Introduction: Recent in-vivo experimental measurements of DaRT seeds have shown that the spread of ²²⁰Rn away from the DaRT seed is greater than expected. This has implications for accurate treatment planning.

Methods: Geant4 and Geant4-DNA have been used to simulate the additional spread of ²²⁰Rn and study the impact on particle and RBE (relative biological effectiveness) weighted dose distribution from the whole DaRT decay chain. A percentage of ²²⁰Rn is assumed to migrate with a higher effective diffusion constant due to transport in the blood flow in the surrounding vasculature. The DNA damage was simulated for the fit to the measurement showing the greatest additional spread.

Results: Good agreement was found between the simulation and experimental in-vivo results. The additional spread of ²²⁰Rn increases the number of α-particles from these decays distally from the DaRT seed. However, the distribution of α-particles emitted from later stages of the decay chain is not as significantly affected. There is an increase in DSB yield and the RBE weighted dose extends further from the DaRT seed with the additional spread. The distance of the 1 Gy RBE weighted isodose from the DaRT seed surface moves from 4.4 mm without the additional spread to 5.7 mm including it.

Conclusion: The increase DSB induction distally from the DaRT seed has the potential to increase treatment efficacy. However, this additional spread cannot easily be predicted. Understanding this uncertainty is important for the dose calculations in treatment planning for tumours and normal tissues.

Keywords: DNA Damage; DaRT; Geant4-DNA.

Copyright © 2025 Associazione Italiana di Fisica Medica e Sanitaria. Published by Elsevier Ltd. All rights reserved.

简介： 最近的体内实验测量显示，²²⁰Rn从DaRT种子扩散出去的程度大于预期。这对准确的治疗计划制定有影响。

方法： 使用Geant4和Geant4-DNA模拟了²²⁰Rn的额外扩散，并研究了整个DaRT衰变链对粒子和RBE（相对生物有效性）加权剂量分布的影响。假设一部分²²⁰Rn由于周围血管中的血液流动而以更高的有效扩散常数迁移。模拟了与实验测量结果相差最大的DNA损伤情况。

结果： 仿真结果和体内实测数据之间有良好的一致性。额外的²²⁰Rn扩散增加了从DaRT种子远处α粒子的数量，然而衰变链后期发出的α粒子分布没有显著影响。DSB产率增加，并且RBE加权剂量更远地延伸到DaRT种子外。包括额外扩散后，1 Gy RBE加权等剂量线距离DaRT种子表面的距离从无额外扩散时的4.4 mm增加到了5.7 mm。

结论： 从DaRT种子远处诱导DSB的增加有可能提高治疗效果。然而，这种额外扩散不易预测。了解这一不确定性对于肿瘤和正常组织剂量计算中的治疗计划制定非常重要。

关键词： DNA损伤；DaRT；Geant4-DNA。