The invention relates to a computer readable medium including software instructions, which when executed by a scaling parameters for processor perform a method. The method includes obtaining a first and a second pre-calculated history, wherein the first and the second pre-calculated history corresponds to a first and a second path of particles through a reference material. The method further includes obtaining a first and a second plurality of phase space points and performing a first and a second set of simulations in parallel on a first and a second GPU. Each simulation uses a distinct one of the first and second plurality of phase space points, the geometry information, and the first and second pre-calculated history. The sets of simulations are performed on the GPU's to obtain a set of simulated histories. The method further includes calculating an absorbed dose of energy in the target using the set of simulated histories.