Mammography-oncogenecity at low doses

Controversy exists regarding the biological effectiveness of low energy x-rays used for mammography breast screening. Recent radiobiology studies have provided compelling evidence that these low energy x-rays may be 4.42 +/- 2.02 times more effective in causing mutational damage than higher energy x-rays. These data include a study involving in vitro irradiation of a human cell line using a mammography x-ray source and a high energy source which matches the spectrum of radiation observed in survivors from the Hiroshima atomic bomb. Current radiation risk estimates rely heavily on data from the atomic bomb survivors, and a direct comparison between the diagnostic energies used in the UK breast screening programme and those used for risk estimates can now be made. Evidence highlighting the increase in relative biological effectiveness (RBE) of mammography x-rays to a range of x-ray energies implies that the risks of radiation-induced breast cancers for mammography x-rays are potentially underestimated by a factor of four. A pooled analysis of three measurements gives a maximal RBE (for malignant transformation of human cells in vitro) of 4.02 +/- 0.72 for 29 kVp (peak accelerating voltage) x-rays compared to high energy electrons and higher energy x-rays. For the majority of women in the UK NHS breast screening programme, it is shown that the benefit safely exceeds the risk of possible cancer induction even when this higher biological effectiveness factor is applied. The risk/benefit analysis, however, implies the need for caution for women screened under the age of 50, and particularly for those with a family history (and therefore a likely genetic susceptibility) of breast cancer. In vitro radiobiological data are generally acquired at high doses, and there are different extrapolation mechanisms to the low doses seen clinically. Recent low dose in vitro data have indicated a potential suppressive effect at very low dose rates and doses. Whilst mammography is a low dose exposure, it is not a low dose rate examination, and protraction of dose should not be confused with fractionation. Although there is potential for a suppressive effect at low doses, recent epidemiological data, and several international radiation risk assessments, continue to promote the linear no-threshold (LNT) model. Finally, recent studies have shown that magnetic resonance imaging (MRI) is more sensitive than mammography in detecting invasive breast cancer in women with a genetic sensitivity. Since an increase in the risk associated with mammographic screening would blur the justification of exposure for this high risk subgroup, the use of other (non-ionising) screening modalities is preferable.

Comparison of measured and calculated spatial dose distributions for a bench-mark 106Ru/106Rh hot particle source

This study is a part of a programme of research to provide validated dose measurement and calculation techniques for beta emitting hot particles by the construction of well-defined model hot particle sources. This enables parallel measurements and calculations to be critically compared. This particular study concentrates on the high-energy beta emitter, (106)Ru/(106)Rh (Emax = 3.54 MeV). This source is a common constituent of failed nuclear fuel, particularly in accident situations. The depth dose distributions were measured using radiochromic dye film (RDF); an imaging photon detector coupled to an LiF thermoluminescent dosemeter (LiF-IPD) and an extrapolation ionisation chamber (ECH). Dose calculations were performed using the Monte Carlo radiation transport code MCNP4C. Doses were measured and calculated as average values over various areas and depths. Of particular interest are the doses at depths of 7 and 30-50 mg cm(-2), and averaged over an area of 1 cm2, as recommended by the International Commission on Radiological Protection for use in routine and accidental over-exposures of the skin. In this case, the average ratios (MCNP/measurement) for RDF, ECH and LiF-IPD were 1.07 +/- 0.02, 1.02 +/- 0.01 and 0.83 +/- 0.16, respectively. There are significantly greater discrepancies between the ECH and LiF-IPD measurement techniques and calculations-particularly for shallow depths and small averaging areas.

Dose distribution measurements and calculations for Dounreay hot particles

Discrete fragments of irradiated nuclear fuel have been discovered on the foreshore at the Dounreay nuclear site in Scotland, offshore on the seabed and at nearby beaches which have public access. The fragments contain mainly (137)Cs and (90)Sr/(90)Y and for particles recovered to date, (137)Cs activities are within the range of 10(3) to 10(8) Bq. The most active particles found at Sandside Beach contain approximately 3 x 10(5)Bq (137)Cs. Direct measurements of the spatial dose distributions from 37 fuel fragments were measured in detail for the first time using radiochromic dye film as part of a national evaluation of the associated potential radiological hazard. Monte Carlo code calculations of the doses are in good agreement with measurements, taking into account variations to be expected due to differences in shape and the increasing importance of self-absorption for the larger, more active fragments. Dose measurements provide little evidence for wide variations in the (137)Cs:(90)Sr/(90)Y ratio between fragments. Specific attention is given to the evaluation of skin dose, averaged over an area of 1 cm(2) at a depth of 0.07 mm, since this is of major radiological concern. There is no obvious dependence of skin dose on the site of origin of the fragments (foreshore, seabed or beaches) for a given (137)Cs activity level. A dose rate survey instrument (SmartION) was shown to provide a rapid and convenient method for skin dose assessment from fuel fragments in the (137)Cs activity range measured (2 x 10(5) to 2 x 10(7) Bq). A conversion factor multiplier of 240 can be applied to the open window SmartION scale reading to estimate the skin dose rate within +/-25%.

