Calibration of the Gamma Knife using a new phantom following the AAPM TG51 and TG21 protocols

To compare calibration of the Leksell Gamma Knife according to the American Association of Physicists in Medicine Task Groups 21 and 51 protocols. A new phantom was fabricated for this purpose. Its design, physical properties, and composition are described.

The Gamma Knife TG-51 calibration phantom is designed to be filled with water and support an ionization chamber positioned at its center. The phantom is thimble-shaped, with a 2 mm plastic wall to contain water. The phantom and chamber assembly was mounted in a Leksell stereotactic frame. The location of the chamber's sensitive volume was determined using computed tomography. The chamber-phantom assembly was attached to the 18 mm helmet in the Gamma Knife by the stereotactic frame. The phantom's geometry allowed radiation beams from each of the 201 Gamma Knife cobalt-60 sources to converge after an 8 cm path to the ionization chamber's sensitive volume. This is similar to the arrangement by which one calibrates the Gamma Knife using the manufacturer-supplied polystyrene phantom.

The phantom was attached to the Gamma Knife so that the ionization chamber was reproducibly positioned at the convergence of the radiation beams. Because of the phantom's design, the phantom could be affixed to either trunnions or the automatic patient positioning system, once mounted in the Leksell stereotectic frame. Comparisons using different phantoms and protocols resulted in the following calibration ratios for TG-21 in the polystyrene sphere phantom, TG-21 in the water phantom, and TG-51 in the water phantom, respectively: 1.000, 1.008, 0.986, when corrected for transmission through the plastic water reservoir wall and using the same ionization chamber. Transmission measurements using a 1 cm thickness of the same material in the Co-60 beam determined that the phantom's 2 mm plastic wall resulted in a reduction in the measured the output by 0.5%.

Calibration of the Gamma Knife can be performed in liquid water using the AAPM TG-51 protocol and this new phantom, thereby eliminating uncertainties with respect to the composition of the manufacturer's phantom. Perturbation of calibration measurements by nonwater materials was characterized and could be corrected. Calibration values for the Gamma Knife that were obtained using the three methods for our phantoms agree to within 1.4%. TG21 and TG51 calibration of the Gamma Knife using the water phantom agreed to within 2.2%.