“…The first exposure encompasses only the actual treatment field while the second exposure is made with an enlarged field with all beam shaping blocks removed, thus making it easier to recognize anatomical landmarks. Because the quality of high energy port films is markedly less than that of diagnostic or simulation films, there has been a steady effort to improve the imaging characteristics of port films (Droege, 1977;Galkin & Wu, 1978;Hammoudah & Henschke, 1977, Reinstein & Orton, 1979and Strubler, Galkin & Suntharalingam, 1977). Although portal films render poorer quality images compared to other radiographs, they are considered to contain sufficient information for determining accuracy of beam placement (Stanton & Stinson, 1992).…”