Electron beam dose distributions near standard inhomogeneities

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Abstract
At a recent workshop on electron beam dose planning, a set of standard geometries was defined to facilitate the comparison of electron beam treatment planning algorithms and dosimetric measurements. The geometries consist of one-, two- or three-dimensional inhomogeneities embedded near the entrance surface of a water phantom. In the three-dimensional case, the inhomogeneities are small cylinders of air or aluminium located on the beam axis. The authors have used a small (1 mm square by 0.1 mm thick) p-type silicon detector to measure the dose distributions behind these inhomogeneities for broad beams of 10 and 20 MeV electrons. The effect of the inhomogeneities is to perturb the dose in their vicinity by as much as 50% over a range of a few millimetres. These results provide a stringent test of techniques for calculating dose distributions. Current clinical algorithms do not accurately predict the dose distributions, but detailed Monte Carlo simulations are shown to be in good agreement with the experimental results.