RECONSTRUCTION OF MEGAVOLTAGE PHOTON SPECTRA BY ATTENUATION ANALYSIS

A simple analytical model for reconstructing photon spectra from the i ndirect measurement of transmission or depth-dose curves is presented, based on the Schiff expression for forward-directed bremsstrahlung di fferential in photon energy with added inherent filtration. Calculated transmission curves for 21 simulated and measured bremsstrahlung spec tra with maximum photon energies in the range 4 to 30 MeV covering a b road range of filtration conditions were used to assess the model's ab ility to represent megavoltage spectra. Input data were reproduced to within 0.1 per cent in all cases, which is the same order of magnitude as the uncertainty involved in its calculation. Inclusion of a fourth parameter in the model is shown to allow the effective maximum photon energy present in the spectrum to be derived to an accuracy of approx imately 3 per cent.