Oral administration of tritiated water (HTO) in mouse. III: Low dose-rate irradiation and threshold dose-rate for radiation risk.
OBJECTIVE
To investigate the biological effect of tritium on mouse at low dose-rates.


METHODS
Mice ([C57BL/6N x C3H/He]F1) 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 to approximately 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.
Yamamoto
O
O
Faculty of Life Science, Yasuda Women College, Hiroshima, Japan.
Seyama
T
T
Itoh
H
H
Fujimoto
N
N
D000284:Administration, Oral; D000818:Animals; D004307:Dose-Response Relationship, Radiation; D005260:Female; D008452:Maximum Allowable Concentration; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D012307:Risk Factors; D014316:Tritium / Q000008:administration & dosage / Q000633:toxicity*; D014867:Water / Q000008:administration & dosage*
D014867:Water; D014316:Tritium