Relative biological effectiveness measurements using murine lethality and survival of intestinal and hematopoietic stem cells after fermilab neutrons compared to JANUS reactor neutrons and 60Co gamma rays

The relative biological effectiveness (RBE) of the 25-MeV (average energy) neutron beam at the Fermi National Accelerator Laboratory was measured using murine bone marrow (LD50/30) and gut (LD50/6) lethality and killing of hematopoietic colony forming units (CFU-S) or intestinal clonogenic cells (ICC). The reference radiation was 60Co gamma rays. The LD50/30 and LD50/6 for mice exposed to the Fermilab neutron beam were 6.6 and 8.7 Gy, respectively, intermediate between those of JANUS neutrons and 60Co gamma rays. The D0 values for CFU-S and ICC were 47 cGy and 1.05 Gy, respectively, also intermediate between the lowest values found for JANUS neutrons and the highest values found after 60Co gamma rays. The split-dose survival ratios for CFU-S at intervals of 1-6 hr between doses were essentially 1.0 for both neutron sources, while the corresponding split-dose survival ratio for 60Co gamma rays was consistantly above 1, reaching a maximum of 1.7 with a 1-hr interval between doses. The 3-hr split-dose survival ratios for ICC were 1.0 for JANUS neutrons, 1.85 for Fermilab neutrons, and 6.5 for 60Co gamma rays. The RBE estimates for LD50/30 were 1.5 and 2.3 for Fermilab and JANUS neutrons, respectively. Based on LD50/6, the RBEs were 1.9 (Fermilab) and 3.0 (JANUS). The RBEs for CFU-S D0 were 1.4 (Fermilab) and 1.9 (JANUS) and for jejunal microcolony D0 1.4 (Fermilab) and 2.8 (JANUS).     

Comet assay to sense neutron 'fingerprint'

The suitability of comet assay to identify DNA damage induced by neutrons of varying energy was tested. For this purpose, monoenergetic neutrons from Hiroshima University Radiobiological Research Accelerator (HIRRAC) were used to induce DNA damage in irradiated human peripheral blood lymphocytes. The level of damage was computed as tail moment for different doses (0.125-1 Gy) and compared with the effects resulting from irradiation with (60)Co gamma. The neutron-irradiated cells exhibited longer comet tails consisting of tiny pieces of broken DNA in contrast to the streaking tails generated by (60)Co gamma. The peak biological effectiveness occurred at 0.37 and 0.57 MeV; a further increase or decrease in neutron energy led to a reduced RBE value. The RBE values, as measured by the comet assay, were 6.3, 5.4, 4.7, 4.3, 2.6, and 1.7 for 0.37, 0.57, 0.79, 0.186, 1, and 2.3 MeV neutrons. The lower RBE value obtained by the comet assay when compared to that for other biological end points is discussed. This study reports the usefulness of the alkaline comet assay for identifying DNA damage induced by neutrons of the same radiation weighting factor. The comet assay is a potential tool for use in neutron therapy, as well as a method for the rapid screening of samples from individuals accidentally exposed to radiation.     

The cytogenetic effect of thermal neutrons inLens esculenta (Moench)

The first information concerning the cytogenetic efficiency of thermal neutrons in lentil are presented in this paper. The range of cytologically effective dosages of thermal neutrons in lentil was determined. This determination enables us to compare the efficiency with the mutagenic effect of thermal neutrons and with their effect on the growth and development of plants of M₁ generation. These effects were already evaluated in previous communications. Thermal neutron irradiation significantly affected all the characters studied. A linear dependence of the effect on the dose of the neutron radiation was found for most of the analyzed characters. From a sample of scored cells, whose nuclei were in the anaphase or early telophase, 9.0 to 72.0% of them had chromosomal bridges and fragments after irradiation with dosages from 3.3×10¹¹ n cm^?2 to 4.5×10¹² n cm^?2. The highest number of rearrangements per one cell reached 2.16 after irradiation with 4.25×10¹² n cm^?2 while the lowest dosage used, 3.3×10¹¹ n cm^?2, induced 0.17 of chromosomal rearrangements per one cell. Irradiation with thermal neutrons is capable of inducing a large number of very complicated chromosome rearragements.     

