Late effects of α-particles on Thorotrast patients in Japan

The results of tissue dose estimation for 138 Thorotrast cases are described. The mean liver dose rate and dose were 0.26 Gy/year and 8.34 Gy, respectively with a mean exposure time of 36.8 year. The average amounts of thorium in the liver, spleen and bone marrow were 2.37 g, 0.52 g and 1.39 g, respectively, whereas, based on relatively fresh samples, the relative amounts of thorium in the three organs were 60%, 19% and 21%, respectively. Among all the diseases, the highest mean dose rate (0.39 Gy/year) and shortest mean exposure time (32.6 year) were seen for liver cirrhosis. There were no significant differences in the subtypes of malignant liver tumours. In plots of cumulative disease frequency versus exposure time, the curves appear to become more curvilinear upward as the liver dose rate decreases. (author)