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DOSIMETRY AND METABOLISM OF INCORPORATED RADIONUCLIDES

Objective


For administering aerosols of high toxicity (eg alpha emitters) to small laboratory animals, a new inhalation facility has been built and commissioned. It comprises a suite of interconnecting gloveboxes which accommodates all the operations involved in an inhalation exposure.

Experimental data obtained on the biokinetics of the plutonium-239 bearing materials in rats indicated that whilst the annual limits on intake (ALI) ranged between those recommended by the International Commission on Radiological Protection (ICRP) for Class W and Y compounds, the rates of transfer of plutonium-239 and americium-241 blood (and hence the doses and risks to liver and skeleton) varied by more than two order of magnitude.

Experiments have been undertaken to investigate the clearance of soluble forms of uranium from the 3 anatomical regions of the respiratory tract defined in the ICRP lung model. Small volumes of uranyl nitrate and bicarbonate solutions were instilled into the nasal passage, trachea and bronchial tree and pulmonary region of the lungs of rats. The amounts of uranium which translocated to the blood by 10 days were, for both compounds, about 8%, 40% and 70% respectively of the initial deposits. We have also investigated the efficacy of the sodium salt of 4,5-dihydroxy-1,3 benzene disulphonic acid (Tiron) after the intratracheal instillation of uranyl nitrate in amounts which correspond to about 20 times the permissible daily intake of uranium by workers (2.5 mg). The body contents of uranium 5 days after exposure were approximately 107, 78 and 65 respectively of those in untreated animals. These results indicate that the development of more effective chelating agents is an important consideration for workers potentially exposed to transportable uranium compounds.

Studies have been undertaken of the skeletal uptake and distribution within bone of radium, thorium, uranium, plutonium and americium, mainly in rodents. The results have shown that while each element is initially deposited on bone surfaces, they differ considerably in their distribution on different bone surfaces and may therefore deliver substantially different doses to sensitive cells on bone surfaces and in the marrow. For plutonium, comparisons have been made between rats, mice and baboons, indicating that the behaviour of the actinide in rodent bone can be used to predict behaviour in primate bone. The distribution of radium in bone was studied using pigs because of the close similarity between pigs and humans in alkaline earth metabolism. The results confirmed the initial deposition of radium on bone surfaces rather than throughout bone mineral. A study is currently in progress to compare the toxicity of plutonium-239, americium-241and uranium-233 in mice. The objective is to relate the differences in the distribution of dose within the skeleton, and the extent of irradiation of different cell types, with the observed incidence and distribution of osteosarcoma. A low incidence of leukaemias is also expected.

The general approach used was to administer by inhalation, inert, monodisperse, radiolabelled particles. Respiratory tract retention and clearance were followed by external gamma ray counting and excretion measurements, complemented in animal studies by serial sacrifices to determine tissue distribution.

At the end of the study, there remain major uncertainties about the kinetics of the clearance of particles from each part of the human respiratory tract, even for healthy adult males. Of particular current concern is the possibility that a significant fraction of material deposited in the bronchial tree region may not be cleared rapidly by mucociliary action, since the bronchial tree epithelium is regarded as being of relatively high radiosensitivity. These uncertainties were highlighted by work on the development of a model to represent mechanical clearance kinetics.

Studies in rodents provided further support for the assumption that mechanical transport rates from the respiratory tract are independent of particle composition. This is important for applying the results of human studies using nontoxic materials to the prediction of clearance of toxic materials. Similar rates of mechanical clearance from the alveolar region to the gastrointestinal tract were found for fused aluminosilicate (FAP), an actinide oxide, and 3 different forms of cobalt-57(3)oxide(4). The alveolar mechanical clearance rate was generally found to be remarkably constant in a given species. It is largely independent of age and gender, and unaffected by voluntary exercise, or even bronchoalveolar lavage. High levels of alpha irradiation were found to impair it, and the rate was found to be higher (but only initially) for particles administered by instillation than by inhalation.

Experimental studies on translocation to blood of material deposited. A new facility for administering radioactive aerosols to small animals was designed and constructed and, as results from interspecies comparisons suggested species differences for translocation rates of the same material, possible reasons for these differences were investigated.

Research has carried out in the following areas:
interspecies comparison of translocation rates;
the effect of animal age on lung clearance kinetics;
the factors affecting in vitro dissolution of cobalt-57(3)oxide(4);
the measurement of intraphagolysosomal pH;
the comparison of the retention of soluble cobalt in the lungs.

The study of the comparative toxicity of plutonium-239, americium-241 and uranium-233 in mice can be considered as consisting of 4 parts: radiochemical measurements of the distribution of the nuclides between the skeleton and soft tissues at times up to 448 days after intraperitoneal injection as their citrate complexes; radiochemical measurements of the retention of the nuclides in individual bones of the skeleton over the same period: autoradiographic studies of the distribution of the nuclides within bone; and comparisons of osteosarcoma induction in groups of mice given intraperitoneal injections of plutonium-239, americium-241 and uranium-233 activity to deliver equivalent average skeletal doses.

