An analytical model of a kilovoltage beam phase space.
Our aim in this project was to quantify a kilovoltage beam phase space for use in dose calculations. The phase space was modeled by incorporating the following analytically derived x-ray beam production properties: the intensity variation due to the heel effect, the energy variation due to the differing amount of target material traversed by the photons, and the finite source size. The initial energy spectrum used was generated using a computer program. A Monte Carlo code was adapted in order to examine the validity of the calculated phase space. Dose distributions calculated using the modeled phase space for 10x10 and 20x20 cm2 fields show agreement with experimental values to within 2% and 4%, respectively, for the central 80% of the field size. Within the field but outside this range a maximum of 6% difference (for the 20x20 cm2 field) was observed, however, these values were in a region of sharp gradient and hence small geometric shift. The "tails" of the profiles were underpredicted by up to 6%. Due to uncertainties in experiment (3%) and Monte Carlo (1.5%), the modeled phase space is deemed acceptable for phantom and in-vivo dosimetric calculations within the field boundaries.