虚拟单点源构建医用直线加速器束流模型的探讨

Study of beam model of medical linear accelerator based on virtual single point source

给出一种医用直线加速器X射线束流的虚拟单点源模型,并通过对实际测量的剂量分布进行拟合建模,探讨了模型的可用性。模型将X射线的束流分为原射线和散射线,原射线建模为各向异性点源,散射线建模为强度与射野相关的点源,而模型的参数通过对实测水体模中的吸收剂量分布进行拟合得到。结果显示:对于Varian Trilogy 6 MV直线加速器,模型给出的剂量分布与实测的剂量分布很好地符合,总散射因子(SCP)的测量值与拟合值差异小于1.5%。对于3 cm×3 cm、10 cm×10 cm、40 cm×40 cm射野,从水下深度1.4 cm(最大剂量深度)到水下深度20 cm,深度剂量分布的拟合值与测量值的平均差异分别为1.3%、1.3%和1.1%。在水下深度10 cm处,离轴剂量比(Off Axis dose Ratio,OAR)大于90%的离轴剂量的拟合值与测量值的平均差异分别为2.35%、0.75%和0.53%。这些结果表明X射线束流虚拟点源模型可很好地重建体模内的吸收剂量。

[Background]The beam model of medical linac is one of the bases of dose calculation.It is of great significance to study the beam model of medical linear accelerator in clinical application of radiation dose calculation.[Purpose]This study aims to provide a virtual point source model of medical linear accelerator X-ray beam,and to discuss the applicability of the model by fitting the measured dose distribution.[Methods]The X-ray beam was divided into primary ray and scattered ray.The primary ray was modeled as an anisotropic point source whilst the scattered ray was modeled as a point source whose intensity was related to the radiation field.The model parameters were obtained by fitting the measured absorbed dose distribution in water phantom.[Results]For Varian Trilogy 6 MV linear accelerator,the dose distribution given by the fitting model is in good agreement with the measured dose distribution,and the difference between the measured value and the fitting value of the total scattering factor(SCP)is less than 1.5%.For 3 cm×3 cm,10 cm×10 cm and 40 cm×40 cm radiation fields,from 1.4 cm(maximum dose depth)to 20 cm underwater depth,the average differences between the fitting and the measured value are 1.3%,1.3%and 1.1%respectively.The average differences between the fitting and measured values of offaxis dose in the area with off axis dose ratio(OAR)greater than 90%are 2.35%,0.75%and 0.53%respectively at the depth of 10 cm.[Conclusions]The virtual point source model of X-ray beam can well reconstruct the absorbed dose in phantom.