Particle track structure and its correlation with radiobiological endpoint

Clustered DNA Damage and its Complexity: Tracking the History

The concept of radiation-induced clustered damage in DNA has grown over the past several decades to become a topic of considerable interest across the scientific disciplines involved in studies of the biological effects of ionizing radiation. This paper, prepared for the 70th anniversary issue of Radiation Research, traces historical development of the three main threads of physics, chemistry, and biochemical/cellular responses that led to the hypothesis and demonstration that a key component of the biological effectiveness of ionizing radiation is its characteristic of producing clustered DNA damage of varying complexities. The physics thread has roots that started as early as the 1920s, grew to identify critical nanometre-scale clusterings of ionizations relevant to biological effectiveness, and then, by the turn of the century, had produced an extensive array of quantitative predictions on the complexity of clustered DNA damage from different radiations. Monte Carlo track structure simulation techniques played a key role through these developments, and they are now incorporated into many recent and ongoing studies modelling the effects of radiation. The chemistry thread was seeded by water-radiolysis descriptions of events in water as radical-containing "spurs," demonstration of the important role of the hydroxyl radical in radiation-inactivation of cells and the difficulty of protection by radical scavengers. This led to the concept and description of locally multiply damaged sites (LMDS) for DNA double-strand breaks and other combinations of DNA base damage and strand breakage that could arise from a spur overlapping, or created in very close proximity to, the DNA. In these ways, both the physics and the chemistry threads, largely in parallel, put out the challenge to the experimental research community to verify these predictions of clustered DNA damage from ionizing radiations and to investigate their relevance to DNA repair and subsequent cellular effects. The third thread, biochemical and cell-based research, responded strongly to the challenge by demonstrating the existence and biological importance of clustered DNA damage. Investigations have included repair of a wide variety of defined constructs of clustered damage, evaluation of mutagenic consequences, identification of clustered base-damage within irradiated cells, and identification of co-localization of repair complexes indicative of complex clustered damage after high-LET irradiation, as well as extensive studies of the repair pathways involved in repair of simple double-strand breaks. There remains, however, a great deal more to be learned because of the diversity of clustered DNA damage and of the biological responses.

A double-strand-break model for the relative biological effectiveness of electrons based on ionization clustering

BACKGROUND

The effectiveness of ionizing radiation regarding DNA damage induction depends on its spatial energy deposition pattern. For electrons an increased effectiveness is observed at low kinetic energies due to the enhanced density of energy deposition events at electron track ends.

PURPOSE

A model is presented, which enables the calculation of the double-strand-break (DSB) yield and the relative biological effectiveness (RBE) for DSB induction of electrons.

METHODS

The model applies the mean free path between two ionizations and the assumption that two ionizations within a certain threshold distance are necessary to potentially lead to a DSB. Next to an expression for the electron RBE according to its common definition, a local RBE is determined, which describes the electrons' local effectiveness at a defined point on their track.

RESULTS

This local RBE allows a better understanding of microscopic processes resulting from radiation and can be used, for instance, to describe the mean effectiveness of the mixed electron radiation field as a function of the radial distance to the center of an ion track.

CONCLUSIONS

The presented model reflects the experimentally observed increased effectiveness of low-energetic electrons. It will be used in a future work to improve RBE predictions for ions performed with the local effect model.

Constraints on Energy Deposition and Target Size of Multiply Damaged Sites Associated with DNA Double-strand Breaks

We suggest that the observed experimental data on relative double-strand break (dsb) yield as a function of radiation quality can act as valuable constraints in defining the type of energy deposition which causes this basic lesion in radiation biology. Both heavy-ion and alpha-particle data show sufficient trends for quantitative comparisons with calculation to be made. We use the technique of track-structure simulation and search for energy-deposition clusters (containing at least a given number of ionizations in a given diameter) whose relative frequencies (compared to sparsely ionizing radiation) correlate with the relative biological effects (RBEs) for dsb induction. We conclude that locally multiply damaged sites (LMDS) which cause dsb are probably energy depositions of at least two to five ionizations localized, respectively, in sites of diameters of 1-4 nm. Although our derived cluster sizes should be viewed in light of the quality of the experimental data and uncertainties in the computer simulations at the nanometre level, it is unlikely that these estimates of cluster sizes would change greatly.

