Do we really need the “detriment” for radiation protection?

The purpose of the ICRP detriment concept is to enable a quantitative comparison of stochastic radiation damage for the various organs. For this purpose, the organ-specific nominal risk coefficients are weighted with a function that is intended to express the amount of damage or, respectively, the severity of a disease. This function incorporates a variety of variables that do not depend on radiation parameters, but on characteristics of the disease itself. The question is raised as to whether the rather subtle way of defining the amount of damage is necessary for radiation protection purposes and whether a much simpler relationship can serve for this purpose as well or even better.

     

Dose limits for occupational exposure to ionising radiation and genotoxic carcinogens: a German perspective

This paper summarises the view of the German Commission on Radiological Protection (“Strahlenschutzkommission”, SSK) on the rationale behind the currently valid dose limits and dose constraints for workers recommended by the International Commission on Radiological Protection (ICRP). The paper includes a discussion of the reasoning behind current dose limits followed by a discussion of the detriment used by ICRP as a measure for stochastic health effects. Studies on radiation-induced cancer are reviewed because this endpoint represents the most important contribution to detriment. Recent findings on radiation-induced circulatory disease that are currently not included in detriment calculation are also reviewed. It appeared that for detriment calculations the contribution of circulatory diseases plays only a secondary role, although the uncertainties involved in their risk estimates are considerable. These discussions are complemented by a review of the procedures currently in use in Germany, or in discussion elsewhere, to define limits for genotoxic carcinogens. To put these concepts in perspective, actual occupational radiation exposures are exemplified with data from Germany, for the year 2012, and regulations in Germany are compared to the recommendations issued by ICRP. Conclusions include, among others, considerations on radiation protection concepts currently in use and recommendations of the SSK on the limitation of annual effective dose and effective dose cumulated over a whole working life.     

Principal inferences from studies of radiobiological effects for radiation protection purposes

The most recent Recommendations (Publication 103) issued by the International Commission for Radiological Protection (ICRP) are based on the data that have been published since 1990 up to now. The basic task of the ICRP Committee 1 was to formulate the key implications of studies on radiobiological effects for the purposes of radiological protection. Presented in the paper are the new achievements in the field of biology, radiobiology and radiation epidemiology which were taken into account by the ICRP in the process of Publication 103 preparation. The Recommendations provide present-day values of weighting factors for radiation exposure and tissue weighting factors, as well as radiation detriment and radiogenic risk factors for cancer and genetic diseases. Also considered are tissue reactions to radiation exposure, consequences of in utero exposure and the risks of developing non-cancer diseases for exposed individuals. It should be noted that the key inferences and recommendations are to a considerable degree related to biological effects accounted for by acute and chronic exposure to ionizing radiation in the range of small doses (up to 100 mSv).     

Solid cancer risk coefficient for fast neutrons in terms of effective dose

Cancer mortality risk coefficients for neutrons have recently been assessed by a procedure that postulates for the neutrons a linear dose dependence, invokes the excess risk of the A-bomb survivors at a gamma-ray dose D(1) of 1 Gy, and assumes a neutron RBE as a function of D(1) between 20 and 50. The excess relative risk (ERR) of 0.008/mGy has been obtained for R(1) = 20 and 0.016/mGy for R(1) = 50. To compare these results to the current ICRP nominal risk coefficient for solid cancer mortality (0.045/Sv for a population of all ages; 0.036/Sv for a working population), the ERR is translated into lifetime attributable risk and is then related to effective dose. The conversion is not trivial, because the neutron effective dose has been defined by ICRP not as a weighted genuine neutron dose (neutron kerma), but as a weighted dose that includes the dose from gamma rays that are induced by neutrons in the body. If this is accounted for, the solid cancer mortality risk for a working population is found to agree with the ICRP nominal risk coefficient for neutrons in their most effective energy range, 0.2 MeV to 0.5 MeV. In radiation protection practice, there is an added level of safety, because the effective dose, E, is-for monitoring purposes-assessed in terms of the operational quantity H*, which overestimates E substantially for neutrons between 0.01 MeV and 2 MeV.     

