1979, British Journal of Radiology, 52, 424-427 Proceedings tend to be balanced for predictor factors, and makes the use of statistical tests of significance valid (Byar et ah, 1976). A useful clinical trial must also have an adequate number of patients to ensure that differences between alternative treatments that would be of interest to those in clinical practice are likely to be detected by the trial. Small clinical trials are of very limited value, whether or not they show significant differences between the treatments, because they allow only vague statements about the likely magnitude of the treatment differences to be made. When to stop a clinical trial often poses an ethical and scientific dilemma, and perhaps the only way of dealing with this problem is for the clinicians treating the patients to remain ignorant of the intermediate results and for a close scrutiny to be kept on the data by a suitably constituted group of assessors (Chalmers e* a/., 1972).
It is also necessary that the question being asked in a clinical trial should not be wrongly put; examples of confounded comparisons have occurred in some major trials in the past, and factorial trials are almost unknown in the field of breast cancer. REFERENCES BYAR, D. P., SIMON, R. M., FRIEDEWALD, W. T., SCHLESSELMAN, J. J., DEMETS, D. L., ELLENBERG, J. H., GAIL, M. H., and WARE, J. W., 1976. Randomized
clinical trials. New England Journal of Medicine, 295, 74-80. CHALMERS, T. C , BLOCK, J. B., and LEE, S., 1972.
Con-
trolled studies in clinical cancer research. New England Journal of Medicine, 287, 75-78.
The scientific basis of radiation protection Abstracts of papers presented at a Radiobiology Work-in-Progress meeting of The British Institute of Radiology held on October20,1978 in the Reid-Knox Hall, Institute House, 32 Welbeck Street, London W1M7PG. An introduction to "The Scientific basis of radiation protection", by R. H. Mole. Chromosome damage: the RBE of (d + T) neutrons from two different generators, by Gillian Clare, C. Oxby and B. Dixon. Low doses of neutron radiation and the frequency of satellite association complexes of human acrocentric chromosomes, by M. T. Khokhar, T. Min and J. A. Houghton. Accurate quantification of radiation-induced chromosome damage in human lymphocytes utilizing the harlequin-banding technique: implications for biological dosimetry, by D. Scott, C. Y. Lyons, J. R. Robinson and A. R. Currie. Fast kinetics of radioprotection by sulphydryl compounds in bacteria, by H. A. Harrop and B. D. Michael. Mechanisms of radioprotection by L-cysteine or cytosine arabinoside in mouse marrow, by J. L. Millar. A method for measuring the effects of plutonium 239 on cells in haemopoietic marrow, by E. R. Humphreys and V. A. Stones. An alternative bone model in the calculation of annual limits of intake for plutonium 239 compounds, by N. D. Priest. The initial deposition and redistribution of plutonium 239 in the mouse skeleton: implications for rodent studies in plutonium 239 toxicology, by D. Green, G. R. Howells and C. M. Thorne. Skin and eye irradiations—examples of some problems of implementing international recommendations in radiological protection, by M. W. Charles. The scientific basis for the ICRP's use of linear extrapolation to obtain best estimates of the risk of radiation-induced cancer at low doses, by J. M. Brown. graded reduction in organ function requires much more precise definition than ICRP has given it. Radiobiological experiments are of applied value only if they lead to defensible generalizations which are applicable to the human By R. H. Mole species. A system of radiological protection, because it must Medical Research Council Radiobiology Unit, Harwell, Oxon be simple enough for practical use, must also involve considerable simplification of inevitable biological complexity Chemical agents reducing effects of radiation could well be but simplification is a matter for judgement, not primarily of called radio-resistizers (by analogy and in contrast with science. The increased emphasis on cost-benefit analysis in radio-sensitizers). Radiation protection is a misnomer for ICRP Publ. 26 (1977) demands a considerably increased effort if all the realistic risk estimates which are needed are their action. Radiobiological protection is an activity maintaining con- to be provided. REFERENCE ditions which minimize exposure to ionizing radiation. Its scientific basis is the field of knowledge, both facts and con- ICRP, 1977. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, cepts, which allows risk to be defined quantitatively for any Publication 26. and every kind of exposure. Since human experience of radiation risk is limited to particular cases, the available information has to be codified before it can be applied to other circumstances, absorbed dose providing the necessary linkCHROMOSOME DAMAGE: THE RBE OF (d + T1) ing concept. The effect of protraction (rather than physical NEUTRONS FROM TWO DIFFERENT GENERATORS dose-rate which is relevant to low LET radiation but not high LET radiation) and of radiation quality are the two By Gillian Clare, C. Oxby and B. Dixon main quantitative problems if the metabolic models in use for assessment of dose from "internal emitters" can be acUniversity Department of Radiotherapy, Cookridge cepted as adequate, but the non-stochastic concept of a Hospital and University Department of Medical Physics The General Infirmary, Leeds Reprints of these abstracts may be obtained from the irradiated in vitro at 22°C with 60Co Managing Editor, British Journal of Radiology, 32 Welbeck Human blood has been y rays at 1.5 Gy min-1 or 0.006 Gy min^1, or with 14 MeV Street, London W1M 7PG (01-935 6867). AN INTRODUCTION TO "THE SCIENTIFIC BASIS OF RADIATION PROTECTION"
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Proceedings neutrons given either continuously (Philips tube, 0.11 Gy min"1) or in 15 /tsec pulses at a rate of two per second (Elliott tube, cumulative dose-rate of 0.006 Gy min"1). Metaphases from lymphocytes stimulated with PHA were scored either at the first division for gross chromosomal aberrations and "extra fragments", or for sister chromatid exchanges (SCE) after two divisions in medium containing 5-bromode-oxyuridine. After y-irradiation (0-8 Gy) the response for dicentrics and rings was composed of a linear and a dose-squared component. For "extra fragments" up to 3.0 Gy, and SCE's (0-7 Gy), the response curves were linear. Cells were also irradiated with 3 MeV 1electrons at 306 Gy min"1, or 3 MeV X rays at 236 Gy min" during each 2 /xsec pulse. The numbers of dicentrics and rings were not significantly greater than those induced by y rays. After neutron irradiation, for gross chromosomal aberrations, "extra fragments" and SCE's, all dose-response curves were linear. The RBE relative to 60Co y rays for gross chromosomal aberrations varied from 5.4 to 2.6 (0-0.8 Gy from the Elliott tube), which was about twice as high as the RBE obtained with the Philips tube (0-2.4 Gy), which varied from 2.9 to 1.4. The RBE, using neutrons from the Philips tube, for SCE's was 2.3 ±0.5. The RBE, using neutrons from both tubes, for "extra fragments" was 2.1 ± 0.4. In conclusion, gross chromosomal aberrations appear to be a more accurate indicator of dose than "extra fragments" and both are four times more sensitive indicators of dose than SCE's.
