AbstractID: 7863 Title: Modeling of Radiotherapy X-ray Sources for Monte Carlo Treatment Planning We present a method that can accurately calculate the energy contents of radiotherapy beams. The method can easily be clinically implemented and the same type of information provided by a Monte Carlo simulation can be obtained by a combination of an analytical and a 2-measurement experimental technique. A computer program was written to perform numerical electron transport in the target material employing the Phase Space Time Evolution model. Bremsstrahlung production is calculated using the Schiff cross section. Attenuation of this primary fluence by the target and filter is calculated through raytracing. A correction factor to account for head scatter is derived by comparing the energy deposited in two films irradiated by the beam in question to a theoretical prediction using our model. This is in contrast with other experimental techniques that require up to 25 measurements. The computer tool includes a Monte Carlogenerated dosimetry database that is used to calculate relevant spectrum-related quantities, such as PDD and TPR, for an arbitrary photon spectrum. The source model produced is a 2-D grid of pixels at the plane of measurement containing the spectrum as well as a weight parameter with respect to the pixel at the central axis. This model has been merged into the MCNPX code.