电子线斜入射对剂量分布影响的分析

目的　量化分析不同能量电子线在斜入射情况下对剂量分布的影响。方法　在水模体中 ,测量束流中心轴上不同相对剂量值的深度 ;并测量 80 %和 5 0 %等剂量线的倾斜角度 ;将电子线在不同斜入射角度时的这些测量数据与垂直入射时的测量数据进行比较。结果　①电子线斜入射角度越大 ,最大剂量点深度和 90 %、80 %、5 0 %及 10 %的深度越小 ;②能量越高 ,斜入射对最大剂量点深度和 90 %、80 %、5 0 %及 10 %的深度影响越大 ;③斜入射对相对剂量分布的影响还与射野大小有关 ,射野越大 ,最大剂量点深度和 90 %、80 %、5 0 %及 10 %的深度变化越小 ;④在相同的斜入射条件下 ,电子线能量的改变比射野大小的改变对相对剂量分布的影响要大 ;⑤在斜入射时 ,80 %和 5 0 %等剂量线会向有空气隙的一侧倾斜 ,并且倾斜角度要比斜入射角度大。结论　斜入射不仅使最大电离深度值减小 ,而且使电子线穿透能力减弱 ;电子线穿透能力的减弱程度与电子线能量和射野大小有关 ;临床上要充分考虑在斜入射时 80 %和 5 0 %等剂量线的横向移动 ,否则很容易造成肿瘤靶区的漏照射 ,从而导致肿瘤的局部复发。

Dose distribution of oblique incidence electron beam

ObjectiveTo analyze the dose distribution of oblique incidence electron beam. MethodsIn water phantom,the relative dose values were measured along the central axis of electron beam. The slope angles of 80% and 50% isodose lines were measured in the penumbra area. All these measured data were compared with those without oblique angle. Results1. When the oblique incidence angle of electron beam increased,the depth of the maximal dose and the isodose lines of 90%,80%,50% and 10% decreased. 2.The higher the beam energy ,the greater the depth of maximal dose and the isodose lines of 90%,80%,50% and 10%. 3.The effect of oblique incidence was also related to field size. When the beam size increased,the depth of maximal dose and the isodose lines of 90%,80%,50% and 10% decreased. 4. For the same oblique incidence angle,the effect of electron energy is more obvious than that of beam size. 5. Because of the oblique incidence of electron beam,the 80% and 50% isodose lines in the penumbral area would slope up to the direction with air gap and the slope angle was wider than the oblique incidence angle of the electron beam. Conclusions The oblique incidence of electron beam not only reduces the depth of maximum ionization,but also weakens the transmissive ability of electron beam. The weakening extent of the transmissive ability is related to electron energy and field size. In the oblique incidence,the movement of 80% and 50% isodose lines in horizontal direction should be noted,otherwise the dose of some tumor will be very low.