Induction of Chromosomal Aberrations in Human Lymphocytes by X Rays and Fission Neutrons: Dependence on Cell Cycle Stage

Human lymphocytes were irradiated in culture with several doses of 250 kVp X rays or fission neutrons (0.85 MeV average energy) either in the unstimulated state ( G₀), at 17 hr after phytohemagglutinin stimulation ( G₁) or at approximately 48 hr after stimulation. In the last case, the cells were labeled with tritiated thymidine prior to irradiation to distinguish S and G₂ cells. All cultures were harvested at two successive intervals between 48 and 58 hr. The frequencies of neutron-induced chromatid deletions in G₂ cells differed significantly among harvest times and definite trends were observed for other aberrations. Dose-response curves for chromosome type aberrations followed the classical aberration production model; chromatid aberrations exhibited more complex kinetics. The differential cell cycle response to aberration induction varied with radiation quality, dose, and aberration type. The relative biological effectiveness of fission neutrons for aberration production also varied with aberration type, cell cycle stage, and neutron dose but remained principally between 2 and 6.