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: che intens ity variation due to the heel effect, the energy variation due to the diffe ring amount of target material traversed by the photons, and the finite sou rce size. The initial energy spectrum used was generated using a computer p rogram. 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 20x20cm(2) fields show agreement with experiment al values to within 2% and 4%, respectively, for the central 80% of the fie ld size. Within the field but outside this range a maximum of 6% difference (for the 20x20 cm(2) field) was observed, however, these values were in a region of sharp gradient and hence small geometric shift. The "tails" of th e profiles were underpredicted by up to 6%. Due to uncertainties in experim ent (3%) and Monte Carlo (1.5%), the modeled phase space is deemed acceptab le for phantom and in-vivo dosimetric calculations within the field boundar ies. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(9 9)00609-4].