In the present work, we have adopted the premature chromosome condensation (PCC) technique to study the initial DNA damage in order to distinguish between the biological and physical components of the dose-effect curves. We have studied G2-chromatid breaks in human lymphocytes, isochromatid-type breaks as well as chromatid exchanges at two different sampling times: directly after irradiation (t0) and 12 hours later (t12). The lymphocyte samples have been exposed to 150 MeV and spread out Bragg peak (SOBP) proton beams, 22 MeV/u 11B ions and for comparison to 60Co γ rays. Dose-response curves for both types of breaks have been determined. We have shown that t0 G2-chromatid breaks follow linear-quadratic dependence for all studied cases and could be used for estimation of the effective ion track radius. A comparison to the expected physical track radii leads to the conclusion that the biological repair mechanism considerably prevails the physical effect of the overlapping ion tracks even at the time t0. The results have been also compared to the dose-effect curves previously obtained in our chromosome aberrations study.