Physical model for photon beams used on the General Electric Target radiotherapy treatment planning system

The tabulated measured data model for external photon beam planning used on the General Electric Target system is very powerful, allowing fast and accurate calculations and is extended to incorporate complex planning techniques. This approach is ideally suited for the incorporation of correction factors, defined at any point as the ratio of the dose in an inhomogeneous phantom to the dose in a homogeneous phantom. The correction factor can be determined using techniques such as a generalized Batho power law or the equivalent tissue air ratio method, and the dose in an homogeneous phantom is taken from the measured data tables. The use of tabulated data gives the benefits of very fast calculation times and, when required, the accuracy of sophisticated scatter corrections. The basic model and methods of correcting for inhomogeneities are described. The extension of the two-dimensional model to a three-dimensional model for non-coplanar treatment planning retains the basic principles of the 2D model.