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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=irab20 International Journal of Radiation Biology and Related Studies in Physics, Chemistry and Medicine ISSN: 0020-7616 (Print) (Online) Journal homepage: www.tandfonline.com/journals/irab19 On the Prediction of Dose-rate Effects for Dicentric Production in Human Lymphocytes by X- and γ- rays A.A. Edwards & D.C. Lloyd To cite this article: A.A. Edwards & D.C. Lloyd (1980) On the Prediction of Dose-rate Effects for Dicentric Production in Human Lymphocytes by X- and γ-rays, International Journal of Radiation Biology and Related Studies in Physics, Chemistry and Medicine, 37:1, 89-92, DOI: 10.1080/09553008014550101 To link to this article: https://doi.org/10.1080/09553008014550101 Published online: 03 Jul 2009. Submit your article to this journal Article views: 158 View related articles https://www.tandfonline.com/action/journalInformation?journalCode=irab20 https://www.tandfonline.com/journals/irab19?src=pdf https://www.tandfonline.com/action/showCitFormats?doi=10.1080/09553008014550101 https://doi.org/10.1080/09553008014550101 https://www.tandfonline.com/action/authorSubmission?journalCode=irab20&show=instructions&src=pdf https://www.tandfonline.com/action/authorSubmission?journalCode=irab20&show=instructions&src=pdf https://www.tandfonline.com/doi/mlt/10.1080/09553008014550101?src=pdf https://www.tandfonline.com/doi/mlt/10.1080/09553008014550101?src=pdf INT. J . RADIAT. BIOL ., 1980, VOL . 37, NO . 1, 89-92 CORRESPONDENCE On the prediction of dose-rate effects for dicentric production in human lymphocytes by X- and y-rays A . A. EDWARDS and D. C . LLOYD National Radiological Protection Board, Harwell, Didcot, Oxon OX11 ORQ (Received 2 .3 Mar 1979 ; accepted 9 August 1979) 1 . Introduction Bauchinger, Schmid and Dresp 1979, have reported that dicentric yields produced in human lymphocytes exposed chronically to cobalt-60 y-rays may be predicted from the relationship between dicentric yield and dose for acute exposure . For this conversion they make use of the G-function of Lea and Catcheside (1942) and a mean repair time derived from fractionated exposures (Schmid, Bauchinger and Mergenthaler 1976) . They show that observed yields at a dose-rate of 1 .7 rad/min agree very closely with those predicted . From this laboratory, we have published data for cobalt-60 y-rays at dose-rates of 50 rad/min and 0 . 3 rad/min (Lloyd, Purrott, Dolphin, Bolton, Edwards and Corp 1975), 250 kVp X-rays at a dose-rate of 100 rad/min (Lloyd et al . 1975) and caesium- 137y-rays at various dose-rates from 0 .03 to 6.7 rad/min (Purrott and Reeder 1976 b). In addition, data for fractionated exposures to X-rays have been published (Purrott and Reeder 1976 a, 1978) . The purpose of this communication is to publish data for 250 kVp X-ray exposures at 0 .3 rad/min and to compare observed and calculated dicentric yields for the three electromagnetic radiations to determine whether the thesis of Bauchinger et al . applies to data obtained in this laboratory . 2 . Biological methods Detailed descriptions of the methods employed in irradiating and culturing the cells and the criteria for scoring have been given in the papers from this laboratory cited above and in Purrott and Lloyd (1972) . In brief the cells were irradiated as whole blood maintained at 37 °C. Separated lymphocyte cultures were set up consisting of 4 ml Eagles MEM medium, 1 .0 ml bovine serum, 0 .15 ml reconstituted phytohaemagglutinin and the buffy coat derived from 1 ml of blood . Colcemid was added after 45 hours . The cultures were terminated at 48 hours, fixed, stained with orcein and scored . 3 . Method of analysis Chromosome aberration yields are usually expressed as a function of dose using equation (1) where Y is the aberration yield, D is the dose and a and /3 are fitted coefficients . For acute exposures the fit to this equation Y=aD+/3D2 (1) is generally very good but for chronic exposures the fit is poor (Edwards 1977) . The aD component, often interpreted as the number of aberrations caused by effects within the same particle track, is expected to be independent of dose-rate . The term 0020-7616/80/3701 0089 502 . 00 is 1980 T., I ., & Francis Ltd 90 Correspondence Table 2 . Observed and predicted dicentric yields for 250 kV X-rays at 0 . 3 rad/min . 