Testing prediction capabilities of an 131I terrestrial transport model by using measurements collected at the Hanford nuclear facility

A model describing transport of 131I in the environment was developed by SENES Oak Ridge, Inc., for assessment of radiation doses and excess lifetime risk from 131I atmospheric releases from Oak Ridge Reservation in Oak Ridge, Tennessee, and from Idaho National Engineering and Environmental Laboratory in southeast Idaho. This paper describes the results of an exercise designed to test the reliability of this model and to identify the main sources of uncertainty in doses and risks estimated by this model. The testing of the model was based on materials published by the International Atomic Energy Agency BIOMASS program, specifically environmental data collected after the release into atmosphere of 63 curies of 131I during 2-5 September 1963, after an accident at the Hanford PUREX Chemical Separations Plant, in Hanford, Washington. Measurements of activity in air, vegetation, and milk were collected in nine counties around Hanford during the first couple of months after the accident. The activity of 131I in the thyroid glands of two children was measured 47 d after the accident. The model developed by SENES Oak Ridge, Inc., was used to estimate concentrations of 131I in environmental media, thyroid doses for the general population, and the activity of 131I in thyroid glands of the two children. Predicted concentrations of 131I in pasture grass and milk and thyroid doses were compared with similar estimates produced by other modelers. The SENES model was also used to estimate excess lifetime risk of thyroid cancer due to the September 1963 releases of 131I from Hanford. The SENES model was first calibrated and then applied to all locations of interest around Hanford without fitting the model parameters to a given location. Predictions showed that the SENES model reproduces satisfactorily the time-dependent and the time-integrated measured concentrations in vegetation and milk, and provides reliable estimates of 131I activity in thyroids of children. SENES model generated concentrations of 131I closer to observed concentrations, as compared to the predictions produced with other models. The inter-model comparison showed that variation of thyroid doses among all participating models (SENES model included) was a factor of 3 for the general population, but a factor of 10 for the two studied children. As opposed to other models, SENES model allows a complete analysis of uncertainties in every predicted quantity, including estimated thyroid doses and risk of thyroid cancer. The uncertainties in the risk-per-unit-dose and the dose-per-unit-intake coefficients are major contributors to the uncertainty in the estimated lifetime risk and thyroid dose, respectively. The largest contributors to the uncertainty in the estimated concentration in milk are the feed-to-milk transfer factor (F(m)), the dry deposition velocity (V(d)), and the mass interception factor (r/Y)dry for the elemental form of iodine (I2). Exposure to the 1963 PUREX/Hanford accident produced low doses and risks for people living at the studied locations. The upper 97.5th percentile of the excess lifetime risk of thyroid cancer for the most extreme situations is about 10(-4). Measurements in pasture grass and milk at all locations around Hanford indicate a very low transfer of 131I from pasture to cow's milk (e.g., a feed-to-milk transfer coefficient, F(m), for commercial cows of about 0.0022 d L(-1)). These values are towards the low end of F(m) values measured elsewhere and they are low compared to the F(m) values used in other dose reconstruction studies, including the Hanford Environmental Dose Reconstruction.     

Medical radiation exposure in the U.S. in 2006: preliminary results

Medical radiation exposure of the U.S. population has not been systematically evaluated for almost 25 y. In 1982, the per capita dose was estimated to be 0.54 mSv and the collective dose 124,000 person-Sv. The preliminary estimates of the NCRP Scientific Committee 6-2 medical subgroup are that, in 2006, the per capita dose from medical exposure (not including dental or radiotherapy) had increased almost 600% to about 3.0 mSv and the collective dose had increased over 700% to about 900,000 person-Sv. The largest contributions and increases have come primarily from CT scanning and nuclear medicine. The 62 million CT procedures accounted for 15% of the total number procedures (excluding dental) and over half of the collective dose. Nuclear medicine accounted for about 4% of all procedures but 26% of the total collective dose. Medical radiation exposure is now approximately equal to natural background radiation.     

