Grevillot L, Stock M, Vatnitsky S
EBG MedAustron GmbH, Marie Curie-Straße 5, A-2700 Wiener Neustadt, Austria.
Phys Med Biol. 2015 Oct 21;60(20):7985-8005. doi: 10.1088/0031-9155/60/20/7985. Epub 2015 Sep 29.
This study aims at selecting and evaluating a ripple filter design compatible with non-isocentric proton and carbon ion scanning beam treatment delivery for a compact nozzle. The use of non-isocentric treatments when the patient is shifted as close as possible towards the nozzle exit allows for a reduction in the air gap and thus an improvement in the quality of scanning proton beam treatment delivery. Reducing the air gap is less important for scanning carbon ions, but ripple filters are still necessary for scanning carbon ion beams to reduce the number of energy steps required to deliver homogeneous SOBP. The proper selection of ripple filters also allows a reduction in the possible transverse and depth-dose inhomogeneities that could appear in non-isocentric conditions in particular. A thorough review of existing ripple filter designs over the past 16 years is performed and a design for non-isocentric treatment delivery is presented. A unique ripple filter quality index (QIRiFi) independent of the particle type and energy and representative of the ratio between energy modulation and induced scattering is proposed. The Bragg peak width evaluated at the 80% dose level (BPW80) is proposed to relate the energy modulation of the delivered Bragg peaks and the energy layer step size allowing the production of homogeneous SOBP. Gate/Geant4 Monte Carlo simulations have been validated for carbon ion and ripple filter simulations based on measurements performed at CNAO and subsequently used for a detailed analysis of the proposed ripple filter design. A combination of two ripple filters in a series has been validated for non-isocentric delivery and did not show significant transverse and depth-dose inhomogeneities. Non-isocentric conditions allow a significant reduction in the spot size at the patient entrance (up to 350% and 200% for protons and carbon ions with range shifter, respectively), and therefore in the lateral penumbra in the patients.
本研究旨在选择并评估一种与紧凑型喷嘴的非等中心质子和碳离子扫描束治疗输送兼容的波纹滤波器设计。当患者尽可能靠近喷嘴出口进行非等中心治疗时，空气间隙会减小，从而提高扫描质子束治疗输送的质量。对于扫描碳离子而言，减小空气间隙的重要性较低，但扫描碳离子束仍需要波纹滤波器来减少输送均匀扩展布拉格峰（SOBP）所需的能量步数。正确选择波纹滤波器还可以减少特别是在非等中心条件下可能出现的横向和深度剂量不均匀性。对过去16年现有的波纹滤波器设计进行了全面回顾，并提出了一种用于非等中心治疗输送的设计。提出了一种独特的与粒子类型和能量无关的波纹滤波器质量指数（QIRiFi），它代表能量调制与诱导散射之间的比率。建议使用在80%剂量水平评估的布拉格峰宽度（BPW80）来关联所输送布拉格峰的能量调制和能量层步长，从而产生均匀的SOBP。基于在CNAO进行的测量，门控/Geant4蒙特卡罗模拟已针对碳离子和波纹滤波器模拟进行了验证，随后用于对所提出的波纹滤波器设计进行详细分析。串联的两个波纹滤波器的组合已针对非等中心输送进行了验证，并且未显示出明显的横向和深度剂量不均匀性。非等中心条件可显著减小患者入口处的光斑尺寸（对于带有射程移位器的质子和碳离子，分别高达350%和200%），从而减小患者体内的侧向半影。