Epidemiological studies of cataract risk at low to moderate radiation doses: (not) seeing is believing

The prevailing belief for some decades has been that human radiation-related cataract occurs only after relatively high doses; for instance, the ICRP estimates that brief exposures of at least 0.5-2 Sv are required to cause detectable lens opacities and 5 Sv for vision-impairing cataracts. For protracted exposures, the ICRP estimates the corresponding dose thresholds as 5 Sv and 8 Sv, respectively. However, several studies, especially in the last decade, indicate that radiation-associated opacities occur at much lower doses. Several studies suggest that medical or environmental radiation exposure to the lens confers risk of opacities at doses well under 1 Sv. Among Japanese A-bomb survivors, risks for cataracts necessitating lens surgery were seen at doses under 1 Gy. The confidence interval on the A-bomb dose threshold for cataract surgery prevalence indicated that the data are compatible with a dose threshold ranging from none up to only 0.8 Gy, similar to the dose threshold for minor opacities seen among Chernobyl clean-up workers with primarily protracted exposures. Findings from various studies indicate that radiation risk estimates are probably not due to confounding by other cataract risk factors and that risk is seen after both childhood and adult exposures. The recent data are instigating reassessments of guidelines by various radiation protection bodies regarding permissible levels of radiation to the eye. Among the future epidemiological research directions, the most important research need is for adequate studies of vision-impairing cataract after protracted radiation exposure.     

Hereditary damage

Summary For the purposes of radiation protection, risk estimates should be based on effects of irradiation at low doses and low dose-rates. Although few genetic studies have been made on effects at low doses those carried out at low dose-rates suggest that the response is generally linear for induction of both gene mutations and chromosome aberrations. For obtaining an overall genetic risk assessment under these conditions a doubling dose of 100 rem (1 Sv) has been used by the ICRP and other bodies, with respect to radiation of low LET. In addition, it is necessary to know frequencies of human hereditary conditions, the extent to which these frequencies are maintained by recurrent mutation and the average number of generations the different categories of hereditary damage persist in the population. By the use of this information, as well as some data on translocation induction obtained directly from human exposures, an estimate of the risk of serious hereditary ill health in the first two generations after low-level radiation exposure was obtained for the Commission by one of its task groups. Thus the estimate of 10^–2 Sv^–1 used in ICRP 26 has a factual basis, although a number of far-reaching assumptions have still to be made when any risk estimation of this nature is attempted.Invited paper, presented at the 14th Annual Meeting of European Society of Radiation Biology, Jülich, Germany, October 8–14, 1978     

Assessment of external dose to inhabitants evacuated from the 30-km zone soon after the Chernobyl accident

According to reports by the former USSR government, IAEA and WHO, no case of acute radiation effects was recognized among inhabitants who were evacuated from the 30-km zone around the Chernobyl site soon after the accident on April 26, 1986. Along with the collapse of the USSR, however, several documents appeared that report the occurrence of acute radiation effects among inhabitants. In order to check the possibility of acute radiation effects among evacuees, we evaluated the external dose of evacuees until their evacuation based on the data about the radiation situation soon after the accident. Our estimates indicate that a substantial number of inhabitants in some villages could have received more than 0.5 Sv that is recognized by ICRP and UNSCEAR as a threshold dose for a clinically significant depression of the blood-forming function of bone marrow. Some of them could have received more than 1 Sv.     

Radiation protection: the NCRP guidelines and some considerations for the future.

The National Council on Radiation Protection and Measurements (NCRP) in the USA and the International Commission on Radiological Protection (ICRP), worldwide, were formed about 1928 and have since made recommendations on appropriate levels of protection from ionizing radiation for workers and for the public. These recommendations and much of the guidance provided by these organizations have usually been adopted by regulatory bodies around the world. In the case of the NCRP, the levels have fallen from 0.1 roentgen per day in 1934 to the current 5 rem per year (a factor of about 5). The present levels recommended by both the ICRP and the NCRP correspond to reasonable levels of risk where the risks of harm from ionizing radiation are compared with the hazards of other, commonly regarded, as safe, industries. Some considerations for the future in radiation protection include trends in exposure levels (generally downward for the average exposure to workers) and improvements in risk estimation; questions of lifetime limits, de minimis levels, and partial body exposures; plus problems of high LET radiations, acceptability of risk, synergisms, and risk systems for protection.     

