Nettelbeck H, Takacs G J, Rosenfeld A B
Centre for Medical Radiation Physics, University of Wollongong, Australia.
Phys Med Biol. 2008 Sep 21;53(18):5123-37. doi: 10.1088/0031-9155/53/18/018. Epub 2008 Aug 22.
The application of a strong transverse magnetic field to a volume undergoing irradiation by a photon beam can produce localized regions of dose enhancement and dose reduction. This study uses the PENELOPE Monte Carlo code to investigate the effect of a slice of uniform transverse magnetic field on a photon beam using different magnetic field strengths and photon beam energies. The maximum and minimum dose yields obtained in the regions of dose enhancement and dose reduction are compared to those obtained with the EGS4 Monte Carlo code in a study by Li et al (2001), who investigated the effect of a slice of uniform transverse magnetic field (1 to 20 Tesla) applied to high-energy photon beams. PENELOPE simulations yielded maximum dose enhancements and dose reductions as much as 111% and 77%, respectively, where most results were within 6% of the EGS4 result. Further PENELOPE simulations were performed with the Sheikh-Bagheri and Rogers (2002) input spectra for 6, 10 and 15 MV photon beams, yielding results within 4% of those obtained with the Mohan et al (1985) spectra. Small discrepancies between a few of the EGS4 and PENELOPE results prompted an investigation into the influence of the PENELOPE elastic scattering parameters C(1) and C(2) and low-energy electron and photon transport cut-offs. Repeating the simulations with smaller scoring bins improved the resolution of the regions of dose enhancement and dose reduction, especially near the magnetic field boundaries where the dose deposition can abruptly increase or decrease. This study also investigates the effect of a magnetic field on the low-energy electron spectrum that may correspond to a change in the radiobiological effectiveness (RBE). Simulations show that the increase in dose is achieved predominantly through the lower energy electron population.
对正在接受光子束照射的体积施加强横向磁场会产生剂量增强和剂量降低的局部区域。本研究使用PENELOPE蒙特卡罗代码，研究均匀横向磁场切片对不同磁场强度和光子束能量的光子束的影响。将剂量增强和剂量降低区域中获得的最大和最小剂量产额与Li等人（2001年）在一项研究中使用EGS4蒙特卡罗代码获得的结果进行比较，Li等人研究了应用于高能光子束的均匀横向磁场切片（1至20特斯拉）的影响。PENELOPE模拟产生的最大剂量增强和剂量降低分别高达111%和77%，其中大多数结果在EGS4结果的6%以内。使用Sheikh-Bagheri和Rogers（2002年）的输入光谱对6、10和15兆电子伏特光子束进行了进一步的PENELOPE模拟，得到的结果与使用Mohan等人（1985年）光谱得到的结果相差在4%以内。EGS4和PENELOPE的一些结果之间的小差异促使人们研究PENELOPE弹性散射参数C(1)和C(2)以及低能电子和光子传输截止的影响。使用更小的计分箱重复模拟提高了剂量增强和剂量降低区域的分辨率，特别是在剂量沉积可能突然增加或减少的磁场边界附近。本研究还研究了磁场对可能与放射生物学效应（RBE）变化相对应的低能电子能谱的影响。模拟表明，剂量的增加主要是通过较低能量的电子群体实现的。