Radon exposure of the skin: I. Biological effects

Radon progeny can plate out on skin and give rise to exposure of the superficial epidermis from alpha emitters Po-218 (7.7 MeV, range approximately 66 microm) and Po-214 (6 MeV, range approximately 44 microm). Dose rates from beta/gamma emitters Pb-214 and Bi-214 are low and only predominate at depths in excess of the alpha range. This paper reviews the evidence for a causal link between exposure from radon and its progeny, and deterministic and stochastic biological effects in human skin. Radiation induced skin effects such as ulceration and dermal atrophy, which require irradiation of the dermis, are ruled out for alpha irradiation from radon progeny because the target cells are considerably deeper than the range of alpha particles. They have not been observed in man or animals. Effects such as erythema and acute epidermal necrosis have been observed in a few cases of very high dose alpha particle exposures in man and after acute high dose exposure in animals from low energy beta radiations with similar depth doses to radon progeny. The required skin surface absorbed doses are in excess of 100 Gy. Such effects would require extremely high levels of radon progeny. They would involve quite exceptional circumstances, way outside the normal range of radon exposures in man. There is no definitive identification of the target cells for skin cancer induction in animals or man. The stem cells in the basal layer which maintain the epidermis are the most plausible contenders for target cells. The majority of these cells are near the end of the range of radon progeny alpha particles, even on the thinnest body sites. The nominal depth of these cells, as recommended by the International Commission on Radiological Protection (ICRP), is 70 microm. There is evidence however that some irradiation of the hair follicles and/or the deeper dermis, as well as the inter-follicular epidermis, is also necessary for skin cancer induction. Alpha irradiation of rodent skin that is restricted to the epidermis does not produce skin cancer. Accelerator generated high energy helium and heavy ions can produce skin cancer in rodents at high doses, but only if they penetrate deep into the dermis. The risk figures for radiation induced skin cancer in man recommended by the ICRP in 1990 are based largely on x and beta irradiated cohorts, but few data exist below absorbed doses of about 1 Gy. The only plausible finding of alpha-radiation induced skin cancer in man is restricted to one study in Czech uranium miners. There is no evidence in other uranium miners and the Czech study has a number of shortcomings. This review concludes that the overall balance of evidence is against causality of radon progeny exposure and skin cancer induction. Of particular relevance is the finding in animal studies that radiation exposure of cells which are deeper than the inter-follicular epidermis is necessary to elicit skin cancer. In spite of this conclusion, a follow-on paper evaluates the attributable risk of radon to skin cancer in the UK on the basis that target cells for skin cancer induction are the cells in the basal layer of the inter-follicular epidermis-since this is the conservative assumption made by international bodies such as the International Commission on Radiological Protection (ICRP) for general radiological protection purposes.

Radon exposure of the skin: II. Estimation of the attributable risk for skin cancer incidence

A preceding companion paper has reviewed the various factors which form the chain of assumptions that are necessary to support a suggested link between radon exposure and skin cancer in man. Overall, the balance of evidence was considered to be against a causal link between radon exposure and skin cancer. One factor against causality is evidence, particularly from animal studies, that some exposure of the hair follicles and/or the deeper dermis, as well as the inter-follicular epidermis, is required-beyond the range of naturally occurring alpha particles. On this basis any skin cancer risk due to radon progeny would be due only to beta and gamma components of equivalent dose, which are 10-100 times less than the alpha equivalent dose to the basal layer. Notwithstanding this conclusion against causality, calculations have been carried out of attributable risk (ATR, the proportion of cases occurring in the total population which can be explained by radon exposure) on the conservative basis that the target cells are, as is often assumed, in the basal layer of the epidermis. An excess relative risk figure is used which is based on variance weighting of the data sources. This is 2.5 times lower than the value generally used. A latent period of 20 years and an RBE of 10 are considered more justifiable than the often used values of 10 years and 20 respectively. These assumptions lead to an ATR of approximately 0.7% (0.5-5%) at the nominal UK indoor radon level of 20 Bq m(-3). The range reflects uncertainties in plate-out. Previous higher estimates by various authors have made more pessimistic assumptions. There are some indications that radon progeny plate-out may be elevated out of doors, particularly due to rainfall. Although average UK outdoor radon levels ( approximately 4 Bq m(-3)) are much less than average indoor levels, and outdoor residence time is on average about 10%, this might have the effect of increasing the ATR several-fold. This needs considerable further study. Ecological epidemiology data for the South West of England provide no evidence for elevated skin cancer risks at radon levels <100 Bq m(-3). Case-control or cohort studies would be necessary to address the issue authoritatively.

