Development of a novel, high contrast, cassette/film/screen system for radiation therapy portal localization imaging

Radiation therapy portal images have traditionally exhibited poor discrimination of areas of interest, due to low subject contrast of anatomical parts being imaged at megavoltage energies, and the contrast capabilities of the image receptors, As a result of this low contrast, positioning of the radiation beam and placement of shielding blocks can be difficult. A novel, high-contrast cassette/film/screen system has been developed and clinically evaluated for portal imaging. This system features a copper front screen, a gadolinium oxysulfide, terbium activated (GdO2S:Tb) intensifying screens and a slow speed film with inherently high contrast. Very high film contrast is achieved by narrow grain size distribution and metal ion doping of the silver halide microcrystals. This high-contrast film is exposed by light from the intensifying screen, further increasing contrast. Sensitometric data indicates this new system to have 3.5X greater contrast than conventional portal localization imaging systems at comparable monitor units (dose). Initial clinical evaluation indicates this new system to yield significantly superior images showing clearer definition of structures and was much easier to read and interpret.