LOW DOSES OF NEUTRON RADIATION AND THE FREQUENCY OF SATELLITE ASSOCIATION COMPLEXES OF HUMAN ACROCENTRIC CHROMOSOMES
By M. T. Khokhar, T. Min and J. A. Houghton Cytogenetics Unit, Department of Microbiology, University College, Galway, Ireland The human acrocentric chromosomes may often be observed associated together or aligning themselves with their satellites directed towards each other and in close proximity. It has been suggested that this phenomenon of satellite association (s.a.) may be one of the determining factors in the causation of non-disjunction in man. Down's syndrome, the most common human autosomal disorder, is usually caused by non-disjunction of chromosome 21. The incidence of Down's syndrome has been found to be higher among the children of parents who have been exposed to ionizing radiation for radiotherapeutic or radiodiagnostic reasons. To determine whether irradiation leads to an increased incidence of s.a. and, therefore, an increased risk of chromosome non-disjunction, the frequency and patterns of satellite associations in metaphase preparations of irradiated blood samples were investigated. Cobalt 60 gamma rays did not indicate a dose response effect on the frequency of s.a. of the acrocentric chromosomes (Khokhar et al., 1978). The effects of neutrons have also been studied. Blood samples were irradiated with Hammersmith neutrons giving doses of 0.04, 0.08,0.16, 0.24, 0.48 and 0.96 Gy and the metaphase preparations were G-banded. The metaphases were analysed to determine if there was any selective effect of neutron irradiation on the individual acrocentric chromosomes participating in the formation of s.a. complexes. Analysis of the preliminary data indicates that chromosome 13 associates with higher frequency than chromosomes 14, 15,21 or 22. REFERENCE KHOKHAR, M. T., MIN, T. and HOUGHTON, J. A., 1978. A
study of the effects of very low doses of cobalt 60 gamma rays on the frequency of satellite association of human
acrocentric chromosomes. British Journal of Radiology, 51, 559-560.
ACCURATE QUANTIFICATION OF RADIATION-INDUCED CHROMOSOME DAMAGE IN HUMAN LYMPHOCYTES UTILIZING THE HARLEQUIN-BANDING TECHNIQUE: IMPLICATIONS FOR BIOLOGICAL DOSIMETRY
By D. Scott, C. Y. Lyons, J. R. Robinson and A. R. Currie Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX The frequency of chromosome structural aberrations in peripheral blood lymphocytes of radiation-exposed individuals is used in biological dosimetry by referring to doseresponse curves constructed from in vitro exposure of blood samples. From the shapes of these dose-response curves, predictions are also made of the effect of dose-rate on aberration frequency and the mechanisms of aberration formation. Lymphocytes must be examined at their first post-irradiation mitosis in order to quantitate accurately the chromosome damage. Using the new harlequin-banding technique of bromodeoxyuridine (BUdR) treated cells we have shown, as have others, that the assumption of a pure first-division population after 48h of culture is erroneous; as many as 50% of cells may be in second division at this time. A further assumption which is made in using a 48h sample is that first-division cells at this time carry the same amount of damage as cells undergoing first division at other times, i.e. that the G0-irradiated lymphocyte population is homogeneous in chromosomal radiosensitivity. This latter assumption has been shown to be correct by culturing irradiated cells in a BUdR-containing medium and sampling cells at various post-irradiation intervals. Correct dose response curves can therefore now be obtained over a wide range of sampling times by utilizing this technique and confining observations to non-harlequin-banded {i.e.firstdivision) cells.
