Small Dose Rate Research Articles

Re-examination of the Linear No Threshold (LNT) hypothesis, and a trial for the public to comprehend the reality of the low-dose effects of radiation

People have imprinted knowledge of the LNT hypothesis. Use of the terms LNT hypothesis, ALARA, or stochastic effect, etc. clouds the understanding of people about the real effects of very low doses of radiation. In this paper, the scientific basis of the LNT hypothesis is re-examined. A denial of the presence of a threshold means that individuals have no protection from even very small doses or dose rates of radiation in the body. As the human population is heterogeneous, a linear dose-response curve is obtained, as even whole majorities have a threshold. Current results from radiation epidemiological studies and experimental studies from cells and animals were reported by experts from USA, Canada and Japan at the Radition Effects Association (REA) symposium in March 2005 in Tokyo. The findings did not support the LNT hypothesis, but the importance of biological reactions and factors involving radiation-carcinogenesis. Even a few antinuclear audiences started to think about the presence of a body-defence mechanism against radiation. Necessary information on radiation-carcinogenesis is summarised in several figures for better comprehensions by the public.

Characterization and real‐time optical measurements of the ionizing radiation dose response for a new radiochromic medium

A new radiochromic film, GafChromic EBT, was investigated for use in a real-time radiation dosimetry system. It was found to be approximately eight times more sensitive to ionizing radiation dose, exhibited less postexposure development and achieved stable readout faster than one of its predecessors, GafChromic MD-55. A clear distinction in change in optical density between exposure and postexposure was observed, but the measurements obtained during exposure were not linear with time or dose. This could not be explained by a shift in wavelength of maximum change in absorbance, as it was stable at approximately 636 nm during the entire exposure range (up to 9.52 Gy). Increasing the spectral window of interest over which calculations were performed did little to correct the nonlinearity. The radiochromic film exhibited small dose rate dependence in real-time measurements, with an increase in standard deviation of change in optical density measurements from 0.9% to 1.8% over a sixfold variation in dose rate. Overall, GafChromic EBT has increased sensitivity and decreased postexposure darkening, and this bodes well for its potential role as a radiation dosimeter, including real-time applications.

SU-FF-T-231: Characterization and Real-Time Optical Measurements of the Ionizing Radiation Dose Response for a New Radiochromic Medium

Purpose: Radiochromic films, generally used for two-dimensional dose verification, have lately been considered for point-based real-time in vivo dosimetry. They have some advantages, including near water-equivalency, over dosimeters used currently. In a recent study, GafChromic® MD-55 showed reasonable performance, but some issues remained unresolved. A new film, GafChromic® EBT, has potential to overcome those issues and is investigated. Method and Materials: An optical fibre-based setup, capable of real-time measurements, was used to detect the increase in optical density over 630–640 nm range of the EBT film during and after exposure (6 MV X-rays, Varian 2100 EX). Results: Change in optical density for EBT film, measured at the end of exposure to 1.9 Gy, was 7.7 times greater than that of MD-55 film. EBT also exhibited less post-exposure darkening, with a 12.5% increase over 18 hours, compared to 25% for MD-55. Change in optical density during exposure for EBT film was non-linear with time or dose. This was not due to a shift of the wavelength of maximum change in absorbance, which was stable at ∼636 nm during the entire exposure to 9.52 Gy. Increasing the spectral window over which optical density calculations were performed had little affect on the non-linearity. The EBT film exhibited a small dose rate dependence for optical density measurements during exposure: standard deviation of change in optical density immediately at the end of a 9.52 Gy exposure increased from 0.9% to 1.8% when a six-fold variation in dose rate was introduced. Conclusion: GafChromic EBT film has potential as a radiation dosimeter, including real-time applications, due to its increased sensitivity, decreased post-exposure darkening and spectral stability, but signal non-linearity needs to be investigated further. This work was funded in part by National Institutes of Health / National Institute on Aging (R21/R33 AG19381).

Coloration of Pentacosadiynoic Acid Polycrystalline Powder Dispersed in Polymer Layer by Gamma-ray Irradiation and UV Illumination.

Coloration of pentacosadiynoic acid (PDA) dispersed in polyalkylacrylate layer (color indicator layer) by gamma and UV irradiations has been studied comparatively in an attempt to obtain basic data for development of radiation color indicator of high sensitivity useful at low dose region (below 50 Gy) where the irradiation is carried out to suppress immunity in blood transfusion. Dispersion of PDA at high concentration in polyalkylacrylate layer gives a color indicator sensitive enough for this purpose. A small positive dose rate dependence and large temperature effect were observed on the reaction rate.

Risk estimates for radiation-induced cancer--the epidemiological evidence.