13D 2 is commonly interpreted as an interaction term between effects from two independent particle tracks and the magnitude of this term depends on the time interval between the two tracks. Thus the term /3D 2 is dose-rate dependent . From fractionation experiments using cobalt-60 y-rays, Schmid et al . (1976) conclude that the interaction coefficient /3 for acute exposure should be reduced by a factor exp (-t t/t o ) when fractionation effects are considered . Here t r is the time between fractions and to is a mean life of active species shown by Schmid et al . (1976) to be 110 min, in close agreement with 2 hours reported by Purrott and Reeder (1976 a) and in reasonable agreement with three hours reported by Liniecki, Bajerska, Syszynska and Cisowska (1977) . Bauchinger et al . (1979) proceed to state that if t is the time for a continuous chronic exposure and t/to is represented by x, /3 is modified by G(x) such that 2 G(x) = x2 [x - 1 +exp (-x)] (2) Thus yields for chronic exposure may be derived from the dose-effect relationship for acute exposures using equation (3) where a and /3 are the fitted coefficients for acute exposures . Y=aD+/3G(x)D2 (3) 4 . Results Tables I and 2 show predicted and observed dicentric yields for chronic exposures to cobalt-60 y-rays and 250 KVp X-rays, respectively . The value of t o has been taken as 110 min. The coefficients a and /3 and the chronic exposure data to Table 1 . Observed and predicted dicentric yields for 0 . 3 rad/min cobalt-60 ,,, -ray irradiation . Dose Predicted Observed rad aD /3D2 x=t/to G(x) yield yield 5 . 3 0 . 0025 0 . 00017 0 . 1611 0 . 949 0.0027 0 .0035+0 .0009 10 . 6 0 . 0050 0 . 00070 0 . 321 0 . 901 0. 0056 0 .0059+_0 . 0011 15 . 9 0. 0076 0 . 00156 0. 482 0.857 0 .0089 0 .0095+0 .0013 25 0 .0119 0 . 00387 0 . 758 0 . 789 0.0150 0 .0096+_0 . 0015 50 0 .0238 0 . 0155 1 . 52 0 . 640 0.0337 0 .025 +_0 .0034 100 0.0476 0 .0619 3 . 03 0.453 0 .0756 0 .083 _+0 .008 150 0. 071 0 . 139 4. 55 0 .344 0 . 119 0 . 110 +_0 .010 200 0 . 095 0 . 248 6 . 06 0 . 271 0 . 162 0206 _+0 .024 250 0. 119 0 . 387 7 . 58 0 . 229 0 . 208 0237 +0 .035 300 0. 143 0 . 557 9 . 09 0 . 196 0 . 252 0 .38 _+0 .059 400 0 . 190 0 .990 12 . 1 0. 151 0 . 340 0 .596 +0 .072 Dose rad aD /3D 2 x=t/to G(x) Predicted yield Observed yield 25 0 . 0039 0 . 0031 0 . 758 0 . 789 0.0063 0 .007 ±0 .001 50 0 . 0079 0 .0125 1 . 52 0 . 640 0.0159 0 .0124±0 .0017 100 0.0157 0 .05 3 . 03 0.453 0 .0384 0.056 ±0 .006 200 0. 031 0 . 2 6 .06 0. 271 0 .0856 0 .176 ±0 .014 400 0.063 0 .8 12 . 1 0 . 151 0 . 184 0 .654 ±0 .040 800 0 . 13 3 . 2 242 0.079 0 . 378 1 .57 +0. 13 Correspondence 91 cobalt-60 y-rays were published by Lloyd et al . (1975). The data for chronic X-ray exposure are published here for the first time . 5 . Discussion For cobalt-60 irradiation (table 1) it can be seen that at 25 and 50 rad the predicted and observed yields agree reasonably well but at doses of 100 rad and greater the predicted yields are considerably less than those observed . For X-rays (table 2) the agreement is reasonable up to 250 rad but at 300 and 400 rad the predicted yields are lower than the observed . The modification factor G(x) is a function of irradiation time alone so that it is more relevant to refer to irradiation times rather than doses . For cobalt-60 there is agreement for irradiation times below about 3 hours but disagreement above about 6 hours . For X-rays the disagreement occurs for irradiation times greater than 14 hours . The reason for the discrepancy between these two times is not clear . Theexistence of a time above which the observed yield exceeds the predicted yield implies that the function G(x) is not accurate and therefore the assumption (Lea 1946) that the initially formed chromosome breaks fall exponentially with time is wrong . Greater yields may only be observed if there is a longer lived component to the lifetime of initial chromosome breaks. The existence of long term breaks which remain available for recombination long after the majority of damaged sites have been rendered unreactive was originally discussed by Lea (1946) and some evidence for them has been obtained more recently by Purrott and Reeder (1976a) . In the experiments of Bauchinger et al . (1979) the longest time of exposure used was 235 min which is not long enough to see such an effect . It is possible that long-lived breaks at higher doses may result from dose dependent damage to the repair system . However, unequal split-dose experiments on Tradescantia (Savage 1966) and human lymphocytes (Purrott and Reeder 1978) indicate that this is unlikely . Liniecki et al . (1977) have used a similar treatment for the 250 rad caesium-137 data of Purrott and Reeder (1976 b) and concluded that for irradiation times up to about 250 min the results are consistent with a rejoining time of about 2 hours . At longer irradiation times the break repair is slower, compatible with rejoining times in the region of 3 to 5 hours . We have analysed the data of Purrott and Reeder (1976 b) for doses of 100 rad and 500 rad and found that similar conclusions