1. A method of commissioning a radiation source for use in radiation therapy comprising the steps of: (a) inputting measured dose data into a data processor, said measured dose data derived from: (i) exposing a phantom to radiation from said source; and (ii) measuring the radiation dose to obtain a measured dose in said phantom resulting from the exposing step, said measured dose measured at a plurality of points within said phantom, at least some of said points being axial points located at positions along a substantially central axis of said radiation source and others of said points being transverse points located at positions along an axis transverse to the central axis, (b) performing a Monte Carlo simulation of said radiation source to determine a simulated dose at said plurality of points; and (c) modeling said radiation source using said simulated dose and said measured dose.