In utero haemopoietic sensitivity to alpha, beta or X-irradiation in CBA/H mice.

PURPOSE: To assess in utero sensitivity to x-rays, alpha-emissions from plutonium-239 and beta-emissions from tritium in terms of induction of chromosomal aberrations in bone marrow cells. MATERIALS AND METHODS: CBA/H mice were exposed to a single dose of X-rays (0.5Gy) on either day 7 or day 14 of pregnancy or given (239)Pu (100 kBq kg(-1)) by intraperitoneal injection on either day 6 or day 13. Tritium was administered to mice throughout pregnancy as either tritiated water, ad libitum in drinking water (total intake averaged 130 MBq), or as homogenized tritiated cress, administered by gastric intubation (total 60 MBq). Irradiated and unexposed control mice and their offspring were sacrificed at 2-8 weeks after birth. Direct metaphase preparations from femoral bone marrow cells from mothers and offspring were used for G-band analysis. RESULTS: The incidence of stable aberrations was significantly and similarly increased in neonatal and maternal marrow samples after exposure to X-rays, (239)Pu or (3)H. The estimated average bone absorbed doses from (239)Pu in pregnant females were similar to the X-ray dose of 0.5 Gy, suggesting a low RBE for alpha-irradiation in adults. The similar levels of damage observed in neonates after X-irradiation and 239Pu exposure are indicative of greater in utero sensitivity to alpha-irradiation since the overall estimated in utero alpha-particle doses to haemopoietic tissue were much lower. In utero doses from (3)H and corresponding maternal doses were around 0.5Gy, showing no evidence of greater in utero sensitivity, no significant difference between the effects of the two forms of tritium, and were consistent with an RBE value of 1-2. CONCLUSIONS: Comparison of stable aberration yields in haemopoietic cells suggests a greater sensitivity to alpha-particles from (239)Pu than X-rays or beta-particles from (3)H for irradiation in utero but a low RBE value in adults.

Tumorigenic target cell regions in bone marrow studied by localized dosimetry of 239Pu, 241Am and 233U in the mouse femur.

PURPOSE: To study the temporal change in microdistribution of plutonium-239, americium-241 and uranium-233 in the mouse distal femur and to compare and combine calculated radiation doses with those obtained previously for the femoral shaft. Also, to relate doses to relative risks of osteosarcoma and acute myeloid leukaemia. MATERIALS AND METHODS: Computer-based image analysis of neutron-induced and alpha-track autoradiographs of sections of mouse femora was used to quantify the microdistribution of (239)Pu, (241)Am and (233)U from 1 to 448 days after intraperitoneal injection. Localized dose-rates and cumulative doses over this period were calculated for different regions of the marrow spaces in trabecular bone. The results were then combined with previous data for doses to the cortical marrow of the femoral shaft. A morphometric analysis of the distal femur was carried out. RESULTS: Initial deposition on endosteal surfaces and dose-rates near to the trabecular surfaces at 1 day were two to four times greater than corresponding results for cortical bone. Burial was most rapid for (233)U, about twice the rate in cortical bone. As in cortical bone, subsequent uptake into the marrow was seen for (239)Pu and (241)Am but not (233)U. Cumulative doses to 448 days for different regions of trabecular marrow were greater than corresponding values for cortical marrow for each radionuclide. Combined doses reflected the greater overall volume of cortical marrow. CONCLUSIONS: Cumulative radiation doses to the 10 microm thick band of marrow adjacent to all endosteal surfaces were in the ratio of approximately 7:3:1 for (239)Pu:(241)Am:(233)U. This ratio is not inconsistent with observed incidences of osteosarcoma induction by the three nuclides. Analysis of doses to different depths of marrow, however, showed that although ratios were probably not significantly different to that for a 10 microm depth, better correlations with osteosarcomagenic risk were obtained with 20-40 microm depths. For acute myeloid leukaemia, the closest relationship between relative risk and doses was obtained by considering only the central 5-10% of marrow, which gave a dose ratio of approximately 12:11:1 for (239)Pu:(241)Am:(233)U respectively.

