APPLICATION OF THE CONSTANT EXPOSURE TIME TECHNIQUE TO TRANSFORMATIONEXPERIMENTS WITH FISSION NEUTRONS - FAILURE TO DEMONSTRATE DOSE-RATE DEPENDENCE

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 ca rried out under identical conditions. Monolayers of exponentially grow ing C3H/10T1/2 cells were exposed at 37 degrees C to reactor-produced neutrons (fluence-mean energy E(n) = 0.68 MeV, less than or equal to 5 % gamma component, frequency mean linear energy y(F) = 21 keV/mu m, do se mean lineal energy y(D) = 42 keV/mu m in an 8-mu m spherical cavity ). Survival or transformation induction were studied at five doses fro m 10.5 to 94 cGy. In low dose-rate irradiations, these doses were prot racted 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. C oncurrently with transformation we studied cell proliferation in contr ol versus cells irradiated at 40 cGy (acute and a 4.5-h protraction) a nd found no evidence of a shift in the cell cycle distribution among t hese cells. At a given dose and dose-rate, the effect of dose protract ion on survival or transformation was assessed by the dose-rate modify ing factor (DRMF), defined as the low:high dose-rate effect ratio at t he same dose. Survival or transformation induction curves were nearly linear with initial slopes, respectively, of about 6.5 x 10(-3) or 6.2 x 10(-6) cGy(-1). Consistent with dose-response curves, DRMFs were in dependent of the dose and dose-rate. The mean values of the DRMF with their uncertainties and 99% confidence intervals, based on measurement s in individual doses and dose-rates for survival or transformation we re, 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 o r transformation, and no dose or dose-rate influence on these end poin ts.