Linear Energy Transfer Measurements and Estimation of Relative Biological Effectiveness in Proton and Helium-Ion Beams Using Fluorescent Nuclear Track Detectors

Purpose To develop a methodology for assessing the linear energy transfer (LET) and relative biological effectiveness (RBE) in clinical proton and helium-ion beams using fluorescent nuclear track detectors (FNTDs). Methods and materials FNTDs were exposed behind solid water to proton and helium- (4He-) ion spread-out Bragg peaks (SOBPs). Detectors were imaged with a confocal microscope, and the LET spectra were derived from the fluorescence intensity. The track- and dose-averaged LET (LETF and LETD, respectively) were calculated from the LET spectra. LET measurements were used as input on RBE-models to estimate the RBE. Human alveolar adenocarcinoma cells (A549) were exposed at the same positions as the FNTDs. The RBE was calculated from the resulting survival curves. All measurements were compared with Monte Carlo simulations. Results For protons, average relative differences between measurements and simulations were 6% and 19% for LETF and LETD, respectively. For helium ions the same differences were 11% for both quantities. The position of the experimental LET spectra primary peaks agreed with the simulations within 9% and 14% for protons and helium-ions, respectively. For the RBE-models using LETD as input, FNTD-based RBE values ranged from 1.02 ± 0.01 to 1.25 ± 0.04 and from 1.08 ± 0.09 to 2.68 ± 1.26 for protons and helium-ions, respectively. The average relative differences between these values and simulations were 2% and 4%. For A549 cells, the RBE ranged from 1.05 ± 0.07 to 1.47 ± 0.09 and from 0.89 ± 0.06 to 3.28 ± 0.20 for protons and helium-ions, respectively. Regarding the RBE-weighted dose (2.0 Gy at the SOBP), the differences between simulations and measurements were below 0.10 Gy. Conclusion This study demonstrates for the first time that FNTDs can be used to perform direct LET measurements and to estimate the RBE in clinical proton and helium-ion beams.