Composite energy deposition kernels for focused point monodirectional photon beams

A 3D volume overlap algorithm has been developed for converting energy depo sition kernels between arbitrary 2D and 3D irradiation geometries. The kern els can be used as convolution kernels in inverse radiation therapy plannin g and as accurate descriptions of the dose distributions for clinically imp ortant beam geometries. The new method of dose calculation combining Monte Carlo and analytical methods has introduced an improved accuracy in dose ca lculation on the fractional per cent level. The comparisons are also made f or a wide range of photon spectra and irradiation geometries from narrow po int monodirectional pencil beams to finite uniform beams, 4 pi steradians i sotropically converging beams and divergent beams from isotropic point sour ces. It is seen that the photon scatter penumbra is highest at low photon e nergies whereas the secondary electron penumbra is widest at high photon en ergies, making low energy beams more interesting for small targets and high energy beams most useful for large deep-seated targets.