Photon energy absorption and exposure buildup factors for deep penetration in human tissues

Using photons in therapeutic and diagnostic medicine requires accurate computation of their attenuation coefficients in human tissues. The buildup factor, a multiplicative coefficient quantifying the ratio of scattered to primary photons, measures the degree of violation of the Beer–Lambert law. In this study, the gamma-ray isotropic point source buildup factors, specifically, the energy absorption buildup factor (EABF) and exposure buildup factor, are estimated. The computational methods used include the geometric progression fitting method and simulation using the Geant4 (version 10.4) Monte Carlo simulation toolkit. The buildup factors of 30 human tissues were evaluated in an energy range of 0.015–15 MeV for penetration depths up to 100 mean free paths (mfp). At all penetration depths, it was observed that the EABF seems to be independent of the mfp at a photon energy of 1.5 MeV and also independent of the equivalent atomic number (\(Z_{\text{eq}}\)) in the photon energy range of 1.5–15 MeV. However, the buildup factors were inversely proportional to \(Z_{\text{eq}}\) for energies below 1.5 MeV. Moreover, the Geant4 simulations of the EABF of water were in agreement with the available standard data. (The deviations were less than 5%.) The buildup factors evaluated in the present study could be useful for controlling human exposure to radiation.