Low Dose Rate Gamma Irradiation Research Articles

Suppression of carcinogenic processes in mice by chronic low dose rate gamma-irradiation

Effects of low dose rate radiation on the process of carcinogenesis induced by a chemical carcinogen were examined. ICR female mice, 35 or 36 mice for each group, were kept and exposed to 137Cs gamma-rays in the long-term low dose rate irradiation facility at the Central Research Institute of Electric Power Industry at a dose rate of 0.3, 0.96, or 2.5 mGy/h. Thirty-five days later, the mice were injected in the groin with 0.5 mg of 20-methylcholanthrene (MC) dissolved in olive oil, and irradiation was continued. Tumours started to appear 2 months after MC injection. Cumulative tumour incidences after 216 days following MC injection were 94% in the mice irradiated at 0.3 mGy/h, 76% at 0.95 mGy/h, 89% at 0.30 mGy/h, and 94% in non-irradiated control mice. The difference between the tumour incidence in the control mice and that in the mice irradiated at 0.95 mGy/h was statistically significant. These results indicate the suppressive effect of low dose rate irradiation on the process of tumour induction initiated by MC with an optimum dose rate of approximately 1 mGy/h.

Mutation induction by continuous low dose rate gamma irradiation in human cells

Human teratocarcinoma cells P3 were subjected to 60Co-gamma-rays in continuous exponential growth using a “chemostat” set-up. Mutations at the HPRT-locus were measured with dose rates of 5 mGy/h, 50 mGy/h and after acute exposure. There was a clear reduction of mutation induction efficiency with 50 mGy/h compared to the high dose rates, however, with 5 mGy/h the same results were obtained as with acute exposure, in other words mutation induction increased again with very low dose-rates. This phenomenon of an “inverse dose rate effect” is restricted to cycling cells, as it was not found in plateau phase cells exposed under identical conditions. These results confirm previous findings by others and us in rodent cells, which were so far not available for human cells. In order to explore a possible contribution of “genomic instability”, cells irradiated acutely with a dose of 3 Gy were grown for up to 40 days in an exponential culture, and the mutant frequencies were determined every second day. A rise in mutant frequency after 20–30 days was consistently found in three independent experiments but time course and mutant numbers were not reproducible and seemed to follow a stochastic behaviour. The results cast doubt on the common assumption that a reduction in the dose rate will always lead to smaller effects, although this somewhat artificial system cannot be generalised to the human situation.

Suppressive effect of long-term low-dose rate gamma-irradiation on chemical carcinogenesis in mice

Female ICR mice, 6 weeks old, 35 in each group, were exposed to gamma-rays from a 137Cs source in the long-term low-dose rate irradiation facility at the Central Research Institute of Electric Power Industry (CRIEPI). The dose rate was 2.6 (A), 0.96 (B), or 0.30 mGy/h (C). Thirty-five days later, the mice were injected in the groin with 0.5 mg of methylcholanthrene (MC) dissolved in olive oil and irradiation was continued. Cumulative tumor incidences after 216 days following MC injection were 89% in group A, 76% in group B, and 94% in group C. The one in the non-irradiated control group was 94%. The difference in the tumor incidence between the control and position B was statistically significant, indicating the suppressive effect of the low-dose rate irradiation on the process of MC-induced carcinogenesis with an optimum dose rate around 1 mGy/h.

The estimation of molecular and cytogenetic effects in mice exposed to chronic low dose gamma-radiation

Molecular and cytogenetic parameters were estimated in male CBA/lac mice exposed to chronic low dose-rate gamma-radiation (62 cGy/year) for 40, 80, 120, 210, and 365 days. After 40 days of exposure (6.7 cGy), spleen lymphocyte susceptibility to hydrogen peroxide was shown to increase. However, beginning from the day 120 of the treatment (20.4 cGy), the opposite effect was observed. An increase in number of the DNA-protein crosslinks was recorded in spleen lymphocytes only on day 40 of the experiment. The number of DNA breaks increased significantly beginning from day 120 of the experiment, as shown by the DNA-comet method. On the day 210 of irradiation, the frequency of abnormal sperm heads in the mice significantly increased. The number of normochromatic micronucleated erythrocytes of the peripheral blood remained unchanged.

