Because of the increasing use of heavy ions in cancer therapy and for the planning of manned space travels, a realistic estimate of the health risks associated with particle exposure is indispensable. The standard method to quantify the exposed dose and to assess the health risks of radiation is the analysis of chromosome aberrations in peripheral blood lymphocytes at the first post-irradiation mitosis at one fixed time, 48 h, after in vitro stimulation. Using this procedure very low RBE values for high LET particles are reported. However, evidence is accumulating that high LET induced cell cycle delays and apoptosis may influence the aberration yield observable in metaphase cells. To address these questions, lymphocytes obtained from a healthy donor were irradiated with X-rays, C-, Fe- and Fe-like particles with LETs ranging from 2-3160 keV/microm and chromosome aberrations were measured in first cycle metaphase cells at multiple 3 h collection intervals from 48 to 84 h post-irradiation. In parallel, aberrations were determined in G2-phase cells and cell cycle progression as well as radiation-induced apoptosis were examined. Analysis of the data sets shows that high LET-induced apoptosis does not affect the observable aberration yield. However, a relationship between high LET induced cell cycle delays and the number of aberrations carried by a cell was found: the delayed entry of heavily damaged cells into mitosis results from a prolonged arrest in G2 and the delay is dose- and LET-dependent. Detailed statistical analysis of the frequency distributions of aberrations among cells revealed a correlation between the selective delay of heavily damaged cells and the number of particle hits per cell nucleus. Altogether, the data demonstrate that the application of the standard metaphase assay 48 h post-irradiation results in an underestimation of the RBE of high LET particles. Application of alternative cytogenetic approaches (G2-PCC analysis, the integration analysis) confirmed this conclusion
