Author: Xiangli Cui, Wei Han, Jie Li, Lingling Liu 👨🔬
Affiliation: Institute of Health and Medical Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Hefei Cancer Hospital, Chinese Academy of Sciences, Institute of Health and Medical Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences 🌍
Purpose: To investigate the influence of tissue heterogeneity on proton beam linear energy transfer (LET) and relative biological effectiveness (RBE) using nine RBE models. This study quantifies the impact of tissue electron densities and compositions on LET and RBE distributions to provide a reference for accurate biological dose calculations in proton therapy.
Methods: A heterogeneous three-layer water-tissue-water phantom (30 × 30 × 30 cm³) was constructed in TOPAS to simulate a 169.23 MeV monoenergetic proton beam traversing various tissues. The phantom included two 10 cm water layers and a 10 cm central layer with 14 materials, including LN-300 Lung, BR-12 Breast, SB3 Cortical Bone, and Titanium. Tissue densities and compositions were based on the Gammex 467 electron density phantom. Proton beam spatial and angular distributions were modeled using ellipsoidally truncated Gaussian profiles, with 10⁹ protons simulated per material. LET distributions, depth doses, and RBE values were calculated using nine RBE models, including LET-based (Carabe, Chen, McNamara, Wedenberg, Wilkens, MKMLET, MKM_Kase) and non-LET-based models (MKM_PIDE, RMF).
Results: Tissue heterogeneity significantly influenced LET and RBE distributions. Low-density tissues exhibited deeper Bragg peaks and lower LET, while high-density tissues produced higher LET and shallower Bragg peaks. RBE correlated strongly with LET, with high-density tissues generating larger RBE peaks. LET-based models showed greater sensitivity to heterogeneity, leading to notable differences in RBE predictions. Secondary RBE peaks in high-density tissues, driven by local LET increases, highlighted the complexity of dose deposition in heterogeneous regions.
Conclusion: Tissue heterogeneity critically affects LET and RBE distributions. High-density tissues generate higher LET and RBE at shallower depths, while low-density tissues produce deeper LET and RBE peaks. Selecting appropriate RBE models and accounting for tissue heterogeneity are crucial for accurate biological dose calculations and optimal treatment planning in proton therapy.