Striking complexity of the photon field in medical devices with heterogeneous density distribution and challenges for industrial irradiators

Despite the emergence of electron beam sterilization, photons are, and most likely will remain the most commonly used agents for radiation sterilization of medical devices. While it is generally accepted that dose gradients have to be carefully addressed in electron beam dose mapping experiments, photon fields are regarded as permissively with respect to performance qualification and aroused little attention for systematic studies. In this paper a computational approach utilizing Monte Carlo simulation is taken to study dose characteristics from cobalt-60 and X-ray sources. The Compton effect as most important interaction mechanism and its impact on dose distribution are studied in detail. The core of the paper deals with attenuation effects by high density objects, particularly steel cylinders, where the dose along the circumference is computed for parallel photons of different energies, colbalt-60 and 7 MeV X-rays. Model results are validated by dedicated experiments in Co-60 and X-ray irradiators. The paper summarizes that dose distributions in photon field show a striking complexity for inhomogeneous materials, especially when photons are collimated in a parallel beam. In general, results suggest that more attention needs to be paid to dose gradients in performance qualification of medical devices with an inhomogeneous mass distribution and generic dosimeter grids inspired from operational qualification are not appropriate. •Gamma irradiation.•X-ray irradiation.•Modelling to support performance qualification.•Photon interaction.•Interpretation of dose gradients in dose maps.