Cleveland Medical Center, University Hospitals, Cleveland, OH, 44106, USA.
Med Phys. 2017 Sep;44(9):4910-4918. doi: 10.1002/mp.12402. Epub 2017 Aug 1.
To develop and commission a Monte Carlo (MC) simulation model for the Leksell Gamma Knife (LGK) Perfexion (PFX) radiosurgery system.
We previously established a source model for MC simulations of the LGK PFX for the purpose of the treatment planning system (TPS) dose verification and plan evaluation. To make practical and effective use of the model in clinic, several issues need to be addressed. First, thorough commissioning procedures are needed to ensure the validity of the model parameters, such as the source-to-focus (STF) distance, the source solid angle. Second, an efficient source particle sampling method is required to facilitate dose calculations for multitarget and multishot configurations in patient treatment plans. Third, inseparably, it is interesting to know the dose difference between the two GK TPS algorithms (TMR and convolution) and the MC method in extreme heterogeneous cases resulting from the inhomogeneous effect. We report our recent development in addressing these issues. Phantoms with the frame fiducials were manually created in the format of DICOM CT image to eliminate the uncertainties associated with scanner artifacts and image registration. The created homogeneous phantom was used to calibrate the model parameters to match the output factors with the manufacturer provided data, and the heterogeneous phantom with multilayer materials was used to study the inhomogeneous effect.
The agreement between the MC calculation and TPS was very good for the homogeneous spherical phantom. The difference of the full width at half maximum (FWHM) of the profiles was less than 1 mm except for the profile for 16 mm collimator along z-axis (less than 2 mm). For the extreme heterogeneous test case, it was shown that the TMR algorithm can overestimate the target dose by up to 22% using the measure of dose volume parameter D95. The agreement between the MC method and the TPS convolution method was better (within 3.6%) for the target near the center of phantom, however, discrepancy (up to 10.7%) existed for the target close to the skull. The difference between the two TPS dose algorithms was about 11%.
Considerable dose difference may result from the effect of heterogeneity, such as in the regions of the air cavities and bones. As the MC method has been extensively used in conventional external beams, it is worthwhile for further investigation in applying the MC method to accurate dose planning in the new GK PFX radiosurgery platform.
开发和委托用于 Leksell Gamma Knife（LGK）Perfexion（PFX）放射外科系统的蒙特卡罗（MC）模拟模型。
我们之前为 LGK PFX 的 MC 模拟建立了一个源模型，用于治疗计划系统（TPS）剂量验证和计划评估。为了在临床上实际有效地使用该模型，需要解决几个问题。首先，需要彻底的调试程序来确保模型参数的有效性，例如源到焦点（STF）距离、源立体角。其次，需要有效的源粒子抽样方法，以方便在患者治疗计划中对多靶点和多射束配置进行剂量计算。第三，不可避免地，了解两种 GK TPS 算法（TMR 和卷积）与 MC 方法在不均匀效应导致的极端不均匀情况下的剂量差异是很有趣的。我们报告了我们在解决这些问题方面的最新进展。使用 DICOM CT 图像格式手动创建带有框架基准的体模，以消除与扫描仪伪影和图像配准相关的不确定性。创建的均匀体模用于校准模型参数，以匹配与制造商提供的数据输出因子，使用具有多层材料的不均匀体模研究不均匀效应。
对于均匀球形体模，MC 计算与 TPS 的一致性非常好。除了 z 轴上 16mm 准直器的轮廓（小于 2mm）外，轮廓的半高全宽（FWHM）差异小于 1mm。对于极端不均匀的测试案例，结果表明 TMR 算法使用剂量体积参数 D95 最多可以高估目标剂量 22%。对于体模中心附近的目标，MC 方法与 TPS 卷积方法的一致性更好（在 3.6% 以内），但是对于靠近颅骨的目标，存在差异（高达 10.7%）。两种 TPS 剂量算法之间的差异约为 11%。
不均匀性会导致相当大的剂量差异，例如在空腔和骨骼区域。由于 MC 方法已广泛应用于常规外照射，因此值得进一步研究将 MC 方法应用于新的 GK PFX 放射外科平台中的精确剂量计划。
