The potential carcinogenic effects of internally deposited α-particle-emitting nuclides, notably plutonium, in the liver in humans are unknown but are of concern in relation to exposures from the nuclear industry. However, patients injected with the radiographic contrast medium Thorotrast are chronically exposed to α-particle radiation from${}^{232}{\rm ThO}{}_{2}$ in the liver. Among 1003 patients injected with Thorotrast, 584 of whom were alive 15 years after the injection and 40 at the end of follow-up, a total of 127 liver cancers were diagnosed, 45 of which were hepatocellular carcinomas, 41 cholangiocarcinomas and 33 hemangiosarcomas. The median time from injection to diagnosis was 35 years (range 18-48) and the cumulative frequency was 55.4% after 48 years. In univariate and multivariate analyses, the cumulative frequency of liver cancer was best described as a function of the estimated mean cumulative α-particle radiation dose to the liver 15 years ago, being independent of age, gender and volume of injected Thorotrast. This may be interpreted to mean that the liver cancer rate is not related to the dose rate and that the period from malignant transformation to diagnosis of cancer is 15 years. The risk of liver carcinogenesis induced by α-particle radiation, assuming 15 years from induction to diagnosis, was estimated to be$712\ \text{cases}/10^{4}$ persons per gray. This value is considerably higher than estimated earlier.

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