Phenomenological dose model for therapeutic photon beams: basic concepts and definitions.

A model for central-axis absorbed dose in therapeutic photon beams is developed. An expression for absorbed dose in a unit density material, including that in the regions of longitudinal and lateral electronic disequilibrium, is derived. The model is based on the concept of primary and scatter. Primary and scatter dose components are approximated using two identical analytical functions. Monte Carlo simulated dose data for 60Co gamma rays and 15 MV x rays in water are used to test the model. The accuracy of the model is demonstrated.