Chromosome aberration yields induced in human lymphocytes by 15 MeV electrons given at a conventional dose-rate and in microsecond pulses.

Yields of unstable chromosome aberration were analysed in human lymphocytes after in vitro exposure to 15 MeV electrons. Two dose-effect curves were prepared. In one, doses of 44 to 742 rad were given at 100 rad/min, and in the other doses of 53 to 764 rad were each delivered in single microsecond pulses. No significant difference could be found between the two sets of data when analysed in terms of the quadratic model of aberration production. Good agreement was observed with other dose-response studies in this laboratory, in which human lymphocytes were exposed to 250 kVp X-rays and 60CO gamma-rays at conventional rates of 100 and 50 rad/min, respectively. Comparison with the results of a low-LET dose-rate experiment shows that the yield of dicentric aberrations remains constant overa wide range, i.e. 25 to 6X 10(9), 100 to 1-5 X 10(10), and 150 to 3 X 10(10) rad/min, respectively, for doses of 100, 250 and 500 rad. Radiochemical consumption of oxygen occuring in the lymphocytes during the single microsecond exposures may amount to less than 5 per cent of the total oxygen present in the blood samples, immediately before irradiation. The data also indicate that the ultra-high dose-rates currently available are insufficient to overcome the therapeutic problem of hypoxic radioresistant tumour cells.     

Chromosome aberrations induced in human lymphocytes by neutron irradiation.

In vitro dose--response curves of unstable chromosome aberrations in human lymphocytes have been obtained for neutron spectra of mean energies 0-7, 0-9, 7-6 and 14-7 MeV. The aberration yields have been fitted to the quadratic function Y = alphaD + betaD2, which is consistent with the single-track and two-track model of aberration formation. However with high-LET radiation, the linear component of yield, corresponding to damage caused by single tracks, predominants, and this term becomes more dominant with increasing LET, so that for fission spectrum neutrons the relationship is linear, Y = alphaD. At low doses, such as those recieved by radiation workers, limiting r.b.e. values between 13 and 47 are obtained relative to 60Co gamma-radiation. At higher doses, as used in radiotherapy, the values are much lower; ranging from 2-7 to 8 at 200 rad of equivalent gamma-radiation. Both sets of r.b.e. values correlate well with track-averaged LET but not with dose-averaged LET. When the numbers of cells without aberrations are plotted against radiation dose, curves are obtained which are similar in shape to those for conventional cell-survival experiments with comparable neutron spectra. The Do values obtained in the present study are close to those from other cell system.     

Chromosome aberrations in human lymphocytes induced by fission neutrons

The dose-response relationships of dicentrics and excess acentrics were analysed after exposure of human lymphocytes to a mixed fission neutron-gamma-ray beam. From the analysis of exclusively first division cells a linear-quadratic relation was obtained for dicentrics with the ratio of linear and quadratic components, zeta, equal to 2.76 Gy. Over the range of doses studied (0.04-1.97 Gy) intratrack events therefore predominated. This also applied to acentrics which were linearly related to dose. At the lowest level of observed effect and dose, r.b.e. values with respect to 60Co gamma-rays of up to about 11 were derived for dicentrics and acentrics. With increasing neutron dose the r.b.e. decreased.     

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.     

Chromosome aberrations induced in vitro by low doses of radiation: nondisjunction in lymphocytes of young adults.

Epidemiological studies suggest radiation exposure as a cause of meiotic non-disjunction in humans, but experimental evidence with cytological proof has been lacking. Our results indicate that mitotic nondisjunction of lymphocyte chromosomes can also be induced by exposure to a low dose of radiation. Abnormal segregation can be induced not only when the cells are irradiated but also when nonirradiated cells are incubated with irradiated cell-free plasma or serum. The X and no. 21 chromosomes appear particularly susceptible to nondisjunction.     

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.     