Model of DNA Damage Induced by Radiations of Various Qualities

A theoretical model permitting estimation of yields of various DNA damages induced by radiations of varying qualities is described. It is based on the Monte-Carlo track structure simulation and DNA structure, and links physical, physicochemical and chemical stages of radiation action. Direct and indirect effects are not strictly distinguished but treated cooperatively. Good agreement between calculated and measured initial yields of double-strand breaks was observed. Other multiple and single damages of DNA are studied. When radiation quality is changed there are quantitative and qualitative transitions in the damage spectrum. The proportion of multiple damage in the damage spectrum is about 30% for low-LET radiations and increases considerably with increasing ionization density.

Calculation of relative biological effectiveness of a low-energy electronic brachytherapy source

Low-energy x-rays are known to have a higher relative biological effectiveness (RBE) than higher energy photons such as the gamma rays from 192Ir and 60Co. In this work the initial yield of single- and double-strand DNA breaks (SSB and DSB) and the RBE was estimated for a novel electronic brachytherapy source (EBS), emitting 40-50 kVp photons. An EGSnrc Monte Carlo model of the source was used in combination with the 'Monte Carlo damage simulation' program (Semenenko and Stewart 2004 Radiat. Res. 161 451-57; 2006 Phys. Med. Biol. 51 1693-706). The results indicate a substantially reduced SSB yield and increased DSB yield for the EBS compared to 60Co or 192Ir, leading to an enhanced RBE by 40-50%. The RBE estimate for the low-energy x-ray EBS was found to be very similar to the low-energy gamma ray brachytherapy isotope 125I. Biological damage was estimated in several human tissues: muscle, breast, calcified breast and cortical bone. SSB and DSB yields were similar in all media, except in bone. These findings should be taken into account if the EBS is intended to replace brachytherapy with the commonly used 192Ir isotope.

Microdosimetric Analysis of Various Mammography Spectra: Lineal Energy Distributions and Ionization Cluster Analysis

In view of recent recommendations on the frequency and the starting age of mammography screening in healthy women, it is desirable to quantify the enhanced relative biological effectiveness (RBE) of mammography X rays compared to hard X rays. While there is little doubt that the former are more potent in inducing biological damage than the latter, the magnitude of the effect is still hotly debated in the literature. We used Monte Carlo simulations and track structure analysis in micrometer and nanometer volumes to investigate differences in distributions of lineal energy and ionization clusters for a range of mammography X-ray qualities. Dose-averaged lineal energies, (yD), in breast tissue for various mammography qualities were found to result in quality factors about 40% higher than unity. Among the various mammography qualities studied, the popular molybdenum/molybdenum target/filter combination was found to have the highest (yD) in 1-microm spheres (about 5.0 keV/microm near the entrance surface of breast tissue). In 10-nm radius spheres, the mean ionization cluster order was found to be about 35% higher in mammography X rays compared to 300 keV electrons (roughly representing 60Co or 192Ir photon radiation). In even smaller spheres (2 nm radius), no significant differences were observed for the mean ionization cluster order between mammography X rays and 300 keV electrons. We conclude that the potential of mammography X rays to induce biological damage is probably not much higher than a factor of two compared to hard X rays.

Theoretical analysis of microdosimetric spectra and cluster formation for103Pd and125I photon emitters

In this work we have compared 125I or 103Pd from a microdosimetric point of view. The photon spectra at different positions around the seeds have first been calculated using EGSnrc Monte Carlo (MC) code. These photon spectra are used as input for the event-by-event MC code TRION to calculate the microdosimetric lineal energy (y) distribution for each isotope. The microdosimetric dose average lineal energy, yD, calculated in a sphere of 1 microm is 3.5 keV microm(-1) for 125I and 4.0 keV microm(-1) for 103Pd, agreeing well with values reported in the literature. yD in a 1 microm sphere diminishes slightly with the distance from the seed for 103Pd. This is due to the spectral hardening caused by the presence of a gamma-ray of 357.5 keV in the initial spectrum of 103Pd. In parallel with the calculation of the microdosimetric spectra, we have analysed the distribution of the size of the energy deposition clusters generated by these low energy photons in structures of 2 and 10 nm of radius. Due to Compton interactions, the fraction of very low energy electrons (<5 keV) generated by 125I photons is 51%, whereas it is only 27% for 103Pd. As these electrons deposit their energy very locally, the pattern of energy depositions contains more clusters of a few nm of radius for 125I than for 103Pd; the mean cluster orders are respectively 3.3 and 3.0 for 10 nm clusters. This is in opposition with the prediction based on the microdosimetric spectrum and the parameter yD and could be of importance for the damage to the cells.

