Low Doses Of high-LET Radiation Research Articles

Simulating the Lunar Environment: Partial Weightbearing and High-LET Radiation-Induce Bone Loss and Increase Sclerostin-Positive Osteocytes.

Exploration missions to the Moon or Mars will expose astronauts to galactic cosmic radiation and low gravitational fields. Exposure to reduced weightbearing and radiation independently result in bone loss. However, no data exist regarding the skeletal consequences of combining low-dose, high-linear energy transfer (LET) radiation and partial weightbearing. We hypothesized that simulated galactic cosmic radiation would exacerbate bone loss in animals held at one-sixth body weight (G/6) without radiation exposure. Female BALB/cByJ four-month-old mice were randomly assigned to one of the following treatment groups: 1 gravity (1G) control; 1G with radiation; G/6 control; and G/6 with radiation. Mice were exposed to either silicon-28 or X-ray radiation. (28)Si radiation (300 MeV/nucleon) was administered at acute doses of 0 (sham), 0.17 and 0.5 Gy, or in three fractionated doses of 0.17 Gy each over seven days. X radiation (250 kV) was administered at acute doses of 0 (sham), 0.17, 0.5 and 1 Gy, or in three fractionated doses of 0.33 Gy each over 14 days. Bones were harvested 21 days after the first exposure. Acute 1 Gy X-ray irradiation during G/6, and acute or fractionated 0.5 Gy (28)Si irradiation during 1G resulted in significantly lower cancellous mass [percentage bone volume/total volume (%BV/TV), by microcomputed tomography]. In addition, G/6 significantly reduced %BV/TV compared to 1G controls. When acute X-ray irradiation was combined with G/6, distal femur %BV/TV was significantly lower compared to G/6 control. Fractionated X-ray irradiation during G/6 protected against radiation-induced losses in %BV/TV and trabecular number, while fractionated (28)Si irradiation during 1G exacerbated the effects compared to single-dose exposure. Impaired bone formation capacity, measured by percentage mineralizing surface, can partially explain the lower cortical bone thickness. Moreover, both partial weightbearing and (28)Si-ion exposure contribute to a higher proportion of sclerostin-positive osteocytes in cortical bone. Taken together, these data suggest that partial weightbearing and low-dose, high-LET radiation negatively impact maintenance of bone mass by lowering bone formation and increasing bone resorption. The impaired bone formation response is associated with sclerostin-induced suppression of Wnt signaling. Therefore, exposure to low-dose, high-LET radiation during long-duration spaceflight missions may reduce bone formation capacity, decrease cancellous bone mass and increase bone resorption. Future countermeasure strategies should aim to restore mechanical loads on bone to those experienced in one gravity. Moreover, low-doses of high-LET radiation during long-duration spaceflight should be limited or countermeasure strategies employed to mitigate bone loss.

Chromosomal aberrations in human lymphocytes and fibroblasts after exposure to very low doses of high-LET radiation