FAST KINETICS OF RADIOPROTECTION BY SULPHYDRYL COMPOUNDS IN BACTERIA
By H. A. Harrop and B. D. Michael Cancer Research Campaign Gray Laboratory, Mount Vernon Hospital, Northwood, Middlesex HA6 2RN We have used a fast mixing and irradiation technique, the gas explosion method, to resolve some of the processes by which serratia marcescens chemically repairs radiation-induced oxygen-dependent damage. The kinetics of the decay of damage (i.e. its chemical repair) display fast and slow stages. These are well fitted by a model in which there are two components of damage, each repaired as an exponential function of time, but differing in repair rate by a factor of ten. In the present work the interaction of these components with the sulphydryl radioprotector dithiothreitol (DTT) has been examined. The addition of DTT was found to increase the rates of decay of both the fast and the slow components of damage. Four concentrations of DTT were tested in the range 0.5 to 5.0 mM and it appeared that the increase of fast and slow decay rates were proportionately the same at each concentration. A linear relationship was found between DTT concentration and repair rate. The reaction rates of DTT1 repair were determined to be 3.6 and 0.36 X 106 dm 1 mol~ s- for the fast and slow components respectively
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assuming that the intracellular and extracellular concentration of D T T are equal. These rates are comparable with, although somewhat slower than, those that have been determined for free radical repair by DTT using the pulse radiolysis technique (Redpath, 1973). When the bacteria were pretreated and irradiated with the sulphydryl binding agent, N-ethylmaleimide (NEM), the lifetimes of both components were decreased by at least a factor of two. The effect of the NEM on the lifetimes of the oxygen-dependent damage could be reversed by adding glutathione to the previously NEM-treated bacteria. It was thus possible to increase or decrease the rate of repair of serratia marcescens by augmenting or depleting the pool of reducing substances in the cell. REFERENCE REDPATH, J. L., 1973. Pulse radiolysis of dithiothreitol. Radiation Research, 54, 364—374. MECHANISMS OF RADIOPROTECTION BY L-CYSTEINE OR CYTOSINE ARABINOSIDE IN MOUSE MARROW
spleens of the hosts is then measured at intervals over several days and the results analysed to give a measure of haemopoietic stem cell numbers in the injected marrow cells (Siegers et al., 1979). Preliminary results comparing 125I activity in spleens measured six and eight days after transplantation of cells from control and 239Pu-treated donors showed that an absorbed dose of about 0.1 Gy depressed marrow cell numbers two or threefold. REFERENCE SIEGERS, M. P., FEINENDEGEN, L. E., LAHIRI, S. K. and
CRONKITE, E. P., 1979. Relative number and proliferation kinetics of haemopoietic stem cells in the mouse. Blood Cells (in press). AN ALTERNATIVE BONE MODEL IN THE CALCULATION OF ANNUAL LIMITS OF INTAKE FOR PLUTONIUM 2 3 9 COMPOUNDS
By N. D. Priest
By J. L. Millar
National Radiological Protection Board, Harwell, Oxon
Institute of Cancer Research, Sutton, Surrey Mice given 1000 rad total body irradiation die 10 to 14 days later as a result of reduced output of essential blood elements from the bone marrow. If, however, the animals are given 200 mg/kg cytosine arabinoside (ARA-C) two days before radiation the animals survive. The improved survival results from a more rapid recovery of peripheral blood elements in pretreated mice. This suggests that the vital bone marrow stem cells which produce these cells are protected by ARA-C pretreatment. This is not the case, however, as ARA-C pretreatment does not alter the radiosensitivity of the stem cells. It is the subsequent recovery of the stem cells in ARAC pretreated animals that is altered. In animals receiving radiation alone, there is a post irradiation lag phase of several days during which there appears to be no increase in haemopoietic stem cell number. ARA-C abolishes this lag phase and recovery of the stem cells begins immediately after the irradiation. This is in contrast to the mechanism by which cysteine protects animals. This compound produces a genuine dose reduction effect, so more cells survive irradiation. These surviving stem cells show no evidence of enhanced recovery and follow the pattern of regeneration in the irradiated controls. We conclude, therefore, that there is a qualitative difference in the ways in which ARA-C and cysteine protect animals from a large dose of radiation that would normally prove lethal.
A METHOD FOR MEASURING THE EFFECTS OF PLUTONIUM 2 3 9 ON CELLS IN HAEMOPOIETIC MARROW
Annual limits for the intake (ALI) of 239Pu are calculated by Committee 2 of ICRP using a dosimetric model which incorporates a skeletal model for bone surface seeking radionuclides (ICRP, 1979). The skeletal model assumes plutonium is evenly deposited as an infinitely thin layer over bone surfaces and retained with a half-time of 100 years. This model is conservative because it takes no account of the burial of plutonium during bone remodelling. This burial greatly reduces the amount of a-energy absorbed by the sensitive cells in the skeleton. Consequently, the ALI's calculated using the ICRP bone model are unnecessarily restrictive. For the present paper ALI's have been calculated using a multicompartment skeletal model which allows for plutonium burial and recycling (Priest and Hunt, 1978). In one compartment of this model, the growing surfaces of cortical bone, it is assumed that plutonium deposits are retained and are not subject to recycling. In other compartments plutonium is taken to be resorbed with subsequent redeposition on to bone surfaces or retention in the bone marrow. The calculations made using this model suggest that 750 Bq (20 nCi) is an appropriate ALI for the inhalation of plutonium compounds and that 830 kBq and 5 MBq (23 /^iCi and 136 fj-Ci) are the appropriate ALI's for the ingestion of soluble and insoluble forms of plutonium respectively. These ALI's are 1.5-3.0 times less restrictive than the ALI's which will be published by Committee 2 of ICRP. REFERENCES ICRP, 1979. Limits for intakes of radionuclides by workers. Report of the ICRP Committee 2. (In press). PRIEST, N. D. and HUNT, B. W., 1978. The calculation of
annual limits of intake for plutonium-239 in man using a bone model which allows for plutonium burial and recycling (unpublished).
