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

To evaluate DNA damage of Auger emitters by numerical modelling at the molecular level.

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.

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.