Kumar Sudhir, Deshpande Deepak D, Nahum Alan E
Department of Physics, Clatterbridge Cancer Centre, Bebington, CH63 4JY, UK. Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK. Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CT & CRS building, Anushaktinagar, Mumbai-400094, India. Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai-400 094, India.
Phys Med Biol. 2015 Jan 21;60(2):501-19. doi: 10.1088/0031-9155/60/2/501. Epub 2014 Dec 30.
The relationships between D, K and Kcol are of fundamental importance in radiation dosimetry. These relationships are critically influenced by secondary electron transport, which makes Monte-Carlo (MC) simulation indispensable; we have used MC codes DOSRZnrc and FLURZnrc. Computations of the ratios D/K and D/Kcol in three materials (water, aluminum and copper) for large field sizes with energies from 50 keV to 25 MeV (including 6-15 MV) are presented. Beyond the depth of maximum dose D/K is almost always less than or equal to unity and D/Kcol greater than unity, and these ratios are virtually constant with increasing depth. The difference between K and Kcol increases with energy and with the atomic number of the irradiated materials. D/K in 'sub-equilibrium' small megavoltage photon fields decreases rapidly with decreasing field size. A simple analytical expression for X̅, the distance 'upstream' from a given voxel to the mean origin of the secondary electrons depositing their energy in this voxel, is proposed: X̅(emp) ≈ 0.5R(csda)(E̅(0)), where E̅(0) is the mean initial secondary electron energy. These X̅(emp) agree well with 'exact' MC-derived values for photon energies from 5-25 MeV for water and aluminum. An analytical expression for D/K is also presented and evaluated for 50 keV-25 MeV photons in the three materials, showing close agreement with the MC-derived values.
在辐射剂量学中，D、K和Kcol之间的关系至关重要。这些关系受到次级电子输运的严重影响，这使得蒙特卡罗（MC）模拟不可或缺；我们使用了MC代码DOSRZnrc和FLURZnrc。本文给出了在50 keV至25 MeV（包括6 - 15 MV）能量范围内，三种材料（水、铝和铜）大野尺寸下D/K和D/Kcol比值的计算结果。在最大剂量深度之后，D/K几乎总是小于或等于1，而D/Kcol大于1，并且这些比值随着深度增加实际上是恒定的。K和Kcol之间的差异随着能量以及被辐照材料的原子序数增加而增大。在“次平衡”小兆伏光子场中，D/K随着场尺寸减小而迅速降低。提出了一个关于X̅的简单解析表达式，X̅是从给定体素到在该体素中沉积其能量的次级电子的平均起源点的“上游”距离：X̅(emp) ≈ 0.5R(csda)(E̅(0))，其中E̅(0)是次级电子的平均初始能量。对于水和铝，这些X̅(emp)与5 - 25 MeV光子能量下由MC得出的“精确”值吻合良好。还给出了D/K的解析表达式，并针对三种材料中50 keV - 25 MeV的光子进行了评估，结果显示与MC得出的值非常吻合。