Lack of detectable transmissible chromosomal instability after in vivo or in vitro exposure of mouse bone marrow cells to 224Ra alpha particles.

Several studies over recent years have highlighted the possibility that radiation can induce transmissible genomic instability. Most of these involve in vitro irradiation and usually in vitro culture. Here it is reported that the short-half-life bone-seeking alpha-particle emitter (224)Ra did not induce excess transmissible chromosomal instability in CBA/H mouse bone marrow cells in a 100-day period after in vivo or in vitro exposure. Similarly, no excess transmissible chromosomal instability could be detected after in vivo whole-body X irradiation. It was noted, however, that short-term culture of murine bone marrow cells elevated yields of aberrations, as did transplantation of untreated marrow into radiation-ablated hosts. These findings emphasize the sensitivity of murine hemopoietic tissue to experimental manipulation and reinforce the importance of appropriate concurrent control experiments in any investigation of transmissible genomic instability.

Comparison of cytogenetic damage in cultured cells from cobalt-60 gamma-radiation and the Auger emitter zinc-65.

PURPOSE: To assess the ability of the Auger-emitting nuclide, zinc-65 (65Zn), relative to gamma-irradiation, to cause chromosomal aberrations in cultured rat prostate cells. MATERIALS AND METHODS: Rat prostate adenocarcinoma cells in culture were exposed to doses of 1, 2, 3 or 5 Gy of external gamma-irradiation for 24h or incubated with 0.7, 1.5, 1.8 or 2.8 MBq of 65Zn for 24 h. The uptake by and clearance from cells of 65Zn was measured. Metaphase spreads prepared from washed cells were scored for chromatid- and chromosome-type aberrations. RESULTS: Following exposure to 65Zn or gamma-irradiation, chromatid-type damage was more commonly observed than chromosome-type aberrations. The relationship between induced chromatid damage and gamma dose (to 3 Gy) was best fitted by a second-order polynomial function, while the activity response relationship for chromatid damage caused by 65Zn appeared to be best fitted by a straight line. Measurements of the uptake of 65Zn by cells showed that average concentrations within cells were about 100 times the concentration in the culture medium. Assuming uniform distribution of 65Zn within cells, with 36% in the nucleus, the dose was estimated as 0.70 Gy per MBq added 65Zn, with Auger electrons contributing most (93%) of the dose. Assuming that 20% of cellular zinc was localized in the nucleus, based on previous measurements, the dose to the nucleus was calculated as 0.44 Gy per MBq added 65Zn. RBE values for chromatid damage induced by 65Zn compared to gamma-radiation range from about 1 to 3 based on a uniform dose throughout the cell and from about 2 to 5 based on 20% of 65Zn in the cell nucleus. CONCLUSION: The observed radiotoxicity of 65Zn is consistent with its behaviour as an Auger-emitting radionuclide that is localized to some extent in the nucleus.

Microdistribution and localized dosimetry of the alpha-emitting radionuclides 239Pu, 241Am and 233U in mouse femoral shaft.

PURPOSE: To analyse the temporal change in microdistribution of 239Pu, 241Am and 233U in mouse femur and to compare the calculated radiation doses with regions of the bone marrow thought to contain target cells for osteosarcoma and leukaemia with relative risk for those diseases. MATERIALS AND METHODS: Neutron-induced and alpha-track autoradiographs were prepared from femora of the CBA/H mouse that had been injected with 40 kBq kg(-1) radionuclide between 1 and 448 days previously. Computer-based image analysis of the autoradiographs was performed and dosimetric methods applied to obtain radiation dose-rates to different regions of the marrow cavity. RESULTS: Initially each radionuclide deposited on endosteal and periosteal bone surfaces; 241Am was additionally deposited on vascular canal surfaces. Redistribution resulted in 233U being incorporated into bone, while 239Pu and 241Am showed transfer into both bone volume and marrow. Accumulation in the central marrow peaked at 112-224 days post-injection, but subsequently was cleared by 448 days. Cumulative doses to both osteosarcomagenic and myeloid leukaemogenic target cell regions showed the trend 239Pu > 241Am > 233U. CONCLUSIONS: Calculation of cumulative doses to a 10-microm layer of marrow adjacent to bone surfaces appears to be a suitable predictor for risk of osteosarcoma. Risks of myeloid leukaemia in the mouse are better predicted by considering the central marrow as the target region rather than average dose to all marrow.

