Bäumer Christian, Koska Benjamin, Lambert Jamil, Timmermann Beate, Mertens Thierry, Takoukam Talla Patrick
Westdeutsches Protonentherapiezentrum Essen (WPE) GmbH.
J Appl Clin Med Phys. 2015 Nov 8;16(6):151–163. doi: 10.1120/jacmp.v16i6.5577.
Acquisition of quasi-monoenergetic ("pristine") depth-dose curves is an essential task in the frame of commissioning and quality assurance of a proton therapy treatment head. For pencil beam scanning delivery modes this is often accomplished by measuring the integral ionization in a plane perpendicular to the axis of an unscanned beam. We focus on the evaluation of three integral detectors: two of them are plane-parallel ionization chambers with an effective radius of 4.1 cm and 6.0 cm, respectively, mounted in a scanning water phantom. The third integral detector is a 6.0 cm radius multilayer ionization chamber. The experimental results are compared with the corresponding measurements under broad field conditions, which are performed with a small radius plane-parallel chamber and a small radius multilayer ionization chamber. We study how a measured depth-dose curve of a pristine proton field depends on the detection device, by evaluating the shape of the depth-dose curve, the relative charge collection efficiency, and intercomparing measured ranges. Our results show that increasing the radius of an integral chamber from 4.1 cm to 6.0 cm increases the collection efficiency by 0%-3.5% depending on beam energy and depth. Ranges can be determined by the large electrode multilayer ionization chamber with a typical uncertainty of 0.4 mm on a routine basis. The large electrode multilayer ionization chamber exhibits a small distortion in the Bragg Peak region. This prohibits its use for acquisition of base data, but is tolerable for quality assurance. The good range accuracy and the peak distortion are characteristics of the multilayer ionization chamber design, as shown by the direct comparison with the small electrode counterpart.
获取准单能（“原始”）深度剂量曲线是质子治疗头调试和质量保证框架中的一项重要任务。对于笔形束扫描输送模式，这通常是通过测量垂直于未扫描束轴的平面中的积分电离来完成的。我们专注于对三种积分探测器的评估：其中两种是平面平行电离室，有效半径分别为4.1厘米和6.0厘米，安装在扫描水模体中。第三种积分探测器是半径为6.0厘米的多层电离室。将实验结果与在宽场条件下使用小半径平面平行室和小半径多层电离室进行的相应测量结果进行比较。我们通过评估深度剂量曲线的形状、相对电荷收集效率以及比较测量范围，研究原始质子场的测量深度剂量曲线如何依赖于检测装置。我们的结果表明，将积分室的半径从4.1厘米增加到6.0厘米，根据束能量和深度，收集效率会提高0% - 3.5%。范围可以由大电极多层电离室常规确定，典型不确定度为0.4毫米。大电极多层电离室在布拉格峰区域表现出小的畸变。这禁止其用于获取基础数据，但对于质量保证是可以容忍的。与小电极对应物的直接比较表明，良好的范围精度和峰畸变是多层电离室设计的特点。
