Are port films reliable for in vivo exit dose measurements?

The possibility of using conventional port films as an in vivo method for obtaining relative exit dose distribution in patients is evaluated. Kodak "Verification" films in "Localization" cassettes are positioned in the usual clinical conditions behind an homogeneous polystyrene phantom as well as behind a phantom containing air, wood and aluminium inhomogeneities. Taking beam divergency into account the densitometric profiles are projected back to the exit side of the phantom. They are compared to the profiles obtained with an ionization chamber used under full backscatter conditions. The agreement between the off-axis ratios determined with either method are mostly better than 2% and never exceed 5%. These phantom measurements are completed by a comparison between off-axis ratios determined on a port film for a head and neck patient and those obtained by diode dosimetry applied on the patient at the exit side of the beam. A similar agreement as between film and ion chamber on the phantoms is obtained. These encouraging results illustrate the possibilities of using conventional port films for in vivo dosimetry.