Cortés-Giraldo M A, Carabe A
Department of Atomic, Molecular and Nuclear Physics, Universidad de Sevilla, Seville, Spain.
Phys Med Biol. 2015 Apr 7;60(7):2645-69. doi: 10.1088/0031-9155/60/7/2645. Epub 2015 Mar 13.
We compare unrestricted dose average linear energy transfer (LET) maps calculated with three different Monte Carlo scoring methods in voxelized geometries irradiated with proton therapy beams with three different Monte Carlo scoring methods. Simulations were done with the Geant4 (Geometry ANd Tracking) toolkit. The first method corresponds to a step-by-step computation of LET which has been reported previously in the literature. We found that this scoring strategy is influenced by spurious high LET components, which relative contribution in the dose average LET calculations significantly increases as the voxel size becomes smaller. Dose average LET values calculated for primary protons in water with voxel size of 0.2 mm were a factor ~1.8 higher than those obtained with a size of 2.0 mm at the plateau region for a 160 MeV beam. Such high LET components are a consequence of proton steps in which the condensed-history algorithm determines an energy transfer to an electron of the material close to the maximum value, while the step length remains limited due to voxel boundary crossing. Two alternative methods were derived to overcome this problem. The second scores LET along the entire path described by each proton within the voxel. The third followed the same approach of the first method, but the LET was evaluated at each step from stopping power tables according to the proton kinetic energy value. We carried out microdosimetry calculations with the aim of deriving reference dose average LET values from microdosimetric quantities. Significant differences between the methods were reported either with pristine or spread-out Bragg peaks (SOBPs). The first method reported values systematically higher than the other two at depths proximal to SOBP by about 15% for a 5.9 cm wide SOBP and about 30% for a 11.0 cm one. At distal SOBP, the second method gave values about 15% lower than the others. Overall, we found that the third method gave the most consistent performance since it returned stable dose average LET values against simulation parameter changes and gave the best agreement with dose average LET estimations from microdosimetry calculations.
我们使用三种不同的蒙特卡罗计分方法，比较了在质子治疗束照射的体素化几何结构中计算得到的无限制剂量平均线能量转移（LET）图。模拟使用Geant4（几何与跟踪）工具包完成。第一种方法对应于文献中先前报道的LET逐步计算。我们发现这种计分策略受到虚假高LET分量的影响，随着体素尺寸变小，其在剂量平均LET计算中的相对贡献显著增加。对于160 MeV的束流，在水模中，体素尺寸为0.2 mm时计算得到的初级质子剂量平均LET值比在平台区体素尺寸为2.0 mm时得到的值高约1.8倍。这种高LET分量是质子步长的结果，在质子步长中，凝聚历史算法确定向材料电子的能量转移接近最大值，而步长由于跨越体素边界而保持有限。为克服此问题，推导了两种替代方法。第二种方法沿每个质子在体素内所描述的整个路径对LET进行计分。第三种方法遵循第一种方法的相同方式，但根据质子动能值在每个步骤从阻止本领表中评估LET。我们进行了微剂量学计算，目的是从微剂量学量中推导参考剂量平均LET值。对于原始布拉格峰或扩展布拉格峰（SOBP），各方法之间均报告了显著差异。对于5.9 cm宽的SOBP，在SOBP近端深度处，第一种方法报告的值系统地比其他两种方法高约15%，对于11.0 cm宽的SOBP则高约30%。在SOBP远端，第二种方法给出的值比其他方法低约15%。总体而言，我们发现第三种方法表现最为一致，因为它针对模拟参数变化返回稳定的剂量平均LET值，并且与微剂量学计算得到的剂量平均LET估计值最为吻合。
