FLUKA Monte Carlo-based Evaluation of Linear Energy Transfer Distributions for Monoenergetic Therapeutic Carbon-ion Beams in Water

Background: This study aims to calculate the distribution of linear energy transfer ( LET ) and its moments such as fluence-averaged LET ( LET f ) and dose-averaged LET ( LET d ) as a function of on-axis depth in water for monoenergetic carbon-ion beams with energies ranging from 80 to 480 MeV/u. Materials and Methods: Depth-dose profiles, LET distributions, LET f , and LET d are calculated as a function of on-axis depth in a 40 cm × 40 cm × 40 cm unit-density liquid water phantom using the FLUKA Monte Carlo code, fully incorporating the contributions of primary ions and all secondary charged fragments. Results: Results show that both LET f and LET d increase with depth and reach a maximum near the Bragg peak region, followed by a rapid fall-off in the distal region as the fluence of heavy ion fragments decreases. Higher beam energies result in deeper LET peaks and more pronounced fragmentation tails in the depth-dose profiles. In the Bragg peak region, depending on beam energy, the LET f and LET d values are in the range of 11.7–106.4 keV/mm and 210.7–384.6 keV/mm, respectively. Conclusion: Overall, the results provide a comprehensive characterization of physical and radiobiological parameters for monoenergetic carbon-ion beams in the therapeutic energy range, essential for benchmarking LET -based RBE models, improving treatment plan robustness, and optimizing biological dose delivery in carbon-ion radiotherapy.