The effects of continuous and single-dose radiation on choline uptake in organotypic tissue slice cultures of rabbit hippocampus

The objective of the present study was to determine the time-dependent course of choline uptake in mature organotypic slice cultures of rabbit hippocampal formation and to assess the effects of continuous and single high-dose irradiation on choline uptake in cultivated slices in vitro. Transverse slices of hippocampus were dynamically incubated in a cerebrospinal fluid-like culture medium for 72 h. To study the changes in choline uptake longitudinally, the slice cultures were processed with 0.1 µ M [3H]-choline, and tritium accumulation was counted. Two different gamma irradiation sources (125I seeds and a clinical 60Co source) were used as representative models of interstitial radiosurgery and other radiosurgical techniques. A total dose of approximately 6000 cGy was delivered to the brain slices in one session or in a continuous, relatively low-dose rate fashion, and their effects on high-affinity choline uptake were examined. In another set of experiments with 125I, 5 µM hemicholinium-3 was used in choline uptake procedures as a competitive high-affinity choline uptake inhibitor. The results can be summarized as follows: (1) in the control group of the hippocampal tissue culture, there was a significant increase in tritium accumulation values from 0 to 48 h and a decrease thereafter; (2) continuous 125I irradiation caused a highly significant depression of the accumulation of tritium compared to that observed in the control group throughout its application for 72 h; (3) there was no significant change in the accumulation of tritium in the slices after single high-dose rate irradiation with a 60Co source; and (4) 5 µM hemicholinium significantly depressed the accumulation of tritium in both the control and the 125I-irradiated groups, and there was no longer a difference between 125I-irradiated and control groups when both groups were treated with hemicholinium. These results demonstrate that the delivery of continuous but relatively low-dose rate gamma irradiation is more efficacious than single high-dose external irradiation on high-affinity choline uptake in hippocampal nervous tissue. The results also indicate that continuous irradiation specifically affected the high-affinity energy-dependent choline uptake mechanism, whereas nonspecific choline uptake did not seem to be disturbed. [Neurol Res 2001; 23: 669-675]

Radiation-induced micronucleus induction in lymphocytes identifies a high frequency of radiosensitive cases among breast cancer patients: a test for predisposition?

Enhanced sensitivity to the chromosome-damaging effects of ionizing radiation is a feature of many cancer-predisposing conditions. We previously showed that 42% of an unselected series of breast cancer patients and 9% of healthy control subjects showed elevated chromosomal radiosensitivity of lymphocytes irradiated in the G2 phase of the cell cycle. We suggested that, in addition to the highly penetrant genes BRCA1 and BRCA2, which confer a very high risk of breast cancer and are carried by about 5% of all breast cancer patients, there are also low-penetrance predisposing genes carried by a much higher proportion of breast cancer patients, a view supported by recent epidemiological studies. Ideally, testing for the presence of these putative genes should involve the use of simpler methods than the G2 assay, which requires metaphase analysis of chromosome damage. Here we report on the use of a simple, rapid micronucleus assay in G0 lymphocytes exposed to high dose rate (HDR) or low dose rate gamma-irradiation, with delayed mitogenic stimulation. Good assay reproducibility was obtained, particularly with the HDR protocol, which identified 31% (12 out of 39) of breast cancer patients compared with 5% (2 out of 42) of healthy controls as having elevated radiation sensitivity. In the long term, such cytogenetic assays may have the potential for selecting women for intensive screening for breast cancer.

O X.4 - O X.4 Chromosomal instability in human lymphocytes after low dose rate gamma-irradiation and delayed mitogen stimulation

O X.4 - O X.4 Chromosomal instability in human lymphocytes after low dose rate gamma-irradiation and delayed mitogen stimulation

Inverse dose-rate effect for mutation induction by gamma-rays in human lymphoblasts.

In order to define further the effects of differences in recombinational proficiency on cell survival and mutation by ionizing radiation, we exposed the syngenic cell lines TK6 and WTK1 to continuous low dose-rate gamma-irradiation. We previously demonstrated that acute X-ray exposure results in lower survival and lower mutation induction at both the thymidine kinase (tk) and the hypoxanthine-guanine phosphoribosyltransferase (hprt) loci in TK6 cells compared with WTK1 cells. These differences were attributed in part to reduced levels of recombination in the TK6 line relative to WTK1. Using a low dose rate 137Cs irradiator, we exposed asynchronous growing populations of these cells to gamma-rays at 14.3, 6.7 and 2.7 cGy/h. Both cell lines exhibited a dose-rate effect on survival. Compared with acute doses, the low dose-rates also protected against mutation induction at the hrpt locus in WTK1, but protection was inversely related to dose-rate. There was also a slight inverse dose-rate effect in TK6, with mutation induction at the lowest dose-rate exceeding that at acute exposures.