Thyroid cancer risk in the population around the Nevada Test Site

Potential thyroid cancer risk in Utah populations due to weapons testing fallout has been examined in two ways. Thyroid cancer incidence rates for Utah for the years 1973-77 were compared to those of the reporting areas of the Surveillance, Epidemiology and End Results Registry. In addition, numbers of lifetime thyroid cancer cases and distribution of probability of causation values for those cases have been estimated for a population of about 23,000 who were in the fallout area of three major shots at the Nevada Test Site during the 1950s. A computer program designed to estimate numbers of expected and excess cases occurring in the remaining lifetime after exposure was used to make the estimates. The estimates of internal dose distribution within various age at exposure groups were made using dose estimates developed by the Off-Site Radiation Exposure Review Project. The potential human risk from 131I at low doses is far from certain. The upper bound risk must be considered to be near that for low doses of acutely delivered external radiation to the thyroid. A practical upper limit for risk from low-dose 131I was taken to be one per million per rad-yr for males and four per million per rad-yr for females (although absolute upper limits would need to be higher). The upper bound estimate of excess cases in the lifetime of the population is about 13, compared to an estimate of about 58 cases expected to occur in the population on the basis of ordinary incidence. If excess risk is assumed to increase linearly with dose, and not to change with age, 12 of 13 excess cases fall into the group aged 18 and below at the time the dose occurred, and 11 of 13 excess cases fall into the group exposed to 10 rad or more.     

Population radiation absorbed dose from nuclear medicine procedures in The Netherlands

The mean radiation absorbed dose per patient and for the whole population per caput per year in the Netherlands from diagnostic nuclear medicine procedures has been estimated using patient data from 10 large hospitals during either 1984 or 1985. The mean effective dose equivalent and the mean gonad dose equivalent per patient were 2.7 mSv and 1.7 mSv, respectively. Extrapolating these figures to all diagnostic nuclear medicine procedures in the Netherlands, the mean effective dose equivalent and the mean gonad dose equivalent per caput per year were 0.037 mSv and 0.024 mSv, respectively.     

Nuclear medicine staff and patient doses in Manitoba (1981-1985)

The number of diagnostic in vivo nuclear medicine (NM) procedures in the Province of Manitoba (population 1 million) has been examined over the period 1981 to 1985. The annual number of procedures performed has remained relatively constant at about 25 per thousand population. The isotope 99mTc accounted for 86% of all the studies performed and the number of NM procedures per imaging system was approximately 1,300 per annum. The total number of NM operators in the province increased from 30 in 1981 to about 40 in 1985. The mean NM operator dose was reduced from 3.8 mSv to 2.5 mSv over this five-year period and the collective operator dose underwent a smaller reduction of 13% to about 100 person-mSv in 1985. The value of the mean patient effective dose equivalent (HE) was relatively constant at 5.2 mSv. The contribution of diagnostic NM procedures to the annual per caput population dose in Manitoba was 0.13 mSv. Three diagnostic procedures (brain, bone and cardiac) accounted for approximately 80% of the collective patient HE. Patient profiles (age, sex and medical history) were obtained for the patients undergoing these three procedures, which showed them to be atypical in comparison to a normal working population. These data suggested that the application of the International Commission on Radiological Protection risk factor of 1.65 X 10(-2) Sv-1 to this patient population would have significantly overestimated the expected radiation detriment.     

甘肃省3家医院核医学科放射工作人员内照射水平分析

目的 对甘肃省3家甲级医院的20名从事核医学工作的医务人员开展甲状腺中¹³¹I内照射监测和剂量估算。方法 使用InSpector 1000型便携式γ谱仪进行体外直接测量法。将谱仪进行能量刻度和效率刻度后，对每位工作人员的甲状腺和大腿部位分别进行一次测量，测量时间均为120 s。大腿部测得结果作为体内本底，计算甲状腺¹³¹I活度，利用甲状腺¹³¹I摄入量，计算甲状腺待积器官剂量并推算年待积有效剂量。结果 3家医院20名核医学工作人员其中8人甲状腺中检出¹³¹I，占总人数的40%，甲状腺中¹³¹I活度范围为：30.29～1271.68 Bq，平均活度为395.39 Bq；甲状腺待积器官剂量范围是0.33～14.00 μSv，平均剂量为4.36 μSv；年待积有效剂量范围是0.02～0.73 mSv，平均剂量为0.23 mSv。结论 调查结果显示，所有人员年待积有效剂量均未超过必须进行内照射监测的1 mSv限值，但也较为接近，可以适当调整监测周期。考虑到每个周期用药量及治疗病人数量的变化，还需要密切关注。     

