Development of a Beam Source Modeling Technique for a Flattening Filter Free (FFF) Beam

This study was focused on a new beam source modeling technique for a flattening filter free (FFF) beam. The model was based on a previous three source model, and improved by introducing off axis ratio (OAR) of photon fluence to the primary and scattered photon sources to generate cone shaped dose profiles. The model parameters and the OAR were optimized from measured head scatter factors and a dose profile with 40 x 40 cm2 field size by using line search optimization algorithm. The model was validated by comparing various dose profiles on 6 and 10 MV FFF beam from a True Beam STx linear accelerator. Planar dose distributions for clinically used radiation fields were also calculated and compared with measured data. All calculated dose profiles were agreed with the measured data within 1.5% for 6 MV FFF beam, and within 1% for 10 MV FFF beam. The calculated planar doses showed good passing rates (> 94%) at 3%/3 mm of gamma indexing criteria. This model expected to be easily applicable to any FFF beams for treatment planning systems because it only required measured PDD, dose profiles and output factors which were easily acquired during conventional beam commissioning process.