wT tote saan bee a! siblebe Mial hats Sates einen st « 110 of the *!°Pb and ?!*Bi atoms observed in the RES are not bound within the particles. The high activities of both these isotopes can be explained by the fact that *8Ra/*3"Th ratios come out somewhat higher those imphed by the figures of Tables 42 and 43. swnably because of the aggregation of Thorotrast ticles that occurs at long times after administrati For individual tissues at long times after Thoro administration, the steady state activity ratio: served by the present authors and those reported viously by other workers, are summarized in Tab: On the basis of these data a collection of current * their biological half-lives within the RES are much longer than their physical half-lives. 1 Best Estimates of the Distribution of Activities throughout the Human Body and the Consequent Radiation Dose-Rates in Long-Term Thorotrast Patients. estimates” (in the present authors’ opinion) o There is unfortunately no completely objective way steady state activity ratios has been assembled i ble 50. In cases where no human data were ava! for this table, the best estimates were derived frat results of the animal experiments reported abov: data are quoted for the ratios ??°Em/**4Ra. How from thoron breath analyses and calculations inve the circulation time of the blood, it is possible to to combine the data reported here with those of other workers to obtain the current “best estimates” of the distribution of Thorotrast activities throughout the hu- man body. Different investigators undoubtedly tend to attach different weighting factors to all the evidence that is available. For the whole body, the most reliable estimates of the steady state activity ratios of the various thorium daughter products are probably derived from excretion measurements. The relations between percent excreted per dayand activity ratio in the whole body are of the form shown for four of the thorium-series radionu- clides in Figure 87. Steadystate activity ratios calcu- lated on this basis for the whole body are summarized in Table 49. It is to be noted in particular that the 100 a rough estimate of the proportion of thoron : generated within the major Thorotrast deposits 1 RES which escape into the general circul. Hursh®® calculated 16%, and Grillmaier et concluded that it might even be as high as 20% tissues other than the RES, the expected thorona ties are more difficult to predict. From Stover’s on #2Pb metabolism ®” there is some evidence TTT TTT] Ty T y TT TTit ACTIVITY RATIO (%) 212Ph /232Th 80 22474 /232 TH 60 40- 228TH /232 Th 2228p, / 2327p 20;+o 0.01 to f i etl 0. | dof py L 1.0 l risa) 10.0 Jet i ttl 100.0 % EXCRETED PER DAY {as fraction of total body content) Fra, 87,—Relation between steady state activity ratios in the whole body and the excretion rates of four thorium serie+ products. TABLE 50. AtctHors’ “Best Estimates” or Steapy Stare Activiry Rarros 1y Loxeg-Team THorotrast PAtTiEeN” Organ Liver 28Ra 22@Th | 28AC/8Ra 0.5 1.00 0.2 1.4) Spleen 0.5 Trabecular bone 1.3%) (1.0) 0.2 0.5 (1.0) (£.0) Kidney Red bone marrow Lung 1,00 28Th/#8Ra | | | 24Ra /28Th wPh /PARa 21273; /22Pb 0.9 0.7 0.5 0.7 1.43 0,9) 4,7) 1.90) 9.9 1.1 1.1 1.0 0.7 1.7 0.6 ~1 0.5 0.9) =1 ~18 0.7 1.0 (1) (1) Source of , | Table 4s "Fable 48 Table 44 Tables 4° Tables 4: Table 46 (a) Except where indicated, the quoted values are based on measurements of tissues from actual long-term Thorotrast p (6) Based on data obtained from experimental animals. ‘c) Values much greater than one are observed in short-term animal experiments (Table 45). @) Assumed value in absence of actual measurement.