Electronic equilibrium and primary dose in collimated photon beams.

Electronic equilibrium conditions are studied in a homogeneous medium irradiated by monoenergetic photons with Compton scattering as a predominant process. Based on the concept of straight charged particle tracks, a geometrical model for spatial distribution of Compton electrons is developed in the limit of primary photon interactions. The model is applied to examine conditions of electronic equilibrium in collimated photon beams and to define equilibrium phase diagrams which establish correlation between various degrees of electronic equilibrium and primary dose. The diagrams predict that in a single direction (longitudinal or lateral) partial electronic equilibrium can be observed in radiation fields of dimensions smaller than the maximum range of secondary electrons. Associated macroscopic effects appear as a variation of the primary dose build-up rate with beam radius and depth in phantom. These effects are observed in the case of both primary and total absorbed dose as judged by the Monte Carlo generated data in waterlike material (1-8 MeV photons).