Displacement corrections used in absorbed dose determination.

When an (exposure) calibrated ionization chamber is used for the determination of absorbed dose from a photon beam, the reading of the instrument must be multiplied by a number of factors, one of which is an attenuation correction for phantom material displaced by the chamber. the magnitude of this correction must depend on the size and shape of the ionization chamber as well as the energy of the radiation beam. For cobalt-60 radiation, a single number, 0.985, has generally been used. Recent measurements, however, and "first scatter" calculations, of kerma suggest that a more appropriate value for a Farmer-type chamber used in a water phantom would be 0.975. Such a change is small but would be important when dose calculations based on "in phantom" measurements are compared to calculations that are based on in air measurements. Values for the attenuation factor for other beam energies have not been generally available. We have carried out "first scatter' calculations for a rather wide range of energies and spectra. Measurements in 60Co beams and in a high-energy (25 MV) photon beam support the calculations. A set of proposed displacement correction factors is presented.