Temporal change in microdosimetry to bone marrow and stromal progenitor cells from alpha-particle-emitting radionuclides incorporated in bone.

The microdistributions of the alpha-particle-emitting bone surface-seeking radionuclides (239)Pu, (241)Am and (233)U in the mouse femoral shaft have been studied using computer-based image analysis of neutron-induced and alpha-particle track autoradiographs, prepared from femora of CBA/H mice which had been injected with 40 kBq kg(-1) of radionuclide (as citrate solution) at times from 1 to 448 days previously. Employing dosimetric methods, radiation dose rates and cumulative radiation doses to regions of the bone marrow thought to contain hemopoietic and stromal progenitor cells susceptible to neoplastic transformation to leukemia and osteosarcomas have been calculated. It has been shown that the three radionuclides differ in their relative deposition on the bone surfaces, and that patterns of changing redistribution with time are also varied. For stromal progenitor cells, which are thought to be targets for induction of osteosarcoma and are found in proximity to the bone surfaces, cumulative doses showed the trend (239)Pu > (241)Am > (233)U, correlating well with incidences of osteosarcoma observed in mice. Cumulative doses to the primitive hemopoietic stem cells, concentrated in the central marrow and thought to be susceptible to neoplastic transformation to myeloid leukemia, were considerably lower and also showed the trend plutonium > americium > uranium.

Chromosome aberrations in Syrian hamsters following very low radiation doses in vivo.

This paper addresses a report of a large increase (approximately 6- to 11-fold) in chromosome aberrations in lymphocytes of persons in Salzburg attributed to their exposure to fallout from the Chernobyl cloud. Their additional exposure, approximately 0.3 mGy in 1 year, comprised about a 30% increase in their normal background radiation dose. The report has attracted considerable attention because, if correct, it seriously challenges assumptions of linearity in the low-dose response for chromosomal damage and, by implication, the linear, no-threshold hypothesis for risk of induced cancer. An experiment has been carried out with Syrian hamsters treated with caesium-137 to produce a range of doses comparable with those calculated for the persons in Salzburg. No significant elevation in lymphocyte aberration yields was found in the hamsters, thus arguing against the conclusions of the Salzburg study.

Transfer of polonium to the embryo and foetus of rat and guinea pig.

Transfer of 210Po to the foetus measured 3 days after administration in rat and 7 days later in guinea pig increased with increasing gestational age to about 0.1% injected activity per rat foetus at birth and 0.6% per guinea pig foetus on day 57, corresponding to whole-body foetus:mother concentration ratios of about 0.1:1 in both species. The greatest concentrations of 210Po were measured in the rat yolk sac during its haemopoietic stage, an order of magnitude greater than concentrations in the placenta and two orders of magnitude greater than foetal concentrations. The results obtained have been used to estimate in utero doses to haemopoietic tissues, taking account of transfer to the blastocyst/egg cylinder, yolk sac, liver and bone marrow. The concentration ratios relative to maternal liver for these tissues were taken to be 1, 3, 0.1 and 0.05 respectively and were applied to periods of human gestation of 0-2.5, 2.5-6, 6-12 and 12-38 weeks respectively. For chronic maternal intake by ingestion of 210Po during the year of pregnancy giving a committed effective dose (CED) to the mother of 1 mSv, the total in utero dose to haemopoietic tissue was about 340 microSv compared with a maternal red bone marrow dose of 2.2 mSv. The yolk sac and bone marrow accounted for 66 and 27% of the in utero dose respectively. In addition, the total CED to the offspring was calculated assuming a whole-body foetus:mother concentration ratio of 0.1:1 and that the distribution of 210Po between tissues was the same in the foetus as in adults and children. For chronic intake of 210Po during the year of pregnancy as assumed above, the CED to the offspring was estimated to be 8% of that to the mother.

