数值方法分析电子线笔形束模型的能量展宽函数

Derivation of Energy Spread Function of Electron Pencil Beam Model

目的:探讨用离轴比曲线分析电子束照射野笔形束模型能量展宽函数的方法。方法:用PTWmp3三维水箱测量Synergy加速器所有电子束能量、限光筒、空气间隙在不同深度的射野离轴比曲线。用数值分析方法对射野离轴比曲线进行分析,得到电子线照射野笔形束模型能量展宽函数σp(z)随电子束标称能量、限光筒大小和限光筒底端面到体模表面空气间隙变化的规律。将计算得到的σp(z)输入到PLATO治疗计划系统,计算吸收剂量,并与相同条件下用0.6cc电离室剂量仪测量的结果进行比较。结果:能量展宽σp(z)随深度增加而变大,接近电子最大射程末端,很快减小,呈液滴状分布。能量展宽和电子的标称能量以及限光筒大小有关,这主要是电子在体模中的单次和多次散射作用引起的。能量展宽随限光筒低端面到体模表面的空气间隙线性变化。标准条件下吸收剂量的计算值和测量值很接近,最大误差小于±5%。结论:电子束照射野笔形束模型充分考虑电子在体模内的作用特点和过程,是比较好的计算模型。用射野离轴比数据分析电子束照射野笔形束模型的特征参数,结果准确可靠。

Objective： To derive the energy spread function of tile electron pencil beam model from the measured beam profiles. Methods： All measurements were completed on a Synergy^TM linear accelerator using PTW mp3 water phantom. Beam profiles at different depths for the given applicator, nominal beam energy, air gap between the end of the applicator and water surface, were measured. The energy spread functions were derived numerically. Characteristic parameters were input into PLATO treatment planning system, absorbed doses at specific points were calculated and compared to the measured values. Results： The energy spread functions increased with the depth, but decreased abruptly near the practical range, having the shape of a water drop. Due to the scattering effect, the spread functions were dependent closely on nominal electron beam energy, applicator size and air gap. The calculated and measured doses were within ±5% for standard SSD without insert. Conclusions： The energy spread function of electron pencil beam model can be derived from measured beam profiles. Electron pencil beam algorithm can calculate dose at a high accurate level owing to its inherent properties.