By E. R. Humphreys and V. A. Stones MRC Radiobiology Unit, Harwell, Oxon OX11 0RD A technique is described which is being used to investigate whether the plutonium which deposits on the endosteum in bone irradiates those marrow cells involved in the early stage of haemopoiesis. Mice were given about 300 Bq 239Pu intravenously and injected three days later with 550 kBq [125I]-5-iodo-2'125 deoxyuridine. The I-labelled marrow cells harvested from the femur shafts of these donors after 24h are then injected into lethally irradiated (10 Gy X rays) syngeneic hosts. The activity in the 125I-labelled colonies established in the
THE INITIAL DEPOSITION AND REDISTRIBUTION OF PLUTONIUM 2 3 9 IN THE MOUSE SKELETON: IMPLICATIONS FOR RODENT STUDIES IN PLUTONIUM 239 TOXICOLOGY
By D. Green, G. R. Howells and M. C. Thorne MRC Radiobiology Unit, Harwell, Oxfordshire, OX 11 0RD Using neutron-induced autoradiography together with computer-based methods of data reduction (Green et al., 1977)
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Proceedings the distribution of 239pu about the endosteal and periosteal surfaces of bone can be measured with a resolution of ± 2.5 /im. Our studies show that in the female CBA mouse the 239 Pu penetrates up to 20 /xm into the bone 24 hours after intravenous injection of monomeric 239Pu-citrate. Studies on the ilium demonstrate that after this period of initial deposition the 239Pu is gradually redistributed throughout the bone and2 its included marrow. It is suggested that the burial of 39pu in bone is governed entirely by the process of bone accretion and that its long term retention in marrow is in fixed tissue macrophages. Blood flow experiments using 18 F have also been performed. 239 These suggest the hypothesis that the initial deposition of Pu in the various bones of the skeleton can be accurately predicted from a knowledge of the surface-tovolume ratios of these bones and the blood flow to them. It is pointed out that if 23yPu deposition in the skeleton is governed by blood flow to and morphological characteristics of the bones and if its redistribution is governed entirely by mechanisms of bone turnover, it should be239 possible to construct a satisfactory physiological model of Pu deposition and redistribution in the human skeleton from clinical data which are already potentially available.
non-stochastic effect, much as Malone (1978) has suggested for benign neoplasia in the thyroid. In the case of the eye the annual maximum permissible dose has been increased from 0.1 5 to 0.3 Sv (15-30 rem). A lifetime dose-equivalent of 15 Sv (1500 rem) is considered to be below the threshold for cataract induction (ICRP 26, para. 61). This value appears to leave little, if any, margin of safety compared with the threshold doses indicated in some clinical studies with low and high LET radiation (Merriam et al., 1972; Roth et al., 1976). The extent of sparing for protracted cataractogenic exposures remains an important parameter for continued study. REFERENCES ICRP, 1977. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, Publication 26. MALONE, J. F., 1978. Thyroid irradiation and the new ICRP dose limits. British Journal of Radiology, 51, 553-556. MERRIAM, G. R., SZECHTER, A., and FOCHT E. F., 1972. The
effects of ionizing radiations on the eye. Frontiers of Radiation Therapy and Oncology, 6, 346-385. ROTH, J., BROWN, N., CATTERALL, M. and BEAL, A., 1976.
Effects of fast neutrons on the eye. British Journal of Ophthalmology, 60, 236-244.
REFERENCE
SHORE, R. E., ALBERT, R. E. and PASTERNACK, B.S.,
GREEN, D., HOWELLS, G. R. and THORNE, M. C , 1977. A 23
SKIN AND EYE IRRADIATIONS—EXAMPLES OF SOME PROBLEMS OF IMPLEMENTING INTERNATIONAL RECOMMENDATIONS IN RADIOLOGICAL PROTECTION
1976.
Follow up study of patients treated by X-ray epilation for tinea capitis. Archives of Environmental Health, 31, 21—28.
new method for the accurate localization of 9p u m bone. Physics in Medicine and Biology, 22, 284-297.
THE SCIENTIFIC BASIS FOR THE ICRP's USE OF LINEAR EXTRAPOLATION TO OBTAIN BEST ESTIMATES OF THE RISK OF RADIATION-INDUCED CANCER AT LOW DOSES
By M. W. Charles
By J. M. Brown
CEGB Berkeley Nuclear Laboratories Berkeley, Glos. GL13 9PB Publication 26 of the International Commission on Radiological Protection (ICRP, 1977) provides a rationalized radiation protection system, through the use of up-to-date stochastic risk factors, which avoids some of the inconsistencies of earlier recommendations. It has introduced the concepts of stochastic and non-stochastic phenomena and the evaluation of detriment rather than just lethality as a measure of harm. These concepts will no doubt continue to evolve as the ICRP's recommendations have done in the past. Some areas where further discussion and experimental radiobiology would be pertinent are sometimes apparent to the health physicist who applies international recommendations in operational situations. In practice, for example, the skin may sometimes be subjected to non-uniform irradiation with radioactive sources of dimensions from several centimetres down to a few microns. Peak1 dose rates in the basal layer may be several tens of Gy h" but, due to the spatial non-uniformity of the exposures,2 the mean dose rate over some arbitrary area such as 1 cm may be several orders of magnitude less. The outcome of such exposures is not wellknown and ICRP 26 (para. 183) gives little guidance on their treatment. Whole-body skin exposures are limited, by nonstochastic effects, to life-time dose equivalents of 20 Sv (2000 rem); skin is considered to be much less liable to develop fatal cancer than other tissues (ICRP 26, para. 63). However, since the incidence of skin cancer may not be negligible (Shore et al., 1976), and since its magnitude is presumably required for the evaluation of detriment, stochastic effects in skin should not be altogether forgotten. Evaluation of detriment is, however, complex and a case can be made for considering basal cell carcinoma as a pseudo
Stanford University and MRC Unit of Clinical Oncology and Radiotherapeutics Hills Road, Cambridge In its latest recommendations on radiation protection, the International Commission on Radiological Protection has published estimates of the risk of developing cancer folowing low doses and dose rates of radiation (ICRP, 26, 1977). These are to be considered "best estimates" of the risk at low doses, and as such represent a break from the traditional view that risk estimates derived from linear extrapolation are likely to overestimate the real risk at low doses. The basis for this judgement is that the shape of the dose response curve at low to moderate doses is likely to be of the form: E=aD + bD2 where E is the effect at dose D and "a" and "b" are constants. To derive values of a/b (dose at which the linear and quadratic terms contribute equally to the response), it is assumed that the initiating event for radiation carcinogenesis is a somatic mutation. A summary of the values of a/b obtained for mutations and 2-hit chromosome aberrations in the literature shows that they cluster around 100 rad for mammalian cells. Thus, if this applied to cancer induction the dose-response curve would be predominantly linear from 0 to 100 rad. An analysis of risk estimates for radiationinduced cancer in man supports this prediction. REFERENCE ICRP, 1977. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, Publication 26.