The risk of low-dose radiation exposures has--for a variety of reasons--been highly politicised. This has led to a frequently exaggerated perception of the potential health effects, and to lasting public controversies. A balanced view requires a critical reassessment of the epidemiological basis of current assumptions. There is reliable quantitative information available on the increase of cancer rates due to moderate and high doses. This provides a firm basis for the derivation of probabilities of causation, e.g. after high radiation exposures. For small doses or dose rates, the situation is entirely different: potential increases of cancer rates remain hidden below the statistical fluctuations of normal rates, and the molecular mechanisms of cancerogenesis are not sufficiently well known to allow numerical predictions. Risk coefficients for radiation protection must, therefore, be based on the uncertain extrapolation of observations obtained at moderate or high doses. While extrapolation is arbitrary, it is, nevertheless, used and mostly with the conservative assumption of a linear dose dependence with no threshold (LNT model). All risk estimates are based on this hypothesis. They are, thus, virtual guidelines, rather than firm numbers. The observations on the A-bomb survivors are still the major source of information on the health effects of comparatively small radiation doses. A fairly direct inspection of the data shows that the solid cancer mortality data of the A-bomb survivors are equally consistent with linearity in dose and with reduced effectiveness at low doses. In the leukemia data a reduction is strongly indicated. With one notable exception -- leukemia after prenatal exposure--these observations are in line with a multitude of observations in groups of persons exposed for medical reasons. The low-dose effects of densely ionizing radiations--such as alpha-particles from radon decay products or high-energy neutrons--are a separate important issue. For neutrons, there is little epidemiological information. This has facilitated exaggerated claims of high neutron effects with reference to alleged dangers from transports of reactor fuel. However, in spite of limited information, it can be shown that the data from Hiroshima exclude the stated claims. New dosimetric information on neutrons may turn out to be highly informative with regard to an upper limit for the potential effects of neutrons and equally with regard to a reassessment--and a possible reduction--of risk estimates for gamma-rays.

Subconstrictor doses of neuropeptide Y potentiate alpha 1-adrenergic venoconstriction in vivo.

The 36-amino acid human neuropeptide Y is a vasoactive compound released after stimulation of the sympathetic nervous system. In addition to its direct and long-lasting vasopressor effects, it may potentiate the constrictor action of catecholamines and other vasoconstrictors at doses that do not per se exert vascular effects. Using the hand vein compliance technique, we have previously shown that neuropeptide Y also constricts superficial hand veins and that its effects may last for several hours. In this study, we investigated the local effect of neuropeptide Y on alpha 1-adrenergic venoconstriction in nine healthy volunteers at dose rates that did not affect venous compliance. On separate days, cumulative dose-response curves to phenylephrine alone and with coadministration of 1 or 30 pmol neuropeptide Y per minute were constructed, and the responses were fitted to a four-parameter logistic equation. Neuropeptide Y dose dependently shifted the phenylephrine curves toward lower dose rates without affecting maximal effects. ED50 values for phenylephrine alone and with 1 or 30 pmol/min neuropeptide Y were 4.0, 4.9 (P = NS versus control), and 1.2 (P < .005) nmol/min, respectively. Comparison with neuropeptide Y dose-response curves revealed that the interaction was synergistic. These are the first data in humans to show that small dose rates of neuropeptide Y may potentiate alpha-adrenergic effects in vivo. Because this interaction occurs at estimated local concentrations nearly achieved in humans, these studies suggest that neuropeptide Y might modulate the filling of this capacitance system in vivo.

Sterilization of health care products by 5 MeV bremsstrahlung (X ray)

Radiation sensitivities of B . pumilus and B. subtilis spores were examined to bremsstrahlung of 5 MeV EB and Cobalt -60 γ rays in order to confirm the effects of radiation and dose rate. Biological indicators (SPORTROL, NAmSA, USA) were irradiated with the X ray in the dose rate range of 4.7–47kGy/h. D-value of B. pumilus spores was 1.4–1.5 kGy, and that of B. subtilis was 1.1–1.3 kGy. The D-values of B.pumilus and B. subtilis have very small dose rate dependences to X ray, and the D-values are similar to those of γ ray. Dose distribution by X-ray and γ irradiation was measured for cartons containing 32 unit dialyzers. The D max./D min. of the X-ray irradiation (1.2) was smaller than that of γ ray (1.3).

Inactivation by ionizing radiation of ion channels formed by polyene antibiotics amphotericin B and nystatin in lipid membranes: an inverse dose-rate behavior

The phenomena reported are part of a study about the effects of ionizing radiation on membrane transport. We found that the conductance of lipid membranes in the presence of the polyene-antibiotics nystatin or amphotericin B is reduced to virtually zero following irradiation. Ion channels formed by these substances seem to represent extremely sensitive structures being inactivated by radiation doses in the range of a few Centigray (1 cGy = 1 rad) at sufficiently small dose rates. Inactivation shows a so-called inverse dose-rate behavior, i.e., at constant radiation dose the effect increases with decreasing dose rate. Similar to radiation-induced lipid peroxidation the phenomenon may be understood on the basis of a radical chain mechanism initiated by free radicals of water radiolysis. The process--via peroxidation of the polyene part of the molecules--is suggested to modify the hydrophobic exterior and to destabilize the barrel-like structure of the ion channels.

