The practical implementation of a scatter model for portal imaging at 10 MV

A detailed validation of a physical model for scattered radiation in portal images at 10 MV is presented. The ratio of the signal due to scattered radiation to the signal due to primary radiation (SPR) in an electronic portal image is defined. A simple physical model for SPR on the central axis (SPR*) was presented by Swindell and Evans for 6 MV and validated for field sizes up to 320 cm2. In this paper, the model is extended to 10 MV and validated for field sizes up to 625 cm2. The model is first compared with Monte Carlo modelled data for field areas A from 40 to 320 cm2, scatterer thicknesses d of 5 to 35 cm water and scatterer to detector distances L2 of 40 to 100 cm. The physical model has one free parameter, which is fitted empirically using these data. Second, experimental measurements are performed with A from 40 to 625 cm2, d from 4.6 to 27.4 cm and L2 fixed at 100 cm. The root mean square (rms) difference between the physical model and the Monte Carlo calculations was less than 0.001 for all L2 greater than 60 cm. Agreement between experimentally measured and physically modelled data amounts to a radiological thickness error of at best 0.7 mm in 273.6 mm and at worst 0.4 in 45.6 mm. The model performs equally well at all field sizes tested. This study shows that the Swindell and Evans SPR* model is accurate at 10 MV for L2 greater than 60 cm for all A up to 625 cm2.