Analytical scatter kernels for portal imaging at 6 MV

X-ray photon scatter kernels for 6 MV electronic portal imaging are investi gated using an analytical and a semi-analytical model. The models are teste d on homogeneous phantoms for a range of uniform circular fields and scatte rer-to-detector air gaps relevant for clinical use. It is found that a full y analytical model based on an exact treatment of photons undergoing a sing le Compton scatter event and an approximate treatment of second and higher order scatter events, assuming a multiple-scatter source at the center of t he 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 cm(2) and air gaps over 30 cm, but shows significant discrepancies for larger field sizes. Monte Carlo results are presented showing that the effective multipl e-scatter source is located toward the exit surface of the scatterer, rathe r than at its center. A second model is therefore investigated where second and higher-order scattering is instead modeled by fitting an analytical fu nction describing a nonstationary isotropic point-scatter source to Monte C arlo generated data. This second model is shown to be accurate to within 1% for air gaps down to 20 cm, for held sizes up to 900 cm(2) and phantom thi cknesses up to 50 cm. (C) 2001 American Association of Physicists in Medici ne.