Hot particle dosimetry and radiobiology--past and present

Small high-activity radioactive particles of nominal diameter ranging from approximately 1 mm down to several microm have been a radiological concern over the last 30 years in and around European and American nuclear reactor facilities. These particles have often been referred to as 'hot particles'. The 'hot particle problem' came into prominent concern in the late 1960s. The potential carcinogenic effects in lungs as the result of irradiation by discrete small particles containing alpha-emitting radionuclides, particularly (239)Pu, were claimed by some to be several orders of magnitude greater than those produced by uniform irradiation to the same mean dose. The phrase 'hot particle problem' was subsequently used to refer to the difficulty of predicting health effects for all microscopic radioactive sources. The difficulty arose because of the paucity of comparative human, animal or cell studies using radioactive particles, and the lack of validated measurement or calculational techniques for dose estimation for non-uniform exposures. Experience was largely restricted to uniform, large-area/volume exposures. The concern regarding cancer induction was extended to deterministic effects when the ICRP in 1977 failed to give adequate dose limits for dealing with 'hot particle' exposures of the skin. Since 1980, considerable efforts have been made to clarify and solve the dosimetric and radiobiological issues related to the health effects of 'hot particle' exposures. The general recommendations of the ICRP in 1991 used the latest radiobiological data to provide skin dose limits which are applicable to 'hot particle' exposures. More recently the NCRP has extended considerations to other organs. This progress is reviewed and applied to the specific case of the recent evaluation of potential health effects of Dounreay fuel fragments commissioned by the Scottish Environment Protection Agency (SEPA). Analyses of possible doses and risks in this case indicate that the principal concern following skin contact, ingestion or inhalation is the possibility of localised ulceration of skin or of the mucosal lining of the colon or extra-thoracic airways.

LNT--an apparent rather than a real controversy?

Can the carcinogenic risks of radiation that are observed at high doses be extrapolated to low doses? This question has been debated through the whole professional life of the author--now nearing four decades. In its extreme form the question relates to a particular hypothesis (LNT) used widely by the international community for radiological protection applications. The linear no-threshold (LNT) hypothesis propounds that the extrapolation is linear and that it extends down to zero dose. The debate on the validity of LNT has increased dramatically in recent years. This is in no small part due to concern that exaggerated risks at low doses leads to undue amounts of societal resources being used to reduce man-made human exposure and because of the related growing public aversion to diagnostic and therapeutic medical exposures. The debate appears to be entering a new phase. There is a growing realisation of the limitations of fundamental data and the scientific approach to address this question at low doses. There also appears to be an increasing awareness that the assumptions necessary for a workable and acceptable system of radiological protection at low doses must necessarily be based on considerable pragmatism. Recent developments are reviewed and a historical perspective is given on the general nature of controversies in radiation protection over the years. All the protagonists in the debate will at the end of the day probably be able to claim that they were right!

Enhanced biological effectiveness of low energy X-rays and implications for the UK breast screening programme

Recent radiobiological studies have provided compelling evidence that the low energy X-rays as used in mammography are approximately four times--but possibly as much as six times--more effective in causing mutational damage than higher energy X-rays. Since current radiation risk estimates are based on the effects of high energy gamma radiation, this implies that the risks of radiation-induced breast cancers for mammography X-rays are underestimated by the same factor. The balance of risk and benefit for breast screening have been re-analysed for relative biological effectiveness (RBE) values between 1 and 6 for mammography X-rays. Also considered in the analysis is a change in the dose and dose-rate effectiveness factor (DDREF) from 2 to 1, women with larger than average breasts and implications for women with a family history of breast cancer. A potential increase in RBE to 6 and the adoption of a DDREF of unity does not have any impact on the breast screening programme for women aged 50-70 years screened on a 3 yearly basis. Situations for which breast screening is not justified due to the potential cancers induced relative to those detected (the detection-to-induction ratio (DIR)) are given for a range of RBE and DDREF values. It is concluded that great caution is needed if a programme of early regular screening with X-rays is to be used for women with a family history of breast cancer since DIR values are below 10 (the lowest value considered acceptable for women below 40 years) even for modest increases in the RBE for mammography X-rays.

Development of an ICCD-scintillator system for measurement of spatial dose distributions around 'hot particles'

An intensified charge coupled device (ICCD)-scintillator system has been investigated for potential use in measuring the spatially non-uniform dose distribution around 'hot particles'. This imaging system is capable of producing real-time measurements considerably quicker than other presently available radiation dosimetry techniques and exhibits good linearity and reproducibility and relatively high spatial resolution (approximately 17.5 microm). The time required for a dose evaluation is less than a hundredth that required for radiochromic dye film measurements. The non-uniformity of the system has been eliminated by applying pixel-to-pixel correction factors. The measurable dose rate range using a 110 microm thick scintillator extends from approximately 2000 down to approximately 6 Gy h(-1). The prototype ICCD-scintillator system has been used in evaluation of the skin dose from some high-activity nuclear fuel fragments. The results agree within a few percentage with radiochromic dye film measurements for 1 cm(2) averaging areas.