The distribution and retention of plutonium, americium and uranium in CBA/H mice.

Groups of male and female CBA/H mice were given intraperitoneal injections of 40 kBq kg-1 of 239Pu, 241Am and 233U citrate solutions and the retention and distribution of the three radionuclides compared at times up to 448 days. Similar results were obtained for males and females and showed that: 1. Whole body retention at 448 days was very similar for 239Pu and 241Am, accounting for about 20% of injected activity for each nuclide; retention of 233U was much lower at about 3%. 2. The skeleton accounted for 85% or more of retained 239Pu, 241Am and 233U activity from 6 weeks after injection. 3. The greatest concentrations of each radionuclide were measured in the main body of the spine, limb girdles and ribs, with lowest concentrations in the paw bones, head bones and caudal vertebrae. The inhomogeneity of distribution was in the order Pu > U > Am; with a trend to more uniform activity with time. 4. Average bone doses to 448 days were calculated as about 1.6 and 1.7 Gy for 239Pu and 241Am, respectively, and 0.3 Gy for 233U, with ranges for individual bones of 0.7-3.0 Gy, 1.1-2.5 Gy and 0.1-0.6 Gy, respectively. Average liver doses to 448 days were calculated as about 0.9 Gy, 0.6 Gy and 0.007 Gy for 239Pu, 241Am and 233U respectively, whilst the dose to the kidney for 233U was about 0.1 Gy. 5. Autoradiographic studies of the distribution of the nuclides in the femur showed differences in their initial distribution and subsequent movement. Initially, concentrations of 239Pu were greater on endosteal than periosteal surfaces while 241Am distributed more evenly on bone surfaces. The initial deposition of 233U on all surfaces was uneven with concentrations probably on active surfaces. Burial of all three nuclides in areas of bone growth was observed. Transfer of activity to the marrow was greatest for 239Pu and least for 233U.

Gastrointestinal absorption and retention of polonium in adult and newborn rats and guinea pigs.

The gastrointestinal absorption of 210Po was determined by comparing tissue retention after oral and systemic administration. The results indicate an increase in absorption in adult rats for 210Po administered in liver compared with 210Po nitrate with estimated absorption of 5 and 13%, respectively. For 210Po citrate, values of about 7% were obtained in 1-day-old neonate and adult rats while absorption in guinea pigs was estimated to be about 23% in 1-day-old neonates, 17% in 5-day-old neonates, and 9% in adults. Gut retention of ingested 210Po in neonates was high in rats but not guinea pigs. In adult animals, but not neonates, the liver accounted for a greater proportion of 210Po reaching the bloodstream after ingestion than after systemic injection. The significance of these results is discussed in relation to current assumptions made in the calculation of doses from 210Po.

The biokinetics of plutonium-239 and americium-241 in the rat after subcutaneous deposition of contaminated particles from the former nuclear weapons site at Maralinga: implications for human exposure.

