Cheung Y C, Yu K N, Ho R T, Yu C P
Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Kowloon, People's Republic of China.
Appl Radiat Isot. 2000 Sep;53(3):427-30. doi: 10.1016/s0969-8043(99)00285-7.
The accuracy of the dose planning system (Leksell GammaPlan) used in Gamma Knife (model-B) radiosurgery was verified using both the GafChromic films MD-55 (improved) and the EGS4 Monte Carlo technique. The Monte Carlo technique was employed to calculate the dose distribution along the x-, y- and z-axes when a single shot with opening of all 201 sources was delivered at the centre of a simulated water phantom with a diameter of 160 mm. Collimator helmets with different size of the Gamma Knife unit were verified. Good consistency (typical discrepancy less than 2%) was obtained between the results of Monte Carlo and GammaPlan. Small discrepancies, however, were obtained by GafChromic films. Discrepancies, as great as 10% when using the 4 mm collimator helmet, at the low percentage isodose curve along the z-axis of the measurement results were probably due to the small energy dependency of the GafChromic films. Significant discrepancies were not observed along x- and y-axes because such small discrepancies were easily over-washed by other gamma beams coming from the x- and y-directions. Similar results showing the discrepancies between the GafChromic films and GammaPlan were obtained when using the 8, 14 and 18 mm collimator helmets. However, the discrepancies along the z-axis became smaller as scattering effect increased when using larger collimator sizes. We suggest that the Monte Carlo technique should also be applied in stereotactic dose planning system verification as it is an ideal and reliable computational technique.
使用GafChromic MD - 55（改进型）胶片和EGS4蒙特卡罗技术，对伽玛刀（B型）放射外科中使用的剂量规划系统（Leksell GammaPlan）的准确性进行了验证。当在直径为160 mm的模拟水模中心进行单次照射且所有201个源均打开时，采用蒙特卡罗技术计算沿x、y和z轴的剂量分布。对伽玛刀装置不同尺寸的准直器头盔进行了验证。蒙特卡罗结果与GammaPlan结果之间具有良好的一致性（典型差异小于2%）。然而，GafChromic胶片得到的差异较小。在测量结果沿z轴的低剂量等剂量曲线上，使用4 mm准直器头盔时差异高达10%，这可能是由于GafChromic胶片的能量依赖性较小。沿x轴和y轴未观察到显著差异，因为来自x方向和y方向的其他伽马射线很容易掩盖这种小差异。使用8、14和18 mm准直器头盔时，也得到了显示GafChromic胶片与GammaPlan之间差异的类似结果。然而，当使用较大尺寸的准直器时，随着散射效应增加，沿z轴的差异变小。我们建议蒙特卡罗技术也应应用于立体定向剂量规划系统验证，因为它是一种理想且可靠的计算技术。
