Department of Biomedical Physics, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90024.
Med Phys. 2013 Nov;40(11):111907. doi: 10.1118/1.4823795.
The purpose of this study is to adapt an equivalent source model originally developed for conventional CT Monte Carlo dose quantification to the radiation oncology context and validate its application for evaluating concomitant dose incurred by a kilovoltage (kV) cone-beam CT (CBCT) system integrated into a linear accelerator.
In order to properly characterize beams from the integrated kV CBCT system, the authors have adapted a previously developed equivalent source model consisting of an equivalent spectrum module that takes into account intrinsic filtration and an equivalent filter module characterizing the added bowtie filtration. An equivalent spectrum was generated for an 80, 100, and 125 kVp beam with beam energy characterized by half-value layer measurements. An equivalent filter description was generated from bowtie profile measurements for both the full- and half-bowtie. Equivalent source models for each combination of equivalent spectrum and filter were incorporated into the Monte Carlo software package MCNPX. Monte Carlo simulations were then validated against in-phantom measurements for both the radiographic and CBCT mode of operation of the kV CBCT system. Radiographic and CBCT imaging dose was measured for a variety of protocols at various locations within a body (32 cm in diameter) and head (16 cm in diameter) CTDI phantom. The in-phantom radiographic and CBCT dose was simulated at all measurement locations and converted to absolute dose using normalization factors calculated from air scan measurements and corresponding simulations. The simulated results were compared with the physical measurements and their discrepancies were assessed quantitatively.
Strong agreement was observed between in-phantom simulations and measurements. For the radiographic protocols, simulations uniformly underestimated measurements by 0.54%-5.14% (mean difference = -3.07%, SD = 1.60%). For the CBCT protocols, simulations uniformly underestimated measurements by 1.35%-5.31% (mean difference = -3.42%, SD = 1.09%).
This work demonstrates the feasibility of using a measurement-based kV CBCT source model to facilitate dose calculations with Monte Carlo methods for both the radiographic and CBCT mode of operation. While this initial work validates simulations against measurements for simple geometries, future work will involve utilizing the source model to investigate kV CBCT dosimetry with more complex anthropomorphic phantoms and patient specific models.
本研究的目的是将最初为常规 CT 蒙特卡罗剂量量化开发的等效源模型适用于放射肿瘤学环境，并验证其在评估集成到直线加速器中的千伏 (kV) 锥形束 CT (CBCT) 系统引起的伴随剂量方面的应用。
为了正确描述来自集成 kV CBCT 系统的射束，作者对先前开发的等效源模型进行了改编，该模型由一个等效谱模块组成，该模块考虑了固有过滤，以及一个等效滤波器模块，用于描述附加的蝴蝶结过滤。为 80、100 和 125 kVp 射束生成等效光谱，通过半值层测量来描述射束能量。从全蝴蝶结和半蝴蝶结的蝴蝶结轮廓测量中生成等效滤波器描述。将每个等效光谱和滤波器的等效源模型合并到蒙特卡罗软件包 MCNPX 中。然后，根据 kV CBCT 系统的射线照相和 CBCT 模式对体模内的测量值进行蒙特卡罗模拟验证。在直径为 32 cm 的体模和直径为 16 cm 的头部 CTDI 体模内的各种位置处，测量各种协议的射线照相和 CBCT 成像剂量。在所有测量位置模拟体模内的射线照相和 CBCT 剂量，并使用空气扫描测量和相应模拟计算的归一化因子将模拟结果转换为绝对剂量。将模拟结果与物理测量值进行比较，并对差异进行定量评估。
体模内模拟与测量值之间存在很强的一致性。对于射线照相协议，模拟值普遍比测量值低 0.54%-5.14%（平均差值=-3.07%，SD=1.60%）。对于 CBCT 协议，模拟值普遍比测量值低 1.35%-5.31%（平均差值=-3.42%，SD=1.09%）。
这项工作证明了使用基于测量的 kV CBCT 源模型来促进使用蒙特卡罗方法进行剂量计算的可行性，适用于射线照相和 CBCT 操作模式。虽然这项初步工作验证了简单几何形状下的模拟与测量值的一致性，但未来的工作将涉及利用源模型研究更复杂的人体模型和患者特定模型的 kV CBCT 剂量学。
