Longevity and tumour incidence in mice exposed to fast neutrons at different ages

Experiments are under way in the authors' laboratory to observe both neoplastic and non-neoplastic late effects in mice irradiated with fission neutrons and X-rays at three different ages. Analysis of data from over 2800 animals is in progress and a preliminary evaluation can be made on the survival and the pathology at spontaneous death of mice irradiated in utero and at 3 months of age. Single doses of 250kV X-rays or of attenuated fission neutrons obtained in the biological channel of the experimental fast reactor RSV TAPIRO of the Casaccia Centre were given to male BC3F1 mice of 3 months of age and to pregnant females on day 17.5 post coitum. Both male and female offspring of the latter group were followed to spontaneous death, along with appropriate untreated controls. As for the 3 month old irradiated animals their mean survival time was decreased by X-rays, the dose-effect relationship being compatible with a linear fitting. Fission neutrons proved to be more efficient than X-rays in the induction of life shortening, but the shape of the dose-effect relationship did not fit a linear model because efficiency is higher at low than at high neutron doses. Reticulum cell sarcoma (RCS) was decreased by increasing X-ray and neutron doses, the latter being more efficient. The final incidences of all other neoplasms, regardless of tumour type and site, indicate that neutrons are more efficient than X-rays in tumour induction at low and intermediate doses. As for prenatally irradiated mice, no detectable effect on mean lifespan was observed for either type of radiation. A low final frequency of RCS was seen after irradiation at all dose levels of both types of radiation. The incidence of all other tumours was practically unchanged in male mice irradiated as foetuses with X-rays, but a significant excess was found after neutron irradiation, showing a frequency peak in the range of 0.27-0.45Gy. Similar results were obtained after irradiation of foetal females. (author)