Threshold Dose Rate Research Articles

Elaboration on a radiological environmental impact assessment methodology for Northern environments

The present work encompasses some key aspects of a 3-year long research project (INTRANOR) where the main focus has been specifically on environmental assessments for radiation exposure through application of existing methodologies and their adaptation to quantify transfer, exposure and effects in Boreal/Arctic ecosystems. Non-parametric statistical methods have been applied in order to estimate the threshold dose rates above which radiation effects can be expected in vertebrate organisms. In addition, industrial areas contaminated by uranium mill tailings and radium production wastes, in the Komi Republic, Russia, were selected as suitable sites to study further the effects of exposure to radiation under boreal conditions. Dose–effect relationships have been established for a few natural populations inhabiting this area. Analyses of data have allowed benchmarks to be established below which no decrease in reproductive capacity could be observed. Other work performed within the project includes the collation of data in relation to naturally occurring radionuclides and application of existing methodologies to characterise background radiation exposures. These dose-rates may be a suitable means of contextualising the exposure attributable to enhanced dose-rates arising from human activities. Finally, combined action of ionizing radiation and low temperature has been analyzed and mathematically modelled.

An approach for the assessment of risk from chronic radiation to populations of phytoplankton and zooplankton

A conceptual model of the effects of chronic radiation on a population of phytoplankton and zooplankton in an oceanic nutrient layer is presented. The model shows that there are distinct threshold dose rates at which the different plankton populations become unsustainable. These are 10,400 microGy h(-1) for phytoplankton and 125 microGy h(-1) for zooplankton. Both these values are considerably greater than the current screening values for protection of 10 microGy h(-1). The model highlights the effects of predator-prey dynamics in predicting that when the zooplankton is affected by the radiation dose, the phytoplankton population can increase. In addition, the model was altered to replicate the dose rates to the plankton of a previous ERICA Irish Sea assessment (24 microGy h(-1) for zooplankton and 430 microGy h(-1) to phytoplankton). The results showed only a 10% decrease in the zooplankton population and a 15% increase in the phytoplankton population. Therefore, at this level of dose, the model predicts that although the dose rate exceeds the guideline value, populations are not significantly affected. This result highlights the limitations of a single screening value for different groups of organisms.

Non-parametric estimation of thresholds for radiation effects in vertebrate species under chronic low-LET exposures

Databases on effects of chronic low-LET radiation exposure were analyzed by non-parametric statistical methods, to estimate the threshold dose rates above which radiation effects can be expected in vertebrate organisms. Data were grouped under three umbrella endpoints: effects on morbidity, reproduction, and life shortening. The data sets were compiled on a simple 'yes' or 'no' basis. Each data set included dose rates at which effects were reported without further details about the size or peculiarity of the effects. In total, the data sets include 84 values for endpoint "morbidity", 77 values for reproduction, and 41 values for life shortening. The dose rates in each set were ranked from low to higher values. The threshold TDR5 for radiation effects of a given umbrella type was estimated as a dose rate below which only a small percentage (5%) of data reported statistically significant radiation effects. The statistical treatment of the data sets was performed using non-parametric order statistics, and the bootstrap method. The resulting thresholds estimated by the order statistics are for morbidity effects 8.1 x 10(-4) Gy day(-1) (2.0 x 10(-4)-1.0 x 10(-3)), reproduction effects 6.0 x 10(-4) Gy day(-1) (4.0 x 10(-4)-1.5 x 10(-3)), and life shortening 3.0 x 10(-3) Gy day(-1) (1.0 x 10(-3)-6.0 x 10(-3)), respectively. The bootstrap method gave slightly lower values: 2.1 x 10(-4) Gy day(-1) (1.4 x 10(-4)-3.2 x 10(-4)) (morbidity), 4.1 x 10(-4) Gy day(-1) (3.0 x 10(-4)-5.7 x 10(-4)) (reproduction), and 1.1 x 10(-3) Gy day(-1) (7.9 x 10(-4)-1.3 x 10(-3)) (life shortening), respectively. The generic threshold dose rate (based on all umbrella types of effects) was estimated at 1.0 x 10(-3) Gy day(-1).