Retrospective evaluation of 131I deposition density and thyroid dose in Poland after the Chernobyl accident

The 131I deposition in Poland after the Chernobyl accident on 26 April 1986 was evaluated from the determined 129I deposition and the estimated 129I/131I ratio at the time of the arrival of fallout. Concentrations of 129I and 127I were determined by neutron activation analysis in uncultivated soils from 16 locations in Poland. Determination of 137Cs in soils was carried out by gamma spectrometry. The atomic ratio of 129I/131I at the time of fallout arrival was estimated using the 129I/131I ratio at the time of the accident, which, on the basis of the core inventory data, was assumed to be 22.8. It was estimated from the time of fallout arrival and from the weighed mean atomic ratio that the 129I/131I ratio for Poland was 32.8. The calculated 131I deposition ranged from 63.2 to 729 kBq m(-2). High deposition of 131I occurred in the locations with rainfall but occasionally also in locations without rainfall. Committed equivalent doses from 131I were evaluated for 5-y-old children, 10-y-old children, and adults using the computer model CLRP for the situations with and without countermeasures including iodine prophylaxis. The highest thyroid doses from inhalation and ingestion without countermeasures were 178 mSv, 120 mSv, and 45 mSv for 5-y-old children, 10-y-old children, and adults, respectively. The countermeasures reduced these doses by about 30%.     

上海市2008年临床核医学的医疗照射水平分析

[目的]掌握上海市临床核医学诊断与治疗的医疗照射水平现状及其发展趋势。[方法]设计统一调查表格,普查上海市开展临床核医学诊疗的43家医疗机构的有关基本现状,具体调查2008年43家医疗机构的临床核医学诊疗工作量及其分布,以及受检者与患者的性别、年龄组分布。[结果]2008年上海市拥有各类核医学相关设备169台,从事临床核医学的工作人员为342人。全市核医学诊断年频率为6．63人次／千人口,居多的检查类型为骨显像、PET肿瘤显像、甲状腺显像等,男性比女性高25．6％。核医学诊断年频率随受检者年龄增长（0～15岁、16～40岁、〉40岁组）而增加。临床核医学的放射性药物治疗频率为0．41人次／千人口,女性比男性高62．5％。多数显像和功能检查的常用放射性核素为^99mTc;PET检查常用的放射性核素为^18F。甲状腺癌和甲亢治疗常用的放射性核素为^13I,二者典型放射性活度分别是5088MBq和237MBq。上海市临床核医学诊断的应用频率比1998年增加了91.8％;是1998年我国大陆平均水平的11．4倍。全市核医学的放射性核素治疗频率比1998年增加了182．8％;是1998年我国大陆平均水平的6．8倍多。[结论]上海市临床核医学诊断和治疗的发展速度很快,应切实加强临床核医学的医疗照射防护工作。     

Reconstruction of the ingestion doses received by the population evacuated from the settlements in the 30-km zone around the Chernobyl reactor.

