Lourenço A, Thomas R, Homer M, Bouchard H, Rossomme S, Renaud J, Kanai T, Royle G, Palmans H
Department of Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, United Kingdom. Division of Acoustics and Ionising Radiation, National Physical Laboratory, Teddington TW11 0LW, United Kingdom.
Phys Med Biol. 2017 Apr 7;62(7):N134-N146. doi: 10.1088/1361-6560/aa6147. Epub 2017 Feb 17.
The aim of this work is to develop and adapt a formalism to determine absorbed dose to water from graphite calorimetry measurements in carbon-ion beams. Fluence correction factors, [Formula: see text], needed when using a graphite calorimeter to derive dose to water, were determined in a clinical high-energy carbon-ion beam. Measurements were performed in a 290 MeV/n carbon-ion beam with a field size of 11 × 11 cm, without modulation. In order to sample the beam, a plane-parallel Roos ionization chamber was chosen for its small collecting volume in comparison with the field size. Experimental information on fluence corrections was obtained from depth-dose measurements in water. This procedure was repeated with graphite plates in front of the water phantom. Fluence corrections were also obtained with Monte Carlo simulations through the implementation of three methods based on (i) the fluence distributions differential in energy, (ii) a ratio of calculated doses in water and graphite at equivalent depths and (iii) simulations of the experimental setup. The [Formula: see text] term increased in depth from 1.00 at the entrance toward 1.02 at a depth near the Bragg peak, and the average difference between experimental and numerical simulations was about 0.13%. Compared to proton beams, there was no reduction of the [Formula: see text] due to alpha particles because the secondary particle spectrum is dominated by projectile fragmentation. By developing a practical dose conversion technique, this work contributes to improving the determination of absolute dose to water from graphite calorimetry in carbon-ion beams.
这项工作的目的是开发并采用一种形式体系，以根据碳离子束中石墨量热法测量结果确定水的吸收剂量。在临床高能碳离子束中确定了使用石墨量热计推导水的剂量时所需的注量修正因子[公式：见正文]。测量是在290 MeV/n的碳离子束中进行的，射野尺寸为11×11 cm，无调制。为了对束流进行采样，选择了平面平行的鲁氏电离室，因为其收集体积与射野尺寸相比很小。关于注量修正的实验信息是从水中的深度剂量测量中获得的。在水体模前放置石墨板重复此过程。还通过实施三种基于（i）能量微分注量分布、（ii）等效深度处水中和石墨中计算剂量的比值以及（iii）实验装置模拟的方法，利用蒙特卡罗模拟获得了注量修正。[公式：见正文]项在深度上从入射处的1.00增加到布拉格峰附近深度处的1.02，实验与数值模拟之间的平均差异约为0.13%。与质子束相比，由于次级粒子能谱以弹丸碎裂为主，不存在因α粒子导致的[公式：见正文]减小情况。通过开发一种实用的剂量转换技术，这项工作有助于改进根据碳离子束中石墨量热法确定水的绝对剂量的方法。
