Technical and clinical evaluation of an improved-contrast screen-film combination for radiation therapy portal localization imaging

Purpose/Objective: A problem with conventional radiation therapy portal images is low image contrast, due in part to the low attenuation of the exposing radiation by the anatomical parts being imaged and the contrast capabilities of the film or screen-film combination. The purpose of this study was to design, develop and clinically evaluate a new screen-film combination for portal localization imaging which provides significantly higher contrast and therefore improved image quality. Materials and Methods: Comparison phantom and clinical images were made at two radiation oncology facilities with the new prototype screen-film combination and a commercial screen-film combination currently used for portal localization imaging. All images were made with linear accelerators at 6MV. Sensitometric data was also obtained. The prototype combination features a 1.0mm copper front screen plus front and back gadolinium oxysulfide fluorescent intensifying screens and a very-slow-speed film having inherently high contrast. The film emulsion layers are coated on a 7 mil Estar base which allows processing in a conventional rapid process film processor. For this combination, the film is exposed primarily by light from the intensifying screens. The current, commercially available screen-film combination was a Kodak X-Omatic L Radiation Therapy Cassette with a 1.0mm copper front screen and a 0.25mm lead back screen and Kodak X-Omat RP film in ready pack envelope. With this combination, the film emulsion is exposed by electrons generated in the metal screens. All films were processed in a Kodak M35A X-Omat processor. Radiation oncologists reviewed the phantom and clinical images. Results: Sensitometric data indicate that the film contrast (average gradient) of the new prototype combination is approximately 4 times higher than the conventional commercially available combination. Phantom and clinical comparisons at St. Mary Cancer Center, Langhorne PA. and the Daisy Marquis Jones Radiation Oncology Center, Highland Hospital, Rochester, NY. indicate the monitor units setting for both methods are similar. Initial clinical results indicate portal images made with the new combination are superior to those obtained with the conventional combination. The new portal images have significantly higher contrast, show clearer definition of structures, and are much easier to read. Conclusion: The prototype screen-film combination provided portal localization images with improved image quality due to significantly higher film contrast compared to the conventional commercially available combination. The prototype combination provided radiation oncologists with more detail, especially in difficult cases such as the pelvis and oblique lung. In addition the images were much easier to read and interpret. Technical factors (monitor units) were similar for both methods. The prototype film can be processed in a conventional rapid process film processor