Comparison of DNA Breaks at Entrance Channel and Bragg Peak Induced by Fast C super(6+) Ions -Influence of the Addition of Platinum Atoms on DNA

When energetic carbon ion beam (GeV range) goes through the matter, inelastic processes such as electronic ionization, molecular and nuclear fragmentation occur. For carbontherapy (hadrontherapy) purpose, it is of interest to compare the number of DNA breaks -single SSB or double DSB- for a given dose at the entrance channel and at the Bragg peak to look for a possible differential effect in the number of DNA breaks induced at these two locations. Samples of free plasmids DNA and complexes of plasmids DNA added with molecules containing platinum have been placed at different locations of an experimental setup simulating penetration depths of the ion beam in water and irradiated by carbon ions 290 MeV/amu. The DNA breaks have been quantified by subsequent electrophoresis on agarose gels. To disentangle the respective role of the direct and indirect effect, a free radical scavenger of hydroxyl radicals HO -dimethyl sulfoxide DMSO- has been added in some of the experiments. In the range of Linear Energy Transfert -LET 13 - 110 keV/km-, the number of the DSB was found to be constant versus the LET for a given dose. Contrary, the number of the SSB decreases at the Bragg peak compared to the entrance channel. In the presence of platinum, the number of single and double breaks was considerably enhanced, and follows a similar behaviour than in the free-DNA experiments. Quantitative results on DNA damages do not show significant enhancement due to the nuclear or to the molecular fragmentation in the present experiments.