Non-Linearity Between Dose and Cancer Risk for Internally Deposited Alpha Emitters in Animals

Abstract

The risk of cancer in experimental animals exposed to low doses and dose rates of internally deposited alpha emitting radionuclides was analyzed in 27 animal experiments comprising 78 groups exposed to specific dose levels (dose groups). In these experiments, 3041 animals were exposed to eight different alpha emitters, by injection or by inhalation. Radiation doses ranged from about 60 mGy to more than 7 Gy. There were 1655 control animals. The target organs were the lung and the skeleton. The cancer incidence in exposed groups was compared to that predicted by the Linear No-Threshold Hypothesis (LNTH). In the 3041 exposed animals, 49 cancers were observed, against 83 predicted by the LNTHT. The LNTH appears to be a reliable risk predictor in 11 of the 78 dose groups, but it overestimates the risk in the 67 other dose groups (71 predicted cases, 8 cases observed). No cancer was observed in 53 dose groups, when 23 were predicted by the LNTH. In these 53 dose groups, the probability of not observing a single case of cancer was extremely small. These observations led to the conclusion that, at least in the case of alpha emitters with long physical and biological half-lives, the LNTH is not a good predictor of the risk of cancer.