Purpose: The invariance of the total direct strand break yield when DNA is irradiated by different types of particles and energies has been reported by previous works. This study is intended to explain the physical causes of this behavior. Methods: The GEANT4-DNA extension of the GEANT4 general purpose Monte Carlo simulation toolkit has been used to determine direct strand break yields induced by protons and alpha particles impacting on a B-DNA geometrical model, including five organization levels of the human genetic material. The linear energy transfer (LET) of such particles ranges from 4.8 keV/mu m (10 MeV protons) to about 235 keV/mu m (2 MeV alpha particles), at 5.225 mu m depth (near the center of the region of interest). Direct total, single and double strand break probabilities have been determined in a liquid water homogeneous medium with a 1.06 g/cm (3) density. The energetic spectra of single strand breaks (SSB), the number of energy deposition events, and the SSB/event ratio were determined. Results: The target-hit probability was found to be independent of both the type and the energy of the incident particle, even if this latter is a secondary electron. This probability is determined by the geometrical properties of the system. The total strand break yield and the number of energy deposition events required to reach a certain absorbed dose were found nearly independent of the type and energy of the incident ion (proton or alpha). In contrast, the double strand break (DSB) yield was found strongly dependent on the LET of the incident radiation. Conclusions: The SSB generation process is homogeneous and independent of the LET of the particles involved, at least within the proton and alpha particle energy range here studied. The target-hit probability is only determined by the ratio between the total volume occupied by targets and that of the ROI where the radiation deposits its energy. The maximum separation distance between two adjacent SSBs to produce a DSB is the parameter that breaks the homogeneity of the target-hit process, making the DSB production process strongly heterogeneous. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3597568]38741474153Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)French Agence Nationale de la Recherche [ANR-09-BLAN-0135-01]European Space Agency [AO6041-22712/09/NL/AT]French Agence Nationale de la Recherche [ANR-09-BLAN-0135-01]European Space Agency [AO6041-22712/09/NL/AT]