Cancer risk from chronic exposures to chemicals and radiation: a comparison of the toxicological reference value with the radiation detriment

This article aims at comparing reference methods for the assessment of cancer risk from exposure to genotoxic carcinogen chemical substances and to ionizing radiation. For chemicals, cancer potency is expressed as a toxicological reference value (TRV) based on the most sensitive type of cancer generally observed in animal experiments of oral or inhalation exposure. A dose–response curve is established by modelling experimental data adjusted to apply to human exposure. This leads to a point of departure from which the TRV is derived as the slope of a linear extrapolation to zero dose. Human lifetime cancer risk can then be assessed as the product of dose by TRV and it is generally considered to be tolerable in a 10^–6–10^–4 range for the public in a normal situation. Radiation exposure is assessed as an effective dose corresponding to a weighted average of energy deposition in body organs. Cancer risk models were derived from the epidemiological follow-up of atomic bombing survivors. Considering a linear-no-threshold dose-risk relationship and average baseline risks, lifetime nominal risk coefficients were established for 13 types of cancers. Those are adjusted according to the severity of each cancer type and combined into an overall indicator denominated radiation detriment. Exposure to radiation is subject to dose limits proscribing unacceptable health detriment. The differences between chemical and radiological cancer risk assessments are discussed and concern data sources, extrapolation to low doses, definition of dose, considered health effects and level of conservatism. These differences should not be an insuperable impediment to the comparison of TRVs with radiation risk, thus opportunities exist to bring closer the two types of risk assessment.

     

Lung cancer risk of Mayak workers: modelling of carcinogenesis and bystander effect

Lung cancer mortality in the period of 1948-2002 has been analysed for 6,293 male workers of the Mayak Production Association, for whose information on smoking, annual external doses and annual lung doses due to plutonium exposures was available. Individual likelihoods were maximized for the two-stage clonal expansion (TSCE) model of carcinogenesis and for an empirical risk model. Possible detrimental and protective bystander effects on mutation and malignant transformation rates were taken into account in the TSCE model. Criteria for non-nested models were used to evaluate the quality of fit. Data were found to be incompatible with the model including a detrimental bystander effect. The model with a protective bystander effect did not improve the quality of fit over models without a bystander effect. The preferred TSCE model was sub-multiplicative in the risks due to smoking and internal radiation, and more than additive. Smoking contributed 57% to the lung cancer deaths, the interaction of smoking and radiation 27%, radiation 10%, and others cause 6%. An assessment of the relative biological effectiveness of plutonium was consistent with the ICRP recommended value of 20. At age 60 years, the excess relative risk (ERR) per lung dose was 0.20 (95% CI: 0.13; 0.40) Sv(-1), while the excess absolute risk (EAR) per lung dose was 3.2 (2.0; 6.2) per 10(4) PY Sv. With increasing age attained the ERR decreased and the EAR increased. In contrast to the atomic bomb survivors, a significant elevated lung cancer risk was also found for age attained younger than 55 years. For cumulative lung doses below 5 Sv, the excess risk depended linearly on dose. The excess relative risk was significantly lower in the TSCE model for ages attained younger than 55 than that in the empirical model. This reflects a model uncertainty in the results, which is not expressed by the standard statistical uncertainty bands.     

Comparison of the gastrointestinal syndrome after total-body or total-abdominal irradiation

In pathogen-free mice, but not standard conventionally housed laboratory rodents, two distinctly different modes of early radiation lethality can be identified by modifying the irradiation technique (total-body versus abdominal irradiation) or by therapeutic intervention such as rescue of total-body-irradiated mice with syngeneic bone marrow or spleen. While damage to the gastrointestinal tract is usually designated as the predominant cause of death occurring within 10 days of radiation exposure, it was demonstrated that damage to the hematopoietic/lymphopoietic system can result in animal lethality over the same period as the gastrointestinal syndrome and that this target cell population is more radiation-sensitive than the gastrointestinal epithelium.     