As a consequence of the Chernobyl accident, about 50,000 people were evacuated from the settlements in the 30-km zone around the reactor in the period 3-11 d after the accident. As no countermeasures were implemented in the early phase, people continued to consume milk and some leafy vegetables. In this paper, average effective ingestion doses are modeled for evacuees. Input data for the assessment are the 137Cs activity per unit area, the ratios of the radionuclides relative to 137Cs, the mean day of evacuation, and intake rates for milk and green vegetables. The transfer of radionuclides from deposition to humans is estimated by modeling radionuclide interception by vegetation, weathering, and the time-dependent transfer of radionuclides to milk taking into account site-specific agricultural practices. Depending on the evacuation day and site, the estimated ingestion doses for the settlements are in the range of 20 to 1,300 mSv and 3 to 180 mSv for infants and adults, respectively. 131I is by far the most important isotope, the ingestion dose due to 133I is more than one order of magnitude lower. The most exposed organ is the thyroid, inducing more than 80% and 50% of the ingestion dose for infants and adults. The ingestion doses are compared to the doses due to inhalation and external exposure. The internal dose exceeds the external by a factor of about 2-10 for adults and 2-40 for 1-y-old infants depending on site and evacuation day. The thyroid doses assessed for the evacuees are consistent with results achieved in studies performed in areas outside the 30-km zone.     

临床核医学诊疗中的辐射剂量与防护研究

目的通过对临床核医学诊疗中辐射剂量的研究,探索核医疗安全管理和辐射防护的具体方法,以保障患者、医疗工作人员以及广大社会群众的身体健康。方法针对最为常见的核医疗方法 131I治疗,选取68例接受131I治疗的患者,并将他们随机分为两组,每组分别由一名医师和两名护士负责治疗工作,一组采用常规治疗防护措施进行治疗,一组采用综合治疗防护措施,并记录两组患者服用131I后的有效剂量和医护人员1年所受的总辐射剂量,然后进行对比分析。结果经过1年的治疗之后,采用常规治疗防护措施的患者受辐射剂量为（59.63±0.56）mSv,医生和护士受辐射剂量为（4.692±0.056）mSv;采用综合治疗防护措施的患者受辐射剂量为（50.21±0.69）mSv,医生和护士受辐射剂量为（1.216±0.037）mSv。结论患者接受131I治疗所受到的辐射量大于公众照射年剂量限值,但比职业照射的年剂量限值低,但通过综合的防护措施,可以有效降低131I治疗对患者和医护人员的辐射剂量,能够保障接受临床核医学诊疗的患者和医护人员的安全健康。     

Effective half-life of 131I in thyroid cancer patients

The oral administration of radioactive 131I is a standard treatment for thyroid carcinoma. One consideration for this therapy is assuring that other people do not receive significant radiation exposure. In particular, federal and state regulatory authorities stipulate that no individual should receive more than 5 mSv (500 mrem) effective dose-equivalent from the released patient. Patients receiving more than 1.11 GBq (30 mCi) of 131I were traditionally confined as in-patients by regulation until their burdens of radioactivity fell below that level or until the external dose rates were less than 50 microSv (5 mrem) per hour at 1 m. Recent regulatory guidance recommends the use of biological elimination as well as physical decay in calculating the confinement time to keep the effective dose-equivalent to members of the public less than 5 mSv. Analysis of a database of more than 250 administrations of 131I for thyroid cancer shows a median effective half-life of at least 14 h, with substantial variation. Thus, discharge criteria for radiation safety purposes should be calculated on the basis of individual measurements. The release of these patients should not always be as prompt as the guidance indicates. The results also challenge some long-used assumptions regarding iodine excretion in this patient population.     

Effects of time of administration and dietary iodine levels on potassium iodide (KI) blockade of thyroid irradiation by 131I from radioactive fallout

