Ottawa Carleton Institute for Physics, Carleton University Campus, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2010 Nov;37(11):5939-50. doi: 10.1118/1.3495537.
To use EGSnrc Monte Carlo simulations to directly calculate beam quality conversion factors, kQ, for 32 cylindrical ionization chambers over a range of beam qualities and to quantify the effect of systematic uncertainties on Monte Carlo calculations of kQ. These factors are required to use the TG-51 or TRS-398 clinical dosimetry protocols for calibrating external radiotherapy beams.
Ionization chambers are modeled either from blueprints or manufacturers' user's manuals. The dose-to-air in the chamber is calculated using the EGSnrc user-code egs_chamber using 11 different tabulated clinical photon spectra for the incident beams. The dose to a small volume of water is also calculated in the absence of the chamber at the midpoint of the chamber on its central axis. Using a simple equation, kQ is calculated from these quantities under the assumption that W/e is constant with energy and compared to TG-51 protocol and measured values.
Polynomial fits to the Monte Carlo calculated kQ factors as a function of beam quality expressed as %dd(10)x and TPR10(20) are given for each ionization chamber. Differences are explained between Monte Carlo calculated values and values from the TG-51 protocol or calculated using the computer program used for TG-51 calculations. Systematic uncertainties in calculated kQ values are analyzed and amount to a maximum of one standard deviation uncertainty of 0.99% if one assumes that photon cross-section uncertainties are uncorrelated and 0.63% if they are assumed correlated. The largest components of the uncertainty are the constancy of W/e and the uncertainty in the cross-section for photons in water.
It is now possible to calculate kQ directly using Monte Carlo simulations. Monte Carlo calculations for most ionization chambers give results which are comparable to TG-51 values. Discrepancies can be explained using individual Monte Carlo calculations of various correction factors which are more accurate than previously used values. For small ionization chambers with central electrodes composed of high-Z materials, the effect of the central electrode is much larger than that for the aluminum electrodes in Farmer chambers.
使用 EGSnrc 蒙特卡罗模拟直接计算 32 个圆柱形电离室在不同束质范围内的束质转换系数 kQ，并量化系统不确定性对 kQ 的蒙特卡罗计算的影响。这些因子是使用 TG-51 或 TRS-398 临床剂量学协议校准外照射放疗束所必需的。
根据蓝图或制造商的用户手册对电离室进行建模。使用 EGSnrc 用户代码 egs_chamber 计算腔室内的空气剂量，使用 11 种不同的表列临床光子谱计算入射束。在腔室不存在的情况下，也在腔室的中心点和中心轴上计算小体积水的剂量。在假设 W/e 与能量恒定的情况下，使用一个简单的方程从这些量中计算 kQ，并与 TG-51 协议和实测值进行比较。
给出了每个电离室的束质转换系数 kQ 的蒙特卡罗计算值与束质表示为 %dd(10)x 和 TPR10(20)的函数的多项式拟合。对蒙特卡罗计算值与 TG-51 协议值或使用 TG-51 计算程序计算的值之间的差异进行了解释。分析了计算 kQ 值的系统不确定性，如果假设光子截面不确定性不相关，则最大不确定性为 0.99%，如果假设它们相关，则最大不确定性为 0.63%。不确定性的最大组成部分是 W/e 的恒定性和水中光子截面的不确定性。
现在可以使用蒙特卡罗模拟直接计算 kQ。大多数电离室的蒙特卡罗计算结果与 TG-51 值相当。可以使用各种校正因子的单独蒙特卡罗计算来解释差异，这些校正因子比以前使用的值更准确。对于具有高 Z 材料中央电极的小型电离室，中央电极的影响比 Farmer 室中的铝电极大得多。