Photocurrent modeling at high dose rates

An analytical steady-state photocurrent model that includes ohmic field effects and treats (nonlinear) ambipolar diffusion and nonlinear recombination effects explicitly is developed. The nonlinear recombination and ambipolar diffusion are solved exactly, and the asymptotic ohmic field is treated as an approximate perturbation with fitted parameters. The model is found to be in good agreement with numerical calculations and with experimental results. Appropriate Taylor series expansions and extensive parametric studies using the model indicate that the superlinear response is largest and occurs at smaller dose rates in relatively high-resistivity materials that have a long excess carrier lifetime and, if the material is n-type, a large tau /sub no// tau /sub po/ ratio. The nonlinearity is more pronounced for infinite-medium devices. For this reason, power devices are expected to exhibit more nonlinear behavior than integrated circuit devices. >

Criteria for Choosing Application Techniques for Desert Locust Control1

Insecticides may be applied in several ways to control desert locusts. The traditional method, of feeding poisoned bait to hopper bands, uses insecticide very efficiently and is inexpensive when put into practice on a limited scale. However, it is both expensive and ineffective when a plague arises. Treating individual bands with contact insecticides is similar and suitable for operations on a modest scale in countries where much of the land is developed for agriculture. Two large‐scale methods, matching the size of a locust plague occurring in uncultivated deserts, are the treatment of swarms of adults and spraying dieldrin onto the natural vegetation of the breeding sites. Despite the small dose rate of dieldrin needed ‐ tens of grams per hectare ‐ the last method is very expensive when breeding occurs over the large areas which are susceptible ‐ hundreds of thousands of square kilometres. Although all methods are suitable and used for particular circumstances, the last alone is the only one of general value and practical feasibility that can match the dimensions of a locust plague. Despite its high cost and the use of small dose rates of a persistent organochloride insecticide, the analysis of benefits and costs indicates that it will be the method mostly used in deserts, if a plague occurs again. The arguments leading to this conclusion are presented to illustrate an example of the type of study required to decide how to apply pesticides in practice for more familiar pest management tasks.

Zeitliche verteilung der dosisrate von impulsen schneller neutronen hinter polyäthylen/eisen-schichten

A very short pulse (fhm = 0.8 ns) of 14 MeV neutrons was sent through plane layers of the material combination polyethylene/ iron. The thickness of this combination was varied from 2 cm + 2 cm to 8 cm + 8 cm in each of the two material sequences. Pulse height and time-of-flight spectra of the outcoming neutrons were recorded by a proton-recoil spectrometer at the backside of the layers in a 256 × 32 matrix. Each of the successive (in time) energy spectra covered a time interval of about 2 ns. A total interval of about 12 ns was analysed. The energy spectra were converted to get dose-rates which are shown as a function of time for every layer thickness. It was found that the material sequence “iron in front of polyethylene” (seen from the neutron source) results in smaller dose-rates behind the layers in the regarded energy and time region.

Clinical radiobiology of low dose-rate radiotherapy.

Investigative irradiations of human skin have shown that the total dose required to obtain a dry desquamation does not increase with the duration of the irradiation as rapidly as it would be expected from the kinetics of repair of the sublethal injuries. This suggests that an additional mechanism, probably accumulation of the cells in a sensitive phase, occurs during a continuous irradiation with dose-rate higher than 150 rads/hour. This effect seems to vanish at smaller dose-rates.

Photovoltaic effects in the ionization response of tantalum capacitors

The response of unbiased tantalum capacitors to ionizing radiation has been measured for both small and large dose rates. The results have been analyzed using a two-trap model for the Ta2O5 dielectric and an equivalent circuit model is presented. A significant reduction in the response of slug capacitors can be achieved by connecting two capacitors in the back-to-back or inverted-parallel configuration.

Lack of recovery from radiation induced chromosome damage in G0 human lymphocytes

Human peripheral blood lymphocyte cultures were irradiated with a dose of X-rays split in two equal fractions. The first fraction of dose was given shortly after culture initiation and the second one at 0, 2, 4, 18, 20, 22 and 24 h thereafter. Dicentric and ring chromosomes were scored at 54 h of cultivation. The aberration yields were essentially the same regardless of whether the radiation was given in a single burst or in two fractions separated by intervals of up to 20 h. The results unexpectedly revealed that no rejoining took place during this period. A drop in aberration yield occurred when the interval between exposures increased to 24 h, suggesting that at this time the system responsible for the rejoining of broken chromosome ends becomes operative. It was also noticed that no rejoining took place in small non-stimulated lymphocytes incubated at 37 °C for 6 h prior to cultivation. These findings may explain the production of two-hit aberrations in peripheral blood lymphocytes of persons chronically exposed to small doses and very small dose rates of ionizing radiations.