Lung compensation in total body irradiation: a radiographic method

Using megavoltage radiography and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the optical film density and the corresponding equivalent thickness of overlying phantom material. Results for Co-60, 4-MV, and 10-MV photons show that the relationship depends on the sensitometric properties of the radiographic film and the photon beam quality. For an actual patient undergoing total body irradiation (TBI), a chest radiograph in treatment geometry provides the optical density information that is used to calculate the tissue deficit in the lung region. The compensators are made of lead whose thickness is chosen to replace the tissue deficit over the lung region. The validity of the method is established both by comparing its results to that obtained from multiple-slice computed tomography (CT) data for 4- and 10-MV photons, and by in-phantom thermoluminescent dosimetry (TLD) for Co-60, 4-MV and 10-MV beams.