ORAL-ADMINISTRATION OF TRITIATED-WATER (HTO) IN MOUSE - III - LOW DOSE-RATE IRRADIATION AND THRESHOLD DOSE-RATE FOR RADIATION RISK

Purpose: To investigate the biological effect of tritium on mouse at l ow dose-rates. Materials and methods: Mice ([C57BL/6N x C3H/He]F-1) we re exposed to beta-rays by continuous administration of tritiated drin king water throughout their lives at low dose-rates of 3.6, 0.9, and 0 .2 mG/day. Results: including the previous study, the tumour frequency was 70 similar to 80% for exposure in the range 240 mGy/day to 9.6 mG y/day. Frequency of rumours decreased with decrease of dose-rate to 50 % comparable to the controls. Restricting to thymic lymphomas, a linea r relationship in a semi-log plot was found between the frequency and the dose-rate above a threshold dose-rate of 12 mGy/day. There was a ' tail' to this relationship down to 0.9 mGy/day. A similar pattern resu lted for the relationship between the life-shortening and the dose-rat e. The threshold dose-rate of H-3 beta-rays, 2 Gy/day (with a tail dow n to 0.2 mGy/day), was much lower than that of gamma-rays, 20 mGy/day (tail down to 2 mGy/day) derived from other studies. Conclusions: Thes e studies suggest that there exists the threshold dose-rate in the bio logical effects of radiation, and that the threshold dose-rate for Co- 60 gamma-irradiation is higher than that for H-3 beta-irradiation.