Exit dose measurements by portal film dosimetry.

Portal in vivo dosimetry is a very attractive tool for patient dose measurements because of the large amount of information that portal film systems can easily collect, once positioned at the exit surface of the patient. The first step in the verification of the reliability of portal films as in vivo dosimeters is the evaluation of the agreement between exit patient dose profiles and optical density profiles measured on the portal film. We checked the possibilities for exit dose measurements of a commercial portal film system (Film Kodak X-Omat V and Localization Kodak Cassette) verifying the agreement between relative exit doses (measured by ionization chamber and film dosimetry, calculated by our treatment planning system (Cadplan Dosetek)) and relative optical densities on portal films in cubic homogeneous and inhomogeneous, cylindrical and humanoid phantoms. In particular, a good agreement (mean difference in absolute value: 2%) between optical densities and calculated exit doses for the Rando phantom were found, once the optical densities values are corrected for an inverse square correction factor, taking into account the variation of the profile of the phantom.