There is currently a push to move away from gamma sterilization for many reasons, such as disruption in the supply chain due to a limited supply of cobalt-60. A promising alternative is X-ray sterilization, which is appealing due to its similarity to gamma sterilization, in that it uses high energy photons.

However, there are fundamental differences between gamma and X-ray sterilization that have significant impact on the resulting dose distributions. In particular, the cobalt-60 used in gamma sterilization produces essentially monoenergetic photons, whereas X-ray sterilization uses a broad energy spectrum produced by a linear accelerator.

In this work we studied the dose delivery mechanisms in gamma/X-ray sterilization. To do so, we used Monte Carlo simulations based on Geant4 which allowed us to turn on/off the interactions that are relevant during sterilization. We simulated the 3D dose distribution delivered to a cube of aluminum by a pencil photon beam of either 1.25 MeV monoenergetic photons (gamma sterilization) or a 7.5 MeV broad spectrum (X-ray sterilization).

The results reveal significant differences in the dominant dose mechanisms. Notably, pair production plays a much bigger role in X-ray sterilization due to the presence of higher energy photons. Similarly, the broad X-ray spectrum also contains more lower energy content, which results in elastic Rayleigh scattering contributing significantly to shaping the dose distribution.

In conclusion, although the dose distributions produced by gamma and X-ray sterilization are similar, there are noteworthy differences in the dose delivery mechanisms due to distinction in the energy content in the spectra. Simulations offer unique insight into the underlying photon-matter interactions.