Boissonnat G, Chesneau H, Barat E, Dautremer T, Garcia-Hernandez J-C, Lazaro D
CEA, LIST, System Modelling and Simulation Lab, Gif-sur-Yvette, F-91191, France.
Med Phys. 2020 Sep;47(9):4531-4542. doi: 10.1002/mp.14311. Epub 2020 Jul 6.
Image-guided radiotherapy (IGRT) improves tumor control but its intensive use may entrain late side effects caused by the additional imaging doses. There is a need to better quantify the additional imaging doses, so they can be integrated in the therapeutic workflow. Currently, no dedicated software enables to compute patient-specific imaging doses on a wide range of systems and protocols. As a first step toward this objective, we propose a common methodology to model four different kV-imaging systems used in radiotherapy (Varian's OBI, Elekta's XVI, Brainlab's ExacTrac, and Accuray's Cyberknife) using a new type of virtual source model based on Monte Carlo calculations.
We first describe our method to build a simplified description of the photon output, or virtual source models (VSMs), of each imaging system. Instead of being constructed using measurement data, as it is most commonly the case, our VSM is used as the summary of the phase-space files (PSFs) resulting from a first Monte Carlo simulation of the considered x-ray tube. Second, the VSM is used as a photon generator for a second MC simulation in which we compute the dose. Then, the proposed VSM is thoroughly validated against standard MC simulation using PSFs on the XVI system. Last, each modeled system is compared to profiles and depth-dose-curve measurements performed in homogeneous phantom.
Comparisons between PSF-based and VSM-based calculations highlight that VSMs could provide equivalent dose results (within 1% of difference) than PSFs inside the imaging field-of-view (FOV). In contrast, VSMs tend to underestimate (for up to 20%) calculated doses outside of the imaging FOV due to the assumptions underlying the VSM construction. In addition, we showed that the use of VSMs allows reducing calculation time by at least a factor of 2.8. Indeed, for identical simulation times, statistical uncertainties on dose distributions computed using VSMs were much lower than those obtained from PSF-based calculations.
For each of the four imaging systems, VSMs were successfully validated against measurements in homogeneous phantoms, and are therefore ready to be used for future preclinical studies in heterogeneous or anthropomorphic phantoms. The cross system modeling methodology developed here should enable, later on, to estimate precisely and accurately patient-specific 3D dose maps delivered during a large range of kV-imaging procedures.
图像引导放射治疗（IGRT）可改善肿瘤控制，但频繁使用可能会带来额外成像剂量导致的晚期副作用。需要更好地量化这些额外的成像剂量，以便将其纳入治疗流程。目前，尚无专门软件能够在多种系统和协议上计算患者特定的成像剂量。作为实现这一目标的第一步，我们提出一种通用方法，使用基于蒙特卡罗计算的新型虚拟源模型，对放射治疗中使用的四种不同千伏成像系统（瓦里安的OBI、医科达的XVI、Brainlab的ExacTrac和安科锐的射波刀）进行建模。
我们首先描述构建每个成像系统光子输出简化描述（即虚拟源模型，VSM）的方法。与通常使用测量数据构建不同，我们的VSM用作对所考虑X射线管首次蒙特卡罗模拟产生的相空间文件（PSF）的总结。其次，VSM用作第二次蒙特卡罗模拟的光子发生器，在该模拟中我们计算剂量。然后，使用XVI系统上的PSF将所提出的VSM与标准蒙特卡罗模拟进行全面验证。最后，将每个建模系统与在均匀体模中进行的轮廓和深度剂量曲线测量进行比较。
基于PSF和基于VSM的计算之间的比较表明，在成像视野（FOV）内，VSM能够提供与PSF等效的剂量结果（差异在1%以内）。相比之下，由于VSM构建所基于的假设，VSM往往会低估成像FOV外的计算剂量（最多低估20%）。此外，我们表明使用VSM可将计算时间至少缩短2.8倍。实际上，在相同模拟时间下，使用VSM计算的剂量分布的统计不确定性远低于基于PSF计算获得的不确定性。
对于这四种成像系统中的每一种，VSM均已成功通过在均匀体模中的测量验证，因此可用于未来在非均匀或人体模型中的临床前研究。这里开发的跨系统建模方法稍后应能够精确且准确地估计在大量千伏成像程序中 delivered（此处原文有误，推测应为“递送”）的患者特定三维剂量图。
