Portal film technique charts

A method to produce consistent radiographic density in portal film images is described. The exposure required to produce the desired film density can be calculated on the basis of tissue depth dose data. The required exposure is tabulated as a function of patient thickness, field size, and air gap between the patient and the cassette. The table, referred to as a "technique chart," helps to maintain consistent image quality by minimizing the frequency of over and under exposed films. Alternatively, treatment planning software may be modified to determine the film exposure required for a specific patient. The calculated exposures are accurate only if the image receptor employs a sufficiently thick front metal screen, a requirement consistent with optimum image contrast.