Oncogenic transformation by fractionated doses of neutrons

Oncogenic transformation was assayed after C3H 10T1/2 cells were irradiated with monoenergetic neutrons; cells were exposed to 0.23-, 0.35-, 0.45-, 5.9-, and 13.7-MeV neutrons given singly or in five equal fractions over 8 h. At the biologically effective neutron energy of 0.45 MeV, enhancement of transformation was evident with some small fractionated doses (below 1 Gy). When transformation was examined as a function of neutron energy at 0.5 Gy, enhancement was seen for cells exposed to three of the five energies (0.35, 0.45, and 5.9 MeV). Enhancement was greatest for cells irradiated with 5.9-MeV neutrons. Of the neutron energies examined, 5.9-MeV neutrons had the lowest dose-averaged lineal energy and linear energy transfer. This suggests that enhancement of transformation by fractionated low doses of neutrons may be radiation-quality dependent.     

Neutron RBEs and the radiosensitive target for mouse immature oocyte killing

The highly radiosensitive immature oocytes of mice were irradiated in vivo with graded doses of 252Cf fission radiation, 0.43- or 15-MeV neutrons, or 60Co gamma rays. Comparisons of oocyte survival for neutrons and for gamma rays demonstrate that neutron RBEs for the killing of these important cells do not reach the high values (30-50 or more) at low doses observed for several other biological end points. Rather, neutrons differ little in effectiveness from gamma rays in killing these extremely sensitive murine oocytes. For 0.43-MeV neutrons, RBEs obtained from fitted survival curves reach only 1.7 at 0.1 rad. For 15-MeV neutrons, they are not significantly different from 1 at any dose tested (lowest, 4.5 rad). For 252Cf fission neutrons (E = 2.15 MeV), RBEs are intermediate between those for 0.43- and 15-MeV neutrons. For all neutron energies tested, the RBEs are particularly low in the juvenile period, a time when murine immature oocytes are especially radiosensitive. With exposure just prior to birth, however, when these cells are much less easily killed, higher, more usual RBEs are found. The minimum size of the lethality target in mouse immature oocytes, estimated from the inactivation constant for 0.43-MeV neutrons and microdosimetric values, is larger than the nucleus but not larger than the cell. This and related analytical considerations suggest that the hypersensitive target in these particular oocytes is the plasma membrane, a finding which is in excellent accord with results from other experiments using different, contrasting radiations and dose deliveries (accelerated Si14+ ions, gamma rays, and beta rays from 3HOH compared with those from [3H]thymidine).     

Measurement of neutron-induced genetic damage in mouse immature oocytes

Recent experimental evidence concerning the nature of radiosensitive targets in mouse immature (resting) oocytes has led to new experimental designs that permit measurement of radiation-induced genetic damage in these important cells. We have previously reported initial results of the detection of genetic damage in mouse immature oocytes using monoenergetic 0.43-MeV neutrons. Here we provide a full report of our data and compare the genetic sensitivity of immature oocytes with those measured by others for maturing oocytes. Until recently, all attempts to detect radiation-induced genetic damage in mouse immature oocytes had failed. This appears to have been because the radiation types and modes of dose delivery used in those studies did not sufficiently spare the hypersensitive lethality target (the plasma membrane) while at the same time deposit enough dose in DNA to produce detectable mutation. Recoil protons from 0.43-MeV neutrons produce short ionization tracks (2.6 micron mean) and can therefore deposit energy in the DNA without simultaneously traversing the plasma membrane. Using these particles, we have obtained dose-response relationships for both chromosome aberrations and dominant lethal mutations in oocytes from females irradiated 8-12 weeks earlier, when oocytes were immature. Results suggest that the intrinsic mutational sensitivity of mouse immature oocytes is not very different from that of maturing oocytes.     

Chromosomal rearrangements induced in mouse spermatogonia by 14.5-MeV neutrons

Inbred CBA male mice were irradiated with 14.5-MeV neutrons. Three acute doses, 75, 150 and 250 rad, and one chronic dose, 250 rad, were given. The percentages of affected spermatocytes as counted from reciprocal translocations which had been induced in spermatogonia were 0.7, 0.8 and 1.6 respectively for the acute series and 2.2 after chronic exposure. The data could be fitted to a linear or concave curvilinear regression line. There seemed to be a slight increase of damage with dose, even if the percentages were generally lower than those reported earlier for fast neutrons with energies around 1 MeV. The existence of dose-rate effects is discussed, and the conclusion drawn so far is that there seems to be no such effect either for 1-MeV fast neutrons or 14.5-MeV high energy neutrons. The term “reversed dose-rate effect”, as used earlier, relates to another phenomenon. The difference between the point estimates for the chronic and acute 250 rad series is not significant. The effectiveness of neutrons with energies around 14 MeV versus neutrons with energies around 1 MeV is discussed.     

