title: Technique for verifying treatment fields using portal images with diagnostic quality.
abstract: The image quality of portal films for megavoltage photon beams, when using the double-exposure technique, is poor compared to diagnostic quality, X ray images. A technique is described to record on a single film a megavoltage portal image superimposed upon a diagnostic X ray image, which provides the radiotherapist with "diagnostic quality" portal images. The technique uses a commercially available X ray tube mounted on the head of a 60Co unit. The alignment procedure, which uses a leveling device to ensure that the X ray focal spot and 60Co source are at the same location for each exposure, is confirmed by registering on film the image of an alignment marker. An evaluation of film-screen combination showed therapy verification film in a rare earth intensifying screen cassette to be best suited for this technique. The relationship between off-axis dose and the penumbral region of the portal image has been evaluated and should be useful in the interpretation of portal verification film relative to the treatment volume.
mesh_terms: D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D006801:Humans; D011786:Quality Control; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000379:methods*; D014965:X-Rays
chemical_list: D003037:Cobalt Radioisotopes