A comparison of radioprotection from three neutron sources and 60Co by WR-2721 and WR-151327

Two thiophosphoroate radiation protectors (WR-2721 and WR-151327) were assessed for their ability to modify the effects of neutron or gamma irradiation on the gastrointestinal tract. Three neutron sources (DOSAR, JANUS, and FERMILAB) were compared to the response obtained after 60Co irradiation. The end points studied were intestinal stem cell survival and LD50(6). DOSAR and JANUS, both fission-spectrum neutrons, showed somewhat different gut sensitivities [LD50(6)] of about 240 and 400 cGy respectively. The intestinal LD50 obtained with FERMILAB neutrons (25 meV) was closer (875 cGy) to that obtained after 60Co (1068 cGy) irradiation. WR-151327 protected against the lethal effects of fission neutron (DOSAR and JANUS) to a greater degree (DMF = 2.2) than with lower LET sources such as FERMILAB neutrons (DMF = 1.7) or 60Co (DMF = 1.7). The results did not correlate with the intestinal stem cell assays where WR-2721 when compared to WR-151327 showed either similar (DOSAR; fission spectrum neutrons) or somewhat better (60Co and FERMILAB neutrons) protection. Possible explanations for the differing results are discussed.     

RBE of nearly monoenergetic neutrons at energies of 36 keV-14.6 MeV for induction of dicentrics in human lymphocytes

We examined the induction of dicentric chromosomes in human lymphocytes irradiated in vitro with nearly monoenergetic neutrons at energies in the range of 36 keV–15.0 MeV. For the assessment of the relative biological effectiveness (RBE) both 220 kV x-rays and ⁶⁰Co -rays were used as reference radiations. To avoid potential confounding factors that would influence the outcome of the experiments, only blood from one individual was used. The neutron RBE culture conditions ensured that the chromosome analysis could be performed exclusively in metaphases of the first cell cycle in vitro. For the reference radiation of 220 kV x-rays, the values of RBE_M were found to increase from 16.6 (E_n=36 keV) to the maximum value of 23.4 (E_n=385 keV). For ⁶⁰Co -rays utilized as the reference radiation, the corresponding RBE_M values were found to be higher by a factor of 4. These results agree well with the previously published large data sets of three laboratories on dose-response relationships for dicentrics or dicentrics plus centric rings. They show a similar dependence of RBE on neutron energy.     

RBE of thermal neutrons for induction of chromosome aberrations in human lymphocytes

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with slow neutrons was examined to assess the maximum low-dose RBE (RBE_M) relative to ⁶⁰Co γ-rays. For the blood irradiations, cold neutron beam available at the prompt gamma activation analysis facility at the Munich research reactor FRM II was used. The given flux of cold neutrons can be converted into a thermally equivalent one. Since blood was taken from the same donor whose blood had been used for previous irradiation experiments using widely varying neutron energies, the greatest possible accuracy was available for such an estimation of the RBE_M avoiding the inter-individual variations or differences in methodology usually associated with inter-laboratory comparisons. The magnitude of the coefficient α of the linear dose–response relationship (α = 0.400 ± 0.018 Gy^?1) and the derived RBE_M of 36.4 ± 13.3 obtained for the production of dicentrics by thermal neutrons confirm our earlier observations of a strong decrease in α and RBE_M with decreasing neutron energy lower than 0.385 MeV (RBE_M = 94.4 ± 38.9). The magnitude of the presently estimated RBE_M of thermal neutrons is—with some restrictions—not significantly different to previously reported RBE_M values of two laboratories.     

