EXTENSION OF A NUMERICAL ALGORITHM TO PROTON DOSE CALCULATIONS .1. COMPARISONS WITH MONTE-CARLO SIMULATIONS

A numerical algorithm originally developed for electron dose calculati ons [Med. Phys. 21, 1591 (1994)] has been modified for use with pluton beams. The algorithm recursively propagates the proton distribution i n energy, angle, and space from one level in an absorbing medium to an other at slightly greater depth until all protons stop. Vavilov's theo ry is used to predict, at any point in the absorber, the broadening of the primary proton energy spectrum. Moliere's theory is applied to de scribe the angular distribution, and it is shown that the Gaussian fir st term of Moliere's series expansion is of sufficient accuracy for do se calculations, These multiple scattering and energy loss distributio ns are sampled using equal probability spacing to optimize computation al speed while maintaining calculational accuracy. Inelastic nuclear c ollisions along the proton trajectories are modeled by a simple expone ntial extinction, Predictions of the algorithm for absolute dose depos ition by a 160 MeV initially monoenergetic proton beam are compared wi th the results of Monte Carlo simulations performed with the PTRAN cod e, The excellent level of agreement between the results of these two m ethods of dose calculation (<5% dose and <3 mm spatial deviations) dem onstrate that dose deposition from proton beams may be computed to hig h accuracy using this algorithm without the need for extensive empiric al measurement as input. (C) 1997 American Association of Physicists i n Medicine.