apply at these doses also . These data therefore support the conclusion with cobalt-60 and 250 kVp X-rays that as irradiation time increases the agreement between prediction and observation becomes worse . An explanation for the discrepancy may be that the sensitivity of the cells changes over the period of protracted exposure . Evidence indicating that this is unlikely was provided by some control data in Purrott and Reeder's fractionation experiments (1976 a) where no significant difference in aberration yields induced by 100 rad was found between whole blood irradiated immediately or held at 37 ° C for 7 or 24 hours and then exposed . Another possible cause for the discrepancy is that the shapes of the dose-response curves may be wrong . Since the curves from Bauchinger's group and from this laboratory were constructed it has been shown that with 48 h cultures a proportion of the cells are in second in vitro metaphase (Scott and Lyons 1979) . These are more likely to be free of aberrations so that the yield per cell scored is low . In this laboratory second division cells in unirradiated blood from 15 donors have ranged from 0 to 16 per cent (average 8 . 9 per cent) and specifically the two donors used for 92 Correspondence the dose-response curves yielded 1 and 10 .5 per cent (work in progress) . The proportion of second divisions falls with increasing dose ( 2 percent at 200 rad, Scott and Lyons 1979) so that the greatest distortion is at the lower doses and will mostly influence the accuracy of the a coefficient of the yield equation . At higher doses there is very little distortion and since the responses at higher doses determine the coefficient /3, the error in /3 must be negligible . Even if the effect of the dilution by second division cells were not negligible both curves would require correcting by increasing the aberration yields and the correction to the chronic curve would be greater. This is because higher acute doses produce greater mitotic delay and therefore fewer second division cells by 48 hours and chronic exposures generally produce lower effects . Consequently the yields in column 7 of both tables 1 and 2 should be increased more than the corresponding values in column 6 . This would increase the difference between prediction and observation and we conclude that the existence of second division cells cannot explain the discrepancy . Conclusion Lea and Catcheside (1942) working with Tradescantia proposed that the dose- rate effect could be predicted by applying a G-function to the dose squared term of the yield equation . This has been applied recently by Bauchinger et al . (1979) to their data with human lymphocyte aberrations . They showed that the yield equation obtained with acute exposures can be used to predict the dose-response relationship for chronic exposures involving irradiation times up to about 4 hours. When applying Lea's formula to dose-response data from this laboratory where observ- ations have been made down to lower dose-rates we confirm the findings of Bauchinger et al . up to an exposure time of about 6 hours . For more protracted exposures however the predictions fail . It is proposed that this is due to a small proportion of lesions which remain available for recombination for much longer than 2 hours . Acknowledgments The authors wish to thank Mr . M. J . Corp, M .R.C . Radiobiological Unit, Harwell, who was responsible for the X-ray dosimetry and Mrs . Dawn Bolton who patiently scored the metaphases . This work was partially supported by Euratom Contract No. 171-76-1 BIO U.K . References BAUCHINGER, M ., SCHMID, E ., and DRESP, J ., 1979, Int . J. Radiat. Biol ., 35, 229 . EDWARDS, A . A ., 1977, Rad. Environm . Biophys., 14, 161 . LEA, D. E ., 1946, Actions of Radiations on Living Cells (Cambridge : Cambridge University Press) . LEA, D. E ., and CATCHESIDE, D . G ., 1942, J. Genet ., 47, 137 . LINIECKI, J ., BAJERSKA, A ., SYSZYNSKA, K ., and CISOWSKA, B ., 1977, Mutation Res ., 43, 291 . LLOYD, D. C., PURROTT, R . J ., DOLPHIN, G . W., BOLTON, DAWN, EDWARDS, A . A ., and CORP, M . J., 1975, Int. J. Radiat . Biol ., 28, 75 . PURROTT, R . J ., and LLOYD, D. C ., 1972, The Study of Chromosome Aberration Yield in Human Lymphocytes as an Indicator of Radiation Dose, Vol . 1, Techniques . N RPB-R2 (Harwell : U.K. National Radiological Protection Board) . PURROTT, R . J ., and REEDER, E . J ., 1976 a, Mutation Res ., 34, 437 ; 1976 b, Ibid ., 35, 437; 1978, Ibid., 52, 291 . SAVAGE, J . R. K ., 1966, Int. J. Radiat . Biol., 11, 287 . SCHMID, E., BAUCHINGER, M ., and MERGENTHALER, W ., 1976, Int . J . Radiat. Biol ., 30, 339 . SCOTT, D ., and LYONS, C. Y ., 1979, Nature, Lond ., 278, 756 . page 1 page 2 page 3 page 4
