EVALUATION OF A CASSETTE-SCREEN-FILM COMBINATION FOR RADIATION-THERAPY PORTAL LOCALIZATION IMAGING WITH IMPROVED CONTRAST

A traditional limitation with radiation therapy portal images is low i mage contrast, due in part to the low attenuation of the exposing radi ation by the tissues being imaged, and the contrast capabilities of th e image receptor. We have developed, and have clinically evaluated, a cassette-screen-film combination for portal localization imaging, whic h features a copper front screen plus Gd2O2S:Tb fluorescent screens an d a slow-speed, fine grain, film emulsion with inherently high contras t coated on both sides of a 7 mil Estar base. The film can be processe d in a conventional rapid-process film processor. Sensitometric data i ndicate that the film contrast (average gradient) for the new combinat ion is approximately 3.5 times higher than the conventional portal loc alization systems in current use. The new combination has been clinica lly compared with two conventional systems. The required monitor unit settings were found to be similar. Initial clinical results indicate p ortal images made with the new combination are superior to those obtai ned with the conventional combinations. The images have much higher co ntrast, subjective impressions of lower noise, show clearer definition of structures, and are much easier to read. (C) 1997 American Associa tion of Physicists in Medicine.