Evidence for radiation hormesis after in vitro exposure of human lymphocytes to low doses of ionizing radiation

Abstract

Previous research has demonstrated that adding a very small gamma-ray dose to a small alpha radiation dose can completely suppress lung cancer induction by alpha radiation (a gamma-ray hormetic effect). Here we investigated the possibility of gamma-ray hormesis during low-dose neutron irradiation, since a small contribution to the total radiation dose from neutrons involves gamma rays. Using binucleated cells with micronuclei (micronucleated cells) among in vitro monoenergetic-neutron-irradiated human lymphocytes as a measure of residual damage, we investigated the influence of the small gamma-ray contribution to the dose on suppressing residual damage. We used residual damage data from previous experiments that involved neutrons with five different energies (0.22-, 0.44-, 1.5-, 5.9-, and 13.7-million electron volts [MeV]). Corresponding gamma-ray contributions to the dose were approximately 1%, 1%, 2%, 6%, and 6%, respectively. Total absorbed radiation doses were 0, 10, 50, and 100 mGy for each neutron source. We demonstrate for the first time a protective effect (reduced residual damage) of the small gamma-ray contribution to the neutron dose. Using similar data for exposure to gamma rays only, we also demonstrate a protective effect of 10 mGy (but not 50 or 100 mGy) related to reducing the frequency of micronucleated cells to below the spontaneous level.

Keywords: Gamma rays; Hormesis; Human Lymphocytes; Micronucleus; Neutrons; X rays.

FIGURE 1

Dose-response curves for the frequency of micronucleated cells after exposure of human lymphocytes to photon radiations with different energy characteristics: Gamma rays (diamonds); 70-kVp X rays (squares); 250-kVp X rays (triangles). Horizontal dashed lines indicated the 95% confidence region for the pooled control data. Error bars for data points are 95% confidence intervals.

FIGURE 2

Dose-response curves for the frequency of micronucleated cells after exposure of human lymphocytes to monoenergetic neutrons: 13.7-MeV (open circles); 1.5-MeV (triangles); 5.9-Mev (closed circles); 0.44-MeV (squares); 0.22-MeV (diamonds). Horizontal dashed lines indicated the 95% confidence region for the pooled control data. Error bars for data points are 95% confidence intervals.

FIGURE 3

Gamma-ray protection factors against neutron-induced micronucleated cells plotted as a function of <z₁>. PROFAC = 0 for the reference group (0.22- and 0.44-MeV neutrons combined) which is plotted (three superimposing data points) at the middle of the range 80–94 mGy/hit for <z₁>.