Neutron dose-rate experiments at the AFRRI nuclear reactor. Armed Forces Radiobiology Research Institute

Some in vitro and in vivo studies with neutrons have shown increased carcinogenic effectiveness following low-dose-rate or fractionated irradiation compared to acute exposure in the low-dose range. This would imply that the risk of cancer for persons exposed occupationally to low doses of neutrons is underestimated at present. The C3H 10T1/2 assay has played a major role in investigating neutron dose-rate effects. We describe three independent series of experiments addressing the question of the influence of dose rate using the AFRRI fission-neutron source with the C3H 10T1/2 cell transformation assay as well as with two mutation assays utilizing human-hamster hybrid AL cells. In the first two series, we focused on performing experiments with fission-neutron doses and dose rates similar to those for which enhancement of neoplastic transformation of C3H 10T1/2 cells was originally reported, and observed no discernible dose-rate effect. In the third series concurrent with the induction of neoplastic transformation in C3H 10T1/2 cells, we also measured mutagenesis at two loci in AL cells. Data for survival, neoplastic transformation, and mutation were obtained at two dose rates in the range of neutron doses 0.005 to 0.9 Gy. Dose-rate effectiveness factors expressed as ratios of the effect for low compared to high dose rate did not differ from one, indicating no influence of dose rate on these end points.