3194 - Comprehensive Evaluation of RBE and LET Dependencies in Proton and Carbon-Ion Spatially Fractionated Lattice Radiation Therapy
Presenter(s)
J. Yin1,2, T. Dai1, and Y. Yin1; 1Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, 2Dalian Medical University, Dalian, China
Purpose/Objective(s): Spatially Fractionated Radiation Therapy (SFRT), particularly lattice radiation therapy (LRT), has demonstrated promising tumor control and has attracted considerable attention in the field of radiation oncology. The combination of LRT with proton or heavy-ion therapy may further enhance therapeutic efficacy. However, in proton or heavy-ion–based LRT, the RBE and LET dependencies within the extremely small lattice vertices have not yet been systematically evaluated. In this work, we quantified the depth- and vertex-size effects on the proton and carbon-ion LET and RBE.
Materials/Methods: Vertices were placed with vertex “radius” of 0.25, 0.5, 0.75, and 1.0 cm in a homogenous water phantom. For protons, targets were positioned at depths of 5, 10, 15, 20, 25, and 30 cm. For each depth–size combination, single-fraction plans were generated prescribing 15 Gy(RBE). For protons, LET at target center (LETcenter) was recorded and RBE was calculated using the Wedenberg model, evaluating a/ß = 10, 6, and 2 Gy and fraction dose D = 15, 8, and 2 Gy. For carbon ions, target-center RBE was calculated by both Microdosimetric Kinetic Model (MKM) and Local Effect Model (LEM).
Results: In proton plans, LETcenter decreased with increasing depth and target size (range 3.1–5.5 keV/µm). Wedenberg model–based proton RBE ranged from 1.033–1.058 (D=15 Gy, a/ß=10 Gy) and increased to 1.210–1.356 (D=2 Gy, a/ß=2 Gy), demonstrating greater sensitivity at low a/ß and low fraction dose. In carbon-ion plans, target-center RBE showed systematic size-dependence under both models. Across the evaluated geometries, MKM-based RBE was approximately 2.65–3.25, whereas LEM-based RBE was approximately 1.72–1.94. For both MKM and LEM, carbon-ion RBE generally decreased with increasing target size, with higher values for smaller targets; the magnitude of variation was more pronounced for MKM than for LEM.
Conclusion: In a standardized water phantom, both modality and geometry (depth and vertex size) drive target-center RBE differences. Proton LET-driven, model-based RBE variability is amplified for low a/ß tissues and lower fraction dose (although large fraction dose was usually considered in LRT), while carbon-ion plans demonstrate higher RBE with consistent target-size dependence and a larger dynamic range under MKM compared with LEM. This work performed a comprehensive evaluation of RBE and LET dependencies in proton and carbon-ion LRT.