An accurate determination of extended source distribution function for the dual-source model of photon beams

A dual-source model for photon beam in-air output factors is derived for Phillips SL20 linear accelerator equipped with MLC. The extended source is assumed to be located at the bottom of the flattening filter. No particular mathematical function for the extended source radial distribution, f(r) is assumed a priori. Instead, it assumed that f(r) can be represented by a power series whose coefficients are obtained from experimentally determined output factors for square fields. The radial function thus obtained is then used to predict the output factors for arbitrary field shapes and sizes. For rectangular fields of different aspect ratios and offsets, the prediction of the dual-source model agrees with the experimental value to within 0.25%. The same model is used to predict the output factors for irregular fields by using superposition of rectangular apertures defined by pairs of MLC leaves. The accuracy obtained from this dual-source model can be significant for MU/field calculations in IMRT delivery, where a large number of fields are used