van Battum L J, Huizenga H
Department of Radiation Oncology, Daniel den Hoed Cancer Center/University of Rotterdam, The Netherlands.
Phys Med Biol. 1999 Nov;44(11):2803-20. doi: 10.1088/0031-9155/44/11/309.
Electron beam radiotherapy treatment planning systems need to be fed with the characteristics of the high-energy electron beams (4-50 MeV) from the specifically applied accelerator. Beams can be characterized by their mean initial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This information is the only input to pencil beam dose calculation models. Newer calculation models like macro Monte Carlo, voxel Monte Carlo and phase space evolution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam component and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to investigate to what extent standard values can be used both for the effective initial angular variance as input to pencil beam models and for the primary beam initial angular variance. Comprehensive benchmark data were obtained on the initial angular variance of various types of accelerator, for various energies and field sizes. The initial angular variance sigma2theta(x) has been derived from penumbra measurements in air by means of film dosimetry at various distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for sigma2theta(x)/T(E) of around 13 cm where T(E) is the ICRU-35 linear angular scattering power in air. This value can be chosen as standard value for the primary beam initial angular variance, only slightly compromising the dose calculation accuracy. As input to pencil beam models, an effective sigma2theta(x)/T(E) should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-10 cm) an effective sigma2theata(x)/T(E) of 20 cm has been found and is recommended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 50% higher for the effective sigma2theta(x)/T(E).
电子束放射治疗治疗计划系统需要输入特定应用加速器产生的高能电子束（4 - 50 MeV）的特性。束流的特征可以通过其平均初始能量、有效初始角方差、虚拟源位置以及在水中产生的中心轴深度剂量分布来描述。这些信息是笔形束剂量计算模型的唯一输入。较新的计算模型，如宏观蒙特卡罗、体素蒙特卡罗和相空间演化模型，需要输入完整的初始相空间或该初始相空间的参数化，通常由一个主束分量和一个或多个散射分量组成。这个主束分量通常由初始能量、主束初始角方差和虚拟源距离来表征。本研究的目的是调查标准值在多大程度上既可以用于笔形束模型输入的有效初始角方差，也可以用于主束初始角方差。针对各种类型的加速器、不同能量和射野大小，获得了关于初始角方差的综合基准数据。初始角方差sigma2theta(x)是通过在离下准直器不同距离处使用胶片剂量学在空气中进行半值层测量得出的。对于当今放射治疗中使用的加速器类型，测量结果显示sigma2theta(x)/T(E)的值约为13 cm，其中T(E)是空气中的ICRU - 35线性角散射功率。这个值可以作为主束初始角方差的标准值，只会对剂量计算精度造成轻微影响。作为笔形束模型的输入，应使用包含来自下准直器散射的有效sigma2theta(x)/T(E)。对于下准直器与患者之间气隙较小（5 - 10 cm）的情况，已发现有效sigma2theata(x)/T(E)为20 cm，并建议作为笔形束模型的标准输入。在所研究的加速器中，仅发现Elekta SL15的有效sigma2theta(x)/T(E)值不同，即高出50%。