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It is also necessary that the question being asked in a clinical trial should not be wrongly put; examples of confounded comparisons have occurred in some major trials in the past, and factorial trials are almost unknown in the field of breast cancer. REFERENCES BYAR, D. P., SIMON, R. M., FRIEDEWALD, W. T., SCHLESSELMAN, J. J., DEMETS, D. L., ELLENBERG, J. H., GAIL, M. H., and WARE, J. W., 1976. Randomized
clinical trials. New England Journal of Medicine, 295, 74-80. CHALMERS, T. C , BLOCK, J. B., and LEE, S., 1972.
Con-
trolled studies in clinical cancer research. New England Journal of Medicine, 287, 75-78.
The scientific basis of radiation protection Abstracts of papers presented at a Radiobiology Work-in-Progress meeting of The British Institute of Radiology held on October20,1978 in the Reid-Knox Hall, Institute House, 32 Welbeck Street, London W1M7PG. An introduction to "The Scientific basis of radiation protection", by R. H. Mole. Chromosome damage: the RBE of (d + T) neutrons from two different generators, by Gillian Clare, C. Oxby and B. Dixon. Low doses of neutron radiation and the frequency of satellite association complexes of human acrocentric chromosomes, by M. T. Khokhar, T. Min and J. A. Houghton. Accurate quantification of radiation-induced chromosome damage in human lymphocytes utilizing the harlequin-banding technique: implications for biological dosimetry, by D. Scott, C. Y. Lyons, J. R. Robinson and A. R. Currie. Fast kinetics of radioprotection by sulphydryl compounds in bacteria, by H. A. Harrop and B. D. Michael. Mechanisms of radioprotection by L-cysteine or cytosine arabinoside in mouse marrow, by J. L. Millar. A method for measuring the effects of plutonium 239 on cells in haemopoietic marrow, by E. R. Humphreys and V. A. Stones. An alternative bone model in the calculation of annual limits of intake for plutonium 239 compounds, by N. D. Priest. The initial deposition and redistribution of plutonium 239 in the mouse skeleton: implications for rodent studies in plutonium 239 toxicology, by D. Green, G. R. Howells and C. M. Thorne. Skin and eye irradiations—examples of some problems of implementing international recommendations in radiological protection, by M. W. Charles. The scientific basis for the ICRP's use of linear extrapolation to obtain best estimates of the risk of radiation-induced cancer at low doses, by J. M. Brown. graded reduction in organ function requires much more precise definition than ICRP has given it. Radiobiological experiments are of applied value only if they lead to defensible generalizations which are applicable to the human By R. H. Mole species. A system of radiological protection, because it must Medical Research Council Radiobiology Unit, Harwell, Oxon be simple enough for practical use, must also involve considerable simplification of inevitable biological complexity Chemical agents reducing effects of radiation could well be but simplification is a matter for judgement, not primarily of called radio-resistizers (by analogy and in contrast with science. The increased emphasis on cost-benefit analysis in radio-sensitizers). Radiation protection is a misnomer for ICRP Publ. 26 (1977) demands a considerably increased effort if all the realistic risk estimates which are needed are their action. Radiobiological protection is an activity maintaining con- to be provided. REFERENCE ditions which minimize exposure to ionizing radiation. Its scientific basis is the field of knowledge, both facts and con- ICRP, 1977. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, cepts, which allows risk to be defined quantitatively for any Publication 26. and every kind of exposure. Since human experience of radiation risk is limited to particular cases, the available information has to be codified before it can be applied to other circumstances, absorbed dose providing the necessary linkCHROMOSOME DAMAGE: THE RBE OF (d + T1) ing concept. The effect of protraction (rather than physical NEUTRONS FROM TWO DIFFERENT GENERATORS dose-rate which is relevant to low LET radiation but not high LET radiation) and of radiation quality are the two By Gillian Clare, C. Oxby and B. Dixon main quantitative problems if the metabolic models in use for assessment of dose from "internal emitters" can be acUniversity Department of Radiotherapy, Cookridge cepted as adequate, but the non-stochastic concept of a Hospital and University Department of Medical Physics The General Infirmary, Leeds Reprints of these abstracts may be obtained from the irradiated in vitro at 22°C with 60Co Managing Editor, British Journal of Radiology, 32 Welbeck Human blood has been y rays at 1.5 Gy min-1 or 0.006 Gy min^1, or with 14 MeV Street, London W1M 7PG (01-935 6867). AN INTRODUCTION TO "THE SCIENTIFIC BASIS OF RADIATION PROTECTION"
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Proceedings neutrons given either continuously (Philips tube, 0.11 Gy min"1) or in 15 /tsec pulses at a rate of two per second (Elliott tube, cumulative dose-rate of 0.006 Gy min"1). Metaphases from lymphocytes stimulated with PHA were scored either at the first division for gross chromosomal aberrations and "extra fragments", or for sister chromatid exchanges (SCE) after two divisions in medium containing 5-bromode-oxyuridine. After y-irradiation (0-8 Gy) the response for dicentrics and rings was composed of a linear and a dose-squared component. For "extra fragments" up to 3.0 Gy, and SCE's (0-7 Gy), the response curves were linear. Cells were also irradiated with 3 MeV 1electrons at 306 Gy min"1, or 3 MeV X rays at 236 Gy min" during each 2 /xsec pulse. The numbers of dicentrics and rings were not significantly greater than those induced by y rays. After neutron irradiation, for gross chromosomal aberrations, "extra fragments" and SCE's, all dose-response curves were linear. The RBE relative to 60Co y rays for gross chromosomal aberrations varied from 5.4 to 2.6 (0-0.8 Gy from the Elliott tube), which was about twice as high as the RBE obtained with the Philips tube (0-2.4 Gy), which varied from 2.9 to 1.4. The RBE, using neutrons from the Philips tube, for SCE's was 2.3 ±0.5. The RBE, using neutrons from both tubes, for "extra fragments" was 2.1 ± 0.4. In conclusion, gross chromosomal aberrations appear to be a more accurate indicator of dose than "extra fragments" and both are four times more sensitive indicators of dose than SCE's.
