Validation of a Monte Carlo model of a large-format kV flat-panel system

IF 3.2 2区医学 Q1 RADIOLOGY, NUCLEAR MEDICINE & MEDICAL IMAGING

Medical physics Pub Date : 2025-09-02 DOI:10.1002/mp.18093

Nicholas Lowther, Marios Myronakis, Thomas Harris, Ross Berbeco, Matt Jacobson, Roshanak Etemadpour, Dianne Ferguson, Rony Fueglistaller, Pablo Corral Arroyo, Vera Birrer, Raphael Bruegger, Daniel Morf, Mathias Lehmann, Yue-Houng Hu

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引用次数: 0

Abstract

Background

Online adaptive radiation therapy (ART) offers a paradigm shift in radiotherapy by enabling adjustments to the planned dose based on daily anatomical variation. In the context of cone-beam computed tomography (CBCT) for online ART on a standard linac, thoracic and abdominal treatment sites in particular present unique challenges due to the typically large treatment volumes, mobile anatomy, scatter-induced image quality degradation, and hounsfield unit (HU) limitations. A recent hardware and software upgrade for a standard linac, Varian TrueBeam (TB) v4.1 HyperSight, seeks to overcome these challenges through implementation of a larger kV imager panel (i.e., 43 × 43 cm), increased gantry speed (i.e., from 6 to 9°/s), and improved HU accuracy. However, investigation of the new upgrade is essential to harness the full potential of these advancements.

Purpose

We report on physical characterization and a digital Monte Carlo (MC) model of the new imaging system hardware.

Methods

The open-source GEANT4 Application for Tomographic Emission (GATE) MC toolkit, which allows scintillation systems, including CBCT, to be accurately modeled, was utilized. All physical components of the new TB upgrade were modeled from vendor-provided geometry and material specifications. The model was validated using physical measurements acquired on the upgraded system. Specifically, the modulation transfer function (MTF), noise power spectrum (NPS), profiles across the physically larger detector, scatter-to-primary ratio (SPR), and loss in spatial resolution as a function of the increased gantry speed and an object's distance from isocenter. The latter was quantified using the pixel distance between the 15% and 85% intensities of the over-sampled edge spread function (ESF) for source-to-edge-phantom distances (SEPDs) of 80, 100, and 120 cm. Focal spot motion was also characterized by the MTF at SEPD of 100 cm.

Results

The MTF₅₀ was 0.901 and 0.889 mm⁻¹ for the measurement and simulation, respectively, for a 125 kVp beam. The normalized root mean square error (nRMSE) was 0.013. While small, the model displayed degraded spatial resolution accuracy for other beam qualities. The general trend of the physically measured normalized noise power spectrum (nNPS) curves was reproduced with the model at all beam energies; however, a small systematic offset was observed. Excellent agreement was observed between central-image x- and y-profiles of measured and MC-generated projections, indicating correct modeling of the larger imaging detector. Simulated SPRs closely agreed with those of measurement. The measured ESF widths for the 6 and 9°/s acquisitions were both 2.5 pixels, indicating no reduction in spatial resolution in the projection space as a result of the increased acquisition speed. The effect of focal spot blurring due to increased gantry speed was accurately modeled, considering ESF width differences no larger than 0.5 pixels were observed for all SEPDs. Decreased spatial resolution in projection images was observed for SEPDs of 80 and 120 cm compared to 100 cm.

Conclusions

The MC model of the novel TB 4.1 HyperSight upgrade accurately reproduced physical measurements acquired on the new system. The model will be used alongside physical testing on the new TB platform, working towards an online CBCT-based ART protocol for thoracic and abdominal treatments.

Abstract Image

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大型kV平板系统蒙特卡罗模型的验证

在线适应性放射治疗（ART）通过能够根据每日解剖变化调整计划剂量，为放射治疗提供了一种范式转变。在锥形束计算机断层扫描（CBCT）在标准直线上用于在线ART的背景下，由于通常的大治疗量、可移动解剖、散射引起的图像质量下降和霍斯菲尔德单元（HU）的限制，胸部和腹部治疗部位尤其面临独特的挑战。瓦里安TrueBeam (TB) v4.1 HyperSight最近对标准直线仪进行了硬件和软件升级，旨在通过实施更大kV成像仪面板（即43 × 43 cm），提高龙门速度（即从6°/s到9°/s）以及提高HU精度来克服这些挑战。然而，对新升级的调查对于充分利用这些进步的潜力至关重要。目的报道新型成像系统硬件的物理特性和数字蒙特卡罗（MC）模型。方法利用开源的GEANT4应用程序层析发射（GATE） MC工具包，对包括CBCT在内的闪烁系统进行精确建模。新TB升级的所有物理组件都是根据供应商提供的几何形状和材料规格建模的。该模型通过在升级后的系统上获得的物理测量数据进行了验证。具体来说，调制传递函数（MTF）、噪声功率谱（NPS）、在物理上较大的探测器上的分布、散射与初级比（SPR）以及空间分辨率损失作为增加的龙门速度和物体与等中心的距离的函数。对于源到边缘幻像距离（sepd）为80、100和120 cm时，后者使用过采样边缘扩展函数（ESF） 15%和85%强度之间的像素距离进行量化。焦点光斑运动也被表征为在SEPD为100 cm处的MTF。结果125 kVp束流的MTF50分别为0.901和0.889 mm−1。标准化均方根误差（nRMSE）为0.013。虽然模型很小，但对其他光束质量的空间分辨率精度却有所下降。用该模型再现了物理测量的归一化噪声功率谱（nNPS）曲线在所有光束能量下的总体趋势；然而，观察到一个小的系统偏移。在测量和mc生成的投影的中心图像x和y轮廓之间观察到非常好的一致性，表明正确建模了更大的成像探测器。模拟的spr与实测值基本一致。6°/s和9°/s采集的测量ESF宽度均为2.5像素，表明由于采集速度的增加，投影空间的空间分辨率没有降低。考虑到所有sepd观察到的ESF宽度差异不大于0.5像素，因此准确地模拟了由于龙门速度增加而引起的焦斑模糊的影响。与100 cm的sepd相比，80和120 cm的sepd在投影图像中的空间分辨率下降。新型TB 4.1 HyperSight升级的MC模型准确再现了在新系统上获得的物理测量值。该模型将与新的结核病平台上的物理测试一起使用，致力于为胸部和腹部治疗提供基于cbct的在线抗逆转录病毒治疗方案。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Medical physics 医学-核医学

CiteScore

6.80

自引率

15.80%

发文量

660

审稿时长

1.7 months

期刊介绍： Medical Physics publishes original, high impact physics, imaging science, and engineering research that advances patient diagnosis and therapy through contributions in 1) Basic science developments with high potential for clinical translation 2) Clinical applications of cutting edge engineering and physics innovations 3) Broadly applicable and innovative clinical physics developments Medical Physics is a journal of global scope and reach. By publishing in Medical Physics your research will reach an international, multidisciplinary audience including practicing medical physicists as well as physics- and engineering based translational scientists. We work closely with authors of promising articles to improve their quality.