Medical Physics Research Group, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
Med Phys. 2014 Feb;41(2):021706. doi: 10.1118/1.4860256.
It is essential to consider large-angle scattered particles in dose calculation models for therapeutic carbon-ion beams. However, it is difficult to measure the small dose contribution from large-angle scattered particles. In this paper, the authors present a novel method to derive the parameters describing large-angle scattered particles from the measured results.
The authors developed a new parallel-plate ionization chamber consisting of concentric electrodes. Since the sensitive volume of each channel is increased linearly with this type, it is possible to efficiently and easily detect small contributions from the large-angle scattered particles. The parameters describing the large-angle scattered particles were derived from pencil beam dose distribution in water measured with the new ionization chamber. To evaluate the validity of this method, the correction for the field-size dependence of the doses, "predicted-dose scaling factor," was calculated with the new parameters.
The predicted-dose scaling factor calculated with the new parameters was compared with the existing one. The difference between the new correction factor and the existing one was 1.3%. For target volumes of different sizes, the calculated dose distribution with the new parameters was in good agreement with the measured one.
Parameters describing the large-angle scattered particles can be efficiently and rapidly determined using the new ionization chamber. The authors confirmed that the field-size dependence of the doses could be compensated for by the new parameters. This method makes it possible to easily derive the parameters describing the large-angle scattered particles, while maintaining the dose calculation accuracy.
在治疗碳离子束的剂量计算模型中，考虑大角度散射粒子是至关重要的。然而，测量大角度散射粒子的小剂量贡献是很困难的。本文作者提出了一种从测量结果中推导出描述大角度散射粒子的参数的新方法。
作者开发了一种由同心电极组成的新型平行板电离室。由于每个通道的灵敏体积与这种类型线性增加，因此可以有效地、轻松地检测到大角度散射粒子的小贡献。从用水测量的新电离室中的铅笔束剂量分布中推导出描述大角度散射粒子的参数。为了评估该方法的有效性，使用新参数计算了剂量的场尺寸依赖性校正，即“预测剂量缩放因子”。
用新参数计算的预测剂量缩放因子与现有参数进行了比较。新校正因子与现有校正因子之间的差异为 1.3%。对于不同大小的靶区，用新参数计算的剂量分布与实测结果吻合良好。
使用新的电离室可以高效快速地确定描述大角度散射粒子的参数。作者证实，剂量的场尺寸依赖性可以通过新参数来补偿。该方法使得描述大角度散射粒子的参数的推导变得容易，同时保持了剂量计算的准确性。