The 15-Country Collaborative Study of Cancer Risk among Radiation Workers in the Nuclear Industry: estimates of radiation-related cancer risks

A 15-Country collaborative cohort study was conducted to provide direct estimates of cancer risk following protracted low doses of ionizing radiation. Analyses included 407,391 nuclear industry workers monitored individually for external radiation and 5.2 million person-years of follow-up. A significant association was seen between radiation dose and all-cause mortality [excess relative risk (ERR) 0.42 per Sv, 90% CI 0.07, 0.79; 18,993 deaths]. This was mainly attributable to a dose-related increase in all cancer mortality (ERR/Sv 0.97, 90% CI 0.28, 1.77; 5233 deaths). Among 31 specific types of malignancies studied, a significant association was found for lung cancer (ERR/Sv 1.86, 90% CI 0.49, 3.63; 1457 deaths) and a borderline significant (P = 0.06) association for multiple myeloma (ERR/Sv 6.15, 90% CI <0, 20.6; 83 deaths) and ill-defined and secondary cancers (ERR/Sv 1.96, 90% CI -0.26, 5.90; 328 deaths). Stratification on duration of employment had a large effect on the ERR/Sv, reflecting a strong healthy worker survivor effect in these cohorts. This is the largest analytical epidemiological study of the effects of low-dose protracted exposures to ionizing radiation to date. Further studies will be important to better assess the role of tobacco and other occupational exposures in our risk estimates.     

Dietary factors and cancer mortality among atomic-bomb survivors

Dietary factors such as fruit and vegetables are thought to reduce the risk of cancer incidence and mortality. We investigated the effect of a diet rich in fruit and vegetables against the long-term effects of radiation exposure on the risk of cancer. A cohort of 36,228 atomic-bomb survivors of Hiroshima and Nagasaki, for whom radiation dose estimates were currently available, had their diet assessed in 1980. They were followed for a period of 20 years for cancer mortality. The joint-effect of fruit and vegetables intake and radiation exposure on risk of cancer death was examined, in additive (sum of effects of diet alone and radiation alone) and multiplicative (product of effects of diet alone and radiation alone) models. In the additive model, a daily intake of fruit and vegetables significantly reduced the risk of cancer deaths by 13%, compared to an intake of once or less per week. Radiation exposure of 1 Sievert (Sv) increased significantly the risk of cancer death by 48-49%. The additive joint-effects showed a lower risk of cancer among those exposed to 1 Sv who had a diet rich in vegetables (49%-13%=36%) or fruit (48%-13%=35%). The multiplicative model gave similar results. The cancer risk reduction by vegetables in exposed persons went from 52% (effect of radiation alone) to 32% (product of effect of vegetables and radiation), and cancer risk reduction by fruit was 52% (radiation alone) to 34% (product of effect of fruit and radiation). There was no significant evidence to reject either the additive or the multiplicative model. A daily intake of fruit and vegetables was beneficial to the persons exposed to radiation in reducing their risks of cancer death.     

Effective dose from radiation exposure in medicine: Past,present, and future

Effective dose (E) has been developed by the International Commission on Radiological Protection (ICRP) as a dose quantity with a link to risks of health detriment, mainly cancer. It is based on reference phantoms representing average individuals, but this is often forgotten in its application to medical exposures, for which its use sometimes goes beyond the intended purpose. There has been much debate about issues involved in the use of E in medicine and ICRP is preparing a publication with more information on this application. This article aims to describe the development of E and explain how it should be used in medicine. It discusses some of the issues that arise when E is applied to medical exposures and provides information on how its use might evolve in the future. The article concludes with responses to some frequently asked questions about uses of E that are in line with the forthcoming ICRP publication. The main use of E in medicine is in meaningful comparison of doses from different types of procedure not possible with measurable dose quantities. However, it can be used, with appropriate care, as a measure of possible cancer risks. When considering E to individual patients, it is important to note that the dose received will differ from that assessed for reference phantoms, and the risk per Sv is likely to be greater on average in children and less in older adults. Newer techniques allow the calculation of patient-specific E which should be distinguished from the reference quantity.     

