Department of Nuclear Medicine, University of Würzburg, Würzburg 97080, Germany.
Med Phys. 2013 Aug;40(8):082502. doi: 10.1118/1.4812684.
In targeted radionuclide therapy, patient-specific dosimetry based on voxel S values (VSVs) is preferable to dosimetry based on mathematical phantoms. Monte-Carlo (MC) simulations are necessary to deduce VSVs for those voxel sizes required by quantitative imaging. The aim of this study is, starting from a single set of high-resolution VSVs obtained by MC simulations for a small voxel size along one single axis perpendicular to the source voxel, to present a suitable method to accurately calculate VSVs for larger voxel sizes.
Accurate sets of VSVs for target voxel to source voxel distances up to 10 cm were obtained for high-resolution voxel sizes (0.5 mm for electrons and 1.0 mm for photons) from MC simulations for Y-90, Lu-177, and I-131 using the radiation transport code MCNPX v.2.7a. To make these values suitable to any larger voxel size, different analytical methods (based on resamplings, interpolations, and fits) were tested and compared to values obtained by direct MC simulations. As a result, an optimal calculation procedure is proposed. This procedure consisted of: (1) MC simulation for obtaining of a starting set of VSVs along a single line of voxels for a small voxel size for each radionuclide and type of radiation; (2) interpolation within the values obtained in point (1) for obtaining the VSVs for voxels within a spherical volume; (3) resampling of the data obtained in (1) and (2) for obtaining VSVs for voxels sizes larger than the one used for the MC calculation for integer voxel ratios (voxel ratio=new voxel size∕voxel size MC simulation); (4) interpolation on within the data obtained in (3) for integer voxel ratios. The results were also compared to results from other authors.
The results obtained with the method proposed in this work show deviations relative to the source voxel below 1% for I-131 and Lu-177 and below 1.5% for Y-90 as compared with values obtained by direct MC simulations for voxel sizes ranging between 1.0 and 10.0 cm. The results obtained in this work show differences between the scored deposited energy and the emitted energy lower than 2% for electron radiation. Higher differences, attributable to the short considered radius of 10 cm in comparison with their penetration, can be found for photons. The authors' results agree well with previously published data obtained by other authors using different methods.
A reliable and fast approach for obtaining accurate VSVs for voxel sizes larger than the voxel size used for the MC calculation of the starting set of high-resolution VSVs was developed and successfully tested for three different radionuclides of interest for targeted radiotherapy: one pure beta (Y-90) and 2 beta-gamma emitters (Lu-177 und I-131). Applying the method of this work allows any interested reader to repeat the calculations for arbitrary radionuclides of interest and∕or smaller high-resolution voxel sizes, provided the means for running MC simulations are available.
在靶向放射性核素治疗中，基于体素 S 值（VSV）的患者特异性剂量学优于基于数学体模的剂量学。为了推导定量成像所需的那些体素大小的 VSV，需要进行蒙特卡罗（MC）模拟。本研究的目的是，从沿垂直于源体素的单个轴获得的小体素尺寸的 MC 模拟获得的一组单个体素的高分辨率 VSV 开始，提出一种合适的方法来准确计算更大体素尺寸的 VSV。
使用辐射输运代码 MCNPX v.2.7a 对 Y-90、Lu-177 和 I-131 进行 MC 模拟，为高分辨率体素尺寸（电子为 0.5mm，光子为 1.0mm）获得高达 10cm 的目标体素到源体素距离的准确 VSV 集。为了使这些值适用于任何更大的体素尺寸，测试了不同的分析方法（基于重采样、插值和拟合）并将其与直接 MC 模拟获得的值进行了比较。结果提出了一种最佳的计算程序。该程序包括：（1）MC 模拟，针对每个放射性核素和类型的辐射，针对小体素尺寸，沿单个体素线获得起始 VSV 集；（2）在（1）中获得的值内进行插值，以获得球体体积内的 VSV；（3）对（1）和（2）中获得的数据进行重采样，以获得整数体素比（新体素尺寸∕体素尺寸 MC 模拟）的大于 MC 计算中使用的体素尺寸的 VSV；（4）在（3）中获得的数据内进行插值，以获得整数体素比。结果还与其他作者的结果进行了比较。
与直接 MC 模拟相比，本工作中提出的方法获得的结果对于 I-131 和 Lu-177 的体素尺寸在 1.0 到 10.0cm 之间，以及 Y-90 的体素尺寸在 1.0 到 10.0cm 之间的体素尺寸的结果显示出相对于源体素的偏差小于 1%。本工作获得的结果表明，对于电子辐射，沉积能量与发射能量之间的差异小于 2%。对于光子，可以发现由于与穿透相比考虑的半径为 10cm 较短而导致的更高差异。作者的结果与其他作者使用不同方法获得的先前发表的数据吻合良好。
为了获得比用于 MC 计算起始高分辨率 VSV 集的体素尺寸更大的体素的准确 VSV，开发并成功测试了一种可靠且快速的方法，该方法适用于靶向放射治疗中感兴趣的三种不同的放射性核素：一种纯β（Y-90）和 2β-γ发射体（Lu-177 和 I-131）。应用本工作的方法允许任何有兴趣的读者重复任意感兴趣的放射性核素和∕或较小的高分辨率体素尺寸的计算，前提是有运行 MC 模拟的手段。