A Lower Dose Threshold for theIn VivoProtective Adaptive Response to Radiation. Tumorigenesis in Chronically Exposed Normal andTrp53Heterozygous C57BL/6 Mice

Low doses of ionizing radiation to cells and animals may induce adaptive responses that reduce the risk of cancer. However, there are upper dose thresholds above which these protective adaptive responses do not occur. We have now tested the hypothesis that there are similar lower dose thresholds that must be exceeded to induce protective effects in vivo. We examined the effects of low-dose/low-dose-rate fractionated exposures on cancer formation in Trp53 normal or cancer-prone Trp53 heterozygous female C57BL/6 mice. Beginning at 6 weeks of age, mice were exposed 5 days/week to single daily doses (0.33 mGy, 0.7 mGy/h) totaling 48, 97 or 146 mGy over 30, 60 or 90 weeks. The exposures for shorter times (up to 60 weeks) appeared to be below the level necessary to induce overall protective adaptive responses in Trp53 normal mice, and detrimental effects (shortened life span, increased frequency) evident for only specific tumor types (B- and T-cell lymphomas) were produced. Only when the exposures were continued for 90 weeks did the dose become sufficient to induce protective adaptive responses, balancing the detrimental effects for these specific cancers and reducing the risk level back to that of the unexposed animals. Detrimental effects were not seen for other tumor types, and a protective effect was seen for sarcomas after 60 weeks of exposure, which was then lost when the exposure continued for 90 weeks. As shown previously for the upper dose threshold for protection by low doses, the lower dose boundary between protection and harm was influenced by Trp53 functionality. Neither protection nor harm was observed in exposed Trp53 heterozygous mice, indicating that reduced Trp53 function raises the lower dose/ dose-rate threshold for both detrimental and protective tumorigenic effects.

Low Doses of Very Low-Dose-Rate Low-LET Radiation Suppress Radiation-Induced Neoplastic TransformationIn Vitroand Induce an Adaptive Response

The purpose of this study was to determine whether adaptation against neoplastic transformation could be induced by exposure to very low-dose-rate low-LET radiation. HeLa x skin fibroblast human hybrid cells were irradiated with approximately 30 kVp photons from an array of (125)I seeds. The initial dose rate was 4 mGy/day. Cell samples were taken at four intervals at various times over a period of 88 days and assayed for neoplastic transformation and the presence of reactive oxygen species (ROS). The dose rate at the end of this treatment period was 1.4 mGy/day. Transformation frequencies and ROS levels were compared to those of parallel unirradiated controls. At the end of 3 months and an accumulated dose of 216 mGy, cells treated with very low-dose-rate radiation were exposed to a high-dose-rate 3-Gy challenge dose of (137)Cs gamma rays, and the effects compared with the effect of 3 Gy on a parallel culture of previously unirradiated cells. Cells exposed to very low-dose-rate radiation exhibited a trend toward a reduction in neoplastic transformation frequency compared to the unirradiated controls. This reduction seemed to diminish with time, indicating that the dose rate, rather than accumulated dose, may be the more important factor in eliciting an adaptive response. This pattern was in general paralleled by a reduction of ROS present in the irradiated cultures compared to controls. The very low-dose-rate-treated cells were less sensitive to the high challenge dose than unirradiated controls, suggesting the induction of an adaptive response. Since there was a suggestion of a dose-rate threshold for induction suppression, a second experiment was run with a fresh batch of cells at an initial dose rate of 1 mGy/day. These cells were allowed to accumulate 40 mGy over 46 days (average dose rate=0.87 mGy/day), and there was no evidence for suppression of transformation frequency compared to parallel unirradiated controls. It is concluded that doses of less than 100 mGy delivered at very low dose rates in the range 1 to 4 mGy/day can induce an adaptive response against neoplastic transformation in vitro. When the dose rate drops below approximately 1 mGy/day, this suppression is apparently lost, suggesting a possible dose-rate-dependent threshold for this process.

