Dose-effect relationships for induction of cell inactivation and asymmetrical chromosome exchanges in three cell lines by photons and neutrons of different energy.

The impairment of clonogenic capacity as well as the induction of dicentrics and centric rings by different types of radiation was investigated in three lines of mammalian cells (R-1,M; RUC-2 and V-79). The experiments were performed with cells in plateau phase cultures which contain 70-80 per cent of cells in the G0 or prolonged G1 phase of the cell cycle. They were irradiated with 137Cs gamma rays, 300 kV X-rays or 0.5, 4.2 or 15 MeV neutrons. The different types of cells show an appreciable variation in sensitivity to induction of both types of effect, being greatest for 0.5 MeV neutrons, intermediate for 4.2 and 15 MeV neutrons and least for photons. The dose-response relationships for both types of effect were analysed for neutrons in terms of a linear dose coefficient only, while the results for photons required analysis in terms of linear and quadratic coefficients. The effectiveness per unit of dose for cell inactivation is greater than that for induction of dicentrics and centric rings by factors of about 4 to 7. However, dicentrics and centric rings constitute only a fraction of the chromosome aberrations. Maximum r.b.e. values of about 12 at low doses were obtained for 0.5 MeV neutrons.