Radioiodines, particularly 131I, may be released into the environment in breach-of-containment nuclear reactor accidents and localize in and irradiate the thyroid with an attendant risk of neoplastic growth and other adverse health effects. Pharmacologic thyroid blockade by oral potassium iodide (KI) (50-100 mg in adults) can substantially reduce thyroid uptake of and irradiation by internalized radioiodine. In the current analysis, computer modeling of iodine metabolism has been used to systematically elucidate the effects of two practically important but highly variable factors on the radioprotective effect of KI: the time of administration relative to exposure to radioiodine and the dietary level of iodine. In euthyroid adults receiving iodine-sufficient diets (250 microg d(-1) in the current analysis), KI administered up to 48 h before 131I exposure can almost completely block thyroid uptake and therefore greatly reduce the thyroid absorbed dose. However, KI administration 96 h or more before 131I exposure has no significant protective effect. In contrast, KI administration after exposure to radioiodine induces a smaller and rapidly decreasing blockade effect. KI administration 16 h or later after 131I exposure will have little effect on thyroid uptake and absorbed dose and therefore little or no protective effect. The 131I thyroid absorbed dose is two-fold greater with insufficient levels of dietary iodine, 2,900 cGy/37 MBq, than with sufficient levels of dietary iodine, 1,500 cGy/37 MBq. When KI is administered 48 h or less before 131I intake, the thyroid absorbed doses (in cGy/37 MBq) are comparably low with both sufficient and insufficient dietary iodine levels. When KI is administered after 131I intake, however, the protective effect of KI is less and decreases more rapidly with insufficient than with sufficient dietary iodine. For example, KI administration 2 and 8 h after 131I intake yields protective effects of 80 and 40%, respectively, with iodine-sufficient diets, but only 65 and 15% with iodine-deficient diets. In conclusion, whether exposed populations receive sufficient or insufficient dietary iodine, oral KI is an effective means of reducing thyroid irradiation from environmentally dispersed radioiodine but is effective only when administered within 2 d before to approximately 8 h after radioiodine intake.     

The relationship of thyroid cancer with radiation exposure from nuclear weapon testing in the Marshall Islands

The US nuclear weapons testing program in the Pacific conducted between 1946 and 1958 resulted in radiation exposure in the Marshall Islands. The potentially widespread radiation exposure from radio-iodines of fallout has raised concerns about the risk of thyroid cancer in the Marshallese population. The most serious exposures and its health hazards resulted from the hydrogen-thermonuclear bomb test, the Castle BRAVO, on March 1, 1954. Between 1993 and 1997, we screened 3,709 Marshallese for thyroid disease who were born before the BRAVO test. It was 60% of the entire population at risk and who were still alive at the time of our examinations. We diagnosed 30 thyroid cancers and found 27 other study participants who had been operated for thyroid cancer before our screening in this group. Fifty-seven Marshallese born before 1954 (1.5%) had thyroid cancer or had been operated for thyroid cancer. Nearly all (92%) of these cancers were papillary carcinoma. We derived estimates of individual thyroid dose proxy from the BRAVO test in 1954 on the basis of published age-specific doses estimated on Utirik atoll and 137Cs deposition levels on the atolls where the participants came from. There was suggestive evidence that the prevalence of thyroid cancer increased with category of estimated dose to the thyroid.     

Estimates of radiation dose and health risks to the United States population following the Chernobyl nuclear plant accident

Estimates of both individual and collective doses received by the United States population following the Chernobyl accident have been made by using the data obtained from the U.S. Environmental Protection Agency's Environmental Radiation Ambient Monitoring System. Radionuclides associated with the debris first were measured in precipitation and surface air particulates at Portland, OR and Olympia, WA on 5 May 1986. Iodine-131 was the most consistently measured nuclide in all media, although several Cs and Ru isotopes also were observed. Strontium and any actinides notably were absent from the samples at the lower level of detection. The highest calculated individual-organ dose due to intake during May and June 1986 was 0.52 mSv to the infant thyroid in the state of Washington. This was predominantly (98%) from the ingestion of milk. The maximum U.S. collective dose equivalent to any organ was calculated to be 3,300 person-Sv to the thyroid. Risk estimates project three excess lung cancer deaths and an additional four deaths due to cancers of thyroid, breast and leukemia in the U.S. population over the next 45 y from exposure during the May-June 1986 interval. The only long-lived radionuclide measured in milk samples following the accident was 137Cs. We estimate 20 excess fatalities from the ingestion of 137Cs in milk during all subsequent years, with six of these due to lung cancer and the majority of the remainder distributed approximately equally among cancers of the thyroid, breast, liver and leukemia. A total of 100 excess fatalities from all dietary components was estimated. Because of the uncertainty of risk estimates from data such as those available for this study, all calculated values carry a range of uncertainty from a minimum of one-half the calculated value to a maximum of two times the calculated value. The estimated excess fatalities given above may be compared with corresponding projected cancer mortality from all other causes: 41,000 fatalities from thyroid cancer and 3,800,000 fatalities from lung cancer are estimated to occur within the U.S. population during the next 45 y.     

