Overview of Biological, Epidemiological, and Clinical Evidence of Radiation Hormesis

Abstract

The effects of low-dose radiation are being increasingly investigated in biological, epidemiological, and clinical studies. Many recent studies have indicated the beneficial effects of low doses of radiation, whereas some studies have suggested harmful effects even at low doses. This review article introduces various studies reporting both the beneficial and harmful effects of low-dose radiation, with a critique on the extent to which respective studies are reliable. Epidemiological studies are inherently associated with large biases, and it should be evaluated whether the observed differences are due to radiation or other confounding factors. On the other hand, well-controlled laboratory studies may be more appropriate to evaluate the effects of low-dose radiation. Since the number of such laboratory studies is steadily increasing, it will be concluded in the near future whether low-dose radiation is harmful or beneficial and whether the linear-no-threshold (LNT) theory is appropriate. Many recent biological studies have suggested the induction of biopositive responses such as increases in immunity and antioxidants by low-dose radiation. Based on recent as well as classical studies, the LNT theory may be out of date, and low-dose radiation may have beneficial effects depending on the conditions; otherwise, it may have no effects.

Keywords: adaptive response; hormesis; low-dose radiation.

Figure 1

Left panel: Survival curves for C57BL/6 male mice after conditioning irradiation at 0, 50, or 100 mGy and challenge irradiation at 5.9 Gy given 6 or 24 h later. Each group consisted of 50 mice. The group receiving 50 mGy 24 h before the challenge dose had higher survival rates than the control group (p = 0.021). Right panel: Survival curves for C57BL/6 male mice after a conditioning dose of 0, 200, or 400 mGy and challenge dose of 5.9 Gy given 6 or 24 h later. Each group consisted of 40 mice. The group receiving 400 mGy 6 h before the challenge dose had lower survival rates than the control group (p = 0.0032). Modified from Reference [29].

Figure 2

Photograph of silkworms on day 44 after the start of the experiment. Left: Radiation-emitting sheet group; right: Control group.

Figure 3

Left panel: Tumor transplantability curves for EMT6 tumors in Balb/c mice receiving 0 to 1500 mGy of whole-body irradiation given 6 h before inoculation of 100 EMT6 cells. Each group consisted of 40 inoculation sites. Right panel: Mean time to tumor appearance in Balb/c mice developing EMT6 tumors as a function of the whole-body dose. Bars represent SE. In the groups inoculated with 1000 EMT6 cells, the differences were significant between the sham-irradiated group and the groups receiving 100, 200, or 1500 mGy (all p < 0.005). In the groups inoculated with 100 EMT6 cells, significant differences were seen between the sham-irradiated group and the groups receiving 200 or 1500 mGy (both p < 0.01). Modified from Reference [51].