Liver Cancer Induction by 239Pu, 241Am, and Thorotrast in the Grasshopper Mouse, Onychomys Leukogaster

Abstract

Forty young adult grasshopper mice (Onychomys leukogaster) of both genders were injected with either 129 or 44 kBq kg-1 of monomeric 239Pu and were maintained for lifetime observation. Average liver doses to death (mean times +/- standard deviation (SD) from injection to death = 405 +/- 133 and 756 +/- 189 d) were calculated as approximately 16 and 9 Gy, respectively. These animals developed a total of 18 primary liver tumors (neoplasms, malignant, and benign). Comparison of these mice to a previously published study involving 49 control animals of the same species, 70 mice given 241Am, and 73 given Thorotrast, indicated that the liver cancer induction of Thorotrast can be attributed almost exclusively to the effects of the radioactivity and not to its nonradiation properties. This suggests that projected risks of liver cancer induction from 239Pu, 241Am, or other liver-seeking actinides in humans probably can be estimated from the liver cancer experience in Thorotrast patients using the calculated radiation doses to liver. For this species, the linear risk coefficient for induction of liver neoplasia (percent of mice with liver tumor) by 241Am or Thorotrast was estimated to be about 14.6 +/- 5.4 times the average liver dose (in Gy) for groups of animals with average liver doses of 5 Gy or less. The lowest average liver dose among groups of these mice given 239Pu was about 9 Gy, the dose was not in the linear range, and it was too high to yield reliable results for determining a risk coefficient for low dose irradiation. However, the estimates for a risk coefficient were similar for the plutonium and americium mice with liver doses of approximately 9 Gy or 16 Gy.