The quality of DNA double-strand breaks: a Monte Carlo simulation of the end-structure of strand breaks produced by protons and alpha particles.

The quality of DNA damage induced by protons and alpha-particles of various linear energy transfer (LET) was studied. The aim was to single out specific lesions in the DNA molecule that might lead to biological endpoints such as inactivation. A DNA model coupled with a track structure code (MOCA-15) were used to simulate the lesions induced on the two helixes. Four categories of DNA breaks were considered: single-strand breaks (ssb), blunt-ended double-strand breaks (dsb, with no or few overlapping bases), sticky-ended double-strand breaks (with cohesive free ends of many bases), and deletions (complex lesions which involve at least two dsb within a small number of base pairs). Calculations were carried out assuming various sets of parameters characterizing the production of these different DNA breaks. No large variations in the yields of ssb and blunt- or sticky-ended dsb were found in the LET range between 10 and 200 keV/mu m. On the other hand, the yield of deletions increases up to about 100 keV/mu m and seems to reach a plateau at higher LET values. In the LET interval from 30 to 60 keV/mu m, protons proved to be more efficient than alpha-particles in inducing deletions. The induction of these complex lesions is thus dependent not simply on LET but also on the characteristics of the track structure. Comparison with RBE values for cell killing shows that this special class of dsb might play an important role in radiation-induced cell inactivation.