Lourenço A, Wellock N, Thomas R, Homer M, Bouchard H, Kanai T, MacDougall N, Royle G, Palmans H
Department of Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK. Division of Acoustics and Ionising Radiation, National Physical Laboratory, Teddington TW11 0LW, UK.
Phys Med Biol. 2016 Nov 7;61(21):7623-7638. doi: 10.1088/0031-9155/61/21/7623. Epub 2016 Oct 14.
Water-equivalent plastics are frequently used in dosimetry for experimental simplicity. This work evaluates the water-equivalence of novel water-equivalent plastics specifically designed for light-ion beams, as well as commercially available plastics in a clinical high-energy carbon-ion beam. A plastic- to-water conversion factor [Formula: see text] was established to derive absorbed dose to water in a water phantom from ionization chamber readings performed in a plastic phantom. Three trial plastic materials with varying atomic compositions were produced and experimentally characterized in a high-energy carbon-ion beam. Measurements were performed with a Roos ionization chamber, using a broad un-modulated beam of 11 × 11 cm, to measure the plastic-to-water conversion factor for the novel materials. The experimental results were compared with Monte Carlo simulations. Commercially available plastics were also simulated for comparison with the plastics tested experimentally, with particular attention to the influence of nuclear interaction cross sections. The measured [Formula: see text] correction increased gradually from 0% at the surface to 0.7% at a depth near the Bragg peak for one of the plastics prepared in this work, while for the other two plastics a maximum correction of 0.8%-1.3% was found. Average differences between experimental and numerical simulations were 0.2%. Monte Carlo results showed that for polyethylene, polystyrene, Rando phantom soft tissue and A-150, the correction increased from 0% to 2.5%-4.0% with depth, while for PMMA it increased to 2%. Water-equivalent plastics such as, Plastic Water, RMI-457, Gammex 457-CTG, WT1 and Virtual Water, gave similar results where maximum corrections were of the order of 2%. Considering the results from Monte Carlo simulations, one of the novel plastics was found to be superior in comparison with the plastic materials currently used in dosimetry, demonstrating that it is feasible to tailor plastic materials to be water-equivalent for carbon ions specifically.
为简化实验，水等效塑料常用于剂量测定。本研究评估了专门为轻离子束设计的新型水等效塑料以及临床高能碳离子束中使用的市售塑料的水等效性。建立了塑料 - 水转换因子[公式：见原文]，以便根据在塑料模体中进行的电离室读数得出水模体中水的吸收剂量。制备了三种原子组成不同的试验塑料材料，并在高能碳离子束中进行了实验表征。使用Roos电离室，在11×11 cm的宽未调制束下进行测量，以测定新型材料的塑料 - 水转换因子。将实验结果与蒙特卡罗模拟进行了比较。还对市售塑料进行了模拟，以便与实验测试的塑料进行比较，特别关注核相互作用截面的影响。对于本研究制备的一种塑料，测量得到的[公式：见原文]校正从表面的0%逐渐增加到布拉格峰附近深度处的0.7%，而对于其他两种塑料，最大校正为0.8% - 1.3%。实验和数值模拟之间的平均差异为0.2%。蒙特卡罗结果表明，对于聚乙烯、聚苯乙烯、兰多模体软组织和A - 150，校正随深度从0%增加到2.5% - 4.0%，而对于PMMA，校正增加到2%。塑料水、RMI - 457、Gammex 457 - CTG、WT1和虚拟水等水等效塑料给出了类似的结果，最大校正约为2%。考虑到蒙特卡罗模拟的结果，发现其中一种新型塑料比目前剂量测定中使用的塑料材料更优越，这表明专门为碳离子量身定制水等效的塑料材料是可行的。
