Quantitative Assessment of the Cataractogenic Potential of Very Low Doses of Neutrons

Abstract

We report on the prevalence and relative biological effectiveness (RBE) for various stages of lens opacification in rats induced by very low doses (2 to 250 mGy) of medium-energy (440 keV) neutrons, compared to those for X rays. Neutron doses were delivered either in a single fraction or in four separate fractions and the irradiated animals were followed for over 100 weeks. At the highest observed dose (250 mGy) and at early observation times, there was evidence of an inverse dose-rate effect; i.e., a fractionated exposure was more potent than a single exposure. Neutron RBEs relative to X rays were estimated using a non-parametric technique. The results were only weakly dependent on time postirradiation. At 30 weeks, for example, 80% confidence intervals for the RBE of acutely delivered neutrons relative to X rays were 8-16 at 250 mGy, 10-20 at 50 mGy, 50-100 at 10 mGy and 250-500 at 2 mGy. The results are consistent with the estimated neutron RBEs in Japanese A-bomb survivors, though broad confidence bounds are present in the Japanese results. Our findings are also consistent with data reported earlier for cataractogenesis induced by heavy ions in rats, mice, and rabbits. We conclude from these results that, at very low doses (<10 mGy), the RBE for neutron-induced cataractogenesis is considerably larger than the RBE of 20 commonly used, and use of a significantly larger value for calculating equivalent dose would be prudent.

Classification of radiation effects for dose limitation purposes: history, current situation and future prospects

“…It should be noted that the lens is historically the only tissue for which ICRP recommended a special w R in 1964–1977 (Supplementary Table 3), and that ICRP-92 could not recommend the w R or RBE values for cataract due to a lack of human data and concern as to how to apply the experimental data (e.g. a high-LET RBE of several hundred [64, 65]—much higher than the commonly used w R and RBE of ≤20). Thus, more studies are necessary in order to identify the biological mechanism whereby high-LET RBE becomes very high for cataract (e.g.…”

Mice and the A-Bomb: Irradiation Systems for Realistic Exposure Scenarios

“…For example, the neutron facility at the Radiological Research Accelerator Facility (RARAF) 5 has had 50 years of experience performing neutron irradiations of small animals (35, 36) and of cells (37–39) using monoenergetic neutrons having energies from 0.22 to 15 MeV (12). …”

Different experimental approaches in modelling cataractogenesis

“…Several experimental treatments aimed at inducing cataracts in rats include streptozotocin-induced diabetes (Kyselova et al ., 2005a; Kyselova et al ., 2005b), galactose feeding (Huang et al ., 1990; Huang et al ., 2000), ionizing radiation (Worgul et al ., 1996), inhibition of cholesterol synthesis and steroid treatment (Dickerson et al ., 1997), overdose of selenite (Shearer et al ., 1997), and finally culture with oxidants or calcium ionophore (Chandrasekher and Cenedella, 1993; Fukiage et al ., 1997; Nakamura et al ., 1999; Mitton et al ., 1999). …”