New Machine Allows Precise Focusing of Radiation Treatment
PHILADELPHIA _ John Donahue lay still on the treatment table while the linear accelerator, as massive as a refrigerator, rotated into position, ready to send beams of lethal radiation into his prostate tumor.
It was an intimidating setup, but Donahue, a retired pharmaceutical researcher from Newtown, Pa., said he was more “fascinated” than scared last week, his third week of treatment at Philadelphia’s Fox Chase Cancer Center.
He knew that the chances of curing his early-stage cancer are high, given the cumulative daily doses he is scheduled to receive in the coming weeks.
He also felt confident that he had reduced his already-low odds of serious side effects: He is among the first patients anywhere to use a new tumor-tracking system that the manufacturer touts as “GPS for the body.”
Permanently implanted within Donahue’s walnut-sized prostate gland are three electromagnetic transponders, each the size of a grain of rice. These “beacons” emit signals _ much like the global positioning system unit in a car _ that therapists use to align the beams of radiation with his prostate. If his prostate moves too much, treatment stops so his body can be readjusted, avoiding potential damage to the nearby rectum and bladder.
How much movement is too much? Five millimeters. Less than a quarter of an inch.
“Any new technology really intrigues me,” Donahue, 65, said before the 15-minute radiation session. “It’s comforting to me to know the radiation is going only to the right place.”
Like most incremental advances in medical technology, the new $500,000 system, made by Calypso Medical Technologies Inc. of Seattle, raises questions about costs versus benefits.
For example, will its greater precision reduce the amount of incontinence and rectal bleeding that are caused by unintended radiation? Currently, for men who are good candidates for the latest types of high-dose radiation, only 1 percent to 2 percent suffer severe urinary or rectal side effects, according to a 2005 study led by Harvard researchers.
“Fifteen years ago, we could never be giving these high doses without causing terrible side effects,” said Eric M. Horwitz, clinical director of radiation oncology at Fox Chase. “Data exist to show that the Calypso system works well, but we need to use it for a while to find out if it’s better.”
The tumor-tracking system was recently approved by the Food and Drug Administration for use in prostate cancer. So far, five centers have installed the system.
About 20 prostate cancer patients here have used it so far _ too few to assess the potential benefits. But already, it is speeding up the process of positioning patients at the start of therapy, and it seems to be improving treatment precision, said Horwitz.
“Almost every treatment, the radiation is exactly where we want it to be,” Horwitz said. “We’re staying within a millimeter of the target.”
More than a million Americans undergo radiation treatment for cancer each year, including many of the 218,000 men with newly diagnosed prostate cancer, according to the American Cancer Society.
The average patient may not think much about internal organs moving around, but radiation oncologists have to. A mere breath or burp can cause a lung to change position. Digestion and urination can shift the prostate more than a third of an inch from one day to the next.
Until now, the primary methods for aligning treatment have involved imaging the tumor _ using implanted gold markers and X-ray images, or ultrasound scans _ before each radiation session. The Calypso system instead provides continuous, real-time positioning.
“It’s the difference between having a snapshot and a video,” Horwitz said.
X-rays and ultrasound scans also require interpretation by technicians, noted Calypso chief executive officer Eric Meier, who hopes his company’s system eventually will be approved to track tumors in lungs and other organs.
At the Mayo Clinic in Rochester, Minn., radiation oncologist Brian Davis believes the system will prove its worth, at least for prostate cancer.
Davis, who said he has no financial interest in Calypso but has evaluated the system, explained that oncologists have to build in a margin of extra radiation around the prostate where the cancer is contained, partly to make sure stray cancer cells are killed, and partly to make up for organ movement. With more precise targeting, the irradiated margin could be reduced by a third, thus sparing healthy tissue.
“I think this definitely adds something in terms of precision that is meaningful,” Davis said.
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