Relative Role of Physical Mechanisms on Complex Biodamage Induced by Carbon Irradiation

The effective use of swift ion beams in cancer treatment (known as hadrontherapy) as well as appropriate protection in manned space missions rely on the accurate understanding of the energy delivery to cells that damages their genetic information. The key ingredient characterizing the response of a medium to the perturbation induced by charged particles is its electronic excitation spectrum. By using linear-response time-dependent density functional theory, we obtained the energy and momentum transfer excitation spectrum (the energy-loss function, ELF) of liquid water (the main constituent of biological tissues), which was in excellent agreement with experimental data. The inelastic scattering cross sections obtained from this ELF, together with the elastic scattering cross sections derived by considering the condensed phase nature of the medium, were used to perform accurate Monte Carlo simulations of the energy deposited by swift carbon ions in liquid water and carried away by the generated secondary electrons, producing inelastic events such as ionization, excitation, and dissociative electron attachment (DEA). The latter are strongly correlated with cellular death, which is scored in sensitive volumes with the size of two DNA convolutions. The sizes of the clusters of damaging events for a wide range of carbon-ion energies, from those relevant to hadrontherapy up to those for cosmic radiation, predict with unprecedented statistical accuracy the nature and relative magnitude of the main inelastic processes contributing to radiation biodamage, confirming that ionization accounts for the vast majority of complex damage. DEA, typically regarded as a very relevant biodamage mechanism, surprisingly plays a minor role in carbon-ion induced clusters of harmful events.