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	Description of Gamma Knife Procedure
	Technical Specs of Gamma Knife
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Technical specifications of the Gamma Knife

There are two current versions of the Leksell Gamma Knife, each made in
Sweden by Elekta Instruments.

Though several versions of the Gamma Knife are in use, the workings of these instruments are largely the same. As the photo above indicates each model has a hydraulic bed for the patient, a donut-shaped helmet that doesn't actually touch the patient's head, and a large radiation unit into which the patient's head is guided.

The unit consists of a permanent 18,000 kilogram housing (radiation unit) and a helmet with a radially-oriented array of 201 cobalt-60 sources. Each beam channel has a source-bushing assembly, a tungsten alloy pre-collimator, and a primary collimator. Secondary collimation is achieved via one of the four helmets containing 201

secondary collimators, or "holes". In each helmet, each of these collimators are used to create different diameter fields at the focal point.

Modification of the distribution can be achieved using multiple isocenters or "shots" with different collimators, or by plugging certain holes (collimators) which stops the beam that would

otherwise pass through that collimator. The only sources of movement during the procedure include the following:

1. Movement of the treatment table in and out of the radiation unit

2. Opening of the shielding door as performed by the hydraulic or
electrical power system

Stereotactic computed tomography (CT), MRI, or angiography can be used for target determination, depending on the indication. The higher resolution provided by MRI with an ability to image in different planes, and the proven accuracy of this technique, have provided significant advantages in comparison to CT. CT, MRI, or MRA imaging is used to supplement conventional angiography for

vascular malformation radiosurgery. The accuracy of dose delivery using the Gamma Knife as tested was found to be approximately 0.25 mm. Mechanical accuracy as dictated by the manufacturer should be less than 0.3 mm.

Multiple isocenters or "shots" using narrow radiation beams are used to create an isodose line that matches the imaging-defined tumor margin and still maintains a steep falloff of radiation into the surrounding structures.

© 2003 MCG	Questions and Comments to Sharon Owens
November 22, 2005	Department of Neurosurgery \| Medical College of Georgia