Age-specific uncertainty of the 131I ingestion dose conversion factor

The production of weapons-grade nuclear materials and their by-products has resulted in a number of releases from United States Department of Energy facilities. 131I, a fission by-product, is one of the most common radionuclides generated and released to the environment. It is known that there are differences in various physiological parameters over all age groups when considering biokinetic modeling of iodine. The establishment of age-specific dose conversion factor uncertainty is necessary for accurate internal dose assessment. The 131I dose conversion factor determined herein is log-normally distributed with varying age-specific distribution characteristics. The two most important parameters for determination of the dose conversion factor, in all age groups, are thyroid mass and iodine uptake fraction. These parameters are assumed to be highly correlated with a relationship that is quite important to dose conversion factor uncertainty. Dose estimates to individuals exposed to radioiodine can be determined more accurately with an increased understanding of the correlation between thyroid mass and uptake fraction. Improved dose estimates following oral intakes of 131I can be made from the consideration of age-specific dose conversion factors and their input parameters.     

Radiation dosimetry for highly contaminated Belarusian, Russian and Ukrainian populations, and for less contaminated populations in Europe

The explosions at the Chernobyl Nuclear Power Plant (CNPP) in Ukraine early in the morning of 26 April 1986 led to a considerable release of radioactive materials during 10 d. The cloud from the reactor spread many different radionuclides, particularly those of iodine (131I) and cesium (134Cs and 137Cs), over the majority of European countries, but the greatest contamination occurred over vast areas of Belarus, the Russian Federation and Ukraine. As the major health effect of Chernobyl is an elevated thyroid cancer incidence in children and adolescents, much attention has been paid to the thyroid doses resulting from intakes of 131I, which were delivered within 2 mo following the accident. The thyroid doses received by the inhabitants of the contaminated areas of Belarus, Russia, and Ukraine varied in a wide range, mainly according to age, level of ground contamination, milk consumption rate, and origin of the milk that was consumed. Reported individual thyroid doses varied up to approximately 40,000 mGy, with average doses of a few to 1,000 mGy, depending on the area where people were exposed. In addition, the presence in the environment of long-lived 134Cs and 137Cs has led to a relatively homogeneous exposure of all organs and tissues of the body via external and internal irradiation, albeit at low rates. Excluding the thyroid doses, the whole-body (or effective) dose estimates for the general population accumulated during 20 y after the accident (1986-2005) range from a few millisieverts (mSv) to some hundred mSv with an average dose of approximately 10 mSv in the contaminated areas of Belarus, Russia, and Ukraine. In other European countries, both the thyroid and the effective doses are, on average, much smaller.     

The third analysis of cancer mortality among Japanese nuclear workers, 1991-2002: estimation of excess relative risk per radiation dose

The present study estimated excess relative risk per sievert (ERR/Sv) of cancer mortality among the cohort of 200?583 male Japanese nuclear workers, with an average individual cumulative dose of 12.2 mSv (<10 mSv, 75.4%; 100 + mSv, 2.6%), conducting Poisson regression using dose category specific observed and expected numbers of deaths, and average doses obtained from the official report of the Radiation Effects Association (REA) on the analysis of mortality of Japanese nuclear industry workers for 1991-2002, which reported the estimates of ERR/Sv for leukaemia but not for all cancers or any other cancer site. The possible confounding biases from drinking alcohol and smoking tobacco were evaluated by examining the association of cumulative radiation dose with the mortality of cancers related to drinking or smoking. For leukaemia (80 deaths), the estimate of ERR/Sv was - 1.93 (95% confidence interval (CI) = - 6.12, 8.57). For all cancers excluding leukaemia (2636 deaths), while the ERR/Sv was estimated to be 1.26 (95%CI = - 0.27, 3.00), confounding by alcohol consumption was suspected since the ERR/Sv estimate of alcohol-related cancers was 4.64 (95%CI = 1.13, 8.91) and the ERR/Sv estimate of all cancers excluding leukaemia and alcohol-related cancers was 0.20 (95%CI = - 1.42, 2.09). In conclusion, confounding by important lifestyle factors related to cancer risk may have a substantial effect on risk estimates, especially when conducting studies of low cumulative dose and, accordingly, low statistical power. Pooled analysis or meta-analysis of nuclear workers for solid cancers needs to take this point into account.     

