A PROTON DOSE CALCULATION ALGORITHM FOR CONFORMAL THERAPY SIMULATIONSBASED ON MOLIERES THEORY OF LATERAL DEFLECTIONS

An algorithm is developed for computing proton dose distributions in t he therapeutic energy range (100-250 MeV). The goal is to provide accu rate pencil beam dose distributions for two-dimensional or three-dimen sional simulations of possible intensity-modulated proton therapy deli very schemes. The algorithm is based on Moliere's theory of lateral de flections, which accurately describes the distribution of lateral defl ections suffered by incident charged particles. The theory is applied to nonuniform targets through the usual pencil beam approximation whic h assumes that all protons from a given pencil beam pass through the s ame material at each depth. Fluence-to-dose conversion is made via Mon te Carlo calculated broad-field central-axis depth-dose curves, which accounts for attenuation due to nuclear collisions and range stragglin g. Calculation speed is enhanced by using a best-fit Gaussian approxim ation of the radial distribution function at depth. Representative pen cil beam and spread-out Bragg-peak computations are presented at 250 M eV and 160 MeV in water. Computed lateral full-widths-at-half-maximum' s in water, at the Bragg peak, agree with the expected theoretical lat eral values to within 1% at 160 MeV and to within 3% at 250 MeV. This algorithm differs from convolution methods in that the effect of the d epth of any inhomogeneities in density or atomic composition are accou nted for in a rigorous fashion. The algorithm differs from Fermi-Eyges based methods by accounting in a rigorous way for the effect of nonsm all-angle scattering and screening due to atomic electrons. The comput ational burden is only slightly greater than that expected using the l ess-rigorous Fermi-Eyges theory. (C) 1998 American Association of Phys icists in Medicine. [S0094-2405(98)02204-4].