LOW DOSES OF NEUTRON RADIATION AND THE FREQUENCY OF SATELLITE ASSOCIATION COMPLEXES OF HUMAN ACROCENTRIC CHROMOSOMES
By M. T. Khokhar, T. Min and J. A. Houghton Cytogenetics Unit, Department of Microbiology, University College, Galway, Ireland The human acrocentric chromosomes may often be observed associated together or aligning themselves with their satellites directed towards each other and in close proximity. It has been suggested that this phenomenon of satellite association (s.a.) may be one of the determining factors in the causation of non-disjunction in man. Down's syndrome, the most common human autosomal disorder, is usually caused by non-disjunction of chromosome 21. The incidence of Down's syndrome has been found to be higher among the children of parents who have been exposed to ionizing radiation for radiotherapeutic or radiodiagnostic reasons. To determine whether irradiation leads to an increased incidence of s.a. and, therefore, an increased risk of chromosome non-disjunction, the frequency and patterns of satellite associations in metaphase preparations of irradiated blood samples were investigated. Cobalt 60 gamma rays did not indicate a dose response effect on the frequency of s.a. of the acrocentric chromosomes (Khokhar et al., 1978). The effects of neutrons have also been studied. Blood samples were irradiated with Hammersmith neutrons giving doses of 0.04, 0.08,0.16, 0.24, 0.48 and 0.96 Gy and the metaphase preparations were G-banded. The metaphases were analysed to determine if there was any selective effect of neutron irradiation on the individual acrocentric chromosomes participating in the formation of s.a. complexes. Analysis of the preliminary data indicates that chromosome 13 associates with higher frequency than chromosomes 14, 15,21 or 22. REFERENCE KHOKHAR, M. T., MIN, T. and HOUGHTON, J. A., 1978. A
study of the effects of very low doses of cobalt 60 gamma rays on the frequency of satellite association of human
acrocentric chromosomes. British Journal of Radiology, 51, 559-560.
ACCURATE QUANTIFICATION OF RADIATION-INDUCED CHROMOSOME DAMAGE IN HUMAN LYMPHOCYTES UTILIZING THE HARLEQUIN-BANDING TECHNIQUE: IMPLICATIONS FOR BIOLOGICAL DOSIMETRY
By D. Scott, C. Y. Lyons, J. R. Robinson and A. R. Currie Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX The frequency of chromosome structural aberrations in peripheral blood lymphocytes of radiation-exposed individuals is used in biological dosimetry by referring to doseresponse curves constructed from in vitro exposure of blood samples. From the shapes of these dose-response curves, predictions are also made of the effect of dose-rate on aberration frequency and the mechanisms of aberration formation. Lymphocytes must be examined at their first post-irradiation mitosis in order to quantitate accurately the chromosome damage. Using the new harlequin-banding technique of bromodeoxyuridine (BUdR) treated cells we have shown, as have others, that the assumption of a pure first-division population after 48h of culture is erroneous; as many as 50% of cells may be in second division at this time. A further assumption which is made in using a 48h sample is that first-division cells at this time carry the same amount of damage as cells undergoing first division at other times, i.e. that the G0-irradiated lymphocyte population is homogeneous in chromosomal radiosensitivity. This latter assumption has been shown to be correct by culturing irradiated cells in a BUdR-containing medium and sampling cells at various post-irradiation intervals. Correct dose response curves can therefore now be obtained over a wide range of sampling times by utilizing this technique and confining observations to non-harlequin-banded {i.e.firstdivision) cells.
