German Cancer Research Center (DKFZ), Heidelberg, Germany. National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO) Heidelberg, Germany. Author to whom any correspondence should be addressed.
Phys Med Biol. 2019 Sep 23;64(19):195005. doi: 10.1088/1361-6560/ab376e.
The recent update of key dosimetric data by the International Commission on Radiation Units and Measurements (ICRU) makes several changes to the computation of beam quality correction factors k with regard to, for example, the mean excitation energies, I, which enter the stopping power computation for water and air, the computation procedure itself, the average energy expended in the production of an ion pair in air, W/e, as well as chamber-specific factors for cobalt-60. With the new recommendations an accurate assessment of the water-to-air stopping-power ratio, [Formula: see text], in reference conditions is necessary to update the dosimetry protocols for carbon ion beams. The ICRU 90 key data were considered for computation of [Formula: see text] for carbon ion beams using Monte Carlo transport simulations for a number of reference conditions, namely monoenergetic carbon ion beams with a range in water from 3 to 30 cm and spread-out Bragg peaks (SOBPs) of different widths and depths in water. New recommendations for [Formula: see text] are presented, namely 1.1247 for the reference condition of depth 1 g cm for monoenergetic carbon ion beams and 1.1273 at the center of physically optimized SOBPs. The recommendation of a constant value (1.126) represents the stopping-power ratio within a 0.3% variation of [Formula: see text] for all reference conditions considered. The impact of these new [Formula: see text] values and the updated key data on k for carbon ion beams was evaluated in a second step. Changes and the difference from experimental data were found to be non-significant, but larger discrepancies to measurements were observed for plane-parallel ionization chambers. The combined uncertainty for k in carbon ion beams decreased to 2.4%. In future, it could be further lowered by using chamber-specific Monte Carlo transport simulations, for which the implementation of ICRU 90 key data as done in this study is a prerequisite.
国际辐射单位和测量委员会（ICRU）最近更新的关键剂量学数据对束质校正因子 k 的计算进行了多项更改，例如，进入水和空气的阻止本领计算的平均激发能 I、计算过程本身、在空气中产生一对离子所消耗的平均能量 W/e 以及钴-60 的专用室因素。根据新的建议，为了更新碳离子束的剂量学协议，有必要对参考条件下水-空气阻止本领比 [Formula: see text] 进行准确评估。使用蒙特卡罗输运模拟，对多种参考条件下的碳离子束的 [Formula: see text] 进行了 ICRU90 关键数据的计算，这些条件包括能量单一的碳离子束，在水中的射程从 3 厘米到 30 厘米，以及不同宽度和深度的扩展布拉格峰（SOBP）。本文提出了新的 [Formula: see text] 建议值，即对于能量单一的碳离子束在水深 1g cm 的参考条件下为 1.1247，在物理优化的 SOBP 中心为 1.1273。推荐使用一个常数（1.126）表示在考虑的所有参考条件下，[Formula: see text] 的变化在 0.3%以内的阻止本领比。在第二步中，评估了这些新的 [Formula: see text] 值和更新的关键数据对碳离子束 k 的影响。结果发现，变化和与实验数据的差异不显著，但对于平行平板电离室，观察到与测量值的较大差异。碳离子束中 k 的合并不确定度降低到 2.4%。未来，通过使用专用室的蒙特卡罗输运模拟，可以进一步降低该值，而这项研究中 ICRU90 关键数据的实施是一个前提条件。
