Perturbation of 6MV photons by radiographic film: measurements and Monte Carlo simulations

Full text: Radiographic film is an established practical tool used in the measurement of dose distributions for radiotherapy purposes. The accuracy and reproducibility of film optical density as an indicator of dose, has been associated with several factors including photon energy, processing conditions and film plane orientation. Few studies have investigated the factors causing variability in film dosimetry, due to the difficulty of separating the individual contributions. The effect that a sheet of radiographic film in a water phantom has on the dose deposited by a 6MV photon beam when orientated perpendicular and parallel to the beam central axis is reported. These measurements are compared with Monte Carlo simulations. The Monte Carlo simulations were performed on a Silicon Graphics Indigo R4000, using the EGS4 code. Measurements were taken using a Varian Clinac 600 C 6MV beam incident on a solid water phantom, with a sheet of Kodak X Omat-V film. The effect of thickness of the film and its orientation with respect to the beam were investigated. Monte Carlo generated spectra were used to calculate collision kerma (KColl) for water and film elements at each depth. Results indicate an expected over response of 3.5% at 30 cm deep compared to 1.5 cm deep. The calculated results suggest an explanation in terms of the predominantly forward directed secondary electrons, for the measured difference in film response at depth for film parallel as compared to perpendicular to the central axis of the beam