Purpose: The relationship between biological effects and low doses of radiation is still uncertain, especially for high-LET radiation exposures. Estimates of risk from exposure to low doses and low dose rates are often extrapolated from the Japanese atomic bomb survivor data using either linear or linear-quadratic models fitted to dose–response data. In this study, we determined the dose–response for chromosome damage after exposure to very low doses of high-LET radiation and assessed the radiation qualities of Fe, Si and Oxygen ions.Materials and methods: Chromosomal aberrations (CA) were measured in human peripheral blood lymphocytes and normal skin fibroblasts after exposure to very low doses (0.01–0.20 Gy) of 77-MeV/u oxygen (LET = 55 keV/µm), 170-MeV/u 28Si (LET = 99 keV/µm), or 56Fe ions with energies of 600- or 450-MeV/u (LET = 180 or 195 keV/µm). These exposures included doses that, on average, produce fewer than one in five direct ion traversals per cell nucleus. Chromosomes were analyzed using the whole-chromosome fluorescence in situ hybridization (FISH) technique during the first cell division after irradiation, and CA were identified as either simple exchanges (translocations and dicentrics) or complex exchanges (involving more than two breaks in two or more chromosomes). The frequencies of CA in the painted chromosome(s) were evaluated as the ratio between aberrations scored and total cells analyzed. The dose–response for simple exchanges was assessed using a generalized linear model assuming binomial errors per number of chromosomes scored. The model coefficients were extrapolated to whole-genome equivalents. The linear dose–response denoted as the targete effects (TE) model considered the mean number of radiation tracks per cell. Two different non-targeted effect (NTE) models, P = P0 + αT + κ × I (NTE1), and P = P0 + αT (1 − e−T) + κe−T × I (NTE2), were compared with the simple linear model, P = P0 + αT. Akaike information criteria (AIC) and Bayes information criteria (BIC) were used to compare TE and NTE models for fitting chromosome aberrations in low dose range.Results: Doses that on average produce more than one ion traversal per cell nucleus showed a linear dose–response for CA in both lymphocytes and fibroblasts. However, for doses that produce fewer than one tracks per cell in fibroblasts, O, Si and Fe particles showed a dose-independent response for CA that was significantly elevated relative to background frequencies. For fibroblasts the NTE model 2, P = P0 + αT (1 − e−T) + κe−T × I, showed improved fit to CA in low dose range compared with TE model or NTE1 model. For lymphocytes, tests of the various models were less clear with TE model optimal for Si and Fe while the NTE2 model optimal for O particles. When low-dose exposures were fractionated with 2-h intervals, increased frequencies of both simple and complex exchanges were observed. Nitric oxide scavenger reduced CA induced by low doses of high-LET irradiation. Inhibition of transforming growth factor-β receptor-1 reduced the frequency of simple exchanges.Conclusions: The results show a non-linear dose–response for CA in fibroblasts after very low doses of high-LET exposure. Possible explanations for this could involve non-targeted effects due to aberrant cell signaling [ 1], perhaps involving nitric oxide and TGF-β, or could be due to delta-ray dose fluctuations [ 2] where CA are induced in cells that receive a significant dose from delta-rays emanating from the multiple ion tracks that do not directly traverse cell nuclei.

Mutagenic adaptive response to high-LET radiation in human lymphoblastoid cells exposed to low doses of heavy-ion radiation

Adaptive response (AR) and bystander effect are two important phenomena involved in biological responses to low doses of ionizing radiation (IR). Furthermore, there is a strong interest in better understanding the biological effects of high-LET radiation. We previously demonstrated the ability of low doses of X-rays to induce an AR to challenging heavy-ion radiation [8]. In this study, we assessed in vitro the ability of priming low doses (0.01 Gy) of heavy-ion radiation to induce a similar AR to a subsequent challenging dose (1–4 Gy) of high-LET IR (carbon-ion: 20 and 40 keV/μm, neon-ion: 150 keV/μm) in TK6, AHH-1 and NH32 cells. Our results showed that low doses of high-LET radiation can induce an AR characterized by lower mutation frequencies at hypoxanthine–guanine phosphoribosyl transferase locus and faster DNA repair kinetics, in cells expressing p53.

High Frequency of Simple and Complex Chromosome Aberrations Detected in the Tokai-mura Survivor four and Five Years after the 1999 Criticality Accident

In September 1999 a criticality accident occurred in a uranium processing plant in Tokai-mura, Japan. During the accident, three workers (A, B and C) were exposed to high acute doses of neutrons and γ-rays: workers A and B fatally and worker C to an estimated whole body absorbed dose of 0.81 Gy neutrons and 1.3 Gy γ-rays. We obtained fixed peripheral blood lymphocytes (PBL) preparations from worker C approximately four and five years after the accident and assayed by 24 colour karyotyping (M-FISH) to determine the frequency and complexity of chromosome aberrations present. We observed a high frequency of simple reciprocal translocations, which we used to provide a rough estimation of dose and, in addition, for the assessment of the emergence of any clinically-relevant clonal exchanges. We did not observe any evidence of clonality but did find some evidence suggesting chromosome 1 as being preferentially involved in exchanges in stable cells. We also detected a relatively high frequency of damaged cells containing complex chromosome aberrations, of both the stable and unstable types. Qualitatively these complex aberrations were consistent with those observed to be induced after exposure to low doses of high-LET radiation or moderate doses of low-LET radiation, supporting the suggestion that heavily damaged cells can be quite long-lived in vivo.