The skin in radiological protection--recent advances and residual unresolved issues

Exposure of the skin is important in radiological protection because, as the most superficial organ of the body, it can often receive the highest absorbed dose from an external exposure. It also has the highest radiation-induced cancer incidence risk factor of any organ (although mortality is very low). The ICRP and NCRP have, particularly over the past 15 y, been able to set dose limits for the exposure of the skin on the basis of an extensive body of radiobiological, clinical and epidemiological data. Some of the main advances in skin dose limitation in radiological protection and some of the remaining unresolved issues are reviewed.

Carcinogenic risk of hot-particle exposures

It has been suggested that spatially non-uniform radiation exposures, such as those from small radioactive particles ('hot particles'), may be very much more carcinogenic than when the same amount of energy is deposited uniformly throughout a tissue volume. This review provides a brief summary of in vivo and in vitro experimental findings, and human epidemiology data, which can be used to evaluate the veracity of this suggestion. Overall, this supports the contrary view and indicates that average dose, as advocated by the ICRP, is likely to provide a reasonable estimate of carcinogenic risk (within a factor of approximately +/- 3). There are few human data with which to address this issue. The limited data on lung cancer mortality following occupational inhalation of plutonium aerosols, and the incidence of liver cancer and leukaemia due to thorotrast administration for clinical diagnosis, do not appear to support a significant enhancement factor. Very few animal studies, including mainly lung and skin exposures, provide any indication of a hot-particle enhancement for carcinogenicity. Some recent in vitro malignant transformation experiments provide evidence foran enhanced cell transformation for hot-particle exposures but, properly interpreted, the effect is modest. Few studies extend below absorbed doses of approximately 0.1 Gy.

Electron spectra measurements and Monte Carlo calculations for a 60Co standardised hot particle source

A benchmark set of measured beta particle spectra for a standardised 60Co hot particle source is presented. The spectra were obtained for conditions similar to those encountered in practical dosimetric applications. The measured spectra were compared with Monte Carlo calculations using the MCNP code. These comparisons provided information to guide the selection of the optimal set-up parameters of the code. Important differences were observed in the MCNP calculated spectra when ITS and the default indexing style algorithm were used. Overall the calculations using the default mode of MCNP version 4B provide the best agreement with the measured electron spectra.

Doses and risks from the ingestion of Dounreay fuel fragments

The radiological implications of ingestion of nuclear fuel fragments present in the marine environment around Dounreay have been reassessed by using the Monte Carlo code MCNP to obtain improved estimates of the doses to target cells in the walls of the lower large intestine resulting from the passage of a fragment. The approach takes account of the reduction in dose due to attenuation within the intestinal wall and self-absorption of radiation in the fuel fragment itself. In addition, dose is calculated on the basis of a realistic estimate of the anatomical volume of the lumen, rather than being based on the average mass of the contents, as in the current ICRP model. Our best estimates of doses from the ingestion of the largest Dounreay particles are at least a factor of 30 lower than those predicted using the current ICRP model. The new ICRP model will address the issues raised here and provide improved estimates of dose.

A wide dynamic range, high-spatial-resolution scanning system for radiochromic dye films

This paper describes the evaluation of an inexpensive, commercially available 35 mm transparency slide scanner as a potential alternative scanning device for GafChromic HD-810 radiochromic dye film. Besides its low cost, the principal advantages of this type of scanner are high spatial resolution and high speed (a typical scan taking less than 1 min). With broad-band illumination the useful dose range using grey-scale imaging of GafChromic HD-810 is limited to about 50-800 Gy. By using the colour-scale imaging capability of the scanner we have been able to achieve a significant extension covering a similar range (15-2000 Gy) to that attainable using monochromatic illumination. The short-term reproducibility of the system is good, with a coefficient of variation of doses estimated from repeat scanning of uniformly exposed calibration films of less than 2%. Long-term stability is ensured by the scanning of a manufacturer-supplied test slide. The slide scanner system has been used in the determination of depth dose distributions from a model 'hot particle' source containing 106Ru/Rh. GafChromic dye film stacks irradiated by the source were read out on both the slide scanner and a conventional Joyce Loebl MDM6 scanning stage microdensitometer. The overall agreement between the dose estimates provided by the two systems was within 10%.

