DNA double-strand breaks induced by low energy protons in V79 cells.

The initial production of DNA double-strand breaks (dsb) was determined in V79 Chinese hamster cells irradiated with proton beams of 3.24, 1.50 and 0.88 MeV, corresponding to values of unrestricted LET evaluated at the cell midplane of 10.9, 20.0 and 30.5 keV/micron, respectively. X-rays were used for comparison. Dsb were measured with the low speed sedimentation technique in neutral sucrose gradients. The initial yield of dsb rose linearly with the dose and did not significantly depend on the proton LET, in contrast with the results obtained in previous studies for cell inactivation and mutation induction. Also, no significant differences for dsb induction were found between protons and X-rays. Two possible explanations, not necessarily mutually exclusive, are proposed: (1) dsb are not the only lesions involved in cellular effects; and (2) the initial number of dsb is not the only important parameter since a fundamental role is played by the degree of clustering, i.e. the association of dsb with other dsb or other types of damage.