[en] For high-energy photon beams, the main interaction process is by Compton scattering, thereby limiting the quality of the obtained portal image (insensitivity to the atomic number of the irradiated tissues). One of the main factors which contributes to the image degradation for high photon energies, are the Compton scattered electrons from the patient, producing a high blurring and a low contrast in the film. In order to significantly reduce the influence of the scattered electrons from the patient, a new cassette was designed, having a front copper screen with 2 gr/sq. cm thickness, coupled with fast films and a rear Cu screen .4 gr/sq. cm thick. A comparison between the designed cassette and a commercial one (RMI-ALDRICH Model 435- 0.4 gr/sq. cm front and rear Cu screens), for our 10 MV x-ray beam (Clinac 12/10 linear accelerator - Varian) is presented, using two types of fast films. An analytical model was developed, which takes into account the presence of the cassette front copper screen. The doses at the film position were then calculated for the SSD and SAD techniques, as function of field size, patients thickness and air-gap between the patient and the film. Dose measurement at the film position compare well with the calculated values. Portal film charts were constructed to deliver the optimum dose on the film. (author). 2 figs, 3 tabs