Ming Xin, Feng Yuanming, Liu Ransheng, Yang Chengwen, Zhou Li, Zhai Hezheng, Deng Jun
Department of Biomedical Engineering, Tianjin University, Tianjin, People's Republic of China.
Phys Med Biol. 2017 Mar 7;62(5):1759-1776. doi: 10.1088/1361-6560/aa5911. Epub 2017 Jan 12.
The goal of this study is to develop a generalized source model for accurate Monte Carlo dose simulations of CT scans based solely on the measurement data without a priori knowledge of scanner specifications. The proposed generalized source model consists of an extended circular source located at x-ray target level with its energy spectrum, source distribution and fluence distribution derived from a set of measurement data conveniently available in the clinic. Specifically, the central axis percent depth dose (PDD) curves measured in water and the cone output factors measured in air were used to derive the energy spectrum and the source distribution respectively with a Levenberg-Marquardt algorithm. The in-air film measurement of fan-beam dose profiles at fixed gantry was back-projected to generate the fluence distribution of the source model. A benchmarked Monte Carlo user code was used to simulate the dose distributions in water with the developed source model as beam input. The feasibility and accuracy of the proposed source model was tested on a GE LightSpeed and a Philips Brilliance Big Bore multi-detector CT (MDCT) scanners available in our clinic. In general, the Monte Carlo simulations of the PDDs in water and dose profiles along lateral and longitudinal directions agreed with the measurements within 4%/1 mm for both CT scanners. The absolute dose comparison using two CTDI phantoms (16 cm and 32 cm in diameters) indicated a better than 5% agreement between the Monte Carlo-simulated and the ion chamber-measured doses at a variety of locations for the two scanners. Overall, this study demonstrated that a generalized source model can be constructed based only on a set of measurement data and used for accurate Monte Carlo dose simulations of patients' CT scans, which would facilitate patient-specific CT organ dose estimation and cancer risk management in the diagnostic and therapeutic radiology.
本研究的目标是开发一种通用源模型，用于仅基于测量数据对CT扫描进行准确的蒙特卡罗剂量模拟，而无需事先了解扫描仪的规格。所提出的通用源模型由位于X射线靶水平的扩展圆形源组成，其能谱、源分布和注量分布是从临床中方便获取的一组测量数据中推导出来的。具体而言，利用在水中测量的中心轴百分深度剂量（PDD）曲线和在空气中测量的锥形输出因子，分别通过Levenberg-Marquardt算法推导能谱和源分布。对固定机架处扇形束剂量剖面的空气膜测量进行反投影，以生成源模型的注量分布。使用经过基准测试的蒙特卡罗用户代码，将开发的源模型作为束输入来模拟水中的剂量分布。在所使用的临床可用的GE LightSpeed和飞利浦Brilliance Big Bore多探测器CT（MDCT）扫描仪上测试了所提出源模型的可行性和准确性。总体而言，对于这两种CT扫描仪，水中PDD以及横向和纵向剂量剖面的蒙特卡罗模拟结果与测量值在4%/1毫米范围内相符。使用两个CTDI体模（直径分别为16厘米和32厘米）进行的绝对剂量比较表明，在两种扫描仪的各种位置，蒙特卡罗模拟剂量与电离室测量剂量之间的一致性优于5%。总体而言，本研究表明，仅基于一组测量数据就可以构建通用源模型，并用于患者CT扫描的准确蒙特卡罗剂量模拟，这将有助于放射诊断和治疗中患者特异性CT器官剂量估计和癌症风险管理。
