Department of Physics, University of Helsinki, Finland.
Med Phys. 2012 Mar;39(3):1335-44. doi: 10.1118/1.3685446.
In this work, accuracy of the mcnp5 code in the electron transport calculations and its suitability for ionization chamber (IC) response simulations in photon beams are studied in comparison to egsnrc and penelope codes.
The electron transport is studied by comparing the depth dose distributions in a water phantom subdivided into thin layers using incident energies (0.05, 0.1, 1, and 10 MeV) for the broad parallel electron beams. The IC response simulations are studied in water phantom in three dosimetric gas materials (air, argon, and methane based tissue equivalent gas) for photon beams ((60)Co source, 6 MV linear medical accelerator, and mono-energetic 2 MeV photon source). Two optional electron transport models of mcnp5 are evaluated: the ITS-based electron energy indexing (mcnp5(ITS)) and the new detailed electron energy-loss straggling logic (mcnp5(new)). The electron substep length (ESTEP parameter) dependency in mcnp5 is investigated as well.
For the electron beam studies, large discrepancies (>3%) are observed between the MCNP5 dose distributions and the reference codes at 1 MeV and lower energies. The discrepancy is especially notable for 0.1 and 0.05 MeV electron beams. The boundary crossing artifacts, which are well known for the mcnp5(ITS), are observed for the mcnp5(new) only at 0.1 and 0.05 MeV beam energies. If the excessive boundary crossing is eliminated by using single scoring cells, the mcnp5(ITS) provides dose distributions that agree better with the reference codes than mcnp5(new). The mcnp5 dose estimates for the gas cavity agree within 1% with the reference codes, if the mcnp5(ITS) is applied or electron substep length is set adequately for the gas in the cavity using the mcnp5(new). The mcnp5(new) results are found highly dependent on the chosen electron substep length and might lead up to 15% underestimation of the absorbed dose.
Since the mcnp5 electron transport calculations are not accurate at all energies and in every medium by general clinical standards, caution is needed, if mcnp5 is used with the current electron transport models for dosimetric applications.
在这项工作中，与 egsnrc 和 penelope 代码相比，研究了 mcnp5 代码在电子传输计算中的准确性及其在光子束中电离室（ic）响应模拟中的适用性。
通过比较在水中水模中使用不同入射能量（0.05、0.1、1 和 10 MeV）的宽平行电子束在薄层中的深度剂量分布来研究电子传输。在三种剂量气体材料（空气、氩气和基于甲烷的组织等效气体）中，在水模中研究 ic 响应模拟，用于光子束（（60）Co 源、6 MV 线性医疗加速器和单能 2 MeV 光子源）。评估了 mcnp5 的两种可选电子输运模型：基于 ITS 的电子能量索引（mcnp5（ITS））和新的详细电子能量损失离散逻辑（mcnp5（new））。还研究了 mcnp5 中的电子子步长（ESTEP 参数）依赖性。
对于电子束研究，在 1 MeV 及更低能量下，MCNP5 剂量分布与参考代码之间存在较大差异（>3%）。对于 0.1 和 0.05 MeV 电子束，差异尤为明显。边界交叉伪影，mcnp5（ITS）众所周知，仅在 0.1 和 0.05 MeV 束能量下观察到 mcnp5（new）。如果通过使用单个评分细胞消除过度的边界交叉，则 mcnp5（ITS）提供的剂量分布比 mcnp5（new）更符合参考代码。如果使用 mcnp5（ITS）或适当地为腔中的气体设置电子子步长长度，则 mcnp5 对气体腔室的剂量估计与参考代码相差在 1%以内。mcnp5（new）的结果发现高度依赖于所选的电子子步长长度，并且可能导致吸收剂量低估高达 15%。
由于根据一般临床标准，mcnp5 电子传输计算在所有能量和每种介质中都不准确，因此如果在当前电子传输模型中使用 mcnp5 进行剂量学应用，则需要谨慎。
