Early changes in cell cycle kinetics after ionizing irradiation below 1 GY.

The present study describes a procedure for quantifying cell cycle alterations within 15 h after radiation with doses below 1 Gy. For detection and assessment of the relevant changes, 5-bromo-2'-deoxyuridine (BrdUrd)-labelling and flow cytometry were used. Using this approach, as early as 6 h after exposure of radiosensitive leukemic HL-60 cells, radiation-induced changes in cell cycle progression could be measured even with radiation doses as low as 0.25 Gy. As a result, a method to define transition rates for a single cell cycle phase or from one phase to another is described. Even minor changes can be described. Moreover, the BrdUrd assay allows for discrimination of cells irradiated in different phases of the cell cycle. Thus, it is possible to follow the progression in the cell cycle of cells either irradiated in G1, S, or G2 + M phase, respectively. Radiation effects on single cell cycle phases can be analysed separately. A detailed evaluation of the cellular response to irradiation regarding dose, time, and effect is described. The value of cell cycle parameters for assessment of various biological indicators of radiation effect is discussed.