Adaptive response: Part II. Further modeling for dose rate and time influences

Purpose: For adaptive response (AR) behavior, to model and analyze the dose rate dependence of AR. To examine and to model the influence of time interval between primer and challenge dose on the magnitude of radioprotection.Method and materials: Microdosimetry is used to examine effects of rate of critical volume specific energy depositions on activation of AR and induction of deleterious damage. The dose rate dependent adaptive response data, Shadley and Wiencke and Shadley et al. on AR fading, are analyzed.Results: We obtain good agreement with observed AR behavior. The 4 – 6 hour activation time for AR is from time necessary for the accumulation of enzymatic resources from the increased radioprotective capability for the recognition and repair of deleterious DNA damage and perhaps resources for reduction of RSO. The Shadley and Wiencke data, for the low 1.0 cGy primer dose, is as a result of a minimum rate of specific energy depositions (hits) for adaptive response to become operative and become fully activated. Increased deleterious damage with increased dose rate for a high 50 cGy primer dose is from a reduction in dose rate sparing as the rate approaches acute exposure.Conclusions: The microdosimetric model substantiates the prior observations in Part I that only several radiation-induced charged particle cell nucleus traversals are sufficient to activate adaptive response. Further however we find here that a minimum dose rate threshold is necessary. The model shows promise to provide insight into adaptive response time and dose rate behavior.

Modeling the microstructural evolution in bcc-Fe during irradiation using kinetic Monte Carlo computer simulation

Kinetic Monte Carlo (KMC) simulations of defect accumulation in neutron-irradiated bcc-Fe are presented. Comparisons of the KMC simulations with the recent experimentals are made to discuss the validity of the proposed KMC model. Then, the effect of environmental variables such as irradiation temperature, dose rate, and neutron spectrum is studied using the technique. The magnitude of dose rate effect is irradiation-temperature dependent, and it is found that a threshold dose rate, below which defect accumulation is not affected by the difference of dose rate, exists when irradiated at 600 K. An evaluation of the number of vacancy jumps of both irradiation-induced vacancies as well as thermal vacancies is done to study a possible effect of dose rate on a solute diffusion in bcc-Fe. The results show that the total number of vacancy jumps is dose rate dependent at very low and high dose rates while there is a dose rate independent region in between.

Effect of radiation-induced emission of schottky defects on the formation of colloids in alkali halides

Formation of vacancy clusters in irradiated crystals is considered taking into account radiation-induced Schottky defect emission (RSDE) from extended defects. RSDE acts in the opposite direction compared with Frenkel pair production, and it results in the radiation-induced recovery processes. In the case of alkali halides, Schottky defects can be produced as a result of the interaction of extended defects with excitons, as has been suggested by Seitz in 1954. We consider a model that takes into account excitonic mechanisms for the creation of both Frenkel and Schottky defects, and which shows that although the contribution of the latter mechanism to the production of primary defects may be small, its role in the radiation-induced evolution of microstructure can be very significant. The model is applied to describe the evolution of sodium colloids and the formation of voids in NaCl, which is followed by a sudden fracture of the material, presenting a potential problem in rock salt-based nuclear waste repositories. The temperature, dose rate and dose dependence of colloid growth in NaCl doped with different types of impurities is analyzed. We have found that colloid growth may become negative below a threshold temperature (or above a threshold dose rate), or below a certain impurity concentration, which is determined by the RSDE, that depends strongly on the type and concentration of the impurities. The results obtained with the model are compared with experimental observations.

