Modelling Ultrasoft X-ray Effects on DNA

Abstract

Between 100 eV and 3 keV, X-rays interact in biological media almost exclusively by photoelectric effect, producing low-energy electrons (photoelectron and Auger electrons), depositing their energy in volumes comparable with DNA dimensions. These electrons are emitted in abundance as secondary particles in all low linear energy transfer (LET) radiations and are supposed to play an important role in the clustering of damages. It has been shown both from experiments [1] and theoretical calculations [2] that these low-energy, track end electrons contribute significantly to critical biological damage. However, recent cell studies with the use of synchrotron-produced ultrasoft X-rays contradict these conclusions with no significant differences in survival results between 273, 860 eV ultrasoft X-rays and 250 kVp X-rays [3]. In this energy range, photon cross-sections present singularities due to ionisation thresholds of Carbon (C), Nitrogen (N), Oxygen (0) and Phosphorus (P) atoms. Enhancement of damage has already been observed from X-rays experiments above the Carbon K (∼ 290 eV) [4] and the Phosphorus K (∼ 2130 eV) [5] thresholds. Monte Carlo track structure calculations were used to study the influence of all DNA atoms ionisation thresholds on strand breakage. The code cpa100 was used to follow the histories of photons and electrons ejected through a hydrated DNA model and the ensuing chemistry was taking into account up to 1 nanosecond. Induced single and double strand breaks were analysed as a function of energy and compared with studies highlighting the effect of track structure on DNA damage [6,7].