Computer program lends new precision to 'Gamma Knife'
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 computer scientist with the University
of Wisconsin-Madison, 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
Ferris presented his work Monday, Feb. 19, at the American Association for the Advancement
of Science (AAAS) 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
"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 (MRI) and positron emission
tomography (PET). U.S. hospitals are seeing a much greater
demand for the technology, since it can greatly improve quality of life over conventional
Contact: Michael Ferris
University of Wisconsin-Madison