Use of semiquantitative reverse transcription-polymerase chain reaction to study gene expression in normal human skin fibroblasts following low dose-rate irradiation.

One way to study the effect of radiation on gene expression is to monitor changes in the levels of specific messenger RNAs. We describe the use of reverse transcription-polymerase chain reaction (RT-PCR) analysis, a faster and more sensitive procedure than the traditional techniques to monitor RNA levels. Using RT-PCR, we confirmed previous results showing increased levels of GADD45 transcripts after high dose-rate X-irradiation in normal human fibroblasts. No differences were observed in the transcript levels of beta-ACTIN, beta-MICROGLOBULIN, Cu-Zn SUPEROXIDE DISMUTASE (SOD-1) and CATALASE. In cells exposed to 3-6 Gy low dose-rate gamma-irradiation we observed increased levels of the GADD45 transcript and lower transcript levels of the genes TOPOISOMERASE II alpha, FACC, CYCLIN A and CYCLIN B. No differences were detected in the transcript levels of beta-ACTIN, beta-MICROGLOBULIN, SOD-1, URACYL-DNA GLYCOSYLASE, CYCLIN C, CYCLIN E, CYCLIN D1, CYCLIN D2, CYCLIN D3, TOPOISOMERASE I and TOPOISOMERASE II beta.

Biological effectiveness of 67-gallium decay in HL60 cells compared with external low dose rate gamma irradiation: Effects on proliferation, G2 arrest, and clonogenic capacity

Purpose: Estimation of the relative biological effectiveness of electron emissions of 67Gallium for cell growth delay and inactivation. Methods and Materials.: Human myeloid HL60 cells were incubated in vitro with 0.74 MBq/mL, 1.48 MBq/mL, or 2.96 MBq/mL of 67Gallium for 4 days. Proliferation (vital cell counts and colorimetric tetrazolium assay), clonogenic survival, and cell-cycle effects were compared with responses of HL60 cells externally irradiated with 0.78, 10.37, and 13.22 Gy by an external 67Gallium source. Dosimetric calculations were performed by Monte Carlo simulations (10,000 events). Results: Proliferation of cells was equally inhibited after 67Gallium incubation with 1.48 and 2.96 MBq/mL compared with 10.37 and 13.22 Gy external irradiation. Irradiation with 10.37 and 13.22 Gy caused a 81% and 89% reduction of Colony Forming Units, compared with 34%, 66%, and 80% reduction after 67Gallium incubation with 0.74, 1.48, and 2.96 MBq/mL, respectively. Peak values for G2/M accumulation were reached on day 4 for the cells externally irradiated with 10.37 and 13.22 Gy (47.5% and 56.7%) and on day 5 after 67Gallium incubation with 0.74 MBq/mL, 1.48 MBq/mL, and 2.96 MBq/mL (26.7%, 43.4%, and 58.2%). Conclusions: 67Gallium incubation exerts a significant cytotoxic effect on human HL60 cells, which, on the basis of dosimetric studies, may be mainly ascribed to conversion electrons (80 KeV) and 8 KeV Auger electrons. Low energy (< 1 keV) Auger electrons do not contribute significantly. The relative biological effectiveness of 67Gallium compared with external low dose rate gamma irradiation is about 1.0 for clonogenic survival and approximately 1.8 and 1.5 for proliferation inhibition and G2 arrest, respectively. For in vivo therapy, this might implicate that higher doses of 67Gallium than 131Iodine or 90Yttrium are necessary for the same biological effect.

Induction of a cytogenetic adaptive response by exposure of rabbits to very low dose-rate gamma-radiation.

Chronic whole-body γ-irradiation of rabbits at a dose-rate of 0·05 Gy/9h (92·6 µGy/min.) induced a cytogenetic adaptive response in peripheral blood lymphocytes with an inductive dose of up to 1·5 Gy when the challenge dose of 1·5 Gy was given either in G0 or in G2 phase.

Effect of gamma radiation at high- and low-dose rate on a novel in vivo mutation assay in mouse intestine.

