Measurement of the stopping power of water in condensed phase for carbon ions in the energy range between 1 MeV and 5 MeV

准确地了解液态水对碳离子在中间能区的停止能力，对重离子剂量测定具有至关重要的意义。在大多数粒子治疗中心使用的充气电离室中，水与空气的停止功率之比是确定吸收剂量的关键参数，这不仅是相对生物有效性的直接决定因素，而且靶体积内的吸收剂量也直接取决于停止功率。目前，将电离室测量值转换为吸收剂量是基于蒙特卡罗模拟的结果。这些模拟采用半经验和理论的停止功率数据，这些数据仅近似于布拉格峰区域中普遍存在的中间离子能量，其结果是，由于水-空气停止功率比[1]的不准确性，对计算的吸收剂量的不确定性贡献最大。研究发现，对于重离子，凝聚态物质和气态物质的阻挡力相差可达48%。对于处于中间能区的碳离子，甚至不清楚物质在液相中的停止力是大于还是小于气相。不能排除的是，使用蒙特卡罗模拟计算的水-空气停止功率比受不确定性的影响，该不确定性高于规定剂量和交付剂量对目标体积的偏差的最大容限[1]。因此，用实验数据来检验停止功率比是必须的。尽管这一点很重要，但目前还没有关于中能区液态水对碳离子停止能力的实验数据。缺乏实验数据的原因是，由于中间离子能量的范围很短，对液体物质的停止能力进行精确的直接测量是不可行的。本项目旨在通过一种复杂的实验技术，即所谓的倒多普勒频移衰减（IDSA）方法，首次通过实验确定能量范围在1至5 MeV之间的碳离子液态水的停止能力。该项目将为临床剂量学和更准确地评估碳离子癌症治疗的相对生物学有效性提供重要数据。此外，它将有助于澄清有关碳离子的凝聚相效应的符号和大小的不确定性。

The accurate knowledge of the stopping power of liquid water for carbon ions in the intermediate energy region is of paramount importance for heavy ion dosimetry. Not only the relative biological effectiveness but also the absorbed dose in the target volume directly depends on the stopping power but also the ratio of the stopping power of water to that of air is the crucial parameter in determining the absorbed dose by means of air-filled ionization chambers that are used in most particle therapy centers. At present, the conversion of the ionization-chamber measurements to absorbed dose is based on the results of Monte-Carlo simulations. These simulations employ semiempirical and theoretical stopping power data which are only approximate for intermediate ion energies prevailing in the Bragg peak area, with the consequence that the greatest contribution to the uncertainty of the calculated absorbed dose arises due to the inaccuracy of the stopping power ratio water-to-air [1]. It has been found that the stopping power of condensed and gaseous substances can differ up to 48% for heavy ions [2]. For carbon ions in the intermediate energy region, it is even not clear whether the stopping power of a substance in liquid phase is smaller or greater than that in gaseous phase. It cannot be excluded that stopping power ratios water-to-air calculated using Monte-Carlo simulations are subject to uncertainties higher than the maximal tolerance [1] for the deviation between the prescribed and delivered dose to the target volume. Therefore, the examination of the stopping power ratios by means of experimental data is mandatory. Despite this importance, there exist no experimental data for the stopping power of liquid water for carbon ions in the intermediate energy region. The reason for this lack of experimental data is that accurate direct measurement of the stopping power of liquid substances for intermediate ion energies is not feasible because of their very short ranges. The present project is aimed to experimentally determine the stopping power of liquid water for carbon ions in the energy range between 1 and 5 MeV for the first time by means of a sophisticated experimental technique, the so-called Inverted Doppler Shift Attenuation (IDSA) Method. The project will provide important data for clinical dosimetry and for a more accurate assessment of the relative biological effectiveness in carbon ion cancer therapy. Furthermore, it will help to clarify the uncertainties with respect to the sign and the magnitude of the condensed phase effect for carbon ions.