A model for evaluating radiological impacts on organisms other than man foruse in post-closure assessments of geological repositories for radioactivewastes

Bioaccumulation and dosimetric models have been developed that allow thecomputation of dose rates to a wide variety of plants and animals in the context ofthe deep geological disposal of solid radioactive wastes. These dose rates can becompared with the threshold dose rates at which significant deleterious effectshave been observed in field and laboratory observations. This provides a generalindication of whether effects on ecosystems could be observable, but does notquantify the level of those effects. To address this latter issue, two indicatororganisms were identified and exposure–response relationships were developed forendpoints of potential interest (mortality in conifers and the induction of skeletalmalformations in rodents irradiated in utero). The bioaccumulation, dosimetryand exposure–response models were implemented and used to evaluate thepotential significance of radionuclide releases from a proposed deep geologicalrepository for radioactive wastes in France. This evaluation was undertaken inthe context of a programme of assessment studies being performed bythe Agence nationale pour la gestion des déchets radioactifs (ANDRA).

Non-endocrine late complications of bone marrow transplantation in childhood: part II.

Non-endocrine late complications of bone marrow transplantation in childhood: part II.

Dose-effect relationship of dicentric and ring chromosomes in lymphocytes of individuals living in the high background radiation areas in China.

Chromosomes of 39 healthy family members (3 generations from 13 families) living both in the high-level background radiation areas (HBRA) and the control areas (CA) were studied. Cumulative dose from birth to the time of blood sampling was estimated by calculating measured exposure rate in each individual. The cumulative doses ranged 30.9-358.9 and 6.0-59.2 mGy for HBRA and CA, respectively. Peripheral lymphocyte chromosome preparations were made according to our improved method. Dicentric and ring chromosomes (Dic + Rc) were scored in average 2,527 cells per individual in HBRA and 2,694 cells in CA under a microscope equipped with an automated stage. A positive correlation between Dic + Rc and age was found in HBRA, while no such dose relationship was clear in CA. The frequency of Dic + Rc linearly increases over lifetime due to chronic low dose exposure and it is likely that the activation of repair enzymes is not triggered in the present HBRA. Threshold dose (rate) of the induction of chromosome aberrations, if any, is below the present dose (rate) level.

Oral administration of tritiated water (HTO) in mouse. III: Low dose-rate irradiation and threshold dose-rate for radiation risk

Purpose: To investigate the biological effect of tritium on mouse at low dose-rates. Materials and methods: Mice (\\[C57BL/6N C3H/He]F) were exposed to beta -rays by continuous administration of tritiated drinking water throughout their lives at low dose-rates of 3.6, 0.9, and 0.2 mG/day. Results: Including the previous study, the tumour frequency was 70 80% for exposure in the range 240 mGy/day to 9.6 mGy/day. Frequency of tumours decreased with decrease of dose-rate to 50% comparable to the controls. Restricting to thymic lymphomas, a linear relationship in a semi-log plot was found between the frequency and the dose-rate above a threshold dose-rate of 12 mGy/day. There was a 'tail' to this relationship down to 0.9 mGy/day. A similar pattern resulted for the relationship between the life-shortening and the dose-rate. The threshold dose-rate of 3H beta -rays, 2 mGy/day (with a tail down to 0.2 mGy/day), was much lower than that of gamma-rays, 20 mGy/day (tail down to 2 mGy/day) derived from other studies. Conclusions: These studies suggest that there exists the threshold dose-rate in the biological effects of radiation, and that the threshold dose-rate for 60Co gamma-irradiation is higher than that for 3H beta-irradiation.

