Department of Medical Physics, University of Wisconsin-Madison, Madison, WI 53705, USA.
Med Phys. 2012 Mar;39(3):1424-34. doi: 10.1118/1.3685577.
Recent advances in the imaging of (90)Y using positron emission tomography (PET) and improved uncertainty in the branching ratio for the internal pair production component of (90)Y decay allow for a more accurate determination of the activity distribution of (90)Y microspheres within a patient. This improved activity distribution can be convolved with the dose kernel of (90)Y to calculate the dose distribution within a patient. This work investigates the effects of microsphere and surrounding material composition on (90)Y dose kernels using egsnrc and mcnp5 and compares the results of these two transport codes.
Monte Carlo simulations were performed with egsnrc and mcnp5 to calculate the dose rate at multiple radial distances around various (90)Y sources. Point source simulations were completed with mcnp5 to determine the optimal electron transport settings for this work. After determining the optimal settings, point source simulations were completed using egsnrc (user code edknrc) and mcnp5 in water and liver [as defined by the International Commission on Radiation Units and Measurements (ICRU) Report 44]. The results were compared to ICRU Report 72 reference data. Point source simulations were also completed in water with a density of 1.06 g[middle dot]cm(-3) to evaluate the effect of the density of the surrounding material. Glass and resin microsphere simulations were performed with average and maximum diameter and density values (based on values given in the literature) in water and in liver. The results were compared to point source simulation results using the same transport code and in the same surrounding material. All simulations had statistical uncertainties less than 1%.
The optimal transport settings in mcnp5 for this work included using the energy-and step-specific algorithm (DBCN 17J 2) and ESTEP set to 10. These settings were used for all subsequent simulations with mcnp5. The point source simulations in water for both egsnrc and mcnp5 were found to agree within 2% of the ICRU 72 reference data over the investigated range. Point source simulations in liver had large differences relative to ICRU 72, approaching -60% near the maximum range of (90)Y. These differences are mostly attributed to the difference in density between water (1.0 g[middle dot]cm(-3)) and liver (1.06 g[middle dot]cm(-3)). Glass and resin microsphere simulations showed a slight decrease in the dose rate near the maximum range of (90)Y relative to the point source simulations. The largest relative differences were approximately -4.2% and -2.8% for the glass and resin microspheres, respectively. Agreement between the egsnrc and mcnp5 simulations results was generally good.
The presence of the microsphere material causes slight differences in the (90)Y dose kernel compared to those calculated with point sources. Large differences were seen between simulations in water and those in liver. For the most accurate calculation of the dose distribution, the density of the patient's liver should be accounted for in the calculation of the dose kernel. Lastly, due to the need to determine the optimal transport settings with mcnp5, electron transport with this code should be used with caution.
最近在使用正电子发射断层扫描（PET）对（90）Y 进行成像方面取得了进展，并且（90）Y 衰变的内对产生分量的分支比的不确定性得到了提高，这使得能够更准确地确定患者体内（90）Y 微球的活性分布。可以将改进后的活性分布与（90）Y 的剂量核函数卷积，以计算患者体内的剂量分布。这项工作使用 egsnrc 和 mcnp5 研究了微球和周围材料成分对（90）Y 剂量核函数的影响，并比较了这两个输运代码的结果。
使用 egsnrc 和 mcnp5 进行蒙特卡罗模拟，以计算各种（90）Y 源周围多个径向距离处的剂量率。使用 mcnp5 完成点源模拟，以确定这项工作的最佳电子输运设置。确定最佳设置后，使用 egsnrc（用户代码 edknrc）和 mcnp5 在水中和肝脏中（由国际辐射单位和测量委员会（ICRU）报告 44 定义）完成点源模拟。结果与 ICRU 报告 72 参考数据进行了比较。还在密度为 1.06 g/cm(-3) 的水中进行了点源模拟，以评估周围材料密度的影响。使用平均和最大直径和密度值（基于文献中给出的值）在水中和肝脏中对玻璃和树脂微球进行了模拟。结果与使用相同输运代码和相同周围材料的点源模拟结果进行了比较。所有模拟的统计不确定性均小于 1%。
这项工作中 mcnp5 中的最佳输运设置包括使用能量和步长特定算法（DBCN 17J 2）和 ESTEP 设置为 10。所有后续 mcnp5 模拟均使用这些设置。在研究范围内，发现 egsnrc 和 mcnp5 的水中点源模拟结果与 ICRU 72 参考数据的差异在 2%以内。在肝脏中的点源模拟与 ICRU 72 相比存在较大差异，在（90）Y 的最大射程附近接近-60%。这些差异主要归因于水（1.0 g/cm(-3))和肝脏（1.06 g/cm(-3))之间的密度差异。玻璃和树脂微球模拟在（90）Y 的最大射程附近显示出与点源模拟相比剂量率略有下降。最大相对差异分别约为玻璃微球和树脂微球的-4.2%和-2.8%。egsnrc 和 mcnp5 模拟结果之间的一致性通常很好。
与使用点源计算相比，微球材料的存在会导致（90）Y 剂量核函数略有差异。在水中的模拟与在肝脏中的模拟之间存在较大差异。为了更准确地计算剂量分布，应在计算剂量核函数时考虑患者肝脏的密度。最后，由于需要使用 mcnp5 确定最佳输运设置，因此应谨慎使用该代码的电子输运。
