A clinical comparison of different film systems for radiotherapy portal imaging

Portal films are an important tool for verification of the shaping and positioning of external radiation fields to the target volume in radiotherapy. One limitation of port films is their inherent lack of contrast, which is due to the low attenuation of the exposing megavoltage radiation by the tissues being imaged. Recently, Kodak introduced a new portal film-cassette system, Kodak EC-L, with much improved contrast compared with conventional film. The aim of this study was to determine if the enhanced contrast of the Kodak EC-L system actually provides better clinical results. To simulate clinical use, port films were taken using an anthropomorphic phantom that was artificially shifted and/or rotated by a predetermined distance. Identical images were taken using a conventional port film system (AGFA-Gevaert Curix MR4 in lead-lined cassette) and the Kodak EC-L system. Twelve different operators (6 physicists and 6 radiation therapists) were asked to diagnose the problem from a total of 20 port films (10 per treatment site), allowing for direct comparison of the 2 types of films. While the diagnosis of the field displacement improved using the Kodak film, it did not speed-up the decision-making process. It was also found that experienced operators were more accurate at evaluating the films. The results indicate that, for the situations studied (head and neck, pelvis), the Kodak system exhibits better contrast and leads to improved decision making.