Verburg J, Shih H, Seco J
Massachusetts General Hospital and Harvard Medical School, Boston, MA.
Med Phys. 2012 Jun;39(6Part20):3860. doi: 10.1118/1.4735758.
The impact of nuclear reaction model differences on simulation of prompt gamma-ray imaging for proton therapy range verification was assessed. Four nuclear reaction models were used to simulate gamma emission in proton beams, and were validated against experimental cross-sections.
Proton-induced nuclear reactions on carbon, oxygen, nitrogen and calcium were investigated with the Monte Carlo toolkits GEANT4 9.5 and MCNPX 2.7, and the dedicated nuclear reaction codes TALYS 1.4 and EMPIRE 3.1. Absolute cross-sections of discrete prompt gamma lines and the total gamma production were obtained for the 1-200 MeV incident proton energy range. They were compared to 34 discrete line measurements reported in literature. Using these cross-sections, we analyzed the gamma production along the path of proton beams passing through various tissues.
The differences in absolute discrete line cross-sections as predicted by the models ranged from almost zero to an order of magnitude, depending on the gamma line and incident proton energy. Overall, the dedicated nuclear reaction codes provided a better fit to most experimental excitation functions. For a 150 MeV proton beam stopping in soft tissue, these differences amount to a variation by a factor of 4 of the gamma emission around the Bragg peak location. The maximum of gamma production near the end of proton range differed by 7 mm, and the change of the 50% emission fall-off position was 4 mm.
There is a clear need for improvement of nuclear reaction models to accurately simulate proton range verification using prompt gamma-rays. Current simulation codes show large uncertainties in both the total gamma yield and the correlation of gamma emission with the proton Bragg peak. GEANT4 and MCNPX in particular appear to have limited predictive power.
评估核反应模型差异对质子治疗射程验证的瞬发伽马射线成像模拟的影响。使用四种核反应模型来模拟质子束中的伽马射线发射，并根据实验截面进行了验证。
利用蒙特卡罗工具包GEANT4 9.5和MCNPX 2.7以及专用核反应代码TALYS 1.4和EMPIRE 3.1，研究了质子与碳、氧、氮和钙的核反应。获得了1-200 MeV入射质子能量范围内离散瞬发伽马射线的绝对截面和总伽马产生量。将它们与文献报道的34条离散线测量结果进行了比较。利用这些截面，我们分析了质子束穿过各种组织路径上的伽马产生情况。
根据伽马射线谱线和入射质子能量的不同，模型预测的绝对离散线截面差异范围从几乎为零到一个数量级。总体而言，专用核反应代码对大多数实验激发函数的拟合效果更好。对于在软组织中停止的150 MeV质子束，这些差异导致布拉格峰位置周围伽马发射变化了4倍。质子射程末端附近的伽马产生最大值相差7 mm，50%发射下降位置的变化为4 mm。
显然需要改进核反应模型，以准确模拟利用瞬发伽马射线进行的质子射程验证。当前的模拟代码在总伽马产额以及伽马发射与质子布拉格峰的相关性方面都存在很大的不确定性。特别是GEANT4和MCNPX的预测能力似乎有限。
