DNA damage estimation of ionizing radiation in the large field of 6 MV x-ray LINAC

Recent studies suggest that relative biological damage (RBD) may change from in to out off field regions for 6 MV photon beams. In this study RBD was calculated in and out of field of 30x30 cm2 and 40x40 cm2 6 MV clinical photon beams including low energy slowing down electrons in track length estimated method. Varian 2100C/D linear accelerator was simulated using MCNPX code. Electron and photon spectra at energies higher than 2 keV were collected in a water phantom at different depths and off-axis points. New extrapolation method was used to estimate the electron spectra at energies lower than 2 keV. These spectra were used as an input to MCDS code to calculate the RBD of induced damage in DNA. There was an observable difference in the energy spectra for photons and electrons for points in the primary radiation field and those points out-of-field. RBD increases up to 10% for 10 MV photons in the out-of-field region. This work supports the hypothesis that in megavoltage treatments at the out-of-field, radiation quality can very enough to have an impact on RBD per unit dose.