小射野条件下高能和低能Χ射线在非均匀介质中的剂量特性的比较研究

利用蒙特卡罗方法模拟计算了临床上常用的高能（15 MV）和低能（6 MV）点源光子束在有限宽非均匀模体中的沉积能量，研究了不同射野条件下中心轴上的剂量分布，并用剂量扰动因子（DPF）定量分析了剂量增强或减弱效应的强度.模拟结果表明：在小射野情况下，异质组织界面处出现了较大的剂量跃变，剂量扰动效应的强弱程度取决于射线的能量、射野的大小和模体介质的不同等多种因素，高能X射线条件下的剂量扰动效应大于低能X射线条件下的剂量扰动；在入射X射线能量相同的条件下，剂量扰动效应随射野尺寸的减小而增强，并呈现一种非线性关系

The dosimetric study on dose perturbation in inhomogeneities with small beamlets for iow and high energy X rays

The absorbed doses in the central axis of different phantoms with inhomogeneity media and finite sizes for small field size beams (beamlets) of 6 MV X rays and 15 MV X rays were simulated with Monte Carlo code system EGSnrc, respectively. The dose enhancement effects and decreasing effects caused by material inhomogeneity were studied quantitatively with a parameter named Dose Perturbation Factor (DPF), which was the ratio of the dose at a point in the heterogeneous phantom to the dose of the same spatial location point in the homogeneous phantom. It was showed that the degrees of the dose enhancement or decreasing depended on the energy of radiation, field sizes of the beamlets and kinds of the phantoms. The dose perturbation for high energy X rays is greater than that for low energy X rays. At the same energy condition, the dose perturbation decreased when the field size increased. And the relationship between them was not linear.