Polf Jerimy C, Newhauser Wayne D
Department of Radiation Physics, Unit 130, The University of Texas M D Anderson Cancer Center, Houston, TX 77030, USA.
Phys Med Biol. 2005 Aug 21;50(16):3859-73. doi: 10.1088/0031-9155/50/16/014. Epub 2005 Aug 2.
Passive beam spreading techniques have been used for most proton therapy treatments worldwide. This delivery method employs static scattering foils to spread the beam laterally and a range modulating wheel or ridge filter to spread the high dose region in depth to provide a uniform radiation dose to the treatment volume. Neutrons produced by interactions of the treatment beam with nozzle components, such as the range modulation wheel, can account for a large portion of the secondary dose delivered to healthy tissue outside the treatment volume. Despite this fact, little is known about the effects of range modulation on the secondary neutron exposures around passively scattered proton treatment nozzles. In this work, the neutron dose equivalent spectra per incident proton (H(E)/p) and total neutron dose equivalent per therapeutic absorbed dose (H/D) were studied using Monte Carlo techniques for various values of range modulation at 54 locations around a passive scattering proton therapy treatment nozzle. As the range modulator wheel step thickness increased from 1.0 to 11.5 cm, the peak values of H(E)/p decreased from approximately 1 x 10(-17) mSv Gy(-1) to approximately 2 x 10(-18) mSv Gy(-1) at 50 cm from isocentre along the beam's central axis. In general, H/D increased with increasing range modulation at all locations studied, and the maximum H/D exposures shifted away from isocentre.
被动束流扩展技术已在全球大多数质子治疗中得到应用。这种输送方法采用静态散射箔来横向扩展束流，并使用射程调制轮或脊形滤波器来在深度上扩展高剂量区域，以便为治疗体积提供均匀的辐射剂量。治疗束与喷嘴部件（如射程调制轮）相互作用产生的中子，可能占输送到治疗体积外健康组织的二次剂量的很大一部分。尽管如此，关于射程调制对被动散射质子治疗喷嘴周围二次中子照射的影响却知之甚少。在这项工作中，利用蒙特卡罗技术研究了被动散射质子治疗喷嘴周围54个位置在不同射程调制值下每个入射质子的中子剂量当量谱（H(E)/p）以及每个治疗吸收剂量的总中子剂量当量（H/D）。随着射程调制轮步长厚度从1.0厘米增加到11.5厘米，在沿束流中心轴距等中心50厘米处，H(E)/p的峰值从约1×10⁻¹⁷毫希沃特每戈瑞（mSv Gy⁻¹）降至约2×10⁻¹⁸毫希沃特每戈瑞（mSv Gy⁻¹）。总体而言，在所研究的所有位置，H/D都随着射程调制的增加而增加，并且最大H/D照射位置偏离了等中心。