Effects of ultraviolet-B radiation on plants during mild water stress. III. Effects on photosynthetic recovery and growth in soybean

Soybean { Glycine max (L.) Merr. ev. Essex} was grown from seed in a greenhouse under ultraviolet-B (UV-B, 280–320 nm) radiation supplied by filtered FS-40 sunlamps. On a weighted, total daily dose basis these plants received either 0 (control) or 2875 effective J m⁻² day⁻¹ UV-B_BE. When weighted with the generalized plant action spectrum (Caldwell 1971), this simulated the solar ultraviolet-B irradiance expected to occur at College Park, Maryland, USA (39°N) in the event the global stratospheric ozone column is reduced by 23%. The effects of ultraviolet radiation on the photosynthetic recovery from water stress were measured with an infrared gas analyzer. These effects were examined in plants which were either well-watered or previously preconditioned to water stress, during two distinct phenological stages of development. During the early stages of soybean growth, enhanced levels of UV-B reduced net photosynthesis by 25%, and water stress also reduced photosynthesis to nearly the same extent (by 20%). The combination of these two stresses resulted in smaller biomass than that produced by plants exposed to either stress independently. Photosynthesis in older, larger plants was much more sensitive to water stress and was reduced by as much as 50–60% in non-preconditioned plants. Although non-irradiated, non-preconditioned (control) plants recovered to only within 60% of their prestressed value, preconditioned plants recovered to within 70–80% during the 3 day recovery period. Both water stress and UV-B radiation affected non-stomatal conductance, while stomatal conductance was primarily affected by water stress.     

Radiation burdens for humans on prolonged exomagnetospheric voyages.

The severity of radiation exposure for astronauts outside the magnetosphere poses a critical unanswered question bearing on the use of manned vehicles in extended exploration of the solar system (moon, Mars). Such prolonged exomagnetospheric voyages (1-3 years) enter a radiologic environment more severe than that of low earth orbit, an annual dose equivalent in the range of 0.3-0.5 Sv (30-50 rem), and a lifetime excess cancer fatality risk of 3-5% due to low linear-energy-transfer components of galactic cosmic radiation alone. To this calculus must be added estimates for high-atomic-number, high-energy particles, the probability of solar particle events, and the limited effectiveness of shielding. For a 3-year Mars voyage these could elevate the dose equivalent to 1.5-2.25 Sv (150-225 rem) total (0.5-0.75 Sv [50-75 rem] annual) and risks to 5-9% excess cancer fatality. Both the mission (civilian scientific research) and the alternatives (unmanned robotic devices) enter the policy decision here. This paper presents a brief review of pertinent physical and biological data and of research urgently needed before reaching a decision on this question.     

Environmental radioactivity damages the DNA of earthworms of Fukushima Prefecture,Japan

Radioactive materials released in 2011 during the accident at Japan's Fukushima nuclear power plant are expected to become concentrated in wildlife via the food web, triggering DNA damage. However, the extent of this wildlife DNA damage remains unknown. We explored the effects of exposure to environmental radiation on the DNA of wild boars (Sus scrofa leucomystax) and earthworms (Megascolecidae) and examined the modes of transfer of radioactivity to these animals. We used a comet assay to analyze the DNA damage in animals collected from regions with low (0.28 μSv/h) and high (2.85 μSv/h) levels of atmospheric radiation. We constructed a model food web featuring the wild boar as the top predator, and we measured the radioactivity levels in soil, plant material, earthworms, and wild boar. The extent of DNA damage in wild boars did not differ significantly between animals captured in the two regions, but earthworms from the “high-dose” region had a significantly greater extent of DNA damage than did those from the “low-dose” region. Radioactive material can be accumulated either directly from the soil or indirectly via plant ingestion. Wildlife inhabiting radioactively contaminated areas is at high risk of DNA damage although the risk varies among species because of the differences in foraging habits. The long-term results of the observed DNA damage must be studied.     