Analysis of risk factors for mandibular bone radionecrosis after exclusive low dose-rate brachytherapy for oral cancer

Background: Brachytherapy is widely adopted as an exclusive treatment of T1/T2 oral cancer with a high probability of definitive cure. Therefore, any major complication, like mandibular bone necrosis, should be avoided. Many risk factors, either clinical or technical, have been considered in the literature. Materials and methods: One hundred consecutive interstitial indium LDR treatments for early cancers of the tongue and floor of the mouth performed from January 1989 to November 1993 were reviewed. An analysis of some simple technical parameters (total dose, doserate, reference volume, linear activity, total reference kerma) was performed in order to identify the main physical risk factors. Moreover, total dose was recalculated as extrapolated responsive dose for normal tissue complications. Results: Bone necrosis was observed in 10 out of 100 patients with a median follow-up of 38 months. No significant incidence of this complication was observed when tumor site (mobile tongue versus floor of the mouth), dental status or total physical dose were considered. A significant correlation between the incidence of bone necrosis and two main parameters was found, i.e. dose-rate ( P < 0.02) and reference volume ( P < 0.05). Conclusions: A threshold value may be suggested both for dose-rate (50 cGy/h) and reference volume (25 000 mm 3). Bone necrosis is clearly related to both these parameters since most cases (i.e. 80%) were observed in the subgroup over the volume and dose-rate threshold.

Pulsed e − beam irradiation of polymers—A comparison of dose rate effects and let effects

Linear Energy Transfer (LET) effects in radiation chemistry are ascribed to the high density of active species in the track structure, resulting in overlapping of spurs. We studied the possibility of spur overlapping in electron beam irradiation at extremely high dose rate, both theoretically and experimentally. Considering differences in overlap mode leads to the concept of a “threshold dose rate”, above which spur overlapping may occur, and an “overlapping dose”, which is necessary to cause overlapping of spurs even at higher dose rate than the threshold, depending on lifetime and effective volume of reactive intermediates. Using Sandia's pulsed power e − beam system, we irradiated cellulose triacetate (CTA), polymethylmethacrylate (PMMA), polycarbonate (PC) and high density polyethylene (HDPE) at a dose rate as high as 4 × 10 10 Gy/s at room temperature in the absence of oxygen. Comparison of the e − beam results with data obtained from gamma irradiation at 0.5 Gy/s showed no dose rate effects based on discoloration sensitivity for CTA, or on scission probabilities of PMMA and PC. For HDPE, the results indicated a slightly lower rate of gel formation under e − beam irradiation, implying that the crosslinking efficiency may be somewhat reduced at the high dose rate.

Relationships between Mitotic Delay and the Dose Rate of X Radiation

Upon exposure of cells to radiation delivered at a continuous low dose rate, cell proliferation may be sustained with the cells exhibiting a constant doubling time that is independent of the total dose. The doubling time or mitotic delay under these conditions has been shown to depend on the dose rate in HeLa, V79 and P388F cells (Mitchell et al., Radiat. Res. 79, 520-536, 1979; Fox and Gilbert, Int. J. Radiat. Biol. 11, 339-347, 1966). Reanalysis of the data for these particular cell lines shows that there is a threshold dose rate for mitotic delay, and that above the threshold there is a linear relationship between the length of mitotic delay and the logarithm of the dose rate which is referred to as the dose-rate response. We have observed the same relationships for L5178Y (LY)-R and LY-S cells exposed to low-dose-rate radiation. The threshold dose rates for LY-R, LY-S and P388F cells are similar (0.01-0.02 Gy/h) and are much lower than for V79 and HeLa cells. The slope of the dose-rate response curve is the greatest for HeLa cells, followed in order by LY-S, V79 and P388F cells, and finally by LY-R cells. The slopes for HeLa and LY-R cells differ by a factor of 35.

Improving the radiation burn-out susceptibility of N-channel power MOSFETs

Computer simulation was used to calculate the dose-rate thresholds of several different power MOSFET structures. Computational results indicate that significant improvements in radiation tolerance and burnout of the device are possible by reducing the lateral size of the power MOSFET unit cell or by using a lower emitter injection efficiency for the bipolar structure. Modification of some other factors, such as the base minority carrier lifetime or the epi/substrate interface shape, shows some minor improvements in device radiation tolerance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A capacitively coupled dose-rate-dependent transient upset mechanism in a bipolar memory

