Accuracy requirements of the primary x‐ray spectrum in dose calculations using FFT convolution techniques

Abstract

The photon beam dose calculation methods using fast Fourier transform (FFT) convolution require that the primary beam energy spectrum is accurately known; however, a quantitative assessment of the required accuracy is not yet provided. In this study, the sensitivity of various parameters such as central axis depth dose and dose under attenuators to the incident photon spectrum was simulated and evaluated for 6- and 18-MV photon beams. The central axis depth-dose data using the modified spectrum were compared with those calculated using the original spectrum. Maximum errors on the central axis between these two sets of depth-dose data were found by changing the weighting of each energy bin of the primary spectrum. The maximum change in percent depth dose is proportional to the change in average incident energy relating to the quality of the x ray. To keep the maximum depth-dose error to less than 1%, the relative change in average energy brought about by changes in single spectral bins should be less than 2%. The accuracy required for each energy bin varies dramatically, dependent on its deviation from the original average energy and its relative weighting. For example, for the 6-MV x-ray spectrum, the energy bins centered around 0.76 MeV (first bin) and 6.8 MeV (fifth bin) need to have accuracies of 5% and 40%, respectively, to obtain a 1% accuracy in percent depth dose.