Lung fibrosis in plutonium workers

There have been few systematic studies of the non-malignant health effects of alpha-particle radiation in humans. Animal studies and a report on plutonium-exposed workers from Russia suggest an association between high doses to the lung from plutonium exposure and the development of fibrotic lung disease. Prompted by a case of lung fibrosis in a retired plutonium worker, we tested the hypothesis that plutonium inhalation increases the risk for developing chest radiograph abnormalities consistent with pulmonary fibrosis. We conducted a retrospective study of nuclear weapons workers that included estimating absorbed doses to the lung with an internal dosimetry model. Our study population consisted of 326 plutonium-exposed workers with absorbed lung doses from 0 to 28 Sv and 194 unexposed workers. We compared the severity of chest radiograph interstitial abnormalities between the two groups using the International Labour Organization profusion scoring system. There was a significantly higher proportion of abnormal profusion scores among plutonium-exposed workers (17.5%) than among unexposed workers (7.2%), P < 0.01. Lung doses of 10 Sv or greater conferred a 5.3-fold risk (95% CI 1.2-23.4) of having an abnormal chest X ray consistent with pulmonary fibrosis when compared with unexposed individuals after controlling for the effects of age, smoking and asbestos exposure. This study shows that plutonium may cause lung fibrosis in humans at absorbed lung doses above 10 Sv.     

Lung cancer mortality among male nuclear workers of the Mayak facilities in the former Soviet Union

An analysis of lung cancer mortality in a cohort of 1,669 Mayak workers who started their employment in the plutonium and reprocessing plants between 1948 and 1958 has been carried out in terms of a relative risk model. Particular emphasis has been given to a discrimination of the effects of external gamma-ray exposure and internal alpha-particle exposure due to incorporated plutonium. This study has also used the information from a cohort of 2,172 Mayak reactor workers who were exposed only to external gamma rays. The baseline lung cancer mortality rate has not been taken from national statistics but has been derived from the cohort itself. For both alpha particles and gamma rays, the results of the analysis are consistent with linear dose dependences. The estimated excess relative risk per unit organ dose equivalent in the lung due to the plutonium alpha particles at age 60 equals, according to the present study, 0.6/Sv, with a radiation weighting factor of 20 for alpha particles. The 95% confidence range is 0.39/Sv to 1.0/Sv. For the gamma-ray component, the present analysis suggests an excess relative risk for lung cancer mortality at age 60 of 0.20/Sv, with, however, a large 95% confidence range of-0.04/Sv to 0.69/Sv.     

Human biokinetics of strontium. Part I: Intestinal absorption rate and its impact on the dose coefficient of 90Sr after ingestion

Intestinal absorption of strontium (Sr) in thirteen healthy adult German volunteers has been investigated by simultaneous oral and intravenous administration of two stable tracer isotopes, i.e. (84)Sr and (86)Sr. The measured Sr tracer concentration in plasma was analyzed using the convolution integral technique to obtain the intestinal absorption rate. The results showed that the Sr labeled in different foodstuffs was absorbed into the body fluids in a large range of difference. The maximum Sr absorption rates were observed within 60-120 min after administration. The rate of absorption is used to evaluate the intestinal absorption fraction, i.e. the f (1) value for various foodstuffs. The equivalent and effective dose coefficients for ingestion of (90)Sr were calculated using these f (1) values, and they were compared with those recommended by the International Commission on Radiological Protection (ICRP). The geometric and arithmetic means of the f (1) values are 0.38 and 0.45 associated with a geometric standard deviation and a standard deviation of 1.88 and 0.22, respectively. The 90% confidence interval of the f (1) values obtained in the present study ranges from 0.13 to 0.98. Expressed as the ratio of the 95 and 50% percentiles of the estimated probability, the uncertainty for the f (1) value corresponds to a factor of 2.58. The effective dose coefficients of (90)Sr after ingestion are 6.1 x 10(-9) Sv Bq(-1) for an f(1) value of 0.05, 1.0 x 10(-8) Sv Bq(-1) for 0.1, 1.9 x 10(-8) Sv Bq(-1) for 0.2, 2.8 x 10(-8) Sv Bq(-1) for 0.3, 3.6 x 10(-8) Sv Bq(-1) for 0.4, 5.3 x 10(-8) Sv Bq(-1) for 0.6, 7.1 x 10(-8) Sv Bq(-1) for 0.8, and 7.9 x 10(-8) Sv Bq(-1) for 0.9, respectively. Taking the effective dose coefficient of 2.8 x 10(-8) Sv Bq(-1) for an f (1) value of 0.3, which is recommended by the ICRP, as a reference, the effective dose coefficient of (90)Sr after ingestion varies by a factor of 2.8 when the f (1) value changes by a factor of 3, i.e. it decreases from 0.3 to 0.1 or increases from 0.3 to 0.9, respectively.