De Vlamynck K, Palmans H, Verhaegen F, De Wagter C, De Neve W, Thierens H
Department of Radiotherapy, University Hospital Gent, Belgium.
Med Phys. 1999 Sep;26(9):1874-82. doi: 10.1118/1.598693.
Small fields where electronic equilibrium is not achieved are becoming increasingly important in clinical practice. These complex situations give rise to problems and inaccuracies in both dosimetry and analytical/empirical dose calculation, and therefore require other than conventional methods. A natural diamond detector and a Markus parallel plate ionization chamber have been selected for clinical dosimetry in 6 MV photon beams. Results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry have been compared with dose measurements. A modification of the existing component module for multileaf collimators (MLCs) allowed the modeling of a linear accelerator SL 25 (Elekta Oncology Systems) equipped with a MLC with curved leaf-ends. A mechanical measurement method with spacer plates and a light-field edge detection technique are described as methods to obtain geometrical data of collimator openings for application in the Monte Carlo system. Generally a good agreement is found between measurements and calculations of depth dose distributions and deviations are typically less than 1%. Calculated lateral dose profiles slightly exceed measured dose distributions near the higher level of the penumbras for a 10x2 cm2 field, but agree well with the measurements for all other cases. The simulations are also able to predict variations of output factors and ratios of output factors as a function of field width and field-offset. The Monte Carlo results demonstrate that qualitative changes in energy spectra are too small to explain these variations and that especially geometrical factors affect the output factors and depth dose curves and profiles.
在临床实践中，未达到电子平衡的小射野正变得越来越重要。这些复杂情况在剂量测定以及分析/经验剂量计算中都会引发问题和误差，因此需要采用非常规方法。已选用天然金刚石探测器和马库斯平行板电离室用于6兆伏光子束的临床剂量测定。利用蒙特卡罗系统BEAM/EGS4对射束几何结构进行建模的模拟结果已与剂量测量结果进行了比较。对现有的多叶准直器（MLC）组件模块进行了修改，从而能够对配备有带弯曲叶端的MLC的直线加速器SL 25（医科达肿瘤系统公司）进行建模。描述了一种使用间隔板的机械测量方法和一种光野边缘检测技术，作为获取准直器开口几何数据以应用于蒙特卡罗系统的方法。一般来说，深度剂量分布的测量值与计算值之间吻合良好，偏差通常小于1%。对于10×2平方厘米的射野，在半值层较高水平附近，计算得到的横向剂量分布略超过测量得到的剂量分布，但在所有其他情况下与测量值吻合良好。模拟还能够预测输出因子的变化以及输出因子比值随射野宽度和射野偏移的变化情况。蒙特卡罗结果表明，能谱的定性变化太小，无法解释这些变化，特别是几何因素会影响输出因子以及深度剂量曲线和分布。