A pattern sensitivity was observed in the threshold dose rate upset response of a bipolar 16 K PROM for radiation pulse widths of 20-100 ns. For the worst case pattern, the upset threshold was a factor of three lower than for the commonly used checkerboard pattern. The mechanism or this pattern sensitivity was found to be a capacitively coupled voltage transient on a sensitive node which caused a low-to-high transition at the output. A design fix was implemented to significantly alter the ratio of the two parasitic capacitances in a capacitive divider which reduced the amplitude of the voltage transient at the sensitive node. It was demonstrated that in the redesign, the pattern sensitivity was eliminated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

HTO Oral Administration in Mice. I: Threshold Dose Rate for Haematopoietic Death

Tritiated water in various concentrations was orally administered continuously to (C57BL/6N and C3H/He)F1 female mice in a closed animal chamber. Tritium radioactivity in various organ tissues was measured periodically after initiating tritiated water intake using an automatic sample combustion system and a liquid scintillation counter. After 7 days the specific radioactivity reached a plateau. These data allowed absorbed organ doses to be estimated. Within a range of 1.48 x 10(11) to 5.92 x 10(11) Bq/dm3 as the concentration of tritiated water in drinking water, the time of death after initiating the administration was about 2 weeks, a typical time for haematopoietic death. A linear relationship of times of death with tritiated water concentrations in drinking water was observed, on a log-log scale, between 1.85 x 10(10) Bq/dm3 and 1.48 x 10(11) Bq/dm3. At concentrations lower than 9.25 x 10(9) Bq/dm3, mice no longer died from haematopoietic failure. We conclude, therefore, that there should be a threshold dose rate for haematopoietic death.

The induction by ionizing radiation of chromosomal aberrations in rhesus monkey pre-meiotic germ cells: effects of dose rate and radiation quality

The induction of reciprocal translocations in rhesus monkey stem-cell spermatogonia was studied using multivalent analysis at metaphase of primary spermatocytes. Animals were exposed to 1 Gy γ-rays at dose rates of 140 and 0.2 mGy/min or to 0.25 Gy acute 2 MeV neutrons. Reduction of the dose rate from 140 mGy/min to 0.2 mGy/min did not result in a lowering of the frequencies of recovered translocations of 0.43%. The neutron data indicated an RBE (neutrons vs. X-rays) of 2.1, which is clearly lower than the value of 4 obtained in the mouse. It is made plausible that in general mammalian species with high sensitivities for the cytotoxic effects of ionizing radiation, such as the rhesus monkey, will exhibit relatively high threshold dose rates below which no further reduction in aberration yield occurs, whereas in more resistant species, such as the mouse, the threshold dose rate will be at a very low level. Similarly, resistant species will show relatively high RBE values for neutron irradiation and sensitive species low ones.

UV-B dose/dose-rate responses of seasonally abundant copepods of Puget Sound.

Refinements in modeling stratospheric processes and recent observational data have generated estimates of moderate future reductions in total global ozone. Nevertheless, considerable uncertainty and concern remain about whether or not the resulting increases in incident solar UV-B could increase the risk of harmful biological effects. The UV-B dose/dose-rate thresholds for five species of copepods collected at the surface in Puget Sound were determined and compared to previously studied zooplankton groups. Generally, species appearing later in the spring and summer were less UV-B sensitive. The UV-B daily-dose and dose-rate thresholds for the five species of copepods, while apparently less than present median incident levels, range from above to below estimated present and future subsurface (1 m) UV-B levels. A separate experiment with a relatively large copepod, Calanus pacificus, in which above and below threshold UV-B dose-rates were administered with various levels of photoreactivating light(UV-A and visible light), indicated the presence of photorepair and its full activation at relatively low intensity. To improve estimates of the extent to which present and predicted UV-B levels affect local populations of copepods and zooplankton, it is essential to more fully understant the natural conditions or processes which influence the composition and intensity of the UV irradiation these animals receive.