Simulation and optimization of cyclic activation Analysis of short-lived isotopes with 14MeV neutron generator

Biological Trace Element Research - A program of simulation and optimization is developed for the case of cyclic activation analysis of short-lived isotopes with 14-MeV neutrons. The background...     

The regulation of human exposure to mutagens amidst scientific controversy

Human peripherial lymphocytes were irradiated with different doses of 15.0-MeV neutrons. The frequency of different aberration types was determined and the dose-response relation was calculated. The data were fitted by least-squared regression analysis to different models. The dicetric, dicentric+centric ring, and different acentric data gave the best fit to the linear quadratic model. The RBE of 15.0-MeV neutrons versus 220 kV X-rays decreased significantly with increasing dose.     

Radiation quality and rat motor performance

The effects of bremsstrahlung, electron, gamma, and neutron radiations were investigated on the motor performance of male Sprague-Dawley rats. Rats were irradiated at a midline tissue dose rate of 20 Gy/min +/- 1 with one of the following: 18.6-MeV electrons (N = 40) or 18.1-MVp bremsstrahlung (N = 57) from a linear accelerator, 60Co 1.25-MeV gamma-ray photons (N = 48), or reactor neutrons at 1.67 MeV tissue-kerma weighted-mean energy (N = 43). Radiation effects were determined by establishing median effective doses (ED50) for rats trained on an accelerod, a shock-avoidance motor performance test. ED50's were based on 10-min postexposure performance. The ED50's were 61 Gy for electrons, 81 Gy for bremsstrahlung, 89 Gy for gamma-ray photons, and 98 Gy for neutrons. In terms of relative biological effectiveness to produce early performance decrement (10 min from the start of irradiation), significant differences existed between the electrons and the other three fields and between the bremsstrahlung and neutron fields. These differences could not be explained by macroscopic dose distribution patterns in the irradiated animals. The data imply that different radiation qualities are not equally effective at disrupting performance, with high-energy electrons being the most effective and neutrons the least.     

Variation of RBE between p(75) + Be and d(50) + Be neutrons determined for chromosome aberrations in Allium cepa.

The RBE of p(75) + Be neutrons relative to d(50) + Be neutrons has been determined for chromosome aberrations induced in Allium cepa (onion) roots. Two biological criteria were selected: the average number of aberrations (mainly fragments) per cell in anaphase and telophase, and the percentage of aberration-free cells. The influence of sampling time (3 to 7 h incubation) between irradiation and fixation was investigated systematically. This factor did not significantly influence the results. The RBE values of p(75) + Be neutrons compared to those of d(50) + Be neutrons were 0.85 (0.79-0.91) and 0.87 (0.80-0.95) for the first and the second criteria, respectively. In previous experiments for the same beams, we found an RBE of 0.90 (0.86-0.94) for survival of V79 cells (D0 ratio), 0.96 (0.93-0.99) for the intestinal crypt cell system, and 0.83 (0.70-0.96) for Vicia faba growth delay.     

Radiobiological intercomparison of clinical neutron beams for growth inhibition in Vicia faba bean roots

Relative biological effectiveness (RBE) and oxygen enhancement ratio (OER) values of different neutron beams produced at the variable energy cyclotron "Cyclone" of Louvain-la-Neuve (Belgium) were determined. The neutrons were obtained by bombarding a beryllium target with 34-, 45-, 65-, or 75-MeV protons or with 50-MeV deuterons. The biological system was growth inhibition in Vicia faba bean roots. Taking the p(65) + Be neutron beam as a reference, RBE values were found equal to 1.36 +/- 0.2, 1.20 +/- 0.1, 1.00 (ref), 0.98 +/- 0.1, and 1.18 +/- 0.1, respectively; the doses corresponding to 50% growth inhibition were 0.39, 0.44, 0.53, 0.54, and 0.45 Gy. For the same beams, OER values were found equal to 1.55 +/- 0.1, 1.38 +/- 0.1, 1.29 +/- 0.1, 1.41 +/- 0.1, and 1.60 +/- 0.2, respectively.     