Studies of UV-cured CR-39 recording properties in view of its applicability in radiobiological experiments with alpha particles

In radiobiology, low doses of high-LET radiation correspond to a few particle traversals through the cell population. Therefore, for studies on cell monolayers irradiated with a low dose of α -particles, it is extremely useful if the number and position of particle traversals can be determined. In this study we describe a new method, based on UV-curing, to obtain a 10 μ m thick CR-39 grafted onto a 2.5 μ m thick PolyEthylene Terephtalate (PET). This thin double polymeric layer, used as a dish base, has a regular and reproducible detector thickness which can be traversed by 3.5 MeV α -particles, with a sufficient residual energy to traverse mammalian cells attached to the base. The recording properties of a PET-CR-39 dish, together with a demonstration of its use for radiobiological experiments, are presented. This new tool allows the precise determination of single-track impact parameters at a sub-cellular level.

The potential impact of bystander effects on radiation risks in a Mars mission.

Densely ionizing (high-LET) galactic cosmic rays (GCR) contribute a significant component of the radiation risk in free space. Over a period of a few months-sufficient for the early stages of radiation carcinogenesis to occur-a significant proportion of cell nuclei will not be traversed. There is convincing evidence, at least in vitro, that irradiated cells can send out signals that can result in damage to nearby unirradiated cells. This observation can hold even when the unirradiated cells have been exposed to low doses of low-LET radiation. We discuss here a quantitative model based on the a formalism, an approach that incorporates radiobiological damage both from a bystander response to signals emitted by irradiated cells, and also from direct traversal of high-LET radiations through cell nuclei. The model produces results that are consistent with those of a series of studies of the bystander phenomenon using a high-LET microbeam, with the end point of in vitro oncogenic transformation. According to this picture, for exposure to high-LET particles such as galactic cosmic rays other than protons, the bystander effect is significant primarily at low fluences, i.e., exposures where there are significant numbers of untraversed cells. If the mechanisms postulated here were applicable in vivo, using a linear extrapolation of risks derived from studies using intermediate doses of high-LET radiation (where the contribution of the bystander effect may be negligible) to estimate risks at very low doses (where the bystander effect may be dominant) could underestimate the true risk from low doses of high-LET radiation. It would be highly premature simply to abandon current risk projections for high-LET, low-dose radiation; however, these considerations would suggest caution in applying results derived from experiments using high-LET radiation at fluences above approximately 1 particle per nucleus to risk estimation for a Mars mission.

Is There Reliable Experimental Evidence for a Chromosomal "Fingerprint" of Exposure to Densely Ionizing Radiation?

A selection of published data on the ratio, F, of interchromosomal to intrachromosomal stable (reciprocal translocations and pericentric inversions) and unstable (dicentric chromosomes and centric rings) exchange aberrations in human lymphocytes has recently been presented as evidence for F values of about 15 for X and gamma rays and about 6 for neutrons and alpha particles (D. J. Brenner and R. K. Sachs, Radiat. Res. 140, 134-142, 1994). On this basis it was proposed that low F values could serve as a chromosomal "fingerprint" of densely ionizing radiation. In the present commentary it is shown that some of the quoted data sets provide little support for this concept. It is further demonstrated that our own data, including a "head-to-head" experiment with gamma rays and alpha particles, reveal no LET dependence, even in the comparison of F values from low-LET radiation with those from low doses of high-LET radiation. In this context it is pointed out that a change in F values cannot be expected at doses of high-LET radiation where the linear component of the dose-effect relationship for exchange aberrations prevails. Additional data for the effects of high- and low-LET radiation which have not been considered in the discussion so far confirm that support of the concept of F-ratio "fingerprinting" by experimental data is insufficient.

Single charged-particle damage to living cells: a new method based on track-etch detectors

Biological effects of ionizing radiation are usually expressed as a function of the absorbed dose. Low doses of high-LET radiation correspond to one or few particle traversals through the cell. In order to study the biological effectiveness of single charged particles, we have developed a new method based on solid state nuclear track detectors. Cells are seeded on mylar and a LR-115 film is stuck below the mylar base. After irradiation, the LR-115 film is etched and cells observed at a phase contrast microscope connected to a video camera and an image analyzer. In this way, it is possible to measure the number of traversals through the cell nucleus or cytoplasm. Coordinates of each cell on the microscope bench are saved. After incubation for about one week, cells are fixed and stained and the colonies observed at the microscope. The fate of each irradiated cell is therefore correlated to the number of traversals. We have tested this method with two different rodent embryo fibroblast cell lines, C3H 10T1/2 and V79, exposed to 3.2 MeV accelerated α-particles (LET=124 keV/ μm). The studied endpoint was cell killing. Preliminary biological results suggest that few α-particle tracks in V79 hamster cells are sufficient to reduce surviving fraction.

