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/10T1/2 cells were exposed at 37 degrees C to reactor-produced neutrons (fluence-mean energy En = 0.68 MeV, < or = 5% gamma component, frequency mean linear energy yF = 21 keV/micron, dose mean lineal energy yD = 42 keV/micron in an 8-micron 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 x 10(-3) or 6.2 x 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.

Rodent cell transformation and immediate early gene expression following 60-Hz magnetic field exposure.

“…Positive control results with X rays are in agreement with observations from related experiments using the C3H/1OT1/2 assay in this and other laboratories (23)(24)(25)(26)(65)(66)(67)(68)(69). The effect of TPA on sham-exposed cells was not significant (sham-irradiated control Figure 4; this demonstrates that approximately equal numbers of cells were collected in each group.…”

“…Then, the groups were blinded using an alphanumeric code according to a computer-generated sequence of random numbers and cells plated into correspondingly coded 100-mm dishes containing culture medium with 0.1 pg TPA/ml or TPA solvent (acetone at 0.05%) for cell survival and transformation determination (24)(25)(26). As in our previous experiments, the growth medium was renewed at weekly intervals (2 weeks for survival assay, 8 weeks for transformation assay) (24)(25)(26)67,69). Cell survival and plating efficiency were determined by colony formation, while neoplastically transformed foci (Fig.…”

“…TPA treatment and X irradiation of C3H/10Ti/2 cells were provided as previously described (24)(25)(26) assay (65); our quality control ofexperiments with C3H/1OTi/2 cells is also described in these reports. A single lot of serum [selected according to the criteria described previously (65,69)] was used in all the experiments reported here. Actively growing cells in passage 10 were used for experimentation.…”