Comparison of Homogeneous and Particulate Lung Dose Rates For Small Mammals

Small, highly radioactive fragments of material incorporated into metallic matrices are commonly found at nuclear weapons test and accident sites and can be inhaled by wildlife. Inhaled particles often partition heterogeneously in the lungs, with aggregation occurring in the periphery of the lung, and are tenaciously retained. However, dose rates are typically calculated as if the material were homogeneously distributed throughout the entire organ. Here the authors quantify the variation in dose rates for alpha-, beta-, and gamma-emitting radionuclides with particle sizes from 0.01-150 μm (alpha) and 1-150 μm (beta, gamma) and considering three averaging volumes-the entire lung (64 cm), a 10-cm volume of tissue, and a 1-cm volume of tissue. Dose rates from beta-emitting particles (e.g., Sr) were approximately one order of magnitude higher than those from gamma-emitting radionuclides (e.g., Cs). Self-shielding within the particle, which reduces the dose rate to the surrounding tissue, was negligible for gammas and minor for betas. For alpha-emitting particles (e.g., Pu), self-shielding in larger particles is substantial, with >90% of emissions captured within particles of +20 μm diameter; but for smaller sizes of the respirable range of 0.01 to 5 μm, an average of 85% of the energy escapes the particle and is deposited in the surrounding tissues. These data provide more detail on respirable particles, which may remain lodged deep in the lung where they represent a considerable contribution to long-term lung dose rates. For practical dose rate calculation purposes, a graph of particle size vs. dose rates for plutonium-containing hot particles is provided. This study demonstrates one possible approach to dose assessments for biota in environments contaminated by radioactive particles, which may prove useful for those engaged in environmental radioprotection.

Plutonium behavior after pulmonary administration according to solubility properties, and consequences on alveolar macrophage activation

The physico-chemical form in which plutonium enters the body influences the lung distribution and the transfer rate from lungs to blood. In the present study, we evaluated the early lung damage and macrophage activation after pulmonary contamination of plutonium of various preparation modes which produce different solubility and distribution patterns. Whatever the solubility properties of the contaminant, macrophages represent a major retention compartment in lungs, with 42 to 67% of the activity from broncho-alveolar lavages being associated with macrophages 14 days post-contamination. Lung changes were observed 2 and 6 weeks post-contamination, showing inflammatory lesions and accumulation of activated macrophages (CD68 positive) in plutonium-contaminated rats, although no increased proliferation of pneumocytes II (TTF-1 positive cells) was found. In addition, acid phosphatase activity in macrophages from contaminated rats was enhanced 2 weeks post-contamination as compared to sham groups, as well as inflammatory mediator levels (TNF-α, MCP-1, MIP-2 and CINC-1) in macrophage culture supernatants. Correlating with the decrease in activity remaining in macrophages after plutonium contamination, inflammatory mediator production returned to basal levels 6 weeks post-exposure. The production of chemokines by macrophages was evaluated after contamination with Pu of increasing solubility. No correlation was found between the solubility properties of Pu and the activation level of macrophages. In summary, our data indicate that, despite the higher solubility of plutonium citrate or nitrate as compared to preformed colloids or oxides, macrophages remain the main lung target after plutonium contamination and may participate in the early pulmonary damage.

Meta-analysis of non-tumour doses for radiation-induced cancer on the basis of dose-rate

Purpose: Quantitative analysis of cancer risk of ionising radiation as a function of dose-rate.

Materials and methods: Non-tumour dose, D_nt, defined as the highest dose of radiation at which no statistically significant tumour increase was observed above the control level, was analysed as a function of dose-rate of radiation.

Results: An inverse correlation was found between D_nt and dose-rate of the radiation. D_nt increased 20-fold with decreasing dose-rate from 1-10⁻⁸ Gy/min for whole body irradiation with low linear energy transfer (LET) radiation. Partial body radiation also showed a dose-rate dependence with a 5- to 10-fold larger D_nt as dose rate decreased. The dose-rate effect was also found for high LET radiation but at 10-fold lower D_nt levels.

