Dose to Endosteal Cells and Relative Distribution Factors for Radium-224 and Plutonium-239 Compared to Radium-226

Abstract

H d t h Physics V d . 35 (July) pp. 91-101 P a w o n Press M.. 1978. Printed in Orcat Britain 0 Health Physics Society 00 I7-W78/78/070 1 -00! 3Abstract-The ICRP model, Alkaline Earth Metabolism in Adult Man, predicts that 10% of injected radium-224 atoms decay on bone surface, and that 1.5% decay in bone volume. The model was rerun for all possible values of the size of the bone surface compartment and of its rate constant in an effort to determine the reliability of the bone surface prediction. These runs together with data for 45Ca, "'Ba, and 526Ra in rabbits and dogs lead to lo+ 2% (probable error) as our best estimate for the bone surface fraction of injected '=Ra in man. This estimate corresponds to a time of maximum radium concentration at bone surface in man of 18+6hr, and a preference for radium compared to calcium in the size of the bone surface compartment of between 2 and 3. New measurements of the total surfacelvolume ratio of bone from 10 human skeletons give a preliminary value of 50 cm2/cm3. Assuming 100% retention of thoron and its daughters at bone surface, these input values, together with integrations reported by Mays, give the estimate that an intravenous injection of 1 pCi of 224Ra in a 70 kg standard man with 5 kg of bone results in an average dose to the endosteal cells 0-10 p m from bone surfaces of 1.5rad. Considering all sources of error we believe this estimate is probably good to within a factor of 2.By calculatin the ratio of the endosteal doses for equal average doses to bone for both 224Ra and ' $u relative to 226Ra in man, we give values for what we have called the Relative Distribution Factor (RDF). This factor predicts the relative toxicity of alphaemitting radionuclides in man relative to 226Ra on the assumption that the dose to endosteal cells is the causative factor. The value of RDF('"Rala6Ra) is 20, practically independently of whether endosteal dose is averaged 0-5 p m , 0-10 p m , 0-20 p m , or 0-30pm. The RDF(23%/p6Ra) is about 28 for a surface source of plutonium, and O.% for a volume source of plutonium. The reference for these factors is taken to be a volume source of 226Ra. Since endosteal doses calculated by averaging exactly 0-10 p m from bone surface are somewhat arbitrary for alpha emitters-we don't really know exactly how far from bone surfaces the target cells lie-it is reassuring to find that RDFs for alpha emitters relative to '=Ra are almost independent of distance so long as the cell targets lie within 3 0 p m of the bone surface. Therefore, the RDF with p6Ra as the standard of reference provides a more secure description of the spatial differences between the dose distributions of alpha emitters in bone than does endosteal dose itself.The observed RBE of 224Ra relative to 226Ra for the induction of osteosarcomas-the ratio of average bone doses (226Ra/mRa) for equal incidences-is about 6, a factor of 3 less than the RDF of 20. In other words, by present calculations, 224Ra appears to be a 'Work performed under the auspices of the U.S. Atomic Energy Commissi...

Autoradiographic Studies of the Distribution of Radium-226 in Rat Bone: Their Implications for Human Radiation Dosimetry and Toxicity

“…Since only accreting bone surfaces would seem to concentrate significant quantities of radium isotopes and in the mature human skeleton such surfaces represent only a few per cent of the total surface area, then in man the vast majority of the radionuclide is likely to be volume distributed at all times after intake. It follows that the use of the known toxicity of 224Ra in man as the convenient bench mark for assessing the likely toxicity of 239pu and other surface seeking radionuclides (Marshall et al, 1978;Mays et al, 1976;Miiller et al, 1978) in the belief that they share similar bone deposition patterns would seem to be unjustified on the basis of the results observed in this rat study. If so, then the explanation for the known differences in the toxicity of zz4Ra and 226Ra must be sought by considering other factors such as the time taken to deliver a given skeletal dose as a consequence of the radioactive half-lives of the isotopes and the comparative distributions and decay patterns of 22oRn and 222Rn and their daughters (Rundo, .1978;Mays et aL 1963).…”

The Radiobiological Significance of the Studies With 224Ra and Thorotrast

“…Any 224Ra which as a consequence reaches the blood stream, will be distributed between tissues in a manner similar to that of 224Ra injected directly into the venous blood (Marshall et al, 1978). The sites of tumours induced by this IZ4Ra should therefore also be the sites of tumours in Thorotrast subjects, granted that the tissue dose is sufficient.…”

Endosteal Dose to Thorotrast Patients

“…Then the endosteal dose-rate was computed by multiplying the average organ dose-rate by the ratio of endosteal doselaverage dose, as tabulated by Marshall et al (1978) or calculated from the dosimetric equations on which their paper was based (Mays and Tueller, 1964). The dose-rates from each source will now be evaluated:…”

“…bone dose of 10.1; while 1.5% decays in bone volume, giving a much lower endosteal/average of 0.49; thus yielding a weighted endosteal/average of I24 ENDOSTEAL DOSE TO THOROTRAST PATIENTS 8.9 (Marshall et al, 1978). Multiplying the average bone dose-rate of 0.57 rad/yr by the endosteal/average of 8.9 gives an endosteal dose-rate from ' "Ra translocating to bone of about 5.1 rad/yr.…”