FAST KINETICS OF RADIOPROTECTION BY SULPHYDRYL COMPOUNDS IN BACTERIA
By H. A. Harrop and B. D. Michael Cancer Research Campaign Gray Laboratory, Mount Vernon Hospital, Northwood, Middlesex HA6 2RN We have used a fast mixing and irradiation technique, the gas explosion method, to resolve some of the processes by which serratia marcescens chemically repairs radiation-induced oxygen-dependent damage. The kinetics of the decay of damage (i.e. its chemical repair) display fast and slow stages. These are well fitted by a model in which there are two components of damage, each repaired as an exponential function of time, but differing in repair rate by a factor of ten. In the present work the interaction of these components with the sulphydryl radioprotector dithiothreitol (DTT) has been examined. The addition of DTT was found to increase the rates of decay of both the fast and the slow components of damage. Four concentrations of DTT were tested in the range 0.5 to 5.0 mM and it appeared that the increase of fast and slow decay rates were proportionately the same at each concentration. A linear relationship was found between DTT concentration and repair rate. The reaction rates of DTT1 repair were determined to be 3.6 and 0.36 X 106 dm 1 mol~ s- for the fast and slow components respectively
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assuming that the intracellular and extracellular concentration of D T T are equal. These rates are comparable with, although somewhat slower than, those that have been determined for free radical repair by DTT using the pulse radiolysis technique (Redpath, 1973). When the bacteria were pretreated and irradiated with the sulphydryl binding agent, N-ethylmaleimide (NEM), the lifetimes of both components were decreased by at least a factor of two. The effect of the NEM on the lifetimes of the oxygen-dependent damage could be reversed by adding glutathione to the previously NEM-treated bacteria. It was thus possible to increase or decrease the rate of repair of serratia marcescens by augmenting or depleting the pool of reducing substances in the cell. REFERENCE REDPATH, J. L., 1973. Pulse radiolysis of dithiothreitol. Radiation Research, 54, 364—374. MECHANISMS OF RADIOPROTECTION BY L-CYSTEINE OR CYTOSINE ARABINOSIDE IN MOUSE MARROW
spleens of the hosts is then measured at intervals over several days and the results analysed to give a measure of haemopoietic stem cell numbers in the injected marrow cells (Siegers et al., 1979). Preliminary results comparing 125I activity in spleens measured six and eight days after transplantation of cells from control and 239Pu-treated donors showed that an absorbed dose of about 0.1 Gy depressed marrow cell numbers two or threefold. REFERENCE SIEGERS, M. P., FEINENDEGEN, L. E., LAHIRI, S. K. and
CRONKITE, E. P., 1979. Relative number and proliferation kinetics of haemopoietic stem cells in the mouse. Blood Cells (in press). AN ALTERNATIVE BONE MODEL IN THE CALCULATION OF ANNUAL LIMITS OF INTAKE FOR PLUTONIUM 2 3 9 COMPOUNDS
By N. D. Priest
By J. L. Millar
National Radiological Protection Board, Harwell, Oxon
Institute of Cancer Research, Sutton, Surrey Mice given 1000 rad total body irradiation die 10 to 14 days later as a result of reduced output of essential blood elements from the bone marrow. If, however, the animals are given 200 mg/kg cytosine arabinoside (ARA-C) two days before radiation the animals survive. The improved survival results from a more rapid recovery of peripheral blood elements in pretreated mice. This suggests that the vital bone marrow stem cells which produce these cells are protected by ARA-C pretreatment. This is not the case, however, as ARA-C pretreatment does not alter the radiosensitivity of the stem cells. It is the subsequent recovery of the stem cells in ARAC pretreated animals that is altered. In animals receiving radiation alone, there is a post irradiation lag phase of several days during which there appears to be no increase in haemopoietic stem cell number. ARA-C abolishes this lag phase and recovery of the stem cells begins immediately after the irradiation. This is in contrast to the mechanism by which cysteine protects animals. This compound produces a genuine dose reduction effect, so more cells survive irradiation. These surviving stem cells show no evidence of enhanced recovery and follow the pattern of regeneration in the irradiated controls. We conclude, therefore, that there is a qualitative difference in the ways in which ARA-C and cysteine protect animals from a large dose of radiation that would normally prove lethal.
A METHOD FOR MEASURING THE EFFECTS OF PLUTONIUM 2 3 9 ON CELLS IN HAEMOPOIETIC MARROW
Annual limits for the intake (ALI) of 239Pu are calculated by Committee 2 of ICRP using a dosimetric model which incorporates a skeletal model for bone surface seeking radionuclides (ICRP, 1979). The skeletal model assumes plutonium is evenly deposited as an infinitely thin layer over bone surfaces and retained with a half-time of 100 years. This model is conservative because it takes no account of the burial of plutonium during bone remodelling. This burial greatly reduces the amount of a-energy absorbed by the sensitive cells in the skeleton. Consequently, the ALI's calculated using the ICRP bone model are unnecessarily restrictive. For the present paper ALI's have been calculated using a multicompartment skeletal model which allows for plutonium burial and recycling (Priest and Hunt, 1978). In one compartment of this model, the growing surfaces of cortical bone, it is assumed that plutonium deposits are retained and are not subject to recycling. In other compartments plutonium is taken to be resorbed with subsequent redeposition on to bone surfaces or retention in the bone marrow. The calculations made using this model suggest that 750 Bq (20 nCi) is an appropriate ALI for the inhalation of plutonium compounds and that 830 kBq and 5 MBq (23 /^iCi and 136 fj-Ci) are the appropriate ALI's for the ingestion of soluble and insoluble forms of plutonium respectively. These ALI's are 1.5-3.0 times less restrictive than the ALI's which will be published by Committee 2 of ICRP. REFERENCES ICRP, 1979. Limits for intakes of radionuclides by workers. Report of the ICRP Committee 2. (In press). PRIEST, N. D. and HUNT, B. W., 1978. The calculation of
annual limits of intake for plutonium-239 in man using a bone model which allows for plutonium burial and recycling (unpublished).
By E. R. Humphreys and V. A. Stones MRC Radiobiology Unit, Harwell, Oxon OX11 0RD A technique is described which is being used to investigate whether the plutonium which deposits on the endosteum in bone irradiates those marrow cells involved in the early stage of haemopoiesis. Mice were given about 300 Bq 239Pu intravenously and injected three days later with 550 kBq [125I]-5-iodo-2'125 deoxyuridine. The I-labelled marrow cells harvested from the femur shafts of these donors after 24h are then injected into lethally irradiated (10 Gy X rays) syngeneic hosts. The activity in the 125I-labelled colonies established in the
THE INITIAL DEPOSITION AND REDISTRIBUTION OF PLUTONIUM 2 3 9 IN THE MOUSE SKELETON: IMPLICATIONS FOR RODENT STUDIES IN PLUTONIUM 239 TOXICOLOGY
By D. Green, G. R. Howells and M. C. Thorne MRC Radiobiology Unit, Harwell, Oxfordshire, OX 11 0RD Using neutron-induced autoradiography together with computer-based methods of data reduction (Green et al., 1977)
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Proceedings the distribution of 239pu about the endosteal and periosteal surfaces of bone can be measured with a resolution of ± 2.5 /im. Our studies show that in the female CBA mouse the 239 Pu penetrates up to 20 /xm into the bone 24 hours after intravenous injection of monomeric 239Pu-citrate. Studies on the ilium demonstrate that after this period of initial deposition the 239Pu is gradually redistributed throughout the bone and2 its included marrow. It is suggested that the burial of 39pu in bone is governed entirely by the process of bone accretion and that its long term retention in marrow is in fixed tissue macrophages. Blood flow experiments using 18 F have also been performed. 239 These suggest the hypothesis that the initial deposition of Pu in the various bones of the skeleton can be accurately predicted from a knowledge of the surface-tovolume ratios of these bones and the blood flow to them. It is pointed out that if 23yPu deposition in the skeleton is governed by blood flow to and morphological characteristics of the bones and if its redistribution is governed entirely by mechanisms of bone turnover, it should be239 possible to construct a satisfactory physiological model of Pu deposition and redistribution in the human skeleton from clinical data which are already potentially available.
