The validity of using radiographic film for radiotherapy dosimetry

Radiographic film is routinely used to obtain dosimetric information about therapy treatment beams. One source of inaccuracy is the variability of the chemical processing of the radiographic film. Several processors, film types and two sources (light and x-rays) were used to investigate the reproducibility of film processing for valid film dosimetry. Particular attention is paid to films commonly used in radiation therapy. The results suggest that for this series of film, given the same exposure, the variability in processing may result in an error of +/- 3.3% in optical density, which would lead to an error of +/- 4.4% in indicated dose. This level of inaccuracy is typical for both point and relative dose estimates. These results indicate that film should not be used as a point or relative dosimeter, unless the combination of film type, exposure, processing and reading have been specifically validated.