Track structure and DNA damage simulation for light ion energies around the bragg peak

Radiation damage induced by low-energy ions significantly contributes to the high relative biological efficiency (RBE) of ion beams around Bragg peak regions. Further, slow light ions released through nuclear interactions of neutrons are responsible for the wide energy dependence of neutrons’ RBE. In the PARTRAC family of biophysical Monte Carlo codes for simulating track structures, DNA damage and its repair [1], cross sections for ions heavier than helium were scaled from proton data [2] by the effective charge according to Barkas [3]. This procedure, however, is applicable only for specific energies above about 1 MeV/u; it leads to an underestimation of the stopping power at lower energies. To solve this issue, the scaling procedure has been modified so that it reproduces in principle the slowing- down behaviour of protons/hydrogen atoms. Charge changing processes (stripping and subsequent electron pick-up) are considered as ionizations with electron emission with the ion’s velocity. The resulting range and linear energy transfer (LET) values for C, N, O, P and Ca ions agree with ICRU data [4] and SRIM calculations [5]. For the determination of DNA damage yields per unit dose due to carbon ion irradiation, the dose inhomogeneity within the cell nucleus has been taken into account for energies around the Bragg peak. The almost constant yield of strand breaks due to direct effects decreases below about 5 MeV/u. The total induction of DSB has a maximum at the highest LET-values at about 0.5 MeV/u. However, if local clusters of DSB with <20 bp distance are scored as one DSB, the highest yield is found at about 4 MeV/u. Further calculations are underway and results will be presented at the meeting. This work has been supported by the FP7-EURATOM project DoReMi (INITIUM).