Biophysics Department, GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany.
Institute for Solid State Physics, Technische Universität Darmstadt, Darmstadt, Germany.
Med Phys. 2022 Jan;49(1):714-726. doi: 10.1002/mp.15343. Epub 2021 Dec 1.
The increased relative biological effectiveness (RBE) of ions is one of the key benefits of ion radiotherapy compared to conventional radiotherapy with photons. To account for the increased RBE of ions during the process of ion radiotherapy treatment planning, a robust model for RBE predictions is indispensable. Currently, at several ion therapy centers the local effect model I (LEM I) is applied to predict the RBE, which varies with biological and physical impacting factors. After the introduction of LEM I, several model improvements were implemented, leading to the current version, LEM IV, which is systematically tested in this study.
As a comprehensive RBE model should give consistent results for a large variety of ion species and energies, the particle irradiation data ensemble (PIDE) is used to systematically validate the LEM IV. The database covers over 1100 photon and ion survival experiments in form of their linear-quadratic parameters for a wide range of ion types and energies. This makes the database an optimal tool to challenge the systematic dependencies of the RBE model. After appropriate filtering of the database, 571 experiments were identified and used as test data.
The study confirms that the LEM IV reflects the RBE systematics observed in measurements well. It is able to reproduce the dependence of RBE on the linear energy transfer (LET) as well as on the α /β ratio for several ion species in a wide energy range. Additionally, the systematic quantitative analysis revealed precision capabilities and limits of the model. At lower LET values, the LEM IV tends to underestimate the RBE with an increasing underestimation with increasing atomic number of the ion. At higher LET values, the LEM IV overestimates the RBE for protons or helium ions, whereas the predictions for heavier ions match experimental data well.
The LEM IV is able to predict general RBE characteristics for several ion species in a broad energy range. The accuracy of the predictions is reasonable considering the small number of input parameters needed by the model. The detailed quantification of possible systematic deviations, however, enables to identify not only strengths but also limitations of the model. The gained knowledge can be used to develop model adjustments to further improve the model accuracy, which is on the way.
与传统光子放射治疗相比，离子放射治疗的一个关键优势是离子的相对生物效应（RBE）增加。为了在离子放射治疗计划过程中考虑离子的增加 RBE，需要一个强大的 RBE 预测模型。目前，在几个离子治疗中心应用局部效应模型 I（LEM I）来预测 RBE，RBE 随生物学和物理影响因素而变化。在引入 LEM I 之后，进行了几项模型改进，从而产生了当前版本的 LEM IV，并在本研究中进行了系统测试。
由于全面的 RBE 模型应该为各种离子种类和能量提供一致的结果，因此使用粒子辐照数据集合（PIDE）系统地验证 LEM IV。该数据库涵盖了 1100 多个光子和离子存活实验的线性二次参数，涉及广泛的离子类型和能量。这使得该数据库成为挑战 RBE 模型系统依赖性的最佳工具。对数据库进行适当过滤后，确定并使用 571 个实验作为测试数据。
研究证实，LEM IV 很好地反映了测量中观察到的 RBE 系统。它能够再现 RBE 对线性能量转移（LET）以及几种离子在广泛能量范围内的α/β 比的依赖性。此外，系统的定量分析揭示了模型的精度能力和限制。在较低的 LET 值下，LEM IV 倾向于低估 RBE，随着离子原子数的增加，低估程度也随之增加。在较高的 LET 值下，LEM IV 高估了质子或氦离子的 RBE，而对于较重的离子，预测结果与实验数据吻合良好。
LEM IV 能够预测几种离子在广泛能量范围内的一般 RBE 特征。考虑到模型所需输入参数的数量较少，预测的准确性是合理的。然而，对可能的系统偏差进行详细量化不仅可以识别模型的优势，还可以识别模型的局限性。所获得的知识可用于开发模型调整，以进一步提高模型的准确性，这正在进行中。
