Bolus下空腔对各向异性分析算法计算精度影响  

Effect of cavity under Bolus to shallow dose calculation precision of anisotropic analytical algorithm

作　　者：孔栋[1] 魏贤顶[1] 惠琳 孔旭东[1] 赵于天 杨波[1] KONG Dong;WEI Xianding;HUI Lin;KONG Xudong;ZHAO Yutian;YANG Bo(Department of Radiation Oncology,Affiliated Hospital of Jiangnan University,Wuxi 214062 China)

机构地区：[1]江南大学附属医院肿瘤放疗科,江苏无锡214062

出　　处：《中国辐射卫生》2021年第1期94-98,共5页Chinese Journal of Radiological Health

摘　　要：目的基于蒙特卡罗算法研究Bolus下不同大小空腔对各向异性分析算法(AAA)计算浅层组织剂量精度的影响。方法在Eclipse中构建上表面位于加速器源轴距、中心与射野中心轴重合的30 cm×30 cm×30 cm水模体,其上表面构建不含或含有不同大小空腔的1 cm厚Bolus,采用AAA分别计算不同空腔下水模体中心轴和不同深度x轴上剂量分布;用Geant4构建加速器模型和相同水模体及补偿膜,验证加速器模型正确性,并计算水模体相同位置剂量分布;以Geant4结果为基准,对比不同空腔下AAA计算精度变化。结果对于1 cm厚Bolus,空腔面积为2 cm×2 cm时,其厚度<5 mm,对AAA精度影响在2%左右,空腔厚度进一步增加,AAA会过高评估浅表剂量,随深度增加对AAA影响迅速减小;空腔面积增加,对AAA计算中心区域影响逐渐减小直至基本无影响,影响较大区域逐渐外移,空腔面积≥射野时,对AAA影响不再变化,影响较大区域移至射野边缘附近。结论计划制定时对于相较治疗子野面积较小的空腔,若其厚度≥5 mm应适当提高空腔下浅表剂量;若空腔面积较治疗子野过大或空腔厚度过大,则需重新定位。Objective To research the effect of cavity under Bolus to anisotropic analytical algorithm(AAA)on calculation precision of dose in shallow tissue based on Monte Carlo method;Methods A 30 cm×30 cm×30 cm water phantom with the upper surface was constructed which was located at the source-axis distance(SAD)of the medical linear accelerator and the center as well as coincided with the central axis of the radiation field in Eclipse treatment panning system.Above the water phantom,a water film of 1 cm thick with or without different cavities was constructed or.AAA was used to calculate the dose distribution on the central axis and the x-axis of different depth of the water model with different cavities respectively.The accelerator model,the same water phantom and the water film were constructed and the dose distributions of the same positions were calculated with Geant4.Based on the Geant4 calculation result,the calculation precision of AAA with different cavity were compared;Results For cavities with area of 2 cm×2 cm,if the thickness is smaller than 0.5 cm,the AAA calculation error is about 2%.with the cavity thickness increase,the AAA would overestimate the dose in the shallow area under the cavity.With the cavity area increase,the area where AAA overestimate the shallow dose gradually moved out until near the edge of the radiation field,and the calculation error on the central area gradually reduced until there is basically no error.Conclusions The shallow dose would be increased according to the cavity size when planning with AAA;If there are cavities with large volume,it is better to reposition.

关 键 词：补偿膜 空腔 各向异性分析算法 GEANT4 剂量 

分 类 号：R811.1[医药卫生—放射医学]