We have previously proposed that an in vivo mutagenicity assay could be based on the detection of mutations affecting the Dlb-1 locus in the small intestine of C57BL/6J x SWR F1 mice. F1 mice are heterozygous Dlb-1b/Dlb-1a and have only a single allele (Dlb-1b) which specifies expression of the binding site for the lectin Dolichos biflorus agglutinin (DBA) in intestinal epithelia. In whole-mount preparations of small intestine stained with a DBA--peroxidase conjugate, mutated stem cells and their progeny can be recognized as ribbons of unstained cells against a stained background. These DBA-negative ribbons can be quantified. The present investigation characterizes the responses of F1 mice to high- and low-dose rate gamma radiation (1.8 and 0.01 Gy/min respectively) and shows that the induction of ribbons is dose dependent in both cases. Dose sparing is evident at the lower dose rate: 20 ribbons/10(4) villi can be induced by 4 Gy at high-dose rate and by 7.1 Gy at low-dose rate, a dose sparing of 1.76. Age-matched untreated mice had a background level of 3.7 ribbons/10(4) villi. As expected, no ribbons were observed in homozygous (Dlb-1b/Dlb-1b) C67BL/6J mice, in which each stem cell has two alleles specifying lectin binding. The results further validate this system as a sensitive in vivo mutagenicity assay and suggests that the target cells are capable of repairing radiation damage.

実効半減期可変型γ線照射装置とその線量分布特性

The gamma irradiation system with variable effective half-life function has been developed, which provides the low dose rate gamma irradiation field by two 137Cs sources (3.7×1010 Bq and 7.4×1011 Bq) . This apparatus was designed to give an exposure at varying low dose rate in accordance with a biological half life of radioactivity in organisms by selecting one of two irradiation methods. The first irradiation mode uses the spatial attenuation by four movable irradiation tables and the other uses the radiation attenuation by an iron filter. Both of these are driven by pulse motors using the personal computer control. The characteristics on spatial gamma-ray irradiation field were measured using calibrated ionization chambers for gamma dose and NaI (Tl) scintillation ionization chambers for gamma dose and Nal (Tl) scintillation counter for gamma spectra. The irradiation field covers the dose rate between 2.5×10-3C kg-1 min-1 and 1.2×10-7 C kg-1 min-1. The energy spectrum information including scattered radiations are also obtained. The apparatus was proved to be useful in the experiments for the irradiation of mice at varying dose rate to simulate tritium in internal irradiation.

Increased mutant induction by very low dose-rate gamma-irradiation.

Effet des radiations γ a des doses allant de 8 mGy h −1 a 4 gy min −1 sur des cultures en suspension de cellules du hamster V79-S: induction de mutants resistants a la 6-thioguanine

Response of Human Fibroblasts to Low Dose Rate Gamma Irradiation

Cells from 11 human strains, including fibroblasts from patients with the genetic diseases of ataxia telangiectasia (AT), xeroderma pigmentosum (XP), and Fanconi's anemia (FA), were exposed to gamma radiation at high (1.6-2.2 Gy/min) and at low (0.03-0.07 Gy/min) dose rates. Survival curves reveal an increase in the terminal slope (D0) when cells are irradiated at low dose rates compared to high dose rates. This was true for all cell lines tested, although the AT, FA, and XP cells are reported or postulated to have radiation repair deficiencies. From the response of these cells, it is apparent that radiation sensitivities differ; however, at low dose rate, all tested human cells are able to repair injury.

Survival of cultured human cells exposed to californium 252Cf radiation at low dose rates.

Human T-1 cells were used to determine the rbe of low dose rate 252Cf (n + gamma) radiation, compared to low dose rate gamma radiation. The T-1 cells were irradiated hypoxically at room temperature, and the dose rates were as follows: a) 252Cf (n + gamma), 0.045, 0.090, and 0.270 Gy/hour; and b) 137Cs gamma radiation, 0.117, 0.246, and 0.765 Gy/hour. The RBE was obtained as the ratio of the initial slope of the 137Cs survival curves (approximately the same for all three dose rates), and the initial slope of the 252Cf survival curves (same for all three dose rates). The RBE was 5.0 +/- 1.0 for all components of the 252Cf radiation and 7.1 +/- 1.7 for the neutron components. As the dose increased, an inverse dose rate effect (more sensitive at lower dose rates) for the 137Cs survival curves was observed.

Effectiveness of continuous low dose-rate gamma-irradiation on rat skin.