As an input to dose assessments, measurements have been made of the clearance of Pu and Am after subcutaneous implantation in rats for six particulate materials and one dust from the Maralinga test sites. The tissue distribution of Pu and Am were measured in groups of six animals at one month and 6 months after implantation. In addition, in vitro solubility tests were carried out on eight different particulate materials. Histological examination of the subcutaneous implantation site was undertaken after one year for selected materials. Autoradiographs of tissue sections showed that particles were surrounded by fibrotic tissue with macrophage and polymorphonuclear cell infiltration, the normal tissue response to foreign materials. The clearance data have been used to make estimates of the likely range in potential radiation doses in humans. To calculate the dose from dissolved 239Pu and 241Am, four different situations have been considered. For the dust, the results suggest that dissolution essentially ceases after the removal of Pu and Am from the surfaces of dust particles. From the values obtained, the acute release of a fraction of 10(-2) of both nuclides from a dust contaminated wound was assumed. For a number of particles the results suggested continued dissolution and the clearance of 10(-3) per year of both nuclides, continuing for a number of years, has therefore been considered. For the least soluble particles, there was no clear evidence of continued clearance and the acute release of 10(-4) has therefore been taken as a lower estimate for dose calculations.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurements of the gastrointestinal absorption and tissue distribution of plutonium, americium and polonium in experimental animals.

In order to set limits on environmental and occupational intakes of radionuclides, information is needed on their uptake and metabolism in man. Human data are very limited, particularly for long-lived alpha-emitting isotopes such as those of the actinides. Animal experiments are therefore an important source of data on the distribution of radionuclides in tissues, and the effects of factors such as subject age and the chemical form of elements on gastrointestinal absorption. The NRPB performs experimental programs using mainly rats and guinea pigs. In order to study the gastrointestinal absorption and tissue distribution of plutonium, americium and polonium, a variety of analytical techniques are employed. These include ion exchange and solvent extraction leading to alpha spectrometry and liquid scintillation counting. The investigation of low specific-activity environmental or industrial materials, and the very low bioavailability of elements such as the actinides, means that very low levels of activity have to be measured. Contamination at the dissection and tissue separation stage, as well as during the radiochemistry, has to be rigorously avoided. Where very detailed information is needed on the location of radionuclides within tissues, such as in the study of alpha-emitter distribution in the intestine, autoradiography is used. The application and relevance of different measurement techniques to animal studies will be discussed and examples of the results presented.

A technique for embedding undecalcified bone samples for detecting alpha-emitters using vacuum impregnation with Spurr's resin.

A method has been developed by which large samples of mineralized bone, containing an alpha-emitter, can be embedded in Spurr's resin in a fraction of the time required by conventional methods. Bone samples were freeze-dried or fixed and dried prior to impregnation with Spurr's resin under vacuum. Sections were cut for the preparation of either alpha-track or fission-track autoradiographs using the solid state detector CR-39. This method is applicable to samples containing a mobile form of a radionuclide that may be translocated during the processes of fixation and dehydration of the specimen.

The incorporation of plutonium by the embryo and fetus of rats and guinea-pigs.

The transfer of 238Pu from the maternal circulation to the developing embryo and fetus was studied in rats and guinea-pigs to provide data for the development of dosimetric models for the human embryo and fetus. Following administration at different stages of gestation, measurements were made after 3 days in the rat and 7 days in guinea-pigs. The amount transferred was greater after administration at later stages of gestation, up to a maximum of about 0.8-0.9% of the injected activity per fetoplacental unit (FPU) and about 0.2% per fetus in both species. In advanced gestation the yolk sac, the initial site of haemopoietic stem cells, contained up to 80% of the total activity in the FPU in rats, compared with about 25% for the guinea-pig; retention in placental trophoblast was greater in the guinea-pig. The concentrations of 238Pu in the yolk sac were generally about two to three orders of magnitude greater than fetal concentrations and of the same order as in maternal liver and bone. In both species, concentrations in the embryo and fetus were greatest after injection early in gestation, reached their lowest around mid-organogenesis and increased again in later gestation. The fetus:mother whole-body concentration ratios in late gestation were about 0.1 and 0.05 in rats and guinea-pigs, respectively. Measurements were also made of the 238Pu retention in neonates at birth. In guinea-pigs the liver accounted for about 6-9% of retained activity; similar values for femora indicated skeletal retention of about 60-80%. For administration at each stage of gestation, and particularly at early stages, transfer of 238Pu to the fetus continued throughout gestation but concentrations decreased due to fetal growth.

