The effect of source to image distance on scattered radiation to the image receptor

In performing experimental or theoretical studies of the effect of source to image distance on dose to the patient, it is conventional to assume that the dose to the image receptor should remain constant at each SID. However, if the proportion of scattered radiation reaching the image receptor depends on the SID, then this assumption will be invalid. In this paper, the effect of SID on the scatter fraction has been determined using a Monte Carlo simulation, using EGSnrcMP. The Monte Carlo code was first benchmarked against three independent experimental studies, and shown to give excellent agreement. In order to obtain agreement with experiment, it was necessary to include Rayleigh scatter and bound Compton effects and to properly simulate multiple scattering. The scatter fraction was then determined for a range of SIDs from 100 to 300 cm, for a range of conditions, including kVp setting, phantom thickness, field size, object to image distance, collimation to entrance surface, centre of phantom and image receptor, and a range of image receptor materials. The results indicate that the scatter fraction is independent of SID within a few percent, except when there is a large object to image distance.