Heterogeneity model for photon beams incorporating electron transport.

A method of calculating photon doses in heterogeneous media incorporating electron transport is studied. The dose is represented as the convolution of kerma with an exponential longitudinal electron spread function which describes the penetration of electrons from one medium to another. At large distances from an interface, the dose approaches an asymptotic value equal to the kerma multiplied by a constant describing the degree of longitudinal and lateral electron equilibrium. For the simple situations studied, this asymptotic dose is adequately described by O'Connor's scaling theorem. The method is compared with both Monte Carlo calculations and measurements for a 15-MV photon beam for various geometries and field sizes. It predicts the dose in regions of electron disequilibrium to within 2% in most cases. In situations of extreme electron disequilibrium, such as within low-density regions at high energies and small field sizes, this represents a significant improvement over many existing techniques.