Separation of primary and scatter components of measured photon beam data

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Abstract
When measured absorbed doses for photon beams are to be separated into their primary and scatter components, the extrapolation method proposed gives consistent results. Information about the head-scatter factor H(s) is required to implement the extrapolation and can be determined by supplementary measurements in air. In this study, H(s) was derived from normalised peak scatter factor for the X-ray beams and already eliminated in the 60Co tissue-air ratio data. Information about the attenuation of the beam is also needed and can be provided by narrow-beam measurements in air. The procedure can be applied to tissue-maximum ratio data, as was done in this study, but also to other representations of doses on the central axis, after transformation to TMR. All doses used for the extrapolation must be measured under electron equilibrium conditions. It simplifies the procedure if all tabulated doses are normalised to one combination of depth and field size, also sufficient for electron equilibrium.