The measurement of thermal neutron flux depression for determining the concentration of boron in blood

Boron neutron capture therapy (BNCT) is a form of targeted radiotherapy that relies on the uptake of the capture element boron by the volume to be treated. The treatment procedure requires the measurement of boron in the patient's blood. The investigation of a simple and inexpensive method for determining the concentration of the capture element 10B in blood is described here. This method, neutron flux depression measurement, involves the determination of the flux depression of thermal neutrons as they pass through a boron-containing sample. It is shown via Monte Carlo calculations and experimental verification that, for a maximum count rate of 1 x 10(4) counts/s measured by the detector, a 10 ppm 10B sample of volume 20 ml can be measured with a statistical precision of 10% in 32 +/- 2 min. For a source activity of less than 1.11 x 10(11) Bq and a maximum count rate of less than 1 x 10(4) counts/s, a 10 ppm 10B sample of volume 20 ml can be measured with a statistical precision of 10% in 58 +/- 3 min. It has also been shown that this technique can be applied to the measurement of the concentration of any element with a high thermal neutron cross section such as 157Gd.

Experimental simulation of A-bomb gamma ray spectra for radiobiology studies

Improved radiation protection of humans requires a better understanding of the mechanisms of radiation action and accurate estimates of radiation risk for both internal and external radiations. The Japanese atomic bomb survivors represent one of the most important sources of human data on the late carcinogenic effects of ionising radiations. The present study was undertaken to investigate whether it would be possible to use hospital radiotherapy/radiobiology equipment to mimic the spectra encountered in Hiroshima and Nagasaki. The estimated total gamma ray fluence spectra (including both prompt and delayed photons) at both Hiroshima and Nagasaki, for distances of 500, 1000, 1500 and 2000 m have been evaluated using DS86 data and previously unpublished information for delayed gamma radiations which constitute the major contribution to survivor doses. Monte Carlo (EGS4) simulations were performed to transport these photons through the body in order to investigate the variation in electron spectra for various body organs. The electron spectra obtained for these fluences at, for example, the colon, have been matched with combinations of electron spectra produced by linear accelerators to within 5% SD. These will, for the first time, enable a direct link to be made between radiobiological studies (for example, on mammography spectra) and the epidemiological data from Japan, which currently underpin radiation risk estimates.

Time- and dose-related changes in the thickness of skin in the pig after irradiation with single doses of thulium-170 beta particles

Time-related changes in skin thickness have been evaluated in the pig using a noninvasive ultrasound technique after exposure to a range of single doses of 0.97 MeV beta particles from (170)Tm plaques. The reduction in relative skin thickness developed in two phases; the separation into two phases was statistically justified only after 120 Gy (P = 0.04). The first phase was between 12 weeks and 24 weeks after irradiation. No further changes were seen until 48-60 weeks after irradiation, when a second phase of skin thinning was observed. No further changes in relative skin thickness were seen in the follow-up period of 104 weeks. The timing of these phases of relative skin thinning was totally independent of the radiation dose; however, the severity of each phase of radiation-induced skin thinning was related to the dose. The pattern of changes was similar to that reported previously after irradiation with 2.27 MeV beta particles from (90)Sr/(90)Y, but the degree of dermal thinning was less for a similar skin surface dose. From a comparison of the depth-dose distribution of the beta particles from the two radionuclides, it was concluded that the target cell population responsible for both the first and second phase of skin thinning in pig skin after irradiation may be located at approximately 800 microm depth. This corresponds to an area in the reticular dermis in pig skin and may be the appropriate site at which to measure the average dose to the dermal tissue.

Describing time and age variations in the risk of radiation-induced solid tumour incidence in the Japanese atomic bomb survivors using generalized relative and absolute risk models

Generalized relative and absolute risk models, in which various functions of time and age modify the excess relative or absolute risk of radiation-induced cancer, are fitted to the Japanese atomic bomb survivor cancer incidence data set. Among generalized relative risk models, those in which a product of powers of time since exposure and attained age modify the relative risk provide the best fit. There are indications that the Armitage-Doll model (in its formulation as a generalized relative risk model) provides a poor fit to the data, possibly in part because of increasing age-adjusted cancer incidence rates in the Japanese cohort. Generalized absolute risk models, and in particular models in which either powers of time since exposure and attained age, or powers of time since exposure and age at exposure modify the excess absolute risk, provide a superior fit to any of the generalized relative risk models for all solid cancer sites analysed together. When six cancer subtypes are examined separately, only for respiratory cancers does this finding remain true, and for two other sites (female breast cancer and thyroid cancer) the generalized relative risk model yields a better fit than the generalized absolute risk model.

