Medical Physics Unit, McGill University, Montreal, QC, H4A 3J1, Canada.
Department of Medical Physics, Karolinska University Hospital, Stockholm, 171 76, Solna, Sweden.
Med Phys. 2018 Apr;45(4):1748-1757. doi: 10.1002/mp.12821. Epub 2018 Mar 23.
To calculate the kQmsr,Q0fmsr,fref factors for nine common ionization chamber types following the small fields dosimetry formalism for the calibration of the Leksell Gamma Knife (LGK) Perfexion using Monte Carlo simulation. This study also provides the first independent comparison of EGSnrc and PENELOPE for the calculation of kQmsr,Q0fmsr,fref correction factors and proposes a practical method to predict these factors based on chamber type, chamber orientation and phantom electron density.
The ionization chambers are modeled using the EGSnrc and PENELOPE Monte Carlo codes based on the blueprints provided by the manufacturers. The chambers are placed in a half-sphere water phantom and five spherical phantoms made of liquid water, solid water, ABS, polystyrene, and PMMA, respectively. Dose averaged over the air cavity of the chambers and a small water volume are calculated using EGSnrc and PENELOPE Monte Carlo codes for both conventional and machine specific reference (msr) setups. Using the calculated dose ratio, the kQmsr,Q0fmsr,fref factor is determined for all phantom materials and two possible orientations of chamber. The calculated kQmsr,Q0fmsr,fref factors are compared to a previous Monte Carlo study. A relationship between the kQmsr,Q0fmsr,fref factor and the electron density of the phantom material is derived to predict the kQmsr,Q0fmsr,fref factor for any phantom material type. Applying the calculated kQmsr,Q0fmsr,fref factors to the measured dose rate of a recent round robin study improves consistency of reference dosimetry of the Leksell Gamma Knife (LGK) Perfexion .
Agreement within uncertainty is observed between kQmsr,Q0fmsr,fref values determined in this study and the previous PEGASOS/PENELOPE study in a liquid water phantom. The difference between kQmsr,Q0fmsr,fref values in parallel and perpendicular detector orientations is most significant for the PTW 31010 (1.8%) chamber. The percentage root-mean-square (%RMS) deviation between EGSnrc and PENELOPE calculated kQmsr,Q0fmsr,fref values for Exradin-A1SL, A14 and A14SL chambers studies in this work was found to be 0.4%. The kQmsr,Q0fmsr,fref values increase linearly with electron density of the phantom material for all chamber types mainly due to the linear dependency of photon energy fluence ratios on electron density. The average percentage difference between the calculated and predicted kQmsr,Q0fmsr,fref values using two methods is found to be 0.15% and 0.16%. Previously measured dose rates corrected with the kQmsr,Q0fmsr,fref values determined in this work leads to absorbed dose values consistent to within 0.8%.
The calculated kQmsr,Q0fmsr,fref values in this work will enable users to apply the appropriate correction for their own specific phantom material only knowing the electron density of the phantom material.
根据小场剂量学公式，使用蒙特卡罗模拟为 Lekell Gamma Knife（LGK） Perfexion 进行校准，计算九种常见电离室类型的 kQmsr、Q0fmsr、fref 校正因子。本研究还首次对 EGSnrc 和 PENELOPE 进行了独立比较，以计算 kQmsr、Q0fmsr、fref 校正因子，并提出了一种基于室类型、室方向和幻���电子密度预测这些因子的实用方法。
使用 EGSnrc 和 PENELOPE 蒙特卡罗代码根据制造商提供的蓝图对电离室进行建模。将腔室放置在半球形水模体和五个分别由液体水、固体水、ABS、聚苯乙烯和 PMMA 制成的球形模体中。使用 EGSnrc 和 PENELOPE 蒙特卡罗代码分别为常规和特定于机器的参考（msr）设置计算腔室空气腔和小体积水中的剂量平均值。使用计算出的剂量比，确定所有幻���材料和两个腔室可能取向的 kQmsr、Q0fmsr、fref 因子。将计算出的 kQmsr、Q0fmsr、fref 因子与之前的蒙特卡罗研究进行比较。得出 kQmsr、Q0fmsr、fref 因子与幻���材料电子密度之间的关系，以预测任何幻���材料类型的 kQmsr、Q0fmsr、fref 因子。将计算出的 kQmsr、Q0fmsr、fref 因子应用于最近的一项循证研究中的测量剂量率，可提高 Lekell Gamma Knife（LGK） Perfexion 的参考剂量一致性。
在液体水幻���中，本研究确定的 kQmsr、Q0fmsr、fref 值与之前的 PEGASOS/PENELOPE 研究在不确定度范围内一致。在平行和垂直探测器方向上，kQmsr、Q0fmsr、fref 值的差异在 PTW 31010（1.8%）腔室中最为显著。本工作中对 Exradin-A1SL、A14 和 A14SL 腔室研究的 EGSnrc 和 PENELOPE 计算的 kQmsr、Q0fmsr、fref 值的均方根偏差（%RMS）百分比为 0.4%。对于所有腔室类型，kQmsr、Q0fmsr、fref 值随幻���材料的电子密度呈线性增加，主要是由于光子能量注量比随电子密度呈线性变化。使用两种方法计算和预测的 kQmsr、Q0fmsr、fref 值之间的平均百分比差异发现为 0.15%和 0.16%。使用本工作中确定的 kQmsr、Q0fmsr、fref 值校正先前测量的剂量率，可使吸收剂量值在 0.8%以内保持一致。
本工作中计算的 kQmsr、Q0fmsr、fref 值将使用户能够仅通过了解幻���材料的电子密度，为其特定的幻���材料应用适当的校正。
