抄録
In order to optimize the design of a beam-delivery system in heavy charged-particle therapy, we have developed an analytical method to calculate the sizes of the penumbra and field widths of a collimated beam. Such an analytical method is effective for optimization, and is earlier to calculate than using the Monte Carlo method. In the theoretical calculation, an exact distribution function was used, which took into account multiple-scattering and field broadening due to beam wobbling. The multiple scattering in the beam-delivery system was calculated using the Fermi-Eyges distribution function. In order to include the influence of field broadening due to beam wobbling, the Fermi-Eyges distribution function was integrated along the trajectory of the wobbled beam. The average and variance of the beam angle of the exact distribution function at the collimator were derived in order to calculate the penumbra and field width for the collimated beam. A simple formula for the penumbra size and field width was also deduced by assuming a uniform field, except for a peripheral region at the irradiation site. The calculated results by both an exact method and a simplified one were compared with the experimental results using therapeutic carbon beams with an energy per nucleon of 290,350 and 400 MeV. These results agreed well for almost all cases of the HIMAC beam.