Describing time and age variations in the risk of radiation-induced solid tumour incidence in the Japanese atomic bomb survivors using generalized relative and absolute risk models

Generalized relative and absolute risk models, in which various functions of time and age modify the excess relative or absolute risk of radiation-induced cancer, are fitted to the Japanese atomic bomb survivor cancer incidence data set. Among generalized relative risk models, those in which a product of powers of time since exposure and attained age modify the relative risk provide the best fit. There are indications that the Armitage-Doll model (in its formulation as a generalized relative risk model) provides a poor fit to the data, possibly in part because of increasing age-adjusted cancer incidence rates in the Japanese cohort. Generalized absolute risk models, and in particular models in which either powers of time since exposure and attained age, or powers of time since exposure and age at exposure modify the excess absolute risk, provide a superior fit to any of the generalized relative risk models for all solid cancer sites analysed together. When six cancer subtypes are examined separately, only for respiratory cancers does this finding remain true, and for two other sites (female breast cancer and thyroid cancer) the generalized relative risk model yields a better fit than the generalized absolute risk model.

Variations with time and age in the risks of solid cancer incidence after radiation exposure in childhood

The Japanese atomic bomb survivor incidence data set and data on five other groups exposed to ionizing radiation in childhood are analysed and evidence found for a reduction in the radiation-induced relative risk of cancers other than leukaemia with increasing time since exposure. Overall, reductions of 5.7-6.1 per cent per year of time since exposure are indicated, depending on the time at which the reduction is presumed to start, and all the reductions are statistically significant at the 5 per cent level. There is no significant heterogeneity in the speed of the reductions in relative risk with time by cohort, by cancer type, sex, or age at exposure group. There is a significant reduction of relative risk with increasing age at exposure, but adjustment for age at exposure does not markedly affect the time trends of relative risk. For all of the groups considered, there is a statistically significant increase in the excess absolute risk with increasing time since exposure. However, by contrast with the relative homogeneity of the time trends of relative risk, there is statistically significant heterogeneity by cancer type within the Japanese cohort (P = 0.05) and between the cohorts (P < 0.0001) in the speed of increase of the excess absolute risk with time since exposure.

The risk of non-melanoma skin cancer incidence in the Japanese atomic bomb survivors

The latest Japanese atomic bomb survivor non-melanoma skin cancer incidence dataset is analysed and indicates substantial curvilinearity in the dose-response curve, consistent with a possible dose threshold of about 1 Sv, or with a dose-response in which the excess relative risk is proportional to the fourth power of dose, with a turning-over in the dose-response at high doses (> 3 Sv). The time distribution of the radiation-induced excess risk is best described by a model in which the relative excess risk is proportional to a product of powers of time since exposure and attained age. The fits of generalized relative risk models with exponential functions of time and age at exposure (and in particular of attained age) to adjust the relative risk are less satisfactory, as also are the fits of other models in which products of powers of time since exposure, age at exposure and attained age adjust the excess absolute risk. Sensitivity analyses indicate the importance of likely adjustments to the Hiroshima neutron doses for the optimal model parameters, particularly if values of the neutron relative biological effectiveness (RBE) of more than 5 are assumed. If adjustments recently proposed are made to the Hiroshima neutron doses, then using the optimal model (in which excess risk is proportional to the fourth power of dose) the best estimate of the neutron RBE is 1.3 (95% CI < 07.1). However, uncertainties in skin dose estimates for the atomic bomb survivors means that the findings with respect to the neutron RBE and the non-linearity in the dose-response curve should be treated with caution.

A review of the risks of leukemia in relation to parental pre-conception exposure to radiation

The apparent risk of childhood leukemia resulting from paternal pre-conception radiation exposure found among children of the Sellafield (West Cumbria, UK) workforce is compared with the apparent risk in a number of other epidemiological studies. In particular, the extent of the incompatibility of the leukemia pre-conception exposure risks in the offspring of the Sellafield workforce born in the village of Seascale with the risks for those born in the rest of west Cumbria, and with the risks in the offspring of the Japanese bomb survivors, the Ontario radiation workers, and the Scottish radiation workers is discussed. A variety of animal data relating to the possibility of leukemia arising as a result of parental pre-conception exposure is also considered. It is concluded that the extent of the inconsistency of the leukemia risks in the Seascale data with this body of epidemiological and experimental data makes it highly unlikely that the association observed in the West Cumbria dataset represents a causal relationship.

