Carcinogenic effects of tritiated water (HTO) in mice

A series of expriments were carried out using female (C57BL/6N x C3H/He)F1 mice (BCF1) to assess the carcinogenic effect of tritiated water (HTO) following a preliminary experiment in which the acute effect of HTO was studied in young female mice of C57BL/6L strain in an attempt to gain basic information on the long-term experiment, and the results were compared with those of fission spectrum neutrons and gamma-rays. The findings obtained are summarized as follows. 1) C57BL/6N mice receiving a single intraperitoneal (i.p.) injection of HTO, 7.4 x 108 Bq (20 mCi) or more, died of bone marrow failure within 20 days. 2) In long-term experiment, BCF1 mice given 1.4 x 108 Bq to 5.6 x 108 Bq developed solid tumors in a variety of tissues but with no obvious dose dependency. 3) The fractionated exposure to 7.4 x 108 Bq (1.9 x 108 each, at 7-day intervals) was highly effective in induction of malignant T-cell lymphomas (85%) with a shorter latency than that of a single exposure (15%). 4) A protracted irradiation of 0.27 Gy of 137Cs, designed to simulate the decreasing absorption rate with time after a single i.p. injection of an equivalent dose of HTO resulted in a drastic reduction in induction rate of ovarian tumors as compared with that of an acute 60Co gamma irradiation. 5) The carcinogenic potential of HTO, given as a single i.p. injection, was quite similar to that of acute 60Co gamma ray irradiation at both 2.7 Gy and 0.27 Gy levels. On the other hand, the effect of HTO was slightly higher than that of protracted gamma-ray irradiation. 6) 252Cf fission neutrons were found to be more potent in tumor induction than gamma-rays or HTO beta-rays under the present experimental conditions. (author)