Electron influence on the reconstruction of a linac 6 MeV photon spectra by unfolding methods

Megavoltage photon sources are normally used for radiotherapy treatments. For these equipments an accurate knowledge of their spectral distribution is essential for accurate dose calculations planning. There are several ways to determine the spectrum of a clinical photon beam: direct measurement, electron source modelling or reconstruction from experimental measures. In this paper we focus on the latter type of spectral reconstruction methods which can be used to provide an independent confirmation of source models for a given machine without any prior knowledge of the spectral distribution. This technique involves measuring the depth dose curve in a water phantom and applying an unfolding method using Monte Carlo simulated depth dose curves for consecutive mono-energetic beams. We illustrate this theory to calculate a 6 MeV photon beam from an Elekta Precise radiotherapy unit using the gradient of depth dose measurements in a cube-shaped water tank.