A skin transplantation technique has been employed for assessment of radiation-induced damage to rat skin. Shrinkage of the area of an irradiated piece of white skin was measured five months after transplantation onto brown F1 hybrid rats. The effectiveness of 137Cs γ rays at a doserate of 85 rads/hour was compared with that of 300 kV X rays applied at 40 rads/minute. Protracted irradiation provides a large sparing effect due to repair of sublethal damage. For a 90 per cent reduction of skin area, the ratio of the doses given at low and high dose-rate, respectively, is 1·8. Comparison of these results with data obtained previously for a rat rhabdomyosarcoma, indicates that protracted irradiation at low dose-rate, when compared with acute irradiations, has a larger sparing effect on skin than on the tumour.

Susceptibility to infection with Pasteurella tularensis and the immune response of mice exposed to continous low dose rate gamma radiation.

Abstract : Mice were exposed to continuous low dose rate (1.4 R/hr) gamma radiation. Immediately after accumulating 1000 R, 2000 R, or 3000 R, they received a subcutaneous injection of the live vaccine strain (LVS) of Pasteurella tularensis. Although the avirulent strain caused no effect in normal mice, a fulminating infection occurred in irradiated mice. As the total radiation dose increased the LD50 of the organism decreased. Animals surviving irradiation and immunization were subjected to an aerosol challenge of either the avirulent LVS or the virulent SCHU S-5 strain of P. tularensis. The LVS strain is virulent for mice when administered as an aerosol, but not when injected subcutaneously. Although an aerosol of avirulent organisms was lethal for normal mice, immune mice, both non-irradiated and irradiated, survived an aerosol infection with the avirulent organism. Irradiated, immunized mice challenged with an aerosol of the virulent SCHU S-5 strain showed a decrease in immunity, especially at the higher levels of radiation. Bacterial counts of the lungs, liver, spleen, and lymph nodes of non-immunized mice exposed to a lethal aerosol dose of the avirulent P. tularensis (LVS) showed increasing numbers of bacteria after the 2nd post-infection day. This increase continued until the death of the animal on about the 10th day. Immune animals, however, were able to clear the lungs 5 days after infection with no involvement of the other tissues. Although continuous low dose rate gamma radiation does not produce the gross radiation pathology that one sees following acute radiation, exposure to a live avirulent Pasteurella tularensis vaccine produces an infection and an impaired immunity.

Homograft response in adult-thymectomized mice:Deficiency with aging and after low dose-rate gamma-irradiation

The homograft response to skin grafts of varying antigenic disparity was measured at 15–20 months of age in mice thymectomized at 8 weeks of age. The response to male isografts was delayed more in thymectomized female recipients than in intact recipients. Exposure of female mice to sublethal X-radiation (at 10–11 weeks of age) resulted in a further delay in intact recipients and abrogated the response in the thymectomized recipients. Other intact but aged mice showed a slight impairment of response to skin grafts differing at non-H-2 loci. In thymectomized groups, this impairment was increased. Exposure of both intact or thymectomized mice (at 9 months of age) to 1133 rad of low dose rate gamma-radiation (1·4 rad/hr) resulted in an increased delay in response relative to the non-irradiated groups. In all cases, the non-H-2 grafts were the most affected by radiation and thymectomy. It is concluded: (1) after adult thymectomy, the homograft response becomes deficient in old mice, particularly as regards male or non-H-2 isoantigens; (2) chronic gamma-irradiation after thymectomy further increases this deficiency, presumably by creating a greater demand for new immunocompetent cells and/or by eliciting defects in their precursors.

Effects of Continuous Low-Level Gamma Irradiation on Circulating and Peritoneal Mononuclear Leukocytes of Mice

Abstract : LAF1 mice were exposed continuously to Co60 gamma radiations at a dose rate of 1.4 rads per hour. The number of lymphocytes in the circulating blood fell sharply during the first week of exposure (190 rads) and decreased thereafter at a very gradual but statistically significant rate for the duration of the experiment (15 weeks, 3450 rads). The disappearance of small lymphocytes (6 microns in diameter) from the peritoneal cavity was also more rapid during the first week of irradiation than during subsequent weeks. Medium peritoneal lymphocytes (8-10 microns in diameter) and peritoneal macrophages disappeared at constant rates over the entire observation period. After the first week of exposure, the disappearance rates of small and medium peritoneal lymphocytes were identical. This rate was greater than that for peritoneal macrophages and that for circulating lymphocytes. Based on the fraction of cells surviving any given exposure, the mononuclear leucocytes may be arranged in the following order of decreasing sensitivity to continuous low dose rate gamma irradiation: circulating lymphocytes, small peritoneal lymphocytes, medium peritoneal lymphocytes, peritonela macrophages. This order is the same as that after acute exposure to X rays. (Author)