3D DOSE CALCULATIONS FOR ELECTRON AND PHOTON BEAMS

A new method has been implemented to calculate rapidly and accurately the three-dimensional dose distribution induced in a heterogeneous volume by one or more electron and/or photon beams. The algorithm is based on the pre-calculation by Monte Carlo of one or more (pencil beam) kernels, where a kernel is the distribution of absorbed energy density within a uniform medium by a unit of beam energy incident normally on a small area of the air-medium surface. Once the intensity and energy distributions of a clinical beam are deduced from measurements, the appropriate kernels can be obtained for that beam. Kernels allow us to estimate how each pencil of a beam deposits its energy in an inhomogeneous medium. When the patient contours and density distribution are known, and beam parameters are specified, the beam energy delivered to each element of the patient surface is calculated and then distributed within the patient volume in accord with the relevant kernels. For photons, our method avoids the assumption of charged particle equilibrium. For electrons, there is no restriction on the inhomogeneities. However, since a precalculated kernel does not contain the directional distribution of secondary electrons as a function of position, assumptions made by the algorithm in distributing absorbed energy may affect the dose distribution around the edges of sharp inhomogeneities. We compare the results of our kernel method with those of full Monte Carlo calculations.