von Voigts-Rhetz Philip, Czarnecki Damian, Zink Klemens
Institut für Medizinische Physik und Strahlenschutz - IMPS, Technische Hochschule Mittelhessen, University of Applied Sciences, Gießen, Germany.
Institut für Medizinische Physik und Strahlenschutz - IMPS, Technische Hochschule Mittelhessen, University of Applied Sciences, Gießen, Germany.
Z Med Phys. 2014 Sep;24(3):216-23. doi: 10.1016/j.zemedi.2013.12.001. Epub 2014 Jan 10.
The presence of an air filled ionization chamber in a surrounding medium introduces several fluence perturbations in high energy photon and electron beams which have to be accounted for. One of these perturbations, the displacement effect, may be corrected in two different ways: by a correction factor pdis or by the application of the concept of the effective point of measurement (EPOM). The latter means, that the volume averaged ionization within the chamber is not reported to the chambers reference point but to a point within the air filled cavity. Within this study the EPOM was determined for four different parallel plate and two cylindrical chambers in megavoltage electron beams using Monte Carlo simulations. The positioning of the chambers with this EPOM at the depth of measurement results in a largely depth independent residual perturbation correction, which is determined within this study for the first time. For the parallel plate chambers the EPOM is independent of the energy of the primary electrons. Whereas for the Advanced Markus chamber the position of the EPOM coincides with the chambers reference point, it is shifted for the other parallel plate chambers several tenths of millimeters downstream the beam direction into the air filled cavity. For the cylindrical chambers there is an increasing shift of the EPOM with increasing electron energy. This shift is in upstream direction, i.e. away from the chambers reference point toward the focus. For the highest electron energy the position of the calculated EPOM is in fairly good agreement with the recommendation given in common dosimetry protocols, for the smallest energy, the calculated EPOM positions deviate about 30% from this recommendation.
在周围介质中存在充气电离室会在高能光子和电子束中引入几种注量扰动，这些扰动必须予以考虑。其中一种扰动，即位移效应，可以通过两种不同的方式进行校正：通过校正因子pdis或应用有效测量点（EPOM）的概念。后者意味着，电离室内的体积平均电离不是报告到电离室的参考点，而是报告到充气腔内的一个点。在本研究中，使用蒙特卡罗模拟确定了兆伏电子束中四种不同的平行板电离室和两种圆柱形电离室的EPOM。将具有此EPOM的电离室放置在测量深度处，会导致在很大程度上与深度无关的残余扰动校正，这是本研究首次确定的。对于平行板电离室，EPOM与初级电子能量无关。而对于高级马克斯电离室，EPOM的位置与电离室的参考点重合，对于其他平行板电离室，EPOM在束流方向下游几十分之一毫米处向充气腔内移动。对于圆柱形电离室，EPOM随着电子能量的增加而向上游方向移动，即远离电离室的参考点朝向焦点。对于最高电子能量，计算出的EPOM位置与常见剂量学协议中的建议相当吻合，对于最小能量，计算出的EPOM位置与该建议偏差约30%。
