Grassberger C, Lomax Anthony, Paganetti H
Department of Radiation Oncology, Massachusetts General Hospital & Harvard Medical School, Boston MA 02114, USA. Centre for Proton Radiotherapy, Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland.
Phys Med Biol. 2015 Jan 21;60(2):633-45. doi: 10.1088/0031-9155/60/2/633. Epub 2014 Dec 30.
The presented work has two goals. First, to demonstrate the feasibility of accurately characterizing a proton radiation field at treatment head exit for Monte Carlo dose calculation of active scanning patient treatments. Second, to show that this characterization can be done based on measured depth dose curves and spot size alone, without consideration of the exact treatment head delivery system. This is demonstrated through calibration of a Monte Carlo code to the specific beam lines of two institutions, Massachusetts General Hospital (MGH) and Paul Scherrer Institute (PSI). Comparison of simulations modeling the full treatment head at MGH to ones employing a parameterized phase space of protons at treatment head exit reveals the adequacy of the method for patient simulations. The secondary particle production in the treatment head is typically below 0.2% of primary fluence, except for low-energy electrons (<0.6 MeV for 230 MeV protons), whose contribution to skin dose is negligible. However, there is significant difference between the two methods in the low-dose penumbra, making full treatment head simulations necessary to study out-of-field effects such as secondary cancer induction. To calibrate the Monte Carlo code to measurements in a water phantom, we use an analytical Bragg peak model to extract the range-dependent energy spread at the two institutions, as this quantity is usually not available through measurements. Comparison of the measured with the simulated depth dose curves demonstrates agreement within 0.5 mm over the entire energy range. Subsequently, we simulate three patient treatments with varying anatomical complexity (liver, head and neck and lung) to give an example how this approach can be employed to investigate site-specific discrepancies between treatment planning system and Monte Carlo simulations.
本研究有两个目标。其一，证明在治疗头出口处精确表征质子辐射场对于主动扫描患者治疗的蒙特卡罗剂量计算的可行性。其二，表明仅基于测量的深度剂量曲线和光斑尺寸即可完成这种表征，而无需考虑确切的治疗头输送系统。这通过将蒙特卡罗代码校准到两个机构（麻省总医院（MGH）和保罗·谢尔研究所（PSI））的特定束流线路得以证明。将MGH全治疗头模拟与使用治疗头出口处质子参数化相空间的模拟进行比较，揭示了该方法用于患者模拟的充分性。治疗头中的次级粒子产生通常低于初级注量的0.2%，但低能电子（对于230 MeV质子，能量<0.6 MeV）除外，其对皮肤剂量的贡献可忽略不计。然而，两种方法在低剂量半影方面存在显著差异，这使得有必要进行全治疗头模拟以研究诸如继发癌症诱导等野外效应。为了将蒙特卡罗代码校准到水模体中的测量值，我们使用解析布拉格峰模型来提取两个机构中与射程相关的能量展宽，因为该量通常无法通过测量获得。测量的深度剂量曲线与模拟的深度剂量曲线的比较表明，在整个能量范围内，两者在0.5 mm内一致。随后，我们模拟了三种解剖结构复杂度不同的患者治疗（肝脏、头颈部和肺部）来举例说明如何采用这种方法来研究治疗计划系统与蒙特卡罗模拟之间特定部位的差异。
