Cytogenetic damage induced in human lymphocytes by low doses of 60Co gamma rays delivered at high and low dose rates.

Chromosomal aberrations were investigated in human peripheral blood lymphocytes after exposure to low doses of 60Co gamma rays delivered acutely or at low dose rates (0.1 or 0.03 Gy/h). Chromosome analysis was performed in cells collected after a 44 to 46 hour culture time in order to avoid scoring of second division cells, and the dose-related induction of aberrations was analysed by the maximum likelihood method. In all cases, the induction of dicentrics was well described by a linear-quadratic dose response model (Y = aD + bD2), the data obtained at a low dose rate being equally well fitted to a linear equation. According to earlier findings on the mechanisms of aberration formation, the two lesions originating from single ionizing tracks have to be produced within a period of approximately 5 hours, in order to interact and to give rise to dicentric aberrations, which could explain the decrease in the quadratic term at the low dose rate since the highest doses were delivered over a period of more than 5 hours.