Conclusions: The cancer risk of ionising radiation varies 1000-fold depending on the dose-rate of radiation and exposure conditions. This analysis explains the discrepancy of cancer risk between A-bomb survivors and radium dial painters.

Low-dose-radiation stimulated natural chemical and biological protection against lung cancer

Research is being conducted world-wide related to chemoprevention of future lung cancer among smokers. The fact that low doses and dose rates of some sparsely ionizing forms of radiation (e.g., x rays, gamma rays, and beta radiation) stimulate transient natural chemical and biological protection against cancer in high-risk individuals is little known. The cancer preventative properties relate to radiation adaptive response (radiation hormesis) and involve stimulated protective biological signaling (a mild stress response). The biological processes associated with the protective signaling are now better understood and include: increased availability of efficient DNA double-strand break repair (p53-related and in competition with normal apoptosis), stimulated auxiliary apoptosis of aberrant cells (presumed p53-independent), and stimulated protective immune functions. This system of low-dose radiation activated natural protection (ANP) requires an individual-specific threshold level of mild stress and when invoked can efficiently prevent the occurrence of cancers as well as other genomic-instability-associated diseases. In this paper, low, essentially harmless doses of gamma rays spread over an extended period are shown via use of a biological-based, hormetic relative risk (HRR) model to be highly efficient in preventing lung cancer induction by alpha radiation from inhaled plutonium.

Comparisons of pulmonary carcinogenesis in rats following inhalation exposure to plutonium dioxide or X-ray irradiation

Radiation-induced pulmonary carcinogenesis was compared in female Wistar rats following either inhalation exposure to alpha-emitting (239)PuO(2) aerosols, whole-body or thoracic X-ray irradiation. Dose-dependent survival reduction was correlated with increased malignant lung tumors at doses over 0.45 Gy, reaching the maximum incidence of 90% at 6.6-8.5 Gy in (239)Pu-exposed rats. While the differential dose responses for each histopathological type of tumors were noted, almost 70-80% were carcinomas among all of the primary tumors from (239)Pu-exposed rats. As the dose response curves for lung carcinomas were compared, the slope of the fit linear equation and the calculated relative effectiveness for 50% incidence of lung carcinomas were approximately 11-times as high in (239)Pu-exposure as those of thoracic X-irradiation. The numbers of tumor lesions distributed in the lung per tumor-bearing animal were about 2-fold more in (239)Pu-exposed rats, while the proportions of their histopathological types were similar between (239)Pu-exposure and X-irradiation. These results indicate that the magnitudes of the relative effectiveness or risk for pulmonary carcinogenesis are greater in (239)Pu-exposure than X-irradiation, and that radiation-induced lung tumors appear to originate mostly from the same target epithelial cells.

Pulmonary neoplasms in rats that inhaled cerium-144 dioxide

The lung neoplasms induced in rats by inhaled, internally deposited 144CeO2 were described and classified using histologic criteria. F344 rats were exposed once or repeatedly by inhalation to 144CeO2 and observed for their life span. There was significant life shortening only in those rats with the highest radiation doses. In these rats, there was a high percentage of squamous cell carcinomas of the lung, as well as much lower percentages of adenocarcinomas of the lung, hemangiosarcomas of the lung, and pleural mesotheliomas. At lower doses, adenocarcinomas were the most predominant tumor. These adenocarcinomas were subdivided based on their histologic pattern: alveolar, papillary, tubular, or undifferentiated. Neither the mode of exposure (single or repeated) nor the sex of the rat influenced the lung tumor incidence or tumor type. The lung neoplasms induced by this beta-emitting radionuclide are similar in nature to those induced by alpha-emitting radionuclides deposited in the lung in rats. However, the radiation-induced squamous cell carcinomas of the lung differ from those induced by heavy particle loads of nonradioactive compounds. The radiation-induced squamous cell carcinomas occur in higher incidence and have a more malignant behavior than those induced by heavy particle loads.

