Lin Yuting, Clasie Benjamin, Lu Hsiao-Ming, Flanz Jacob, Shen Tim, Jee Kyung-Wook
Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA. Department of Radiation Oncology, Duke University Medical Center, Durham, NC, USA.
Phys Med Biol. 2017 Jan 21;62(2):344-357. doi: 10.1088/1361-6560/aa5084. Epub 2016 Dec 20.
While proton beam models in treatment planning systems are generally assumed invariant with respect to the beam deliveries at different gantry angles. Physical properties of scanning pencil beams can change. The gantry angle dependent properties include the delivered charge to the monitor unit chamber, the spot position and the spot shape. The aim of this study is to investigate the extent of the changes and their dosimetric impacts using historical pencil beam scanning (PBS) treatment data. Online beam delivery records at the time of the patient-specific qualify assurance were retrospectively collected for a total of 34 PBS fields from 28 patients treated at our institution. For each field, proton beam properties at two different gantry angles (the planned and zero gantry angles) were extracted by a newly-developed machine log analysis method and used to reconstruct the delivered dose distributions in the cubic water phantom geometry. The reconstructed doses at the two different angles and a planar dose measurement by a 2D ion-chamber array were compared and the dosimetric impacts of the gantry angle dependency were accessed by a 3D γ-index analysis. In addition, the pencil beam spot size was independently characterized as a function of the gantry angle and the beam energy. The dosimetric effects of the perturbed beam shape were also investigated. Comparisons of spot-by-spot beam positions between both gantry angles show a mean deviation of 0.4 and 0.7 mm and a standard deviation of 0.3 and 0.6 mm for x and y directions, respectively. The delivered giga-protons per spot show a percent mean difference and a standard deviation of 0.01% and 0.3%, respectively, from each planned spot weight. These small deviations lead to an excellent agreement in dose comparisons with an average γ passing rate of 99.1%. When each calculation for both planned and zero gantry angles was compared to the measurement, a high correlation in γ values was also observed, also indicating the dosimetric differences are small when a field is delivered at different gantry angles. Utilizing the online beam delivery records, the gantry angle dependencies of the PBS beam delivery were assessed and quantified. The study confirms the variations of the physical properties to be sufficiently small within the clinical tolerances without taking into account the gantry angle variation.
虽然治疗计划系统中的质子束模型通常被假定为相对于不同机架角度的束流输出是不变的。但扫描笔形束的物理特性可能会发生变化。与机架角度相关的特性包括传递到监测单元腔室的电荷量、束斑位置和束斑形状。本研究的目的是利用历史笔形束扫描（PBS）治疗数据，研究这些变化的程度及其剂量学影响。回顾性收集了在我院接受治疗的28例患者共34个PBS射野的患者特定质量保证时的在线束流输出记录。对于每个射野，通过一种新开发的机器日志分析方法提取两个不同机架角度（计划机架角度和零机架角度）下的质子束特性，并用于在立方水体模几何结构中重建传递的剂量分布。比较了两个不同角度下重建的剂量以及二维电离室阵列的平面剂量测量结果，并通过三维γ指数分析评估了机架角度依赖性的剂量学影响。此外，笔形束斑尺寸被独立表征为机架角度和束流能量的函数。还研究了束斑形状扰动的剂量学效应。两个机架角度之间逐点束斑位置的比较显示，x和y方向的平均偏差分别为0.4和0.7 mm，标准差分别为0.3和0.6 mm。每个束斑传递的千兆质子与每个计划束斑权重相比，平均百分比差异和标准差分别为0.01%和0.3%。这些小偏差导致剂量比较中的极佳一致性，平均γ通过率为99.1%。当将计划机架角度和零机架角度的每次计算结果与测量结果进行比较时，还观察到γ值之间的高度相关性，这也表明当在不同机架角度输送射野时，剂量学差异很小。利用在线束流输出记录，评估并量化了PBS束流输出的机架角度依赖性。该研究证实，在不考虑机架角度变化的情况下，物理特性的变化在临床公差范围内足够小。
