The present paper describes the development of a model linking the optical density of a radiographic film to the applied dose. This model takes into account the kind of bremsstrahlung used in radiotherapy, and uses fewer parameters compared with the models developed so far. The measurements showed an influence of the field size on the model parameters. Monte Carlo simulations showed that different field sizes and depths in solid water can change the spectral distributions. The spectrum hardens with decreasing field size and increasing depth in phantom material. This has an influence on the model parameters. Nevertheless, the model can still be used to describe the relation between optical density and applied dose for films that are irradiated with fields of different size. A dose-rate dependence was not observed. The size and form of the fields used in calibration and application can therefore cause a systematic error in verification with film.
