Radiotherapy and Radiosurgery Department, Humanitas Research Hospital and Cancer Center, Milan-Rozzano, Italy.
PTW-Freiburg GmbH, Freiburg, Germany.
Radiat Oncol. 2018 Jul 11;13(1):126. doi: 10.1186/s13014-018-1070-6.
Linac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.
Beams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions. Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.
The depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.
A better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.
直线加速器的输出剂量作为射野大小的函数，包含体模散射和头部散射分量。后一种分量可以在空气中使用合适的组织等效物进行测量，以确保完全的电子平衡和不存在污染电子。可以使用组织等效物帽或微型体模来实现平衡条件。本研究通过在模拟微型体模的虚拟体模中进行蒙特卡罗模拟，分析测量准直器散射因子（直线加速器输出因子的头部散射分量）的设置条件，以便更好地理解。
在长 20cm、直径从 0.5 到 4cm 的不同圆柱状有机玻璃 phantom 中，模拟了来自 TrueBeam 的 6 和 15MV 射线，射野大小从 4×4cm2 到 40×40cm2。采用基于 PENELOPE 蒙特卡罗代码的 PRIMO 程序包，使用直线加速器供应商提供的瓦里安 TrueBeam 直线加速器的相空间文件进行直线加速器头部模拟。分析了深度剂量曲线，并在不同 phantom 条件下估算了不同深度处的准直器散射因子。此外，在空气条件下，使用有机玻璃和黄铜组织等效物帽以及瓦里安 Clinac DHX 的微型有机玻璃体模，对 6 和 18MV 射线进行了测量。
沿圆柱体的深度剂量曲线进行比较，在每个 phantom 中，对于所有分析的射野大小，都显示出非常相似的曲线，证明了在微型体模中估算准直器散射因子的正确性，前提是将探测器放置在足够深的位置以排除电子污染。这些结果得到了测量的验证，其中有机玻璃组织等效物帽显示无法正确估算准直器散射因子，而微型体模和黄铜帽则给出了合理的测量结果。
通过对深度剂量曲线的分析，更好地理解了虚拟微型体模内的束特性，指出了使用有机玻璃组织等效物帽的关键点，并建议在中大型射野范围内使用微型体模测量准直器散射因子。
