For portal films, describe the principles of acquisition, assessment, and consequences of a positioning error being found.
Portal films are the most straightforward of the treatment verification films. A radiographic film is placed on the distant side of the patient to the beam. Two small amounts of radiation (typically 2 – 3 MU) are given off by the machine, one with collimation ‘on’ and one with an open beam. This double exposure allows the position of the collimation relative to the patient’s anatomy to be checked.
Portal images suffer from reduced contrast due to reliance of Compton interactions on electron density rather than atomic number.
Assessment of Portal films
Unlike newer technologies, portal films must be developed before being viewed. This makes them unsuitable for treatment verification prior to delivery of treatment. Instead, portal films were used to ensure that treatments were being delivered accurately after the treatment had been delivered; if an error existed then corrections would be made for future treatment.