Determination of the thermal and epithermal neutron sensitivities of an LBO chamber

An LBO (Li₂B₄O₇) walled ionization chamber was designed to monitor the epithermal neutron fluence in boron neutron capture therapy clinical irradiation. The thermal and epithermal neutron sensitivities of the device were evaluated using accelerator neutrons from the ⁹Be(d, n) reaction at a deuteron energy of 4 MeV (4 MeV d-Be neutrons). The response of the chamber in terms of the electric charge induced in the LBO chamber was compared with the thermal and epithermal neutron fluences measured using the gold-foil activation method. The thermal and epithermal neutron sensitivities obtained were expressed in units of pC cm², i.e., from the chamber response divided by neutron fluence (cm^?2). The measured LBO chamber sensitivities were 2.23 × 10^?7 ± 0.34 × 10^?7 (pC cm²) for thermal neutrons and 2.00 × 10^?5 ± 0.12 × 10^?5 (pC cm²) for epithermal neutrons. This shows that the LBO chamber is sufficiently sensitive to epithermal neutrons to be useful for epithermal neutron monitoring in BNCT irradiation.     

Effects of single and split doses of cobalt-60 gamma rays and 14 MeV neutrons on mouse stem cell spermatogonia

The long-term effects of ionizing radiation on male gonads may be the result of damage to spermatogonial stem cells. Doses of 10 cGy to 15 Gy (60)Co gamma rays or 10 cGy to 7 Gy 14 MeV neutrons were given to NMRI mice as single or split doses separated by a 24-h interval. The ratios of haploid spermatids/2c cells and the coefficients of variation of DNA histogram peaks as measures of both the cytocidal and the clastogenic actions of radiation were analyzed by DNA flow cytometry after DAPI staining. The coefficient of variation is not only a statistical examination of the data but is also used here as a measure of residual damage to DNA (i.e. a biological dosimeter). Testicular histology was examined in parallel. At 70 days after irradiation, the relative biological effectiveness for neutrons at 50% survival of spermatogonial stem cells was 3.6 for single doses and 2.8 for split doses. The average coefficient of variation of unirradiated controls of elongated spermatids was doubled when stem cells were irradiated with single doses of approximately 14 Gy (60)Co gamma rays or 3 Gy neutrons and observed 70 days later. Split doses of (60)Co gamma rays were more effective than single doses, doubling DNA dispersion at 7 Gy. No fractionation effect was found with neutrons with coefficients of variation.     

Dose-response modeling of life shortening in a retrospective analysis of the combined data from the JANUS program at Argonne National Laboratory

Life shortening was investigated in both sexes of the B6CF1 (C57BL/6 x BALB/c) mouse exposed to fission neutrons and 60Co gamma rays. Three basic exposure patterns for both neutrons and gamma rays were compared: single exposures, 24 equal once-weekly exposures, and 60 equal once-weekly exposures. Ten different dose-response models were fitted to the data for animals exposed to neutrons. The response variable used for all dose-response modeling was mean after-survival. A simple linear model adequately described the response to neutrons for females and males at doses less than or equal to 80 cGy. At higher neutron dose levels a linear-quadratic equation was required to describe the life-shortening response. An effect of exposure pattern was observed prior to the detection of curvature in the dose response for neutrons and emerged as a potentially significant factor at neutron doses in the range of 40-60 cGy. Augmentation of neutron injury with dose protraction was observed in both sexes and began at doses as low as 60 cGy. The life-shortening response for all animals exposed to gamma rays (22-1918 cGy) was linear and inversely dependent upon the protraction period (1 day, 24 weeks, 60 weeks). Depending on the exposure pattern used for the gamma-ray baseline, relative biological effectiveness (RBE) values ranged from 6 to 43. Augmentation, because it occurred only at higher levels of neutron exposure, had no influence on the estimation of RBEm.     

RBE of quasi-monoenergetic 60 MeV neutron radiation for induction of dicentric chromosomes in human lymphocytes

The production of dicentric chromosomes in human lymphocytes by high-energy neutron radiation was studied using a quasi-monoenergetic 60 MeV neutron beam. The average yield coefficient of the linear dose–response relationship for dicentric chromosomes was measured to be (0.146±0.016) Gy⁻¹. This confirms our earlier observations that above 400 keV, the yield of dicentric chromosomes decreases with increasing neutron energy. Using the linear-quadratic dose–response relationship for dicentric chromosomes established in blood of the same donor for ⁶⁰Co γ-rays as a reference radiation, an average maximum low-dose RBE (RBE_M) of 14±4 for 60 MeV quasi-monoenergetic neutrons with a dose-weighted average energy of 41.0 MeV is obtained. A correction procedure was applied, to account for the low-energy continuum of the quasi-monoenergetic spectral neutron distribution, and the yield coefficient α for 60 MeV neutrons was determined from the measured average yield coefficient . For α, a value of (0.115±0.026) Gy⁻¹ was obtained corresponding to an RBE_M of 11±4. The present experiments extend earlier investigations with monoenergetic neutrons to higher energies.     

