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Program lends new precision to ‘gamma knife’

February 19, 2001 By Brian Mattmiller

If ever a medical procedure seemed in dire need of technological support, this would be it.

In treating brain tumors with radiation surgery, doctors face this manual task: Develop a treatment plan that bombards the entire tumor, minimizes exposure outside the target and avoids sensitive brain structures. The job must be done by analyzing scores of two-dimensional brain images and completed within 40 minutes, as the patient waits in an uncomfortable head frame.

In a promising new research effort, a mathematical program is helping automate and fine-tune this arduous process. Michael Ferris, a UW–Madison computer scientist , is working with medical physicists and oncologists at the University of Maryland Medical School on a computer program to reduce the threat of human error in setting radiation treatment plans.

The team’s work is optimizing a unique technology called the “Gamma Knife,” a device that is designed exclusively for treating brain tumors. The Gamma Knife uses 201 radiation sources that combine simultaneously to create a “spherical ball” of treatment. About 200 Gamma Knife machines are in operation worldwide.

In the Maryland clinical trial, a doctor with nearly two decades experience with the Gamma Knife is going head to head with the computer program, with each one developing an optimal treatment strategy for a real patient. In each of several occasions so far, the doctor has opted to use the computerized treatment plan.

“We’re very excited by the idea of bringing uniformity to these treatment plans,” says Ferris, a scientist with UW–Madison’s Data Mining Institute. “Now if a patient is lucky, they will get a doctor who’s been doing this for 15 years, rather than someone who just joined the program. But with this program, both will be equally effective.”

Ferris presented his work Feb. 19, at the American Association for the Advancement of Science annual meeting in San Francisco, along with several other U.S. scientists who are using the power of mathematics to solve medical problems. There are growing examples of how computer algorithms and mathematical programming are taking disease treatment and drug design to new levels.

With the Gamma Knife, each radiation shot is acting like a scalpel that burns out the tumor. When designing a treatment, doctors are mapping out a series of radiation shots within the tumor, akin to filling a bag with marbles. But the spaces between those marbles end up eluding radiation exposure.

The computer-based optimization by Ferris, however, takes a non-linear approach to the problem, to better reflect how a radiation dosage truly behaves within a tumor. Radiation attenuates or tapers out when it goes through material. The group recognized that the radiation is not 100 percent within the ball and zero outside of it, but is instead more of a bell-shaped curve with the highest dosage in the center.

Using this bell-shaped design, the computer model was able to overlap different radiation shots to provide enough intensity to kill tumor cells in the spaces between doses. The entire tumor needs to receive at least 50 percent of the maximum dosage to destroy the tumor. The computer program was also able to reduce the total number of doses needed for a completed treatment, which frequently can require ten or more trips through the Gamma Knife.

“The mathematics involved is much better able to handle this three-dimensional problem,” Ferris says. “Doctors are trying to cover a three-dimensional target by just looking at many two-dimensional slices.”

Another major benefit of the computer program is speed. Since it is able to map out a treatment plan in 20 minutes or less, neurosurgeons can experiment with several alternative setups to find the most effective one.

The procedure also holds the promise of treating larger tumors, Ferris says. The Gamma Knife has normally been reserved for smaller tumors because the planning challenge for larger tumors becomes virtually insurmountable. The University of Maryland hospital is pioneering the use of this machine for tumors that are larger, have irregular shapes or are close to a sensitive structure, such as the spinal cord.

The Gamma Knife has been around since 1967 and was built by a Swedish medical company. But it has only come into its own recently as a brain surgery tool, thanks in part to huge advances in imaging such as magnetic resonance imaging and positron emission tomography. U.S. hospitals are seeing a much greater demand for the technology, since it can greatly improve quality of life over conventional radiation therapy.

Ferris’ work is supported by the National Science Foundation, the Air Force Office of Scientific Research and Microsoft Corporation.