小射野条件下^(60)Coγ射线及6–MV Χ射线在非均匀介质中的剂量特性研究

Dosimetry perturbation in inhomogeneous phantoms with small beamlets of ^(60)Coγ-rays and 6-MV photon beams

用蒙特卡罗应用程序EGSnrc模拟计算了60Coγ射线、6–MV的点源光子束在有限宽非均匀模体中,不同射野条件下中心轴上的剂量分布情况,利用剂量扰动因子(Dose perturbation factor,DPF)定量研究了剂量增强或减弱效应的强度。DPF即不均匀介质模体中某点的剂量与均匀水模体中同一空间位置点处的剂量值之比。模拟结果表明,剂量扰动效应的强弱程度取决于多种因素,如射线的能量、射野的尺寸和入射介质的尺寸。另外,无论是在骨等效介质中,还是在肺等效介质中,剂量增强或减弱效应随射野尺寸的变化并不是线性的。

Dosimetry distribution in central axis of irthomogeneous phantoms with small field size beams （beamlets） of ^60Co γ-rays and 6-MV X rays were simulated with Monte Carlo code system EGSnrc. The dose enhancement effects and decreasing effects caused by material inhomogeneity were studied dose perturbation factor （DPF）, a parameter defined as the ratio of the dose at a point in a heterogeneous phantom to the dose of the same spatial location point in a homogeneous phantom. The results showed that the degrees of the dose enhancement or decreasing depended on energy of the radiation, field sizes of the beamlets and sizes of the phantoms. And this kind of dosimetry change was not linear.