Variation of sensitometric curves of radiographic films in high energy photon beams

Film dosimetry is an important tool for the verification of irradiation tec hniques. The shape of the sensitometric curve depends on the type of film a s well as on the irradiation and processing conditions. Existing data conce rning the influence of irradiation geometry on the sensitometric curve are conflicting. In particular the variation of optical density, OD, with field size and depth in a phantom shows large differences in magnitude between v arious authors. This variation, as well as the effect of beam energy and fi lm plane orientation on OD, was therefore investigated for two types of fil m, Kodak X-Omat V and Agfa Structurix D2. Films were positioned in a solid phantom, either perpendicular or (almost) parallel to the beam axis, and ir radiated to different dose levels using various photon beams (Co-60, 6 MV, 15 MV, 18 MV, 45 MV). It was found that the sensitometric curves of the Kod ak film derived at different depths are almost identical for the four x-ray beams. For the Kodak film the differences in OD with depth are less than 2 %, except for the Co-60 beam, where the difference is about 4% at 10 cm dep th for a 15 cm X 15 cm field. The slope of the sensitometric curve of the A gfa film is somewhat more dependent on photon beam energy, depth and field size. The sensitometric curves of both types of film are almost independent of the film plane orientation, except for shallow depths. For Co-60 and fo r the same dose, the Kodak and Agfa films gave at dose maximum an OD lower by 4% and 6%, respectively, fur the parallel compared to the perpendicular geometry. Good dosimetric results can be obtained if films from the same ba tch are irradiated with small to moderate field sizes (up to about 15 cm x 15 cm), at moderate depths (up to about 15 cm), using a single calibration curve, e.g., fur a 10 cm x 10 cm field. (C) 2001 American Association of Ph ysicists in Medicine.