Oral administration of tritiated water (HTO) in mouse. III: Low dose-rate irradiation and threshold dose-rate for radiation risk

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Abstract
Purpose: To investigate the biological effect of tritium on mouse at low dose-rates. Materials and methods: Mice ([C57BL/6N C3H/He]F) were exposed to beta -rays by continuous administration of tritiated drinking 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 80% for exposure in the range 240 mGy/day to 9.6 mGy/day. Frequency of tumours decreased with decrease of dose-rate to 50% comparable to the controls. Restricting to thymic lymphomas, a linear 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 resulted for the relationship between the life-shortening and the dose-rate. The threshold dose-rate of 3H beta -rays, 2 mGy/day (with a tail down 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: These studies suggest that there exists the threshold dose-rate in the biological effects of radiation, and that the threshold dose-rate for 60Co gamma-irradiation is higher than that for 3H beta-irradiation.

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