoos that become the targets for the radiation beam.

Now, the Cancer Center's computers can not only aim the beam at the tattooed dots but can synchronize it with the patient's breathing, turning the radiation on at the exact moment the lungs are at a given expansion point, improving accuracy.

Because the lungs are so close to other critical organs, the prior inability to achieve pinpoint accuracy required technicians to reduce the intensity of the radiation in order to limit collateral damage. "You can easily destroy half a lung with radiation," Hug says. The typical lung cancer patient can't spare that much tissue. She is elderly, Hug says, perhaps 70, and a smoker. It's very likely that she also suffers from asthma and emphysema, which make her lungs "medically inoperable," Hug explains. Because of all these complications, this average patient has had just a 40% chance that radiation therapy will succeed.

But with radiation pulsing's ability to "give enough radiation to the right point," Hug adds, "we can control malignant tumors, increase survival, and decrease side effects." The chance of survival doubles to 80%. "The patient goes from being not likely to survive lung cancer to likely" to survive, says Hug.

John Milne

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Dartmouth Medical SchoolDartmouth-Hitchcock Medical CenterWhite River Junction VAMCNorris Cotton Cancer CenterDartmouth College