The collapsed cone superposition algorithm applied to scatter dose calculations in brachytherapy

Methods for scatter dose calculations in brachytherapy have been developed based on the collapsed cone superposition algorithm. The methods account fo r effects on the scatter dose caused by the three-dimensional distribution of heterogeneities in the irradiated volume and are considerably faster tha n methods based on straightforward superposition of kernels or direct Monte Carlo simulations. Use of a successive-scattering approach, in which the d ose contribution from once- and multiply scattered photons are calculated s eparately, was found superior to conventional superposition using a single point kernel for all scatter generations. Use of the successive-scattering approach significantly reduces artifacts stemming from steep fluence gradie nts, typical of the brachytherapy geometry and critical for the collapsed c one approximation. The algorithm is tested versus Monte Carlo simulations f or point sources of energies 28.4, 100, 350, and 662 keV. Results agree wel l for both a homogeneous water phantom and an air-water half-phantom. (C) 2 000 American Association of Physicists in Medicine. [S0094-2405(00)00210-8] .