WASHINGTON - Despite all the promising news about protons, proponents of the Gamma Knife (GK) have a lot to celebrate as well, as made clear in a presentation at last week's conference of the International Brain Mapping and Intraoperative Surgical Planning Society by David Larson, PhD, professor of radiation oncology at the University of California at San Francisco (UCSF). He gave attendees a closer look at the Perfexion GK, made by Elekta (Stockholm).

Larson said that all told, half a million people have been treated worldwide to date with the GK, and the pace is picking up, with 50,000 patients now going under this virtual knife each year. He said that since the introduction of the GK in 1968, the machine has undergone four substantial redesigns. The first four iterations used up to 201 sources of the cobalt isotope, cobalt 60, and in "treatment with prior models, numerous maneuvers were necessary" to irradiate the entire tumor.

"There was a time penalty to make more and more conformal treatments" because the cobalt sources are in a fixed position in the conventional collimator, Larson said.

Elekta assembled an advisory panel a couple of years ago to suggest a way to deal with this, he said. "The key was to throw away the helmets" in favor of a cone-shaped collimator with the cobalt sources mounted in several collimator panels instead of a single hemispherical helmet, he said. The sliding collimator panels not only provide a greater range of positions for the cobalt radiation but also allow the individual cobalt sources to be turned off.

"The overall [collimator] volume was increased so that one could treat the entire frame without a lot of gymnastics," Larson said, adding that the manufacturer is using tungsten to decrease the thickness of the shielding.

Beam diameters in the Perfexion vary from 4, 8 to 16 mm, and the unit offers only 192 sources of cobalt, but the redesign allows the lower number of cobalt sources to do more work.

"Changing from one configuration to another takes less than one second," Larson said, and the conformity index, a measure of the degree to which the radiation matched the exact shape of the tumor, was as high as .95 for the Perfexion, a good jump from the .7 to .8 conformity index of earlier GK models.

Residual radiation is still an issue with photons, so the number of target lesions is still limited with radioactive treatment modes. Larson said that most doctors "get uncomfortable" treating more than 25 targets of 8 mm or larger. Still, the new machine induces less exposure than its antecedents.

The dose decrease represents, Larson said, "a significant reduction" at the thyroid, in breasts and in the lower body.

Larson said that Elekta has already installed four units, the latest at metropolitan Salt Lake City at the Jon and Karen Huntsman Cancer Center (Murray, Utah). Other units are in operation in London and Marseilles, France (Medical Device Daily, July 14, 2006), and Hospital CUF Infante Santo in Lisbon (MDD, June 1, 2007) has a unit on order.

According to Larson, sales are doing will, with units to be installed at UCSF, the University of Virginia (Charlottesville) and a half-dozen other locations in the U.S. and abroad.

- MARK McCARTY

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