The Effects of Dose and Radiation Quality on the Shape and Power Saturation of the EPR Signal in Alanine

Variations in the shape and the power saturation of EPR spectra of L-alanine irradiated with photons, electrons, neutrons and protons are reported. It is shown that the ratio of the intensities of the satellite lines attributable to "spin flips" and the central line depend on the EPR microwave power, and it is proposed as a quantitative measure of the signal saturation effect. Dependence of this ratio on the microwave power is affected by the radiation quality and for doses in excess of 10 kGy by the absorbed dose. At high doses of low-LET radiation these changes are attributed to a high local density of free radicals, while for low doses of high-LET radiation these are due to changes induced in the crystal lattice. Consequently, the conventional peak-to-peak amplitude measurement of the EPR signal intensity is inaccurate when used for high doses and for comparison between radiations with different beam quality. The possibility to determine radiation quality from an EPR measurement is discussed.

Neutron-Induced Tumors in BC3F 1 Mice: Effects of Dose Fractionation

An experimental study of the biological effectiveness of multifractionated low doses of high-LET radiation was carried out using BC3F1 male mice. They were treated with whole-body irradiation with five equal daily fractions of fission neutrons to yield cumulative doses of 0.025, 0.05, 0.10, 0.17, 0.25, 0.36, 0.535 and 0.71 Gy at the RSV-TAPIRO reactor (mean neutron energy 0.4 MeV, in terms of kerma, y D = 51.5 keV/microns, dose rate 0.004 Gy/min) and were followed for their entire life span. The statistical method described by Peto et al. (IARC Monograph, Suppl. 2, 1980) to establish the existence of a carcinogenic effect in long-term animal experiments was applied to the data sets. This analysis was done for myeloid leukemia and for the presence of selected solid tumors. Myeloid leukemia was absent in the control group and was rarely found in irradiated animals. However, a positive significant trend was found in the dose ranges 0-0.17 Gy and higher. Epithelial tumors were induced at doses from 0.17 Gy on. Tumor occurrence was evaluated further as final incidences with age adjustment for the differences in mortality rates. Survival and incidence data for selected classes of tumors after 0.17, 0.36 and 0.71 Gy were compared with those from a previous experiment at corresponding doses given acutely (dose rate between 0.05 and 0.25 Gy/min). This indicated no marked overall influence of the time regimen of neutron irradiation on survival and tumor induction.

Radiation Doses and LET Distributions of Cosmic Rays

Among cosmic rays, the heavy nuclei ( HZE particles) like iron provide the dominant contribution to the dose equivalent during exposures in space. The LET distributions and radiation doses of cosmic-ray components have been calculated--with and without the quality factors--for a set of shielding and tissue self-shielding penetration depths. The relative contributions of heavy ions among solar flare particles to the dose equivalent are also explored. The transport calculations of the nuclei in air, shielding materials, and biological tissue-like material were carried out using the partial and total nuclear cross-section equations and nuclear propagation codes of Silberberg and Tsao . Outside the magnetosphere , at solar minimum, the product of the unshielded dose and the quality factors of cosmic-ray protons and heavy nuclei with atomic number Z greater than or equal to 6 are about 5 and 47 rem/year, respectively. With 4 g/cm2 aluminum shielding and at a depth of 5 cm in a biological phantom of 30 cm diameter, the respective values of the dose equivalents are about 4 and 11 rem/year. Due to the hard spectrum of cosmic rays, the attenuation of protons thus is relatively modest, while that of heavy nuclei is larger due to the larger interaction cross section. The dose equivalent of neutrons in the shielded case mentioned above is similar to that of protons. The biological risks are tentatively assessed in terms of the BEIR 1980 report. Uncertainties in risks due to possible large RBE values at low doses of high-LET radiation and due to the microbeam nature of damage by heavy ions are pointed out. Certain experiments and studies by radiobiologists are suggested for reducing the uncertainties in the estimates of the risks.