Chromosome aberrations induced in human lymphocytes by radiation from 252Cf.

In vitro dose--response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 252Cf neutron radiation. The aberration yields fitted best to the linear function Y=aD, which is consistent with the single-track model of aberration formation for high LET radiation. The curves have been compared with others previously produced in this laboratory for several energies of neutrons and for 60Co gamma radiation. The r.b.e. for 252Cf with respect to 60Co is 27 at very low doses, decreasing to 6 at an aberration yield equivalent to 400 rad of 18 rad/hour gamma radiation. A profile of chromosome-aberration induction with depth in a perspex phantom was obtained by placing blood samples at several distances over the range 0.65-2.0 cm from the californium source. This profile was compared with depth-damage calculations for a radium needle. The r.b.e. of 252Cf radiation relative to 226Ra gamma radiation increased with the distance from the source, implying that californium is more effective at greater distances in destroying the ability of cells to divide, which may be an advantage in the treatment of large tumours.     

Thermal and resonance neutrons generated by various electron and X-ray therapeutic beams from medical linacs installed in polish oncological centers

BackgroundHigh-energy photon and electron therapeutic beams generated in medical linear accelerators can cause the electronuclear and photonuclear reactions in which neutrons with a broad energy spectrum are produced. A low-energy component of this neutron radiation induces simple capture reactions from which various radioisotopes originate and in which the radioactivity of a linac head and various objects in the treatment room appear.AimThe aim of this paper is to present the results of the thermal/resonance neutron fluence measurements during therapeutic beam emission and exemplary spectra of gamma radiation emitted by medical linac components activated in neutron reactions for four X-ray beams and for four electron beams generated by various manufacturers’ accelerators installed in typical concrete bunkers in Polish oncological centers.Materials and methodsThe measurements of neutron fluence were performed with the use of the induced activity method, whereas the spectra of gamma radiation from decays of the resulting radioisotopes were measured by means of a portable high-purity germanium detector set for field spectroscopy.ResultsThe fluence of thermal neutrons as well as resonance neutrons connected with the emission of a 20 MV X-ray beam is ～10⁶ neutrons/cm² per 1 Gy of a dose in water at a reference depth. It is about one order of magnitude greater than that for the 15 MV X-ray beams and about two orders of magnitude greater than for the 18–22 MeV electron beams regardless of the type of an accelerator.ConclusionThe thermal as well as resonance neutron fluence depends strongly on the type and the nominal potential of a therapeutic beam. It is greater for X-ray beams than for electrons. The accelerator accessories and other large objects should not be stored in a treatment room during high-energy therapeutic beam emission to avoid their activation caused by thermal and resonance neutrons. Half-lives of the radioisotopes originating from the simple capture reaction (n,γ) (from minutes to hours) are long enough to accumulate radioactivity of components of the accelerator head. The radiation emitted by induced radioisotopes causes the additional doses to staff operating the accelerators.     

In Vitro Effects of Gamma Radiation from 60Co and 137Cs on Plasmid DNA

The effects of gamma radiation from ⁶⁰Co and ¹³⁷Cs on DNA in aqueous solution are studied experimentally. Using an improved plasmid purification protocol and improved electrophoretic gel analysis techniques provided results with relatively small uncertainties. The results are compared with both theoretical and experimental results. In particular, the results obtained here are discussed in the light of recent discussion on supposed differences of the effects induced by gamma radiation from ⁶⁰Co and ¹³⁷Cs. We find that the effects of both types of gamma radiation are similar.     

