FLASH radiotherapy delivers a high dose (≥10 Gy) at a high rate (≥40 Gy/s). In this way, particles are delivered in pulses as short as a few nanoseconds. At that rate, intertrack reactions between chemical species produced within the same pulse may affect the heterogeneous chemistry stage of water radiolysis. This stochastic process suits the capabilities of the Monte Carlo method, which can model intertrack effects to aid in radiobiology research, including the design and interpretation of experiments. In this work, the TOPAS-nBio Monte Carlo track-structure code was expanded to allow simulations of intertrack effects in the chemical stage of water radiolysis. Simulation of the behavior of radiolytic yields over a long period of time (up to 50 s) was verified by simulating radiolysis in a Fricke dosimeter irradiated by ⁶⁰Co γ rays. In addition, LET-dependent G values of protons delivered in single squared pulses of widths, 1 ns, 1 µs and 10 µs, were obtained and compared to simulations using no intertrack considerations. The Fricke simulation for the calculated G value of Fe³⁺ ion at 50 s was within 0.4% of the accepted value from ICRU Report 34. For LET-dependent G values at the end of the chemical stage, intertrack effects were significant at LET values below 2 keV/µm. Above 2 keV/µm the reaction kinetics remained limited locally within each track and thus, effects of intertrack reactions remained low. Therefore, when track structure simulations are used to investigate the biological damage of FLASH irradiation, these intertrack reactions should be considered. The TOPAS-nBio framework with the expansion to intertrack chemistry simulation provides a useful tool to assist in this task.

闪光放射治疗以高速率（≥40 Gy/s）提供高剂量（≥10 Gy）。通过这种方式，粒子以短至几纳秒的脉冲传输。以该速率，同一脉冲内产生的化学物质之间的间轨反应可能会影响水辐射分解的异质化学阶段。这种随机过程适合蒙特卡罗方法的功能，该方法可以模拟轨迹间效应以帮助放射生物学研究，包括实验的设计和解释。在这项工作中，TOPAS-nBio 蒙特卡罗轨迹结构代码得到了扩展，可以模拟水辐射分解化学阶段的轨迹间效应。^{通过在 Fricke 剂量计中模拟60} Co γ 射线照射的辐射分解，验证了长时间（最多 50 秒）辐射分解产率行为的模拟。此外，还获得了以宽度为 1 ns、1 µs 和 10 µs 的单平方脉冲传递的质子的 LET 相关 G 值，并与不考虑轨道间因素的模拟进行了比较。Fricke 模拟计算出的 Fe ³⁺离子在 50 s 时的 G 值与 ICRU 报告 34 中接受的值的误差在 0.4% 以内。对于化学阶段结束时与 LET 相关的 G 值，轨迹间效应在 LET 值下显着低于 2 keV/μm。高于 2 keV/μm 时，反应动力学在每个轨道内仍然受到局部限制，因此轨道间反应的影响仍然很低。因此，当利用轨道结构模拟研究FLASH辐照的生物损伤时，应考虑这些轨道间反应。TOPAS-nBio 框架具有跨轨道化学模拟的扩展功能，为协助完成这项任务提供了一个有用的工具。