Derivation of electron and photon energy spectra from electron beamcentral axis depth dose curves

Abstract

A method for deriving the electron and photon energy spectra from electronbeam central axis percentage depth dose (PDD) curves has been investigated. ThePDD curves of 6, 12 and 20 MeV electron beams obtained from the Monte Carlofull phase space simulations of the Varian linear accelerator treatment headhave been used to test the method. We have employed a `random creep' algorithmto determine the energy spectra of electrons and photons in a clinicalelectron beam. The fitted electron and photon energy spectra have beencompared with the corresponding spectra obtained from the Monte Carlo fullphase space simulations. Our fitted energy spectra are in good agreement withthe Monte Carlo simulated spectra in terms of peak location, peak width,amplitude and smoothness of the spectrum. In addition, the derived depth dosecurves of head-generated photons agree well in both shape and amplitude withthose calculated using the full phase space data. The central axis depth dosecurves and dose profiles at various depths have been compared using anautomated electron beam commissioning procedure. The comparison hasdemonstrated that our method is capable of deriving the energy spectra for theVarian accelerator electron beams investigated. We have implemented thismethod in the electron beam commissioning procedure for Monte Carlo electronbeam dose calculations.