Point kernels and superposition methods for scatter dose calculations in brachytherapy

Point kernels have been generated and applied for calculation of scatter do se distributions around monoenergetic point sources for photon energies ran ging from 28 to 662 keV. Three different approaches for dose calculations h ave been compared: a single-kernel superposition method, a single-kernel su perposition method where the point kernels are approximated as isotropic an d a novel 'successive-scattering' superposition method for improved modelli ng of the dose from multiply scattered photons. An extended version of the EGS4 Monte Carlo code was used for generating the kernels and for benchmark ing the absorbed dose distributions calculated with the superposition metho ds. It is shown that dose calculation by superposition at and below 100 keV can be simplified by using isotropic point kernels. Compared to the assump tion of full in-scattering made by algorithms currently in clinical use, th e single-kernel superposition method improves dose calculations in a half-p hantom consisting of air and water. Further improvements are obtained using the successive-scattering superposition method, which reduces the overesti mates of dose close to the phantom surface usually associated with kernel s uperposition methods at brachytherapy photon energies. It is also shown tha t scatter dose point kernels can be parametrized to biexponential functions , making them suitable for use with an effective implementation of the coll apsed cone superposition algorithm.