The gastrointestinal absorption and retention of niobium in adult and newborn guinea pigs.

The gastrointestinal absorption of 95Nb ingested in milk by adult guinea pigs on a milk-supplemented diet was estimated as 0.8 +/- 0.2% (SEM; n = 4) and a value of 1.4 +/- 0.2% was obtained for guinea pigs fasted for 24 h before and 2 h after the oral administration of 95Nb in a citrate solution. The absorption in 2-day-old animals given the 95Nb-citrate solution was estimated as 1.5 +/- 0.2% (SEM; n = 3). These results support the values currently used for radiological protection purposes to calculate doses from the ingestion of niobium isotopes; these are 1% for adults and 2% for infants in the first year of life. Intestinal retention of niobium in newborn guinea pigs was low, unlike retention in other species, but consistent with observations of the retention of other elements in guinea pigs. It is considered that retention in human neonates is likely to be most similar to that in guinea pigs. Whole-body retention of 95Nb after systemic administration in citrate solution was slightly lower in 2-day-old guinea pigs than in adults, with rapid excretion of about 50% over the first day compared with 40% in adults. Subsequent clearance was similar in the two age groups with a retention half-time of about 30 days.

Autoradiographic studies of the distribution of radium-226 in rat bone: their implications for human radiation dosimetry and toxicity.

A solution containing 226Ra chloride was injected into young female rats via the saphenous vein. Subsequently, the distribution and retention of the 226Ra in the skeleton was studied. The results show that: 226Ra is initially deposited in the rat femur as a volume deposit and is fairly evenly distributed throughout the bone matrix. Much of the 226Ra initially deposited in the skeleton is lost within a few days of its administration. During the first week 226Ra gradually accumulates at sites of bone deposition including accreting surfaces. Subsequent bone growth results in the burial of contaminated bone surfaces and Following bone resorption some of the 226Ra released from individual bones is recycled systemically so that all skeletal components tend towards a uniform 226Ra concentration per unit of bone mineral. Of the two models conventionally used for radiation dosimetry purposes, the results reported here for rats suggest that though neither is ideal, the volume distribution model is preferable to the surface model at all times after the uptake of radium by the skeleton.

Pattern of uptake of americium-241 by the rat skeleton and its subsequent redistribution and retention: implications for human dosimetry and toxicology.

The distribution and retention of intravenously injected 241Am in the skeleton of the female rat has been investigated using autoradiographic and radiochemical techniques. The studies were designed to assess the dosimetric and toxicologic implications of an 241Am intake by man. They showed that in the rat approximately one third of the intravenously injected 241Am was deposited in the skeleton where it appeared to be retained with a long biological half-time. The studies also showed: 1 241Am is initially deposited onto all types of bone surface including endosteal surfaces, periosteal surfaces and those of the vascular canals within cortical bone, but seems to be preferentially deposited onto those that are resorbing, 2 Bone accretion results in the burial of surface deposits of 241Am, 3 Bone resorption causes the removal of 241Am from surfaces, 4 Resorbed 241Am is retained by phagocytic cells (probably macrophages) in the bone marrow, 5 The transfer of 241Am from the phagocytic cells in the marrow to adjacent bone surfaces seems to occur, (local recycling). 6 The possibility that some of the 241Am removed from the bone surfaces enters the blood and is redeposited in bone, (systemic recycling) cannot be dismissed. These results show that 241Am deposition and redistribution in bone shares many characteristics with other 'bone surface-seeking radionuclides' typified by 239Pu. Consequently, it is suggested that a similar model to that used to calculate annual limits of intake for 239Pu in man would be suitable for the calculation of corresponding values for the 241Am isotopes.