102 Rundo®>? observed that the estimated *°°Th content of tion of °Th during the days immediately foll and 75% of that of the whole liver. Experiments with dogs reported by Zilversmit et al.“5) suggest that such the administered dose in 17 days, and 0.1% i the whole spleen varied between the wide limits of 8 administration of Thorotrast to two patients (0. days) can be taken as evidence that a small p tion of the thorium in Thorotrast mayexist in «: ble form. Thus, it may well be that the small sk differences are unlikely to be due to the use of Thorotrast batches of different mean particle sizes. It is more likely that the explanation can be found in the different pathological conditions from which these pa- burden of thorium in Thorotrast patients is d from such a soluble pool, in which case, like *** long times after injection,'2) it would be expec tients suffered, and which were, in part, the reason for the administration of the Thorotrast. As reported by have a relatively diffuse distribution. Rankin et al.,“!® wide variations in the relative uptake of colloidal material by liver, spleen and bone marrow, Tissue Distribution of *7P5 Many of the thorium daughter products ar are observed in patients with liver disease. The estimate of the thorium content of the whole marrow-free skeleton obtained in this investigation was 3.1% of the whole-body burden of **Th. This is somewhat higher than the values reported by other workers (Table 37, mean 0.6%). These discrepancies tributed throughout the body quite differently the parent nuclide, 7**Th. This is the conseque various mechanisms which operate to bring abc lective translocation from one organ to another the excreta. The phenomenonis probably most m are not surprising since all the presently available es- for 21°2Pb and the other daughter products whic low thoron in the deeay series. Despite the 54 half-life of thoron, its being an inert gas allows : ciable translocation to occur, even to the exten timates of skeletal burden are based on extrapolations from very small, and perhaps nonrepresentative, sam- ples of bone and bone marrow. A more thorough investigation, embracing manydifferent parts of the skeleton, is definitely required. Concerning its distribution approximately 10% is exhaled.‘1% 13.14 _212Pb is of little direct significance in the dos) in bone marrow, a recent autoradiographic study by Simmons‘2°) has shown that Thorotrast (*2°Th) is de- of Thorotrast since it emits only low energy 8 rays, and, therefore, contributes very little to th: energy deposition within the tissues. Neverthelean interesting element to study, for reasons less posited in rat bone marrow as a “hot line” at the time of injection and is not translocated further with subsequent bone growth. The chemical form of the skeletal 7°°Th is of some interest since it has a bearing on whether the distribu- tion is likely to be diffuse or ‘hot spot’’, 1.e. concen- trated in colloidal particles. The observation by Hursh and his colleagues’ of a small but significant excreTABLE 38. ficial than that its activity happens to be r measurable. In most of the tissues its biological life is considerably longer than its physical he of 10.6 hr.“® Therefore, there are good groun expecting that 7!°Pb can be used to estimate the ities of its dosimetrically important precursors, Tissurp DistrinutTion oF #?2Pb™ Animal Ageof Code Liver Spleen Kidney Rat 35d 38d 4ld 72 73 67 160 100 100 111 200 185 19 32 237 TH3 100 228 228 18m TH2 22m 12m TH4 100 100 120 — 311 THS5 100 241 Dog 21d 78 100 17 Man 48d 85 100 59 8 100 144 2 | ‘) Concentration of 2“Pb/g of tissue as “% of liver concentration. ) Kidney medulla. (e} Kidney cortex. Cortical Redbone Bloo 400 22m Mean values Lung : 202 8°) bog 9 BGC) =| 35 8 30 2 85 9 30 2