DNA DOUBLE-STRAND BREAKS INDUCED BY LOW-ENERGY PROTONS IN V79 CELLS

The initial production of DNA double-strand breaks (dsb) was determine d in V79 Chinese hamster cells irradiated with proton beams of 3.24, 1 .50 and 0.88 MeV, corresponding to values of unrestricted LET evaluate d at the cell midplane of 10.9, 20.0 and 30.5 keV/mu m, respectively. X-rays were used for comparison. Dsb were measured with the low speed sedimentation technique in neutral sucrose gradients. The initial yiel d of dsb rose linearly with the dose and did not significantly depend on the proton LET, in contrast with the results obtained in previous s tudies for cell inactivation and mutation induction. Also, no signific ant differences for dsb induction were found between protons and X-ray s. Two possible explanations, not necessarily mutually exclusive, are proposed: (1) dsb are not the only lesions involved in cellular effect s; and (2) the initial number of dsb is not the only important paramet er 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.