Abstract
A microchamber, PTW Pinpoint 31006, was used to measure transverse beam profiles for an Elekta Gamma Knife, Model B, and compared with profiles measured with film dosimetry. The microchamber sensitive volume has a diameter of 2 mm, which is smaller than the gamma knife beams, and a length of 5 mm. The chamber was mounted in a custom cassette in a spherical plastic phantom, supplied by Elekta, and oriented in a sagittal plane with the 2-mm dimension at right angles to the transverse plane. The phantom was manually moved across the beam, using the gamma knife x-coordinate trunnions, to measure the profiles. Profiles were also measured with V-film placed in a cassette mounted in the spherical plastic phantom. The films were scanned with a Scanditronix film scanner and converted to dose with a density to dose calibration curve. The results were superimposed for comparison. The beam width at the 50 intensity was measured from the film profiles to give the dimensions of the beams in the orthagonal planes. The ion chamber measurements are compared with the film results for the transverse, x profiles. Good agreement between the film and ion chamber transverse profiles is observed. 2002 American College of Medical Physics.
Highlights
The Elekta Leksell Gamma Knife has 201 Co-60 sources to form the clinical treatment volume
Beam profiles of the transverse plane measured by the two methods are shown in Figs. 4, 5, 6, and 7 for the 18, 14, and 8% 4-mm helmets, respectively
The transverse profiles show a region of flat, 100% dose for the 18, 14, and 8-mm helmets, while the 4-mm helmet is sharply peaked with no plateau
Summary
The Elekta Leksell Gamma Knife has 201 Co-60 sources to form the clinical treatment volume. The 201 sources are positioned on the surface of a hemisphere with the radiation beams directed towards the isocenter of the hemisphere. Primary collimators, fixed in the main body of the gamma knife, shape the primary beam, while interchangeable helmets provide secondary collimation to shape the final treatment beam. Each helmet has 201 individual secondary collimators mated to each of the primary collimators and the associated 201 Co-60 sources. Four helmets with different size collimators are available to form the clinical beam utilizing the 201 individual beams with nominal clinical beam sizes of 4, 8, 14, and 18 mm. The clinical beam is a spherical volume and these nominal sizes refer to the width of the 50% diameter
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