基于笔形束散射核的非均匀模体透射平面散射线分析

目的:利用蒙特卡罗方法分析透射平面上散射光子的物理性质以及非均匀模体厚度对散射核的影响，为基于电子射野影像设备（EPID）的在体剂量验证研究提供基础。方法:利用EGSnrc建立笔形束散射核模型，并模拟获得X射线穿过非均匀模体（水肺水/水骨水模体）以及相应等效厚度水模后30 cm处透射平面上的多种散射线能量注量分布，并分析水肺水/水骨水模体与其等效厚度水模体在散射线能量注量分布上的差异。结果:散射核中一阶康普顿散射线最大能量注量在1×10-4 MeV·cm-2数量级，当离轴距离为8~12 cm时下降至最大值的一半，而散射核中其它散射线能量注量最大值在1×10-5 MeV·cm-2数量级附近或以下。对于水肺水/水骨水模体，散射核能量注量相对偏差变化为±1.2%~±11.5%，且随模体非均匀层厚度增大而增大。结论:散射核中一阶康普顿散射线占比最大，同时也贡献了大部分能量注量相对偏差，在通过散射核来重建非均匀模体后EPID平面上的射线分布时，应着重考虑一阶康普顿散射线对重建结果的影响，并对其进行有效的修正。

Scattered photons on inhomogeneous phantom transmission plane: an analysis based on pencil beam scatter kernels

Objective To analyze the physical characteristics of scattered photons on transmission planes and the effects of inhomogeneous phantom thickness on scatter kernels with the use of Monte Carlo method,and to provide a basis for the studies on electronic portal imaging devices-based in vivo dosimetric verification.Methods Pencil beam scatter kernel model was established by EGSnrc.After the X-rays passing through inhomogeneous phantoms(water-lung-water phantoms and water-bonewater phantoms)and their equivalent homogeneous phantoms(EHPs),the energy fluence distributions of different scattered photons on transmission plane which was 30 cm below phantoms were obtained.The energy fluence deviation of scattered photons between inhomogeneous phantoms(water-lung-water phantoms and water-bone-water phantoms)and their EHPs were analyzed.Results The maximum energy fluence of the first order Compton scatter of scatter kernels was 1×10-4 MeV·cm-2 orders of magnitude,and fell to the half when the off-axis distance was 8 to 12 cm.For other scattered photons of scatter kernels,the maximumenergy fluence was near or lower than 1×10-5 MeV·cm-2.The relative deviation of energy fluence of scatter kernels was in the range of±1.2%to±11.5%between inhomogeneous phantoms(water-lung-water phantoms and water-bone-water phantoms)and EHPs,and the deviation was larger with the increase of inhomogeneous phantom thickness.Conclusion The first order Compton scatter of scatter kernels accounts for the largerst and contributed to the most of the relative deviations of energy fluence.Therefore,the effect of the first order Compton scatter should be considered and corrected when reconstructing the rays on the EPID plane after the reconstruction of inhomogeneous phantom by scatter kernels.