Printz Ringbæk Toke, Simeonov Yuri, Witt Matthias, Engenhart-Cabillic Rita, Kraft Gerhard, Zink Klemens, Weber Uli
Technische Hochschule Mittelhessen (THM), Gießen-Friedberg, Germany. Department of Radiotherapy and Radiation Oncology, Philipps-University, Marburg, Germany. Department of Experimental Clinical Oncology, Aarhus University Hospital, Denmark.
Phys Med Biol. 2017 Apr 7;62(7):2892-2909. doi: 10.1088/1361-6560/aa5c28. Epub 2017 Jan 31.
Porous materials with microscopic structures like foam, sponges, lung tissues and lung substitute materials have particular characteristics, which differ from those of solid materials. Ion beams passing through porous materials show much stronger energy straggling than expected for non-porous solid materials of the same thickness. This effect depends on the microscopic fine structure, the density and the thickness of the porous material. The beam-modulating effect from a porous plate enlarges the Bragg peak, yielding similar benefits in irradiation time reduction as a ripple filter. A porous plate can additionally function as a range shifter, which since a higher energy can be selected for the same penetration depth in the body reduces the scattering at the beam line and therefore improves the lateral fall-off. Bragg curve measurements of ion beams passing through different porous materials have been performed in order to determine the beam modulation effect of each. A mathematical model describing the correlation between the mean material density, the porous pore structure size and the strength of the modulation has been developed and a new material parameter called 'modulation power' is defined as the square of the Gaussian sigma divided by the mean water-equivalent thickness of the porous absorber. Monte Carlo simulations have been performed in order to validate the model and to investigate the Bragg peak enlargement, the scattering effects of porosity and the lateral beam width at the end of the beam range. The porosity is found to only influence the lateral scattering in a negligible way. As an example of a practical application, it is found that a 20 mm and 50 mm plate of Gammex LN300 performs similar to a 3 mm and 6 mm ripple filter, respectively, and at the same time can improve the sharpness of the lateral beam due to its multifunctionality as a ripple filter and a range shifter.
具有泡沫、海绵、肺组织和肺替代材料等微观结构的多孔材料具有特殊的特性，这些特性与固体材料不同。穿过多孔材料的离子束表现出比相同厚度的无孔固体材料预期更强的能量离散。这种效应取决于多孔材料的微观精细结构、密度和厚度。多孔板的束调制效应扩大了布拉格峰，在减少照射时间方面产生了与波纹滤波器类似的效果。多孔板还可以用作射程移位器，因为对于体内相同的穿透深度可以选择更高的能量，这减少了束线处的散射，从而改善了横向剂量下降。已经对穿过不同多孔材料的离子束进行了布拉格曲线测量，以确定每种材料的束调制效应。已经建立了一个描述平均材料密度、多孔孔隙结构尺寸和调制强度之间相关性的数学模型，并定义了一个名为“调制功率”的新材料参数，它等于高斯标准差的平方除以多孔吸收体的平均水等效厚度。已经进行了蒙特卡罗模拟，以验证该模型并研究布拉格峰的扩大、孔隙率的散射效应以及束射程末端的横向束宽。发现孔隙率对横向散射的影响可以忽略不计。作为实际应用的一个例子，发现20毫米和50毫米厚的Gammex LN300板分别与3毫米和6毫米厚的波纹滤波器性能相似，同时由于其作为波纹滤波器和射程移位器的多功能性，可以提高横向束的清晰度。
