Application of the Constant Exposure Time Technique to Transformation Experiments with Fission Neutrons: Failure to Demonstrate Dose-rate Dependence

Abstract

A direct comparison of the effectiveness of fission neutrons at high (11·0–31·3 cGy/min) or several low dose-rates (0·14–3·2 cGy/min) was carried out under identical conditions. Monolayers of exponentially growing C3H/10T½ cells were exposed at 37°C to reactor-produced neutrons (fluence-mean energy E_n = 0·68 MeV, ≤ 5% γ component, frequency mean linear energy y_F = 21 keV/μm, dose mean lineal energy y_D = 42 keV/μm in an 8-μm spherical cavity). Survival or transformation induction were studied at five doses from 10·5 to 94 cGy. In low dose-rate irradiations, these doses were protracted over 0·5, 1, 3 or 4·5 h, resulting in 17 different dose-rates. Up to six experiments were performed at each of five exposure times. Concurrently with transformation we studied cell proliferation in control versus cells irradiated at 40 cGy (acute and a 4·5-h protraction) and found no evidence of a shift in the cell cycle distribution among these cells. At a given dose and dose-rate, the effect of dose protraction on survival or transformation was assessed by the dose-rate modifying factor (DRMF), defined as the low:high dose-rate effect ratio at the same dose. Survival or transformation induction curves were nearly linear with initial slopes, respectively, of about 6·5 × 10⁻³ or 6·2 × 10⁻⁶ cGy⁻¹. Consistent with dose-response curves, DRMFs were independent of the dose and dose-rate. The mean values of the DRMF with their uncertainties and 99% confidence intervals, based on measurements in individual doses and dose-rates for survival or transformation were, respectively: 1·01 ± 0·03 (0·92 1·09) or 0·98 ± 0·04 (0·83 1·08) indicating a similar precision in determining DRMF for survival or transformation, and no dose or dose-rate influence on these end points.