The initial study of the distribution and retention of the nuclides in the skeleton and soft tissues after systemic injection of 40 kBq/kg at times up to 448 days is complete. The results obtained were used to calculate average bone doses over the period of 0.52 Gy for plutonium-239, 0.45 Gy for americium-241. A second study examining the retention and distribution of plutonium-239, americium-241 and uranium-233 in the individual bones of the skeleton with time has also been completed. The nuclides deposited preferentially in the main body of the spine, limb girdles and ribs with lower concentrations in the lower limbs, paws and caudal vertebrae. The distribution pattern was similar to that observed in rats by other workers. The differences in the relative concentrations of the nuclides in the individual bones was reduced as time progressed and remodelling occurred, leading to a more homogeneous distribution. The inhomogeneity function, which describes the deviation of the relative concentration of the individual bones from the concentration of the whole skeleton, was calculated. Inhomogeneity was greatest for plutonium and least for americium. The decrease in inhomogeneity with time was more pronounced for plutonium and uranium than americium.

Autoradiographic s tudies using the femur, lumbar vertebrae and mandibular condyle are in progress. Alpha track autoradiographs have been examined to qualitatively determine the gross distribution of each radionuclide at 1, 7, 28, 112, 224 and 448 days after intraperitoneal injection. The autoradiographs showed that plutonium was deposited fairly evenly on endosteal bone surfaces and to a lesser extent on periosteal surfaces. At later times, some burial in the form of lines of activity was apparent as well as areas with more diffuse activity indicative of redeposition during bone remodelling. Progressive accumulation of plutonium in marrow was also observed. Americium was deposited more evenly on all bone surfaces including those of vascular canals within bone mineral. Again, some burial occurred in the form of lines of activity and some accumulation of americium in marrow macrophages was also seen but less than for plutonium. Uranium was deposited on all bone surfaces but not evenly, concentrating preferentially on some parts of the surface, with burial at later times after injection. Little uranium was seen in the marrow.

To quantify the distribution of alpha activity within the bone, fission track autoradiographs of femur sections have been produced. Track counts on random areas of sections have been used to make preliminary estimates of dose. The initial calculations show that while the doses to endosteal surfaces from plutonium-239 and americium-241 were greater than average bone doses at both 1 day and 224 days after administration, the dose from uranium-233 was greater than the average bone dose initially but lower at 224 days. The bone marrow dose from plutonium-239 was about the same as the average bone dose at 1 day and slightly higher at 224 days. Marrow doses from americium-241 and uranium-233 were lower than those for plutonium-239.

To compare osteosarcoma incidence, groups of mice (50 to 100 per group) were injected with either plutonium-239, americium-241 or urani um-233 at 1 of 3 dose levels. For plutonium-239 the dose levels were 5, 15, or 25 kBq/kg. The corresponding amounts for americium-241 and uranium-233, to give equivalent average bone doses, were 6, 17 and 29 kBq for americium-241 and 40, 118, and 197 kBq/kg for uranium-233. To date, approximately 70% of the animals have died. 18 bone tumours have been observed, 10 in animals given plutonium. 8 myeloid leukaemias have been identified and several additional suspected myeloid leukaemias are currently being histopathologically evaluated.

The alpha particle induction of acute myeloid leukaemia (AML) in the CBA/H mouse is characterized by deletion and rearrangement as specific sites of chromosome 2. It is now known that these chromosomal changes are induced directly by ionizing radiation in mouse haemopoietic cells and probably represent initiating events for AML. 2 of the myeloid leukaemias induced in this studies have been karyotyped using conventional methods and both of these have shown the chromosome 2 rearrangements characteristic of AML.

A study of the distribution and retention of polonium-210 in the skeleton of rats is in progress. Tissue distribution results show a skeletal retention of about 10% at 7 days. Autoradiographs of femurs and vertebrae show a fairly uniform distribuiton of activity throughout the marrow at 7 and 200 days after administration; the values for tissue retention at later times are not yet available. A study of the distribution and retention of plutonium-239 and polonium-210 in the marmoset has been carried out. 4 male marmosets were given injections of plutonium-239 and polonium-210 in citrate solution; 2 animals were killed at 1 week and 2 at 1 month. The results obtained for the distribution of plutonium in liver and bone were reasonably consistent with the current International Commission on Radiological Protection (ICRP) dosimetric model. Results for the tissue distribution of polonium at 1 week were similar to the values obtaine d for rats, with about 10% of the total activity retained in the skeleton. Autoradiographs prepared from the femurs of marmosets showed activity on bone surfaces, attributable to plutonium-239 and throughout the marrow, attributable to polonium-210.

The current ICRP biokinetic model for polonium assumes that for polonium entering the blood stream, 10% is deposited in each of the liver, kidneys and spleen and the remaining 70% is uniformly distributed throughout the rest of the body. The results obtained in these studies, together with other available data, suggest that appropriate values for the initial distribution of systemicpolonium might be 30% in liver, 10% in kidneys and 5% in spleen. The results for uptake and distribution of the polonium-210 in the skeleton suggest that it would be appropriate to make specific allowance for the uptake of 10% of systemic polonium-210 in red bone marrow; this would increase the dose to this tissue by a factor of about 6.
1. THE DOSIMETRY OF INHALED RADIONUCLIDES.
2. METABOLISM AND DOSIMETRY OF RADIONUCLIDES IN BONE.
3. MECHANICAL TRANSPORT OF PARTICLES FROM THE RESPIRATORY TRACT.
4. TRANSLOCATION OF MATERIAL FROM PARTICLES DEPOSITED IN THE RESPIRATORY TRACT.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

NATIONAL RADIOLOGICAL PROTECTION BOARD (NRPB)
Address
Chilton
OX11 0RQ Didcot
United Kingdom