Lung compensator design using an electronic portal imaging device

Using a liquid filled electronic portal imaging device (EPID) installed on a linear accelerator and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the pixel values of the imaging detector and the corresponding equivalent thickness of the overlying phantom material. Results for 6 and 10 MV photons show that the relationship depends on the so-called input/output characteristics of the imaging device for a particular photon energy. For a chest irradiation, an EPID image obtained under treatment geometry provides the pixel value information that is used to calculate the tissue deficit over the lung region. The compensators are made of lead whose thickness is calculated from the established empirical relationship to replace the tissue deficit over lungs. The effectiveness of the method is demonstrated with thermoluminescent dosimetry (TLD) for 6 and 10 MV beams. With compensators in place, the dose uniformity was found to be within +/- 5%.