Feasibility of an epidemiologic study of thyroid neoplasia in persons exposed to radionuclides from the Hanford nuclear facility between 1944 and 1956

Between 1944 and 1956, approximately 19.6 PBq (530,000 Ci) of 131I were released to the atmosphere during Pu reprocessing for nuclear weapons at the Hanford nuclear facility in southeastern Washington state. For these years, we summarized historical records of quarterly 131I atmospheric releases and vegetation concentrations measured in nearby communities. We used these data and other reported environmental measurements to make preliminary estimates of maximum doses to the thyroid for the general public. We also computed the statistical power for an epidemiologic study of thyroid neoplasia in birth cohorts of children born in two counties near Hanford during the years of highest exposure. These estimates suggest that an epidemiologic study would be feasible if the actual average radiation doses in the exposed population were no less than one-tenth the preliminary maximum doses. Our analyses also suggest that it may be more appropriate to stratify the exposed population by cumulative dose in order to examine the relation between radiation exposure and thyroid neoplasia.     

The 1986 and 1988 UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) reports: findings and implications

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has published a substantive series of reports concerning sources, effects, and risks of ionizing radiation. This article summarizes the highlights and conclusions from the most recent 1986 and 1988 reports. The present annual per person effective dose equivalent for the world's population is about 3 mSv. The majority of this (2.4 mSv) comes from natural background, and 0.4 to 1 mSv is from medical exposures. Other sources contribute less than 0.02 mSv annually. The worldwide collective effective dose equivalent annually is between 13 and 16 million person-Sv. The Committee assessed the collective effective dose equivalent to the population of the northern hemisphere from the reactor accident at Chernobyl and concluded that this is about 600,000 person-Sv. The Committee also reviewed risk estimates for radiation carcinogenesis which included the new Japanese dosimetry at Hiroshima and Nagasaki. These data indicate that risk coefficient estimates for high doses and high dose rate low-LET radiation in the Japanese population are approximately 3-10% Sv-1, depending on the projection model utilized. The Committee also indicated that, in calculation of such risks at low doses and low dose rates, a risk-reduction factor in the range of 2-10 may be considered.     

Thyroid cancers in France and the Chernobyl accident: risk assessment and recommendations for improving epidemiological knowledge

From 1975 to 1995, the incidence of thyroid cancer in the French population increased by a factor of 5.2 in men and 2.7 in women, thereby raising public concerns about its association with the nuclear accident at Chernobyl. A study performed at the request of French health authorities sought to quantify the potential risk of thyroid cancer associated with the Chernobyl fallout in France in order to determine if this risk could be observed through an epidemiological approach. The study focused on the most exposed population: those living in eastern France and younger than 15 y at the time of the Chernobyl nuclear power plant accident (26 April 1986). The number of spontaneous thyroid cancers in this population was predicted from French cancer registry data, and the thyroid doses were estimated from all available data about contamination in France. Associated risks were calculated with different risk models, all based on a linear no-threshold dose-effect relationship. Under this hypothesis, from 1.3 to 22 excess thyroid cancer cases were predicted for the 1991-2000 period, compared with the 212 spontaneous cases (0.5 to 10.5%) predicted, and from 11.2 to 55.2 excess cases were predicted for 1991-2015, compared with the 1,342 spontaneous cases (0.8 to 4.1%) predicted. These risk calculations indicate that the Chernobyl fallout cannot explain the entire increase in thyroid cancers in France, and that it is improbable that an epidemiological study could demonstrate such an excess. The surveillance of thyroid cancers in France should be enhanced.