Chromosome aberrations induced in human lymphocytes by in vitro and in vivo X-rays

A dose-effect curve is presented obtained by analysis of dicentric chromosomes and centric ring chromosomes in lymphocyte metaphase spreads of three healthy volunteers after in vitro 100 kV X-ray-irradiation of peripheral blood samples. This calibration curve follows a linear quadratic equation, y=c+alpha D+beta D(2), with the coefficients: y=(0.0005+/-0.0001)+(0.0355+/-0.0066)D+(0.0701+/-0.0072)D(2). The model is based on 13.231 first-division metaphases analyzed after in vitro exposure to doses ranging from 0.1 to 2.0 Gy at a dose rate of 0.4 Gy min(-1). Significant overdispersion of the observed chromosomal aberrations was evident for dose points 1.0 and 2.0 Gy, respectively. The calibration curve was applied to derive equivalent whole body doses of three subjects after suspected extensive exposure to diagnostic X-rays.     

Chromosome aberrations induced in human lymphocytes by 3.45 MeV alpha particles analyzed by premature chromosome condensation.

Premature chromosome condensation (PCC) experiments using human lymphocytes with centromere staining have shown that after exposure to 3.45 MeV alpha-particle radiation, the full number of dicentric chromosomes appears when the cell fusion protocol is applied immediately after irradiation. In this case, the time available for repair and misrepair of DNA damage is only about 30 min. The number of dicentrics does not change with a further increase in the time available for chromatin rearrangement. This fast response confirms the expectation based on our previous experiments using PCC with 150 kV X rays in which the alpha component of the yield of dicentrics was found to appear when the cell fusion protocol was applied immediately after irradiation, whereas the beta component was delayed by several hours. The time constant for rejoining of the excess acentric chromosome fragments is found to be donor-specific and not to differ for alpha particles and X rays, but alpha-particle radiation leaves a larger fraction of the excess acentric fragments unrejoined. The RBEs of the 3.45 MeV alpha-particle radiation compared to 150 kV X rays, evaluated for the alpha component for the yield of dicentrics and for the yield of unrepaired acentric fragments, have almost equal values of about 4. This is consistent with data in the literature on chromosome aberrations observed in metaphase that show the equality of the RBE values for production of dicentrics and acentric fragments. Our experimental results concerning the fast kinetics of the alpha component of the yield of exchange-type chromosome aberrations are not consistent with Lea's pairwise lesion interaction model, and they support the proposed alternative mechanism of lesion-nonlesion interaction between chromatin regions carrying clustered DNA damage and intact chromatin regions.     

Frequencies of chromosomal aberrations induced in human blood lymphocytes by low doses of X-rays

The dose-response for radiation-induced chromosome aberrations in human lymphocytes is usually fitted to the quadratic model. This assumes that the slope is essentially linear at low doses. Empirical observations of linearity at less than 200 mGy are, however, sparse. Some data have been published indicating a non-linear (threshold) response and these are reviewed. In particular one study with X-rays showed a plateau in response up to 50 mGy and with a significant dip below the control level at 4 mGy. The mechanism proposed to explain non-linearity is that low doses stimulate the enzymic repair capability of lymphocytes. Preliminary data are presented from a large experiment by six laboratories in which the low dose-response for X-rays has been re-examined. The plateau in the dose-response relationship, if it exists, does not extend to doses above approximately 10 mGy. No irradiated cells yielded aberration levels significantly below the control. Over the range 0-300 mGy the response can be fitted to a linear regression. There are, however, variations in sensitivity between cells from different donors. An unexpected finding was that some lymphocytes contained greater than 1 exchange aberrations. This may indicate a small subset of cells that are especially susceptible to the induction of aberrations by low doses.     

Chromosome damage induced in human lymphocytes by 5-methoxypsoralen and 8-methoxypsoralen plus UV-A

The induction of structural chromosome aberrations and sister chromatid exchanges (SCE) was studied in human lymphocytes in vitro after treatment with the two bifunctional furocoumarins 5-methoxypsoralen (5-MOP) and 8-methoxypsoralen (8-MOP) in the presence of UV-A. The results show that both psoralens induce a dose-dependent increase in the SCE rate as well as in structural chromosome aberrations. 5-MOP was 2.0-2.5 times more effective for the induction of chromosome breaks and had a slightly stronger effect with respect to SCE induction. A significant influence on proliferation kinetics could be observed only with 5-MOP plus UV-A.     

