Abstract
Analytical and numerical models for reconstructing clinical bremsstrahlung spectra with maximum photon energies in the range 4 to 30 MeV from indirect measurements such as transmission or depth-dose curves are investigated and compared. The iterative and Laplace transform models of Huang et al and Archer and Wagner are extended to the energy region of interest and a further two models based on thin and thick target bremsstrahlung theory developed. Calculated transmission curves for a set of 21 simulated and measured bremsstrahlung spectra covering a broad range of filtration conditions are used to assess each model's ability to represent photon spectra. A model based on the Schiff expression for forward-directed thin-target bremsstrahlung differential in photon energy with added inherent filtration is shown to provide a promising method for reliably reconstructing megavoltage spectra in terms of 3 parameters. For this model, input data was reproduced to within 0.1 percent, this being the same order of magnitude as the accuracy of its generation and within expected uncertainties for measured input data. Inclusion of a fourth parameter in the model is shown to allow the effective maximum photon energy present in the spectrum to be derived. The considerations involved in the practical use of reconstruction models to derive clinical photon spectra from measured transmission and depth-dose data and likely applications of these spectra are discussed.