Calculated strand breaks from (125)I in coiled DNA.

PURPOSE: DNA single strand breaks (SSB) and double-strand breaks (DSB) induced by Auger electrons from incorporated (125)I decay were calculated using a B-DNA model to assess contributions from direct and OH damage and effects of higher-order structure. Three decay sites, linker DNA, nucleosome, and two adjacent nucleosomes, were assessed and compared to experimental data. METHOD: A Monte Carlo track structure code for electron was used to track electrons, OH and H radicals through linear and a higher-order model of B-DNA. Direct and indirect DNA hits were scored and used to determine SSB and DSB. RESULTS: The three different (125)I decay locations produced different number of DSBs and fraction of radical damage. The average number of DSB per (125)I decay was 0.83, 0.86 and 1.33, respectively, for the three sites. OH radical attack contributed to or exclusively caused 70%, 57%, and 50%, of the DSBs located in the entire model. When only 10 base pairs on either side of the incorporation site were considered, radical damage contributions were 40%, 25% and 67%, respectively. Locations distant from the site of incorporation, however, consistently yielded 70-80% of the DSB from radical attack. CONCLUSIONS: Coiling of DNA can greatly change both the absolute number of DSB per incorporated (125)I decay and the relative contributions of radical damage to the local site of decay and, to a lesser extent, the average over all DNA. Higher order structure only slightly affects the number and quality of DNA damage to distant locations, which is mostly from radical attack.

On the biological efficiency of I-123 and I-125 decay on the molecular level.

PURPOSE: To evaluate DNA damage of Auger emitters by numerical modelling at the molecular level. MATERIAL AND METHODS: Energy emission spectra of I-123 and I-125 were used as input data for a computer code that simulates the complete transport of electrons and photons from the physical stage up to the primary chemical stage at 10(-7) s. The simulation was performed in a complex environment of liquid water, DNA structures and scavengers. Electron and photon interactions with the DNA molecules were carefully managed. Simulations were carried out with both I-123 and I-125 bound to a pBR322 plasmid or free in its vicinity. RESULTS: The distributions of direct and indirect single strand breaks (SSB) and double strand breaks (DSB) as a function of the kinetic energy of the emitted Auger electrons show that damage is caused primarily by electrons with energies lower than 800 eV, while higher energy electrons are mainly involved in indirect effects. The yields per unit energy emitted strengthen this fact. When compared to experimental values, the calculated yields of linearization (LE) and relaxation (RE) events show good agreement as well as does the ratio LE/RE for each radionuclide and the ratio I-125/I-123 in the case of LE.

Analysis of computational problems expressing the overall uncertainties: photons, neutrons and electrons.

In the frame of the EU Coordination Action CONRAD (coordinated network for radiation dosimetry), WP4 was dedicated to work on computational dosimetry with an action entitled 'Uncertainty assessment in computational dosimetry: an intercomparison of approaches'. Participants attempted one or more of eight problems. This paper presents the results from problems 4-8-dealing with the overall uncertainty budget estimate; a short overview of each problem is presented together with a discussion of the most significant results and conclusions. The scope of the problems discussed here are: the study of a (137)Cs calibration irradiator; the manganese bath technique; the iron sphere experiment using neutron time-of-flight technique; the energy response of a RADFET detector and finally the sensitivity and uncertainty analysis for the recoil-proton telescope discussed in the companion paper.

Molecular basic data calculation for radiation transport in chromatin.

Differential and integral electron impact ionisation cross sections were calculated using the binary-encounter-Bethe theoretical model for each core particle molecules: the four DNA bases, the backbone (sugar phosphate), and the 19 amino acids constituent of histone proteins. The binding energies and populations of molecular orbitals were computed using General Atomic Molecular Electronic Structure System. At present, there are neither experimental nor other theoretical results on amino acid electron impact ionisation cross sections. Regarding DNA bases and backbone, our results show good agreement with those published in journals.