Department of Radiation Oncology, University of California, San Francisco, CA, United States of America. Author to whom any correspondence should be addressed.
Phys Med Biol. 2018 Nov 28;63(23):235015. doi: 10.1088/1361-6560/aaeeee.
Details of the pattern of ionization formed by particle tracks extends knowledge of dose effects on the nanometer scale. Ionization detail (ID), frequently characterized by ionization cluster size distributions (ICSD), is obtained through time-consuming Monte Carlo (MC) track-structure simulations. In this work, TOPAS-nBio was used to generate a highly precise database of biologically significant ID quantities, sampled with randomly oriented 2.3 nm diameter cylinders, 3.4 nm (10 base pairs) long, inside a chromatin-size cylinder, irradiated by 1-1000 MeV/u ions of Z = 1-8. A macroscopic method developed to utilize the database using condensed-history MC was used to calculate distributions of the ICSD first moment [Formula: see text] and cumulative probability [Formula: see text] in a 20 × 20 × 40 cm water phantom irradiated with proton and carbon spread-out Bragg peak (SOBP) of 10.5 cm range, 2 cm width. Results were verified against detailed MC track-structure simulations using phase space scored at several depths. ID distributions were then obtained for intensity modulated proton and carbon radiotherapy plans in a digitized anthropomorphic phantom of a base of skull tumor to demonstrate clinical application of this approach. The database statistical uncertainties were 0.5% (3 standard deviations). Fluence-averaged ID as implemented proved unsuitable for macroscopic calculation. E -averaged ID agreed with track-structure results within 0.8% for protons. For carbon, maximum absolute differences of 2.9% ± 1.6% and 5.6% ± 1.9% for [Formula: see text], 1.7% ± 0.8% and 1.9% ± 0.4% (1 standard deviation) for [Formula: see text], were found in the plateau and SOBP, respectively, up to 11.5% ± 5.6% in the tail region. Macroscopic ID calculation was demonstrated for a realistic treatment plan. Computation times with or without ID calculation were comparable in all cases. Pre-calculated nanodosimetric data may be used for condensed-history MC for nanodosimetric ID-based treatment planning in ion radiotherapy in the future. The macroscopic approach developed has the calculation speed of condensed-history MC while approaching the accuracy of full track structure simulations.
粒子径迹形成的电离模式细节扩展了对纳米尺度剂量效应的认识。通过耗时的蒙特卡罗（MC）轨迹结构模拟获得电离细节（ID），通常通过电离簇大小分布（ICSD）来表征。在这项工作中，TOPAS-nBio 被用于生成具有生物意义的 ID 数量的高精度数据库，该数据库是通过在一个直径为 2.3nm、长 3.4nm（10 个碱基对）的随机取向的圆柱体中进行采样获得的，该圆柱体位于一个染色质大小的圆柱体内部，由 Z=1-8 的 1-1000 MeV/u 离子辐照。开发了一种宏观方法来利用数据库，使用凝聚历史 MC 进行计算，以计算在 20×20×40cm 水模体中用质子和碳扩展布拉格峰（SOBP）辐照时 ICSD 第一矩 [公式：见正文]和累积概率 [公式：见正文]的分布，质子 SOBP 范围为 10.5cm，宽度为 2cm。使用在几个深度处得分的相空间对详细的 MC 轨迹结构模拟进行了验证。然后，在数字化的颅底肿瘤人体模型中为强度调制质子和碳放射治疗计划获得 ID 分布，以展示该方法的临床应用。数据库统计不确定性为 0.5%（3 个标准差）。实施的平均剂量 ID 不适合宏观计算。对于质子，E 平均 ID 与轨迹结构结果的偏差在 0.8%以内。对于碳，在平台和 SOBP 中，[公式：见正文]的最大绝对值差异分别为 2.9%±1.6%和 5.6%±1.9%，[公式：见正文]的差异分别为 1.7%±0.8%和 1.9%±0.4%（1 个标准差），在尾部区域高达 11.5%±5.6%。为现实的治疗计划演示了宏观 ID 计算。在所有情况下，带或不带 ID 计算的计算时间都相当。预计算的纳米剂量数据可用于未来的离子放射治疗中的凝聚历史 MC 进行基于纳米剂量 ID 的治疗计划。所开发的宏观方法具有凝聚历史 MC 的计算速度，同时接近全轨迹结构模拟的精度。