Radiotoxicity of plutonium in NTA-degrading Chelatobacter heintzii cell suspensions

The radiotoxicity of plutonium in NTA-degrading Chelatobacter heintzii cell suspensions was investigated as part of a more general study to establish the key interactions between actinide-organic complexes and microorganisms in the subsurface. The radiation tolerance of C. heintzii, based on ⁶⁰Co gamma irradiation experiments, was 165 ± 30 Gy. No bacteria survived irradiation doses greater than 500 Gy. In the presence of plutonium, where alpha particle decay was the primary source of ionizing radiation, the observed toxicity was predominantly radiolytic rather than chemical. This was evident by the greater effect of activity, rather than concentration, on the toxicity noted. Bioassociation of plutonium with C. heintzii was postulated to be an important and necessary step in the observed loss of cell viability since this was the best way to account for the observed death rate. The radiotoxicity of plutonium towards bacteria is a potentially important consideration in the bioremediation of sites contaminated with radionuclide-organic mixtures and the bioprocessing of nuclear waste.     

Evidence for similarities between radiation damage expressed by beta-araA and damage involved in the interaction effect observed after exposure of V79 cells to mixed neutrons and gamma radiation

Plateau-phase V79 cells were exposed sequentially to fast neutrons and gamma rays. A dose-dependent reduction in the shoulder width of the gamma-ray survival curve was observed after preexposure of cells to neutrons. A similar effect was demonstrated on the neutron survival curve when cells were preirradiated with gamma rays. Treatment of cells with 150 microM beta-araA after either gamma or neutron irradiation reduced primarily the shoulder of the survival curve. When beta-araA was given to the cells after exposure to mixed radiation modalities, survival curves similar to those observed after exposure to a single radiation modality and treatment with beta-araA were obtained. The kinetics of loss of the interaction observed after exposure of cells to gamma rays following neutron irradiation was similar to the kinetics of loss of sensitivity to beta-araA (T1/2 = 1 h) measured by delaying drug administration after exposure to gamma rays. The results suggest that the PLD expressed by beta-araA is at least partly involved in the interactive effect observed after combined exposure of plateau-phase V79 cells to neutrons and gamma rays.     

Dosimetric response of tooth enamel to 14 Mev neutrons

The electron paramagnetic resonance (EPR) response of tooth enamel in a monenergetic neutron beam of 14 MeV was studied, with the aim to evaluate the relative neutron to ⁶⁰Co sensitivity. Three samples of tooth enamel powder were irradiated in air. A whole tooth and a powdered sample were irradiated in a geometrical PMMA phantom, in order to simulate the real exposure of a tooth inside a human head. The measured dose in enamel was compared to the dose calculated by Monte Carlo simulation. The relative neutron to ⁶⁰Co sensitivity using different reference materials (air, water and enamel) was evaluated as well. Large differences in sensitivity values were found depending on the reference material: the obtained relative neutron to ⁶⁰Co sensitivity was 0.47±0.09 for enamel and 0.15±0.03 for water. A comparison with results in fast neutron fields is reported.     

Effect of Ionizing Radiations on Rice

Rice seed and natural brown rice were irradiated with cobalt-60 gamma rays and cathode rays. Development of primary leaf and root was highly inhibited by irradiation more than 5×l0⁴ rep, whereas the growth of root was somewhat accelerated by 5×10³ rep. More than 1.6×l0⁶ rep was necessary for the complete destruction of the germination. Thiamine and riboflavin contents of rice seed irradiated with 10⁵ rep of cobalt-60 gamma rays were almost the same magnitude as the non-irradiated one, and 87 to 96% of microorganism on brown rice could be killed with 1×l0⁵ rep.     