Dose- and source-size-related changes in the late response of pig skin to irradiation with single doses of beta radiation from sources of differing energy

Late radiation damage to pig skin has been assessed at 104 weeks after exposure to sources of 90Sr/90Y (Emax 2.2 MeV) and 170Tm (Emax 0.9 MeV). Damage was assessed from measurements of dermal thickness in histological sections of irradiated skin and was compared with that of unirradiated skin to establish the relative reduction in dermal thickness. The size of the source varied from 0.1 to 40.0 mm in diameter; this covered the range of source sizes designed to simulate exposure to radioactive "hot" particles (< or = 1.0 mm diameter) up to the lower range of field size that patients may be exposed to in radiotherapy treatments. Radiation doses were measured using an extrapolation chamber with a collecting electrode of 1.1 mm2, and thus the quoted doses represent an average dose over this area. For the larger 90Sr/90Y sources of > or = 5 mm diameter and the larger 170Tm sources of > or = 2 mm diameter there was no evidence, based on levels of damage consistent with a > or = 10, > or = 20, > or = 30, and > or = 40% reduction in relative dermal thickness, for any effect of source size on the ED50 value for each of these specified levels of effect. However, there was a marked effect of beta-particle energy; the skin surface doses associated with the ED50 values (+/- SE) for a > or = 20% reduction in relative dermal thickness were approximately 12 and approximately 40 Gy for 90Sr/90Y and 170Tm, respectively. This difference in skin surface dose for an equivalent level of dermal injury reflects the variation in the depth dose from these two beta-particle emitters. These skin surface doses, for what was considered to be a clinically detectable dermal effect, were below the ED10 for the early skin response of moist desquamation. This supports the selection of late dermal thinning as the effect on which to base dose limits in radiation protection for more generalized larger area skin exposures. In comparison, single exposures to a small area, from sources designed to simulate those from hot particles, reinforced the view that acute ulceration should be the effect on which dose limitation is based. Either the isoeffect doses for a clinically detectable reduction in relative dermal thickness of > or = 20%, following a single exposure to a small area, were higher than the ED10 for acute ulceration or the area of skin showing dermal thinning was so small that it was not considered to be detrimental.

Fitting the Armitage-Doll model to radiation-exposed cohorts and implications for population cancer risks

The Armitage-Doll model of carcinogenesis is fitted to Japanese bomb survivors with the DS86 dosimetry and to three other radiation-exposed cohorts. The model is found to provide an adequate description of solid cancer incidence and also, to a lesser extent, of that of leukemia as a function of radiation dose when up to two radiation-affected stages are assumed. For non-leukemias the optimal model is one in which there are two radiation-affected stages separated by two additional stages. In the case of leukemia one radiation-affected stage or two adjacent stages provide suitable fits. There appear to be significant differences between the optimal models fitted to each cohort, although there is no heterogeneity within the Japanese data set by sex, by cancer type, or by age at exposure. Low-dose and low-dose-rate population risks for a population having the cancer and overall mortality rates of the current UK population are calculated on the basis of the optimal models fitted to the Japanese data to be about 8.3 x 10(-2) excess cancer deaths person-1 Sv-1, 10.1 x 10(-2) radiation-induced cancer deaths person-1 Sv-1, or 1.40 years of life lost person-1 Sv-1. Risks for a population having the mortality rates of the current Japanese population are about 6.5 x 10(-2) excess cancer deaths person-1 Sv-1, 7.8 x 10(-2) radiation-induced cancer deaths person-1 Sv-1, or 0.89 years of life lost person-1 Sv-1. It is a feature of the Armitage-Doll model, and other multistage models of carcinogenesis, that if radiation acts at more than one stage then (inverse) dose-rate effects may arise as a result of interactions between the effects of a protracted dose at the various radiation-affected stages. However, it is shown in this paper that these three measures of cancer risk in general display fairly slight dependence on administered dose in the range 0.001 to 1.0 Sv and on the length of the time over which the dose is administered in the range 1 to 100 years. Dose-rate effects resulting from the protraction of a radiation exposure over many years acting on (the same) cells at various stages of a multistep process of carcinogenesis are therefore expected to be slight. Dose-rate effects which have been observed in epidemiological studies and cellular radiobiology may thus find their explanation in other phenomena such as short-term intracellular repair.

Time variations in the risk of cancer following irradiation in childhood

The Japanese atomic bomb survivors and three other cohorts of children exposed to radiation are analyzed, and evidence is found for a reduction in the radiation-induced relative risk of cancers other than leukemia with time following exposure. Multiplicative adjustments to the excess risk either of the form exp[-delta.(time since exposure)] or of the form [time since exposure] gamma give equivalent goodness of fit. Using the former type of adjustment an annual overall reduction of 6.9-8.6% in excess relative risk is indicated (depending on the year after which this reduction might take effect). Using the second type of multiplier an adjustment to the excess relative risk varying between [time after exposure]-2.0 and [time after exposure]-3.2 fits best overall. All these reductions are statistically significant at the 5% level. There is no significant variation by cohort, by sex, by cancer type, or by age at exposure group in the degree of annual reduction in excess relative risk. Although time-adjusted relative and absolute risk models give equivalently good fits within each cohort, there is significant variation between cohorts in the degree of increase of risk with time in the absolute risk formulation, in contrast to the lack of such heterogeneity for the relative risk formulation. It is shown that if the range of observed reductions in relative risk is assumed to operate 40 or more years after exposure in the youngest age groups, the calculated UK population risks would be reduced by 30-45% compared to those based on a constant relative risk model.