non-stochastic effect, much as Malone (1978) has suggested for benign neoplasia in the thyroid. In the case of the eye the annual maximum permissible dose has been increased from 0.1 5 to 0.3 Sv (15-30 rem). A lifetime dose-equivalent of 15 Sv (1500 rem) is considered to be below the threshold for cataract induction (ICRP 26, para. 61). This value appears to leave little, if any, margin of safety compared with the threshold doses indicated in some clinical studies with low and high LET radiation (Merriam et al., 1972; Roth et al., 1976). The extent of sparing for protracted cataractogenic exposures remains an important parameter for continued study. REFERENCES ICRP, 1977. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, Publication 26. MALONE, J. F., 1978. Thyroid irradiation and the new ICRP dose limits. British Journal of Radiology, 51, 553-556. MERRIAM, G. R., SZECHTER, A., and FOCHT E. F., 1972. The
effects of ionizing radiations on the eye. Frontiers of Radiation Therapy and Oncology, 6, 346-385. ROTH, J., BROWN, N., CATTERALL, M. and BEAL, A., 1976.
Effects of fast neutrons on the eye. British Journal of Ophthalmology, 60, 236-244.
REFERENCE
SHORE, R. E., ALBERT, R. E. and PASTERNACK, B.S.,
GREEN, D., HOWELLS, G. R. and THORNE, M. C , 1977. A 23
SKIN AND EYE IRRADIATIONS—EXAMPLES OF SOME PROBLEMS OF IMPLEMENTING INTERNATIONAL RECOMMENDATIONS IN RADIOLOGICAL PROTECTION
1976.
Follow up study of patients treated by X-ray epilation for tinea capitis. Archives of Environmental Health, 31, 21—28.
new method for the accurate localization of 9p u m bone. Physics in Medicine and Biology, 22, 284-297.
THE SCIENTIFIC BASIS FOR THE ICRP's USE OF LINEAR EXTRAPOLATION TO OBTAIN BEST ESTIMATES OF THE RISK OF RADIATION-INDUCED CANCER AT LOW DOSES
By M. W. Charles
By J. M. Brown
CEGB Berkeley Nuclear Laboratories Berkeley, Glos. GL13 9PB Publication 26 of the International Commission on Radiological Protection (ICRP, 1977) provides a rationalized radiation protection system, through the use of up-to-date stochastic risk factors, which avoids some of the inconsistencies of earlier recommendations. It has introduced the concepts of stochastic and non-stochastic phenomena and the evaluation of detriment rather than just lethality as a measure of harm. These concepts will no doubt continue to evolve as the ICRP's recommendations have done in the past. Some areas where further discussion and experimental radiobiology would be pertinent are sometimes apparent to the health physicist who applies international recommendations in operational situations. In practice, for example, the skin may sometimes be subjected to non-uniform irradiation with radioactive sources of dimensions from several centimetres down to a few microns. Peak1 dose rates in the basal layer may be several tens of Gy h" but, due to the spatial non-uniformity of the exposures,2 the mean dose rate over some arbitrary area such as 1 cm may be several orders of magnitude less. The outcome of such exposures is not wellknown and ICRP 26 (para. 183) gives little guidance on their treatment. Whole-body skin exposures are limited, by nonstochastic effects, to life-time dose equivalents of 20 Sv (2000 rem); skin is considered to be much less liable to develop fatal cancer than other tissues (ICRP 26, para. 63). However, since the incidence of skin cancer may not be negligible (Shore et al., 1976), and since its magnitude is presumably required for the evaluation of detriment, stochastic effects in skin should not be altogether forgotten. Evaluation of detriment is, however, complex and a case can be made for considering basal cell carcinoma as a pseudo
Stanford University and MRC Unit of Clinical Oncology and Radiotherapeutics Hills Road, Cambridge In its latest recommendations on radiation protection, the International Commission on Radiological Protection has published estimates of the risk of developing cancer folowing low doses and dose rates of radiation (ICRP, 26, 1977). These are to be considered "best estimates" of the risk at low doses, and as such represent a break from the traditional view that risk estimates derived from linear extrapolation are likely to overestimate the real risk at low doses. The basis for this judgement is that the shape of the dose response curve at low to moderate doses is likely to be of the form: E=aD + bD2 where E is the effect at dose D and "a" and "b" are constants. To derive values of a/b (dose at which the linear and quadratic terms contribute equally to the response), it is assumed that the initiating event for radiation carcinogenesis is a somatic mutation. A summary of the values of a/b obtained for mutations and 2-hit chromosome aberrations in the literature shows that they cluster around 100 rad for mammalian cells. Thus, if this applied to cancer induction the dose-response curve would be predominantly linear from 0 to 100 rad. An analysis of risk estimates for radiationinduced cancer in man supports this prediction. REFERENCE ICRP, 1977. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, Publication 26.
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