Prematurely condensed chromosome fragments in human lymphocytes induced by high doses of high-linear-energy-transfer irradiation

This study provides a useful biodosimetry protocol for radiation accidents that involve high doses of heavy particle radiation. Human peripheral blood lymphocytes (PBLs) were irradiated in vitro with high doses (5–50 Gy) of charged heavy-ion particles (carbon ions, at an effective linear-energy-transfer (LET) of 34.6 keV/μm), and were then stimulated to obtain dividing cells. PBLs were treated with 100 nM calyculin A to force chromosomes to condense prematurely, and chromosome spreads were obtained and stained with Giemsa. The G2 prematurely condensed chromosome (G2-PCC) index and the number of G2-PCC including fragments (G2-PCC-Fs) per cell for each radiation dose point were scored. Dose-effect relationships were obtained by plotting the G2-PCC indices or G2-PCC-Fs numbers against radiation doses. The G2-PCC index was greater than 5% up to doses of 15 Gy; even after a 30 Gy radiation dose, the index was 1 to 2%. At doses higher than 30 Gy, however, the G2-PCC indices were close to zero. The number of G2-PCC-Fs increased steeply for radiation doses up to 30 Gy at a rate of 1.07 Gy⁻¹. At doses higher than 30 Gy, the numbers of G2-PCC-Fs could not be accurately indexed because of the limited numbers of cells for analysis. Therefore, the number of G2-PCC-Fs could be used to estimate radiation doses up to 30 Gy. In addition, a G2-PCC index close to zero could be used as an indicator for radiation doses greater than 40 Gy.     

Chromosome aberration in human lymphocytes after X-irradiation in vitro. II. Analysis of primary processes in the formation of dicentric chromosomes

The rejoining distance for the formation of dicentric chromosomes in human lymphocytes has been derived on the basis of microdosimetric concepts. For the formation of a dicentric chromosome, primary lesions produced by absorption events can interact within the nucleus over a distance of at least 1 μm. The dispersion of dicentrics is near to 1 and corresponds to a site number between 17 and 120.     

Chromosome aberration frequency and radiation dose to lymphocytes by alpha-particles from internal deposit of Thorotrast

Summary Frequencies of chromosome aberrations in peripheral blood lymphocytes from 63 Thorotrast patients were analysed basing on the age distribution of lymphocytes. The frequency and distribution of chromosome aberrations among lymphocytes are best explained if we assume that the lymphocytes are renewed as an exponential function of time and spend most of their lifetime in the distributive pool where, while exchange of lymphocytes is taking place, the lymphocytes are hit by-particles from Thorotrast aggregates resulting in the formation of chromosome aberrations and killing at specific rates per hit. The model predicts that the aberration frequency is rather insensitive to the fluence rate because of modulation by cell killing by hit. Fitting the observed data to the model showed that approximately 0.8 dicentrics and rings were produced by a single path of-particle and average fluence rate to lymphocytes in a group of patients with the highest aberration frequency was estimated to be about 1.5 hits or 87 rad/lymphocyte/year.     

Chromosome aberrations induced in human lymphocytes by 8.7 MeV protons and 23.5 MeV helium-3 ions

This paper describes the irradiation of thin samples of blood with 8.7 MeV protons and 23.5 MeV helium-3 ions in the track segment mode. Chromosome aberrations in human lymphocytes have been scored. The relationship between dicentric yield and dose in Gy was Y = 0.044 D + 0.058 D2 for protons and Y = 0.394 D for helium ions. These results are compared with data from other laboratories using protons and an attempt is made to reconcile differences. An unexpected observation was that the ratio of the linear coefficients for helium ions and protons was about 9 whereas the ratio of the l.e.t. values was 4.5. This disagrees with current theory which predicts that the linear coefficients should be proportional to l.e.t. Possible sources of error in our experiments are discussed but do not adequately account for the discrepancies.