General considerations of the choice of dose limits, averaging areas and weighting factors for the skin in the light of revised skin cancer risk figures and experimental data on non-stochastic effects

In recent years considerable clinical and experimental data have become available which can form the basis of a better understanding of the response of skin to radiation exposure and should lead to improved radiological protection criteria. Recent biological data from man and pig on the non-stochastic effects following exposure with a range of beta-emitters are combined with recent epidemiological analyses of skin cancer risks in man to form a basis for suggested improved protection criteria for the skin following whole- or partial-body skin exposures. Specific consideration is given to the choice of an organ weighting factor for the evaluation of effective dose-equivalent. Since the stochastic and non-stochastic end-points involve different cell types, which reside at different depths in the skin, the design of an ideal physical dosemeter may depend on the proportion of the body skin that is exposed and the penetrating power of the radiation. Possible choices of design parameters for skin dosemeters are discussed. The limitation of skin exposure from small radioactive sources ('hot particles') is an important practical problem, which is addressed for the first time using animal data. Dose limitation on the basis of an average dose to an area of skin in the vicinity of the particle or on the basis of number of beta-particles emitted could be used. Animal data, for several energies and sizes of beta-emitting sources, indicate that limitation of the average dose to 1 Gy (average dose to 1 cm2 of skin at a depth between 100 and 150 microns) should prevent the occurrence of transient acute ulceration from small sources with dimensions of less than about 1 mm.

Late nonstochastic changes in pig skin after beta irradiation

Late radiation-induced changes in pig skin have been assessed following irradiation with beta-rays from a 22.5- or 15-mm-diameter 90Sr/90Y source and a 19- or 9-mm-diameter 170Tm source. Late damage, in terms of dermal atrophy, was assessed 2 years after irradiation from measurements of dermal thickness in irradiated and normal skin. After 90Sr irradiation maximum atrophy, a dermal thickness of 40-50% of the control value, occurred at a dose of approximately 40 Gy from the 22.5-mm source and approximately 75 Gy from the 15-mm source. In the case of 170Tm the 19- and 9-mm sources produced similar degrees of atrophy at equal doses. Maximum atrophy occurred at approximately 70 Gy, when the dermis was approximately 70% of the thickness of normal skin. Significant late tissue atrophy was seen at doses, from both types of radiation, which only produced minimal erythema in the early reaction. Such late reactions need to be taken into account when revised radiological protection criteria are proposed for skin.

The influence of the hair cycle on the thickness of mouse skin

The data on mouse skin thickness reported here was prompted by the need to know the true position of basal cells of the epidermis and hair follicles as these are important "cells at risk" for a variety of skin reactions including carcinogenesis following exposure to radiation. There is little reliable data in the literature and most previous reports have ignored the shrinkage of skin that occurs because of its natural elasticity. The values determined for mouse flank skin in telogen--the resting phase of the hair cycle for the different skin layers--are epidermis 10 micron, corium 250 micron, adipose layer 150 micron, and hair follicle depth 150 micron. Three days after chemical depilation which triggers the hair follicles into active cycle (anagen) the epidermis doubles in thickness, remains at this value for 7 days, and then gradually returns to telogen values by day 18. The corium and adipose layers also increase significantly to reach approximately 390 micron and approximately 260 micron, respectively, by day 10 and then return to control values from day 15 onward. The change in hair follicles depths are more dramatic with active follicle basal cells reaching approximately 450-550 micron into the adipose layer between days 7 and 15. One important finding is that chemical depilation does not affect the telogen thickness of skin-the teleogen values for the epidermis and dermis immediately prior to and immediately after depilation were similar to those 23 days later at the beginning of the next telogen phase.

Changes in the DNA labelling index after beta irradiation of the mouse epidermis

This study looked at the changes in the interfollicular DNA labelling index (LI) with time after strontium-90/yttrium-90 beta irradiation of approximately 100 mm2 of mouse flank skin, after a dose of 100 Gy which produces transitory moist desquamation. Within 24 hr of such a dose the LI of the irradiated area was essentially zero (0.07 +/- 0.03%), whilst those of the side area and of the control area were 15.0 +/- 2.6% and 21.4 +/- 2.7%, respectively. The LI of the side and the control areas then fell within 3-5 days to approximately 4% and approximately 2% respectively, whilst that of the irradiated area rose rapidly to a peak value of 30.2 +/- 1.7% at 10 days post-irradiation. There was a 20% reduction in the diameter of the area with detectable radiation damage within 5 days, and this is primarily due to cell proliferation and migration from the unirradiated margins of the field. In contrast, between days 10 and 20 the major source of repopulation is probably derived from local migration and proliferation of surviving hair follicle basal cells within the irradiated field.