Photon beam energy deposition kernels for inverse radiotherapy planning

A method for calculating energy deposition kernels for planar convergent isotropic irradiations of a point target has been developed. A geometric overlap algorithm is applied to pencil beam energy deposition kernels calculated by convolution of Monte Carlo generated point spread functions with the energy released by a point monodirectional photon beam. The geometric overlap interpolation technique is energy conservative and hence, for dosimetric purposes, more precise than interpolations based on smoothing techniques. Furthermore, due to the integration processes involved, most of the statistical noise in the original Monte Carlo data is eliminated. The resulting kernels have a precision and noise level corresponding to 10(9) - 10(10) Monte Carlo simulated photons. The kernels are useful for inverse conformation therapy planning, and for dose planning with small target volumes.