Interpretation of cytogenetic damage induced in the germ line of male mice exposed for over 1 year to 239Pu alpha particles, fission neutrons, or 60Co gamma rays

The relative biological effectiveness (RBE) of 239Pu alpha particles, fission neutrons (0.85 MeV), and 60Co gamma rays has been evaluated for the induction of reciprocal chromosome translocations in spermatogonia and of chromosome/chromatid fragments and chromatid rearrangements in the primary spermatocyte of adult male B6CF1 mice. Age concurrency was maintained for both internal and external radiations which were delivered at about 1 rad/week for 239Pu (single intravenous dose of 10 microCi/kg), 0.67, 1.67, and 2.67 rad/week for neutrons, and 6.95, 17.4, and 32 rad/week for gamma rays for at least 60 weeks. In terms of frequency of translocations, the response to the alpha emitter was nonlinear (concave downward) with little dose-response predictability; to cumulative neutron exposures the response was linear, without evidence of a dose-rate effect; and to gamma radiation the responses were linear, and a significant dose-rate effect was seen. RBE estimates are variable. For translocations, the n/gamma ratio is between 10 and 24, depending upon weekly dose level, and the ratio is 1 or less for the alpha particle relative to the neutron. For fragments, the n/gamma ratio is 18 to 22, depending upon age factors, and alpha/n is 1.5. For chromatid rearrangements, n/gamma is 7 and alpha/n is essentially indeterminate, but much below one. The overall response to the alpha emitter is interpreted to be a complex function of (a) microdosimetric heterogeneity, (b) a nearly invariant deposition pattern in the gonad, (c) the high sensitivity of differentiating spermatogonia to cell killing, and (d) the capacity of stem cells in relatively radiation-free areas to progressively assume the major spermatogenic role.     

Physiological responses of the halophilic archaeon Halobacterium sp. strain NRC1 to desiccation and gamma irradiation

We report that the halophilic archaeon Halobacterium sp. strain NRC-1 is highly resistant to desiccation, high vacuum and ⁶⁰Co gamma irradiation. Halobacterium sp. was able to repair extensive double strand DNA breaks (DSBs) in its genomic DNA, produced both by desiccation and by gamma irradiation, within hours of damage induction. We propose that resistance to high vacuum and ⁶⁰Co gamma irradiation is a consequence of its adaptation to desiccating conditions. Gamma resistance in Halobacterium sp. was dependent on growth stage with cultures in earlier stages exhibiting higher resistance. Membrane pigments, specifically bacterioruberin, offered protection against cellular damages induced by high doses (5 kGy) of gamma irradiation. High-salt conditions were found to create a protective environment against gamma irradiation in vivo by comparing the amount of DSBs induced by ionizing radiation in the chromosomal DNA of Halobacterium sp. to that of the more radiation-sensitive Escherichia coli that grows in lower-salt conditions. No inducible response was observed after exposing Halobacterium sp. to a nonlethal dose (0.5 kGy) of gamma ray and subsequently exposing the cells to either a high dose (5 kGy) of gamma ray or desiccating conditions. We find that the hypersaline environment in which Halobacterium sp. flourishes is a fundamental factor for its resistance to desiccation, damaging radiation and high vacuum.     

The effectiveness of monoenergetic neutrons at 565 keV in producing dicentric chromosomes in human lymphocytes at low doses

The induction of dicentric chromosomes in human lymphocytes from one individual irradiated in vitro with monoenergetic neutrons at 565 keV was examined to provide additional data for an improved evaluation of neutrons with respect to radiation risk in radioprotection. The resulting linear dose-response relationship obtained (0.813 +/- 0.052 dicentrics per cell per gray) over the dose range of 0.0213-0.167 Gy is consistent with published results obtained for irradiation with neutrons from different sources and with different spectra at energies lower than 1000 keV. Comparing this value to previously published "average" dose-response curves obtained by different laboratories for (60)Co gamma rays and orthovoltage X rays resulted in maximum RBEs (RBE(m)) of about 37 +/- 8 and 16 +/- 4, respectively. However, when our neutron data were matched to low-LET dose responses that were constructed several years earlier for lymphocytes from the same individual, higher values of RBE(m) resulted: 76.0 +/- 29.5 for (60)Co gamma rays and 54.2 +/- 18.4 for (137)Cs gamma rays; differentially filtered 220 kV X rays produced values of RBE(m) between 20.3 +/- 2.0 or 37.0 +/- 7. 1. The results highlight the dependence of RBE(m) on the choice of low-LET reference radiation and raise the possibility that differential individual response to low-LET radiations may need to be examined more fully in this context.