Analytical scatter kernels for portal imaging at 6 MV

X-ray photon scatter kernels for 6 MV electronic portal imaging are investigated using an analytical and a semi-analytical model. The models are tested on homogeneous phantoms for a range of uniform circular fields and scatterer-to-detector air gaps relevant for clinical use. It is found that a fully analytical model based on an exact treatment of photons undergoing a single Compton scatter event and an approximate treatment of second and higher order scatter events, assuming a multiple-scatter source at the center of the scatter volume, is accurate within 1% (i.e., the residual scatter signal is less than 1% of the primary signal) for field sizes up to 100 cm2 and air gaps over 30 cm, but shows significant discrepancies for larger field sizes. Monte Carlo results are presented showing that the effective multiple-scatter source is located toward the exit surface of the scatterer, rather than at its center. A second model is therefore investigated where second and higher-order scattering is instead modeled by fitting an analytical function describing a nonstationary isotropic point-scatter source to Monte Carlo generated data. This second model is shown to be accurate to within 1% for air gaps down to 20 cm, for field sizes up to 900 cm2 and phantom thicknesses up to 50 cm.