An assessment of galactic cosmic radiation quality considering heavy ion track structures within the cellular environment

Beyond the magnetic influence of the Earth, the flux of galactic cosmic radiation (GCR) represents a radiological concern for long-term manned space missions. Current concepts of radiation quality and equivalent dose are inadequate for accurately specifying the relative biological "efficiency" of low doses of such heavily ionising radiations, based as they are on the single parameter of Linear Energy Transfer (LET). Such methods take no account of the mechanisms, nor of the highly inhomogeneous spatial structure, of energy deposition in radiation tracks. DNA damage in the cell nucleus, which ultimately leads to the death or transformation of the cell, is usually initiated by electrons liberated from surrounding molecules by the incident projectile ion. The characteristics of these emitted "delta-rays", dependent primarily upon the charge and velocity of the ion, are considered in relation to an idealised representation of the cellular environment. Theoretically calculated delta-ray energy spectra are multiplied by a series of weighting algorithms designed to represent the potential for DNA insult in this environment, both in terms of the quantity and quality of damage. By evaluating the resulting curves, and taking into account the energy spectra of heavy ions in space, a relative measure of the biological relevance of the most abundant GCR species is obtained, behind several shielding configurations. It is hoped that this method of assessing the radiation quality of galactic cosmic rays will be of value when considering the safety of long-term manned space missions.

Molecular and cell models of biological effects of heavy ion radiation

Many quantitative models have been developed for the biological effectiveness of radiation of different quality. They differ substantially in their assumptions, and a lack of firm knowledge remains as to the detailed nature of the critical early molecular damage. Analyses of microscopic features of the stochastic structures of radiation tracks have led to hypotheses on the importance of clustered damage in DNA and associated molecules. Clustered damage of greater complexity or severity is suggested to be less repairable and therefore to dominate the biological consequences.

Repair of DNA double-strand breaks and its effect on RBE

DNA double-strand breaks (DSB) are induced linearly with absorbed dose both for sparsely and densely ionizing radiations. By enzymatic repair the linear relationship between the number of DSB and absorbed dose is converted into a non linear one. Furthermore, the RBE-values of high LET radiations for residual DSB increase with increasing amount of DSB repair especially in the low dose range. Unrepaired and/or misrepaired DSB are supposed to be responsible for chromosomal aberrations, cell killing, oncogenic cell transformation and gene mutation. At low doses, for these endpoints much higher RBE-values than those for initial DSB are observed. However, with increasing doses the RBE-values for these endpoints approach those for initial DSB. These observations are likely to be interpreted using the following two parameters of the energy deposition structure: 1. The distribution of clusters with respect to their size at the nm-scale and to the number of ionizations per cluster (cluster distribution). 2. The distribution of distances between clusters of definite size and with definite number of ionizations (distance distribution of clusters). For the induction of DSB solely the ionization density in clusters of nm-dimensions (i.e. the cluster distribution) is important. For unrepaired or misrepaired DSB (responsible for chromosome aberrations, cell killing, oncogenic cell transformation and gene mutation) both the cluster distribution and the distance distribution of clusters are relevant. At low doses the distance distribution of clusters along a single particle track determines the RBE-value. However, with increasing dose the distribution of clusters produced by all particles traversing the cell nucleus becomes increasingly determinant. Here, solely the cluster distribution is important as it is the case for the induction of DSB.

DNA double-strand breaks induced by low energy protons in V79 cells

The initial production of DNA double-strand breaks (dsb) was determined in V79 Chinese hamster cells irradiated with proton beams of 3.24, 1.50 and 0.88 MeV, corresponding to values of unrestricted LET evaluated at the cell midplane of 10.9, 20.0 and 30.5 keV/micron, respectively. X-rays were used for comparison. Dsb were measured with the low speed sedimentation technique in neutral sucrose gradients. The initial yield of dsb rose linearly with the dose and did not significantly depend on the proton LET, in contrast with the results obtained in previous studies for cell inactivation and mutation induction. Also, no significant differences for dsb induction were found between protons and X-rays. Two possible explanations, not necessarily mutually exclusive, are proposed: (1) dsb are not the only lesions involved in cellular effects; and (2) the initial number of dsb is not the only important parameter since a fundamental role is played by the degree of clustering, i.e. the association of dsb with other dsb or other types of damage.

Estimation of double-strand break quality based on track-structure calculations

The aim of this study was to investigate whether double-strand break (DSB) quality is related to the complexity of radiation-induced damage or to the spatial distribution of initial DSBs. The analysis was based on track structure calculations and DNA damage modelling. The results obtained indicate that the quality of DSBs is related to their initial spatial distribution rather than to their complexity.