Perturbation of radiotherapy beams by radiographic film: measurements and Monte Carlo simulations

Radiographic film is an established practical tool used in the measurement of the dose distribution for radiotherapy purposes. The accuracy and reprod ucibility of film optical density as an indicator of dose has been associat ed with several factors including photon energy, processing conditions and film plane orientation. Few studies have investigated the factors causing v ariability in film dosimetry, due to the difficulty of separating the indiv idual contributions. The effect that a sheet of radiographic film in a wate r phantom has on its response to a 6 MV photon and a cobalt-60 teletherapy beam, when orientated perpendicular and parallel to the beam central axis, is reported. Monte Carlo generated spectra were used to calculate collision kerma (K-coll) for water and film elements. Measured and calculated result s indicate a potential over-response at 25 cm depth of the order of 14 +/- 2.4% and 18 +/- 6.0% respectively for 6 MV photons and 15 +/- 3.4% and 32 /- 4.5% respectively for a cobalt beam. For film exposed parallel as compared to perpendicular to the central axis of the beam, the calculated results suggest an explanation in terms of the predominantly forward directed secondary electrons for the measured differe nce in film response at depth. It is proposed that the difference in respon se of the parallel as compared to perpendicular exposed film be due to the predominantly 'upstream' photon interactions giving rise to energy depositi on in film. The simulations indicate that the variation with depth of relat ive energy imparted in film and water elements correlates with the observed variation in film response with depth.