A novel estimation method of water-equivalent thicknesses of secondary particle tracks using secondary electron bremsstrahlung emitted from therapeutic ion beams for attenuation correction

摘要

Monitoring methods of therapeutic beams based on the measurement of secondary particles have been proposed and studied worldwide. Since secondary particles are influenced by substances in the human body before they reach detectors, estimation methods of the water-equivalent thickness of the tracks of the secondary particles are required for attenuation correction. In this work, we studied a novel estimation method of the water-equivalent thickness based on the observation of energy distribution of secondary electron bremsstrahlung (SEB) emitted from the trajectories of the therapeutic beams by Monte Carlo simulations. A ~(12)C-ion beam with an energy of 290 MeV/u was injected into a water phantom. The phantom was surrounded by a collimator having a CdTe detector behind the collimator. A distribution of energy deposition due to SEB emission was recorded during beam injection. The simulation was iterated by varying the radius of the phantom. The simulation results were plotted as an R_1 vs. R_2 scatter plot, where R_1 was defined as the ratio of the yield having a deposition energy between 30 and 40 keV to that between 20 and 30 keV, and R_2 was defined as the ratio of the yield having a deposition energy between 60 and 70 keV to that between 20 and 30 keV. We found that clusters are formed at different positions on the scatter plot by different radii of the phantom and that the water-equivalent thickness of the secondary particle tracks could be estimated using the R_1 vs. R_2 plot.