Sauer O A
Strahlenabteilung, Universitäts-Frauenklinik, Würzburg, Germany.
Med Phys. 1995 Oct;22(10):1685-90. doi: 10.1118/1.597433.
In the vicinity of interfaces between materials of different atomic number Z, extremes in absorbed dose occur for high-energy photon irradiations. The spatial extension of the effects is within the range of 1 cm, which may not be ignorable from the radiobiological point of view. At the front side of a high-Z slab a maximum is observed, whereas at the exit side a small buildup zone of the dose occurs, e.g., for a 5 MV beam, in front of a water/iron interface, the enhancement is about 30% of that to the homogeneous medium. The reduction at the back of the iron slab is about 16% for this energy, but vanishes with increasing energy. For high-energy photons this effect is mainly caused by the strong atomic number dependence of the scattering power for secondary electrons. The amount and extent of the scattering effects have been measured for aluminum and for iron slabs embedded in water or PMMA. The experimental data are in good agreement with Monte Carlo calculated values. Therefore the data form a reliable base to test the performance of commonly used treatment planning algorithms. The convolution or superposition method is used to calculate dose distributions. To account for the Z dependence of the scattering and the stopping power of the secondary electrons, corrections are applied to the energy deposition kernels. The boundary crossing of energy deposition kernels can be treated only in an approximate manner. However, the algorithm developed improves the accuracy of the dose calculation in the vicinity of interfaces significantly.
在不同原子序数Z的材料界面附近，高能光子辐照会出现吸收剂量的极值情况。这些效应的空间扩展范围在1厘米以内，从放射生物学角度来看这可能不容忽视。在高Z值平板的前侧会观察到剂量最大值，而在出射侧会出现剂量的小剂量积累区，例如，对于5兆电子伏特的射束，在水/铁界面之前，增强量约为均匀介质的30%。对于该能量，铁平板后侧的剂量减少约16%，但随着能量增加而消失。对于高能光子，这种效应主要是由二次电子散射能力对原子序数的强烈依赖性引起的。已经测量了嵌入水或聚甲基丙烯酸甲酯中的铝和铁平板的散射效应的量和范围。实验数据与蒙特卡罗计算值吻合良好。因此，这些数据为测试常用治疗计划算法的性能提供了可靠依据。采用卷积或叠加方法来计算剂量分布。为了考虑二次电子散射和阻止本领对Z的依赖性，对能量沉积核进行修正。能量沉积核的边界穿越只能以近似方式处理。然而，所开发的算法显著提高了界面附近剂量计算的准确性。