Effect of pulsed dose in simultaneous and sequential irradiation of V-79 cells by 14.8-MeV neutrons and 60Co photons

The effect of irradiating V-79 Chinese hamster cells with a mixture of 40% 14.8-MeV neutrons and 60% 60Co photons with simultaneous or sequential exposures is investigated. Sample doses are obtained by irradiating cells with alternating 3-min pulses of neutrons and photons (in the sequential case) or with mixed neutrons and photons followed by equal beam-off periods to ensure equal total exposure times for sequential and simultaneous irradiations. Differences between the survival results under each beam configuration that are consistent with previous observations with nonpulsed irradiations are observed.     

Cytogenetic effects induced in vitro in human peripheral blood lymphocytes by neutrons. II. Relative biological effectiveness of neutrons having different energies]

Human lymphocytes were irradiated in vitro during Go stage by graded doses of thermal neutrons and neutrons having an average energy of 0.04; 0.09; 0.35; 0.85 and 14,7 MeV as well as by 60Co gamma rays, and RBE of neutrons relative to gamma-rays was calculated for the frequency of total and different types of aberrations. It was found that the RBE has the most value at the low doses and decreases when the exposition dose increases. 0.35 MeV neutrons have the maximum RBE in comparison with neutrons having other energies. When comparing the RBE values calculated for different types of chromosome aberrations, it was found out that dicentrics and dicentrics plus centric rings had more RBE than acentric aberrations (pair fragments and minutes).     

Chromosomal damage induced in human lymphocytes by low doses of D-T neutrons

Unstable chromosome aberrations induced by in vitro irradiation with zero plus seven low doses of 14.8 MeV D-T neutrons in the range 3.55-244 mGy have been analysed in human peripheral blood lymphocytes. In order to obtain the required large numbers of scored cells for such low doses, fourteen laboratories participated in the experiment. The dose responses for dicentrics, excess acentrics and total aberrations, fitted well to the Y = alpha D model. The alpha coefficient of yield for dicentrics, 1.60 +/- 0.07 X 10(-2) Gy-1, compares well with the values obtained in previous studies with D-T neutrons at somewhat higher doses. Results from a previous collaborative study using 250 kVp X-rays over a comparable dose range indicated the possible existence of a threshold below 50 mGy. In the present study there is no clear evidence for neutrons for such a threshold. However, the data were insufficient to permit the rejection of a possible threshold below approximately 10 mGy.     

Hyperthermia increases chromosome breakage and loss induced by fission neutrons in Drosophila melanogaster

Pretreatment with hyperthermia did significantly increase the breakage and chromosome loss induced by 0.85-MeV fission neutrons from the JANUS biological research reactor in spermatozoa, early and late spermatids and spermatocytes. Radiation-induced breakage of chromosomes was also enhanced by hyperthermia in early spermatogonia.     

Effects of low-dose neutrons applied at reduced dose rate on human melanoma cells

Human melanoma cells that are resistant to gamma rays were irradiated with 14 MeV neutrons given at low doses ranging from 5 cGy to 1.12 Gy at a very low dose rate of 0.8 mGy min(-1) or a moderate dose rate of 40 mGy min(-1). The biological effects of neutrons were studied by two different methods: a cell survival assay after a 14-day incubation and an analysis of chromosomal aberrations in metaphases collected 20 h after irradiation. Unusual features of the survival curve at very low dose rate were a marked increase in cell killing at 5 cGy followed by a plateau for survival from 10 to 32.5 cGy. The levels of induced chromosomal aberrations showed a similar increase for both dose rates at 7.5 cGy and the existence of a plateau at the very low dose rate from 15 to 30 cGy. The existence of a plateau suggests that a repair process after low-dose neutrons might be induced after a threshold dose of 5-7.5 cGy which compensates for induced damage from doses as high as 32.5 cGy. These findings may be of interest for understanding the relative biological effectiveness of neutrons and the effects of environmental low-dose irradiation.     

Induction of chromosome aberrations in G0 human lymphocytes by low doses of ionizing radiations of different quality

Human peripheral blood lymphocytes from two donors were exposed to low doses (0.05 to 2.0 Gy) of gamma rays, X rays, or fast neutrons of different energies. Chromosome aberrations were analyzed in metaphase of first-division cells after a culture time of 45-46 hr. At this time, less than 5% of the cells were found in second division. Different dose-response relationships were fitted to the data by using a maximum likelihood method; best fits for radiation-induced dicentric aberrations were obtained with the linear-quadratic law for all radiations. The linear component of this equation predominated, however, for neutrons in the range of doses studied, and the frequency of dicentrics induced by d(16)+Be neutrons up to 1.0 Gy could also be described by a linear relationship. The relative biological efficiency (RBE) of X rays and d(16)+Be, d(33)+Be, and d(50)+Be neutrons compared to 60Co gamma rays in the low dose range was calculated from the dose-effect relationships for the dicentrics produced. The RBE increased with decreasing neutron dose and with decreasing neutron energy from d(50)+Be to d(16)-+Be neutrons. The limiting RBE at low doses (RBEo) was calculated to be about 1.5 for X rays and 14.0, 6.2, and 4.7 for the d(16)+Be, d(33)+Be, and d(50)+Be neutrons, respectively.     

