At least 60 case studies of leukemia among people exposed to chronic low-dose α-particle radiation from injections with the radiographic contrast medium Thorotrast and 115 cases from follow-up studies have been described in the literature. In the present study, malignant hematological diseases among 1003 Danish patients injected during 1935-1947 and followed to 1992 accruing 20,433 person-years were assessed and available histopathological specimens revised. The mean cumulative bone marrow α-particle radiation dose (1.34 Gy) was estimated from records of the amount of Thorotrast injected (mean 18.7 ml). Sixteen cases of acute myelogenous leukemia (AML) and seven cases of myelodysplastic syndrome (MDS) were diagnosed 8-40 years after injection, the cumulative frequency reaching 7.6%. No significant relationship was seen between the cumulative frequency of AML + MDS and the age at injection, gender, or amount of Thorotrast injected, but a multivariate analysis described data best by a model with the bone marrow dose and the power of the attained age. The risk estimate for AML + MDS was <tex-math>$173\ \text{cases}/10^{4}$</tex-math> persons per Gy. If also considering cases of acute lymphocytic leukemia (1), chronic myelogenous leukemia (3), non-Hodgkins lymphoma (4), and multiple myeloma (2), the risk estimate became <tex-math>$248\ \text{cases}/10^{4}$</tex-math> persons per Gy. It is suggested that RBE of α particles from thorium may be lower than 20.

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