医用发泡胶对粒子剂量影响的模拟研究

Monte Carlo simulation of the influence of medical foam on particle dose

目前发泡胶在放疗定位中运用广泛,但其对加速器基础剂量影响并无相关报道。研究不同厚度的发泡胶在医用电子直线加速器均整(Flattening Filter,FF)和非均整(Flattening Filter Free,FFF)两种模式对粒子剂量的影响,为临床使用提供参考。利用蒙特卡罗程序EGSnrc进行建模和计算,首先根据厂家提供的发泡胶参数建立PEGS4材料库与截面数据,利用BEAMnrc建立Varian True Beam系列不同射野的加速器机头模型并进行计算,生成对应的相空间文件;然后利用BEAMdp分析不同模型结构产生的射线能谱、能量注量的差异。最后利用Dosexyznrc计算分析不同厚度发泡胶在固定射野和固定厚度发泡胶在不同射野对粒子剂量的影响。结果表明:在有发泡胶时两种模式下的百分深度剂量(Percentage Depth Dose,PDD)都有不同程度的向前移动的现象,导致表面剂量增加,但对射线则影响不大。FF模式下当发泡胶厚度≤5 cm时,同一深度最大剂量偏差≤2%,而当发泡胶厚度达到10 cm时,同一深度最大剂量偏差高达6%,且两种模式下PDD0都超过90%。在两种模式下发泡胶对射野离轴比和半影都有较大影响,射野越小对粒子剂量影响也越小。临床使用发泡胶时尽量将厚度控制在5 cm以内,当需要使用较厚发泡胶时,建议使用FFF模式;使用立体定向放射治疗时发泡胶对剂量影响不大。

[Background]At present,styrofoam is widely used in radiotherapy positioning,but there is no relevant report on its impact on accelerator basic dose.[Purpose]This study aims to investigate the effects of different thicknesses of styrofoam on the particles dose of medical electronic linear accelerator flattening filter(FF)and flattening filter free(FFF),and provide guidance for clinical use.[Methods]The Monte Carlo program EGSnrc was used for modeling and calculation.First of all,the PEGS4 material library and cross-section data were established according to the parameters of the styrofoam provided by the manufacturer,and the BEAMnrc was used to establish accelerator head models of different shooting fields of Varian True Beam series and perform calculations to generate corresponding phase space files.Then,the BEAMdp was employed to analyze the differences in ray energy spectrum and energy fluence generated by different model structures.Finally,the effect of different thickness foams on the fixed field and fixed thickness foams on the field was calculated and analyzed by using Dosexyznrc.[Results]In the presence of styrofoam,the percentage depth dose(PDD)in both FF and FFF modes has a phenomenon of moving forward to varying degrees,resulting in an increase in the surface dose whilst little effect on the quality of the radiation.In the FF mode,the maximum dose deviation at the same depth is≤2%when the thickness of the foam is≤5 cm.When the thickness of the form reaches 10 cm,the maximum dose deviation at the same depth increases to 6%,and the PDD0 exceeds 90%in both modes.In both modes,the presence of styrofoam has a greater effect on the off-axis ratio and penumbra of the shot,the smaller the shooting field,the smaller the effect on the particle dose.[Conclusions]When using styrofoam for clinical purposes,try to control the thickness within 5 cm,and FFF mode is recommended for more thicker styrofoam.Styrofoam has little effect on dose of stereotactic radiation therapy.