Kainz K K, Antolak J A, Almond P R, Bloch C D, Hogstrom K R
Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX 77030-4009, USA.
Phys Med Biol. 2005 Mar 7;50(5):755-67. doi: 10.1088/0031-9155/50/5/002. Epub 2005 Feb 17.
The laser wakefield acceleration (LWFA) mechanism can accelerate electrons to energies within the 6-20 MeV range desired for therapy application. However, the energy spectrum of LWFA-generated electrons is broad, on the order of tens of MeV. Using existing laser technology, the therapeutic beam might require a significant energy spread to achieve clinically acceptable dose rates. The purpose of this work was to test the assumption that a scattering foil system designed for a mono-energetic beam would be suitable for a poly-energetic beam with a significant energy spread. Dual scattering foil systems were designed for mono-energetic beams using an existing analytical formalism based on Gaussian multiple-Coulomb scattering theory. The design criterion was to create a flat beam that would be suitable for fields up to 25 x 25 cm2 at 100 cm from the primary scattering foil. Radial planar fluence profiles for poly-energetic beams with energy spreads ranging from 0.5 MeV to 6.5 MeV were calculated using two methods: (a) analytically by summing beam profiles for a range of mono-energetic beams through the scattering foil system, and (b) by Monte Carlo using the EGS/BEAM code. The analytic calculations facilitated fine adjustments to the foil design, and the Monte Carlo calculations enabled us to verify the results of the analytic calculation and to determine the phase-space characteristics of the broadened beam. Results showed that the flatness of the scattered beam is fairly insensitive to the width of the input energy spectrum. Also, results showed that dose calculated by the analytical and Monte Carlo methods agreed very well in the central portion of the beam. Outside the useable field area, the differences between the analytical and Monte Carlo results were small but significant, possibly due to the small angle approximation. However, these did not affect the conclusion that a scattering foil system designed for a mono-energetic beam will be suitable for a poly-energetic beam with the same central energy. Further studies of the dosimetric properties of LWFA-generated electron beams will be done using Monte Carlo methods.
激光尾波场加速（LWFA）机制能够将电子加速至治疗应用所需的6 - 20 MeV能量范围内。然而，LWFA产生的电子能谱很宽，达数十MeV量级。利用现有的激光技术，治疗束可能需要显著的能量展宽才能达到临床上可接受的剂量率。本研究的目的是检验这样一个假设：为单能束设计的散射箔系统对于具有显著能量展宽的多能束是否适用。基于高斯多库仑散射理论，利用现有的解析形式为单能束设计了双散射箔系统。设计标准是创建一个扁平束，该束在距初级散射箔100 cm处适用于最大25×25 cm²的射野。采用两种方法计算了能量展宽范围为0.5 MeV至6.5 MeV的多能束的径向平面注量分布：（a）通过对一系列单能束穿过散射箔系统的束分布进行求和进行解析计算，以及（b）使用EGS/BEAM代码通过蒙特卡罗方法进行计算。解析计算有助于对箔设计进行精细调整，而蒙特卡罗计算使我们能够验证解析计算的结果，并确定展宽束的相空间特性。结果表明，散射束的平整度对输入能谱宽度相当不敏感。此外，结果表明，通过解析方法和蒙特卡罗方法计算得到的剂量在束的中心部分吻合得非常好。在可用射野区域之外，解析结果与蒙特卡罗结果之间的差异虽小但显著，这可能是由于小角度近似所致。然而，这些并不影响这样的结论：为单能束设计的散射箔系统对于具有相同中心能量的多能束将是适用的。将使用蒙特卡罗方法对LWFA产生的电子束的剂量学特性进行进一步研究。