Analysis of biological samples using prompt gamma radions induced by 14.7-MeV neutrons

We investigate a method for determining the elemental composition of biological samples that uses prompt gamma rays induced by 14.7-MeV neutrons. Alpha particles are produced simultaneously with the neutrons, which exit opposite the alpha detector through the vacuum chamber wall. The sample under investigation is irradiated and emits gamma radiations in a spectral energy distribution characteristic of the material. Barium-fluoride (BaF₂) and high-purity germanium (HPGe) gamma detectors view the sample and record the spectrum of gamma radiation.

     

The effects of ionizing radiations on the chromosomes ofTradescantia bracteata. A comparison between neutrons and X-rays

Journal of Genetics - 1. Neutrons produce qualitatively the same types of chromosome aberrations as X-rays, but more of all types of aberration are produced, per ionization, by neutrons. 2. The...     

Biological effectiveness of fast neutrons with a mean energy of 22 MeV

Biological effectiveness of fast neutrons of a mean energy of 22 MeV obtained by the reaction d[50 MeV]----Be, measured by the death rate, was substantially lower than that of division spectrum neutrons of a mean energy of 1.2 MeV. LD50/30 of the division spectrum neutrons was within 2.57 +/- 0.07 Gy and that of 22 MeV fast neutrons 4.79 +/- 0.13 Gy. The RBE coefficient for the studied neutrons was 1.34 +/- 0.05 as estimated by LD50/30 and 1.5 +/- 0.1 as determined by D37 for a cell model of radiation affection.     

Effect of interferon on the yield of chromosome aberrations in cultured human lymphocytes irradiated with fast neutrons

A study was made of the influence of leucocytic interferon on the yield of chromosome aberrations in a human lymphocyte culture after irradiation of cells with fast neutrons (0.85 MeV) at the G0- and G1-stages. It was shown that in cells treated with interferon (50 UE/ml) prior to irradiation the total yield of aberrations and of some of their types was invariable as compared to the irradiated control.     

The mouse splenocyte assay, an in vivo/in vitro system for biological monitoring: studies with X-rays, fission neutrons and bleomycin.

A modified mouse splenocyte culture system was standardized after testing different mitogens (i.e., phytohemagglutinin (PHA), concanavalin A (Con A)). The mitotic index was determined for comparison between different mitogens. Following selection of appropriate mitogen (PHA 16, Flow), a series of experiments were conducted to evaluate the application of a cytokinesis-block for scoring micronuclei and assays for chromosomal aberrations produced by treatment in G0 and G2 for the purposes of biological dosimetry following in vivo and/or in vitro exposure to X-rays, fission neutrons and bleomycin. In the X-irradiation studies, the frequencies of micronuclei and chromosomal aberrations (i.e., dicentrics and rings) increased in a dose-dependent manner. These data could be fitted to a linear-quadratic model. No difference was observed between irradiation in vivo and in vitro, suggesting that measurement of dicentrics and micronuclei in vitro after X-irradiation can be used as an in vivo dosimeter. Following in vivo irradiation with 1 MeV fission neutrons and in vitro culturing of mouse splenocytes, linear dose-response curves were obtained for induction of micronuclei and chromosomal aberrations. The lethal effects of neutrons were shown to be significantly greater than for a similar dose of X-rays. The relative biological effectiveness (RBE) was 6-8 in a dose range of 0.25-3 Gy for radiation-induced asymmetrical exchanges (dicentrics and rings), and about 8 for micronuclei in a dose range of 0.25-2 Gy. Furthermore, the induction of chromosomal aberrations by bleomycin was investigated in mouse G0 splenocytes (in vitro) and compared with X-ray data. Following bleomycin treatment (2 h) a similar pattern of dose-response curve was obtained as with X-rays. In this context a bleomycin rad equivalent of 20 micrograms/ml = 0.50 Gy was estimated.