112 Thorotrast. At earlier times than the 20-25 years considered in Table 51, the dose rates may be different on account of the different status of activities in the decay chain (Figure 84) and the different aggregation of particles within the tissues. However, these two effects partially compensate each other, and the quoted figures are probably reasonable measures of the average dose rates for Thorotrast burdens of long standing (>10 years). Further information on the expected time dependence of the dose rate has been given by Kaul.“ A comparison between the above best estimates of dose rates and those reported previously by other workers is contained in Table 52. Lest the apparently good agreement suggest that the dosimetry of Thoro- trast no longer presents any problems, it should be pointed out that the present authors’ calculations have, of course, relied extensively on measurements reported by previous workers in this field. The principal new Mr. L. D. Marinelli, Radiological Physies DL for his advice and encouragementin carrying o1 studies; to Dr. D. J. Simmons and Mr. J. E. F. for assistance with the animal experiments; anc F. Markun for other technical assistance. On (RMP) would also like to thank Dr. R. A. (IAEA) for the benefit of many useful discussio for permission to quote the results for patient 1 was one of the IAEAseries of patients. REFERENCES 1. Marinelli, L. D. Trans. 9th Int. Congr. of Radiol: B. Rajewsky. Georg Thieme Verlag, Stuttgai p. 1234. . Rundo, J. Phys. Med. Biol. 1, 138 (1956). co ot A GO bo applied equally to the whole proportion of the activity of each radionuclide, the dose-rate estimates for liver, spleen, and bone marrow would have been reduced respectively to 55, 104 and 17 Rads/year from 50 mI of . Rundo, J. Acta Radiol. 47, 65 (1957). . Rundo, J. Ph.D. Thesis, London, 1958. . Rotblat, J. and Ward, G. Phys. Med. Biol. 1, 125 | . Kaul, A. IAEA Symposium in Heidelberg on the ment of Radioactivity in Man, Proc.[IAEA Symp., berg. Int. Atomic Energy Agency, Vienna, 1964 p. 491. . Stablhofen, W. and Kaul, A. Radiological Health an in Mining and Milling of Nuclear Materials, Prox point of interest is perhaps the dose to the lung since . mates have relied on calculations from the activity of thoron in the breath. . the present estimate is based on an actual activity measurement of lung tissue itself, while previous esti- CONCLUDING REMARKS Although the overall picture of Thorotrast metabolism presented here is unlikely to undergo major revision, there remain many important points of detail that need to be settled. Undoubtedly the greatest in- adequacy in our current estimates of dose rate is their expression in the form of mean organ dose. On the mi- croscopic scale, as shown by Rotblat et al.,"“42 it is to be expected that doses delivered to small regions of an organ may be a factor of ten or more higher than the average. The elucidation of these inhomogeneities re- mains one of the most challenging of the problemsstill to be solved. However, even if this problem is discounted, mean dose rates in the range of 3 to 100 Rads per year for a 50-m! Thorotrast injection are still] dis- turbingly high. By comparison, the maximum permissible body burden of 7?8Ra (0.1 wCi), uniformly dis- tributed m the skeleton, would yield a dose of about 3 Rads per year, It 1s clear, therefore, that Thorotrast patients constitute an important and unique population for the study of low-level and long-term irradia- tion in man. Their numbers are fast diminishing and within a few more years suchstudies will no longer be possible. The authors wish to express their sincere thanks to 10. 11. 12, Symp., Vienna. Int. Atomic Energy Agency, 1964, Vol. 2, p. 475. Kaul, A. IAEA-WHO Panel Meeting on Dosime Toxicity of Thorotrast, Vienna, October, 19 Atomic Energy Agency, Vienna, 1968, Tech. 106, pp. 30 and 69. Hursh, J. B., Steadman, L. 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Med. 40, 255 (1952). 19. Rankin, J. G., Playoust, M. R., and Beal, R. W. « Clin. Med. 68, 920 (1961). 20. Simmons, D. J., Cummins, H., and Nirdlinger, E. . Roentgenol. 103, 902 (1968). 21. Jee, W. 5S. 8. Assessment of Radioactivity in Man. IAEA Symp., Heidelberg. Int. Atomic Energy A Vienna, 1964, Vol. 1, p. 369. 22, Tange, J. D., Hayward, N. J., and Bremner, D. A. A Ann. Med, 14, 49 (1965).