DOSE EQUIVALENT PER UNIT FLUENCE NEAR THE SURFACE OF THE ICRU PHANTOM BY INCLUDING THE SECONDARY ELECTRON TRANSPORT FOR PHOTONS

摘要

The dose equivalent for photon irradiation should be calculated by depositions of the secondary electrons because a photon cannot directly transfer its energy to a medium. The transport of the secondary electron is usually ignored by the kerma approximation under an assumption of secondary electron equilibrium. Ferrari and Pelliccioni calculated conversion coefficients at a depth of 10 mm in the ICRU sphere with consideration of the secondary electron transport. They showed that the conversion coefficients recommended by the ICRP were overestimated due to the kerma approximation. The electron equilibrium becomes more incomplete at depths closer to the surface. This paper considers that overestimation by assuming the kerma approximation exists not only at a depth of 10 mm but also near the surface of a phantom. Distributions of conversion coefficients in the ICRU sphere and the ICRU slab were recalculated by including the secondary electron transport and the kerma approximation. Conver- sion coefficients with no electron transport were overestimated at photon energies higher than 1 and 0.2 MeV at depths of 3 and 0.07 mm, respectively.