Björk Peter, Knöös Tommy, Nilsson Per
Department of Radiation Physics, Lund University Hospital, Sweden.
Phys Med Biol. 2002 Nov 21;47(22):4019-41. doi: 10.1088/0031-9155/47/22/308.
The least known parameters in a Monte Carlo simulation of a linear accelerator treatment head are often the properties of the initial electron beam directed onto the exit vacuum window. Several initial beams with different spatial fluence distributions, angular divergences and energy spectra have been transported through the geometry of a scattering foil accelerator. The electron beam characteristics (energy spectrum and angular distribution) at the phantom surface and the subsequent relative absorbed dose distribution in a water phantom were calculated. The dose distribution was found to be insensitive to the geometrical properties of the initial beam. Furthermore, the lateral dose profiles are unaffected by the energy spectrum of the initial beam. The effect on the depth-dose curve is negligible if the initial energy spectrum is symmetric (e.g., Gaussian shaped) and its full width at half maximum (FWHM) is less than approximately 10% of the most probable energy. A larger FWHM will decrease the normalized dose gradient, but will not affect the dose in the build-up region. An asymmetric wedge shaped spectrum with a low-energy extension simultaneously increases the dose in the build-up region and decreases the dose gradient. The relationship between the energy spectral width and the normalized dose gradient is, however, smaller than published analytical expressions indicate. Some well-established energy-range relationships were shown to be accurate for most of the initial beams studied. The energy spectrum at the phantom surface was also derived from a measured depth-dose curve through different methods. The extracted spectrum depends on the beam model and the spectral reconstruction algorithm. Even though the depth-dose curve is fairly independent of initial beam characteristics, a correct description of the low-energy tail of the energy spectrum is important to obtain good agreement between measured and Monte Carlo calculated doses in the build-up region.
在直线加速器治疗头的蒙特卡罗模拟中，最鲜为人知的参数往往是射向出口真空窗的初始电子束的特性。几种具有不同空间注量分布、角发散度和能谱的初始束已通过散射箔加速器的几何结构进行传输。计算了模体表面的电子束特性（能谱和角分布）以及随后在水模体中的相对吸收剂量分布。发现剂量分布对初始束的几何特性不敏感。此外，横向剂量分布不受初始束能谱的影响。如果初始能谱是对称的（例如高斯形状）且其半高宽（FWHM）小于最可几能量的约10%，则对深度剂量曲线的影响可忽略不计。较大的FWHM会降低归一化剂量梯度，但不会影响剂量建成区的剂量。具有低能延伸的不对称楔形谱会同时增加剂量建成区的剂量并降低剂量梯度。然而，能谱宽度与归一化剂量梯度之间的关系比已发表的解析表达式所表明的要小。对于大多数所研究的初始束，一些已确立的能量-范围关系被证明是准确的。模体表面的能谱也通过不同方法从测量的深度剂量曲线推导得出。提取的能谱取决于束模型和谱重建算法。尽管深度剂量曲线相当独立于初始束特性，但对能谱低能尾部的正确描述对于在剂量建成区获得测量剂量与蒙特卡罗计算剂量之间的良好一致性很重要。
