Age-related susceptibility of mouse liver to induction of tumors by neutrons.

The induction of liver tumors has been studied in BC3F1 male mice after acute whole-body irradiation with fission neutrons and X rays, given at different ages. In prenatally irradiated mice, a small effect is seen after doses of 0.3 to 2.1 Gy of X rays, and a more pronounced effect is found after neutron doses of 0.09 to 0.62 Gy. At 3 months of age the animals show a higher incidence after X-ray doses from 2 Gy, and for neutrons from 0.17 Gy. At 19 months of age, liver tumors are rarely induced by either type of radiation. These findings are confirmed by the statistical analysis of trend. The possibility of deriving dose-response relationships for liver tumors was also investigated. In the dose ranges where the risk appears to increase as a function of the increase in dose, the data points for neutrons were well fitted by a linear expression. A pure quadratic relationship best fitted the X-ray data. Using these expressions, the RBE for neutrons was 28 at 0.09 Gy for prenatal irradiation and 13 at 0.17 Gy for irradiation at 3 months. This suggests the existence of a risk of developing liver tumors after exposure to radiation, and fetal liver seems to be particularly sensitive to neoplastic transformation. This risk may be negligible at low doses (less than 1 Gy) of low-LET radiation, or with exposure at older ages.