Background: The stopping power and range tables published by the National Institute of Standards and Technology (NIST) were obtained by assuming continuous slowing down approximation (CSDA). This study examined more detail depth dose characteristics of ideal proton beams using the particle therapy simulation framework (PTSim) Monte Carlo technique.
Methods: Simulation for parallel broad field geometry (PBFG) was replaced by the pencil beam geometry (PBG) for improved simulation efficiency. Depth dose distributions (Bragg peak, BP) for beam energies from 69.44 to 230.71 MeV at 5 mm range interval were obtained. This study used seven parameters, Rpeak, R90, R80, R50, full width at half maximum (FWHM), W(80-20), and peak-to-entrance ratio to represent BP characteristics. The resulting energy-range relationships were fitted into third order polynomial formulae. In addition, initial beam energy spreads at 0-1% (1σ) of the mean incident energies at 70, 110, 150, 190, and 230 MeV were added into the simulation to uncover their impact on BP shapes.
Results: The study results reveal deeper penetration, broader FWHM and decreased peak-to-entrance dose ratio at increasing beam energy. Study results for beams with initial energy spreads show that R 80 can be a good indicator to characterize initial mean energy. They also suggest FWHM is more sensitive than the width of 80-to-20% distal fall-off in finding initial energy spread.
Conclusion: Detail depth dose characteristics for monoenergetic proton beams and beams with initial energy spreads within therapeutic energy ranges were reported. These data can serve as a good reference for a clinical practitioner in their daily practice.