Microscopic dose distribution around PuO2 particles in lungs of hamsters, rats, and dogs

Syrian hamsters, Fischer rats and Beagle dogs inhaled monodisperse aerosols of PuO2 and were sacrificed during the first 16 days after exposure. The microscopic distribution of radiation dose and tissue-at-risk to alpha irradiation around individual particles in lung was studied using autoradiographs of lung tissue sections. The dose distributions in dogs and rats were more diffuse than in hamsters. A slightly greater tumor incidence was calculated for rats and dogs than for hamsters on the basis of dose distribution using the same dose-effect model for all three species. The small differences in tumor incidence predicted on this basis do not explain the extremely large differences in tumor incidences observed in these species after inhalation of PuO2.

Further experiments to study whether localised fission fragment irradiation of rat lung causes tumours

Male albino rats inhaled an aerosol of 235UO2 (mass median aerodynamic diameter = 2.8 micrometers and geometric standard deviation = 1.6). Approximately 20 h or 7 d post-inhalation the rats were exposed briefly to 10(12) slow neutrons cm-2 in a nuclear reactor, causing the retained 235UO2 particles of approximate mass 40 or 400 micrograms to emit fission fragments which irradiated the lungs. The mean absorbed doses from the fission fragments were either 80 ot 800 cGy approximately and in addition the lungs were exposed to a background of alpha-rays throughout the rats' life-time from the retained 235UO2 which gave mean doses of about half that from the fission fragments. The animals were kept for their life-time and killed when they became moribund. Malignant tumours were found in the lungs (squamous cell carcinoma and adenocarcinoma) which were probably induced by the alpha-rays rather than the fission fragments. Because of insufficient numbers of animals in the experimental groups, however, some statistical uncertainty exists as to whether the fission fragments were in fact less effective than the alpha-rays per unit absorbed dose in causing malignant tumours of the lung.

A short term effect of homogeneous internal irradiation of the mouse lung with 170Tm2O3 particles

The short term effects on the free macrophage population in the mouse lung, following inhalation of thulium-170 oxide, have been studied, with a view to using the system as a model with which to compare the short term effects of inhomogeneous alpha irradiation. The clearance and distribution of thulium-170 was followed up to 44 days post inhalation. The mice had an initial lung burden (+/- SE) of 6.7 +/- 0.5 microCi 170Tm/g of lung and the pattern of removal of this could be described by a single exponential of half time 31 +/- 4 days. This exposure resulted in a uniform dose of about 24 Gy to the lung. The macrophage population was studied by counting cells lavaged from the lungs at intervals over the period of the experiment. The lung burden of 170Tm2O3 was found to decrease the free macrophage numbers by about a factor of 2 in 14 days with some slight recovery at 6 weeks post inhalation. It was found that nearly 50% of the activity in the lung lobes was removed by the lavages and that this fraction remained constant throughout.

The carcinogenic effect of localized fission fragment irradiation of rat lung

In a preliminary investigation of 'hot particle' carcinogenesis uranium oxide particles were introduced into the lungs of rats either by intubation of a liquid suspension of the particles or by inhalation of an aerosol. Subsequently the animals were briefly exposed to slow neutrons in a nuclear reactor, resulting in localized irradiation of the lung by fission fragments emitted from 235U atoms in the oxide particles. The uranium used in the intubation experiments was either enriched or depleted in 235U. Squamous cell carcinomas developed at the site of deposition of the enriched uranium oxide in many cases but no lung tumours occurred in the rats with the depleted uranium oxide, in which the lung tissue was exposed to very few fission fragments. Only enriched uranium oxide was used in the inhalation experiments. Pulmonary squamous cell carcinomas occurred after the fission fragment irradiation but were fewer than in the intubation experiments. Adenocarcinomas of the lung were seen in rats exposed to uranium oxide without subsequent irradiation by neutrons in the reactor and in rats irradiated with neutrons but not previously exposed to uranium oxide. It is concluded that (i) fission fragments were possibly implicated in the genesis of the squamous cell carcinomas, which only developed in those animals exposed to enriched uranium oxide and neutrons and (ii) the adenocarcinomas in the rats inhaling enriched uranium oxide only were likely to have been caused by protracted irradiation of the lung with alpha-rays emitted from the enriched uranium.