Monte Carlo studies on photon interactions in radiobiological experiments

X-ray or γ-ray photons are widely used to study radiobiological effects of ionizing radiations. Photons are indirectly ionizing radiations so they need to set in motion electrons to perform the ionizations. When the photon dose decreases to below a certain limit, the number of electrons set in motion will become so small that not all cells in the “exposed” cell population will get at least one electron hit. When some cells in the cell population are not irradiated, there will be rescue effect between the irradiated and non-irradiated cells, and the overall radiobiological effect observed for the cell population will be different. In the present paper, the mechanisms underlying photon interactions in radiobiological experiments were studied using our developed NRUphoton computer code, which was benchmarked against the MCNP5 code by comparing the photon dose delivered to the cell layer underneath the water medium. The following results were obtained: (1) Interaction fractions decreased in the order: ¹⁶O > ¹²C > ¹⁴N > ¹H. Bulges in the interaction fractions (versus water medium thickness) were observed, which reflected the changes in the energies of the propagating photons as a result of the water medium thickness as well as the energy-dependent photon interaction cross-sections. (2) Photoelectric interaction and incoherent scattering dominated for lower-energy (10 keV) and higher-energy (100 keV and 1 MeV) incident photons. (3) The fractions of electron ejection from different nuclei were mainly governed by the photoelectric effect cross-sections, and the fractions from the 1s subshell were the largest. (4) The penetration fractions in general decreased with increasing medium thickness, and increased with increasing incident photon energy, the latter being explained by the corresponding reduction in interaction cross-sections. (5) The area under the angular distributions of photons exiting medium layer and then interacting with cell layer decreased with increasing incident photon energy. (6) The number of cells suffering at least one electron hit increased with the administrated dose. For larger incident photon energies, the numbers of cells suffering at least one electron hit became smaller, which was attributed to the reduction in the photon interaction cross-section. These results highlighted the importance of the administered dose in radiobiological experiments. In particular, the threshold administrated doses at which all cells in the array suffered at least one electron hit might provide thoughts on the intriguing observation that radiation-induced cancers can be statistically detected only above the threshold value of ~100 mSv, and thus on controversies over the linear no-threshold model.