Direct measurement and analytical modeling of scatter in portal imaging

In this study a direct measurement of scatter in portal imaging for various air Saps and scatterer thicknesses at a beam energy of 6 MV is presented. The experimental data are compared with results from a Monte Carlo (MC) sca tter model. In the regime where the air gap is larger than 9.3 cm the MC an d the experiment agree. Based on this MC model an analytical model is devel oped, which takes all important interaction processes into account. It comp rises a rigorous treatment of first order scattering and an estimation of p hotons scattered more than once within the phantom. This estimation is base d on the assumption that higher order scattering can be considered as isotr opically distributed around a certain scatter origin located in the midplan e of the phantom. It is found that relative deviations between the MC model and the analytical model are of 2% to 3% in regions where scattering is ve ry large. (C) 2000 American Association of Physicists in Medicine. [S0094-2 405(00)01203-7].