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Year : 2018  |  Volume : 41  |  Issue : 3  |  Page : 160-161  

Excerpts of UNSCEAR white paper on “evaluation of data on thyroid cancer in regions affected by the Chernobyl accident”


Editor, RPE, Ex. Head, IDS, RSSD, BARC, Mumbai, Maharashtra, India

Date of Web Publication19-Nov-2018

Correspondence Address:
D D Rao
Editor, RPE, Ex. Head, IDS, RSSD, BARC, Mumbai, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_72_18

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How to cite this article:
Rao D D. Excerpts of UNSCEAR white paper on “evaluation of data on thyroid cancer in regions affected by the Chernobyl accident”. Radiat Prot Environ 2018;41:160-1

How to cite this URL:
Rao D D. Excerpts of UNSCEAR white paper on “evaluation of data on thyroid cancer in regions affected by the Chernobyl accident”. Radiat Prot Environ [serial online] 2018 [cited 2018 Dec 10];41:160-1. Available from: http://www.rpe.org.in/text.asp?2018/41/3/160/245802



The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has brought out a white paper on the evaluation of data on thyroid cancer in the regions affected by the Chernobyl accident. The committee has previously assessed in detail the radiation exposures that resulted from the nuclear power plant accident at Chernobyl in 1986 and analyzed the associated risks and effects in various reports in the years 2000, 2008, and 2012. The committee decided to utilize the recent data generated until 2016 and prepare an update in the form of a short white paper.

The primary objectives of the paper are as follows: (a) to provide an authoritative report on the numbers of thyroid cancer cases observed to date, primarily among people who were children or adolescents at the time of the accident and (b) to make an expert judgment of the fraction that can be attributed to radiation exposure resulting from the accident. Secondary objectives are to clarify, where possible, the scientific basis for and the reliability of risk projections, considering the levels and patterns of radiation dose to the exposed populations.

The white paper is based on data on annual incidence of thyroid cancer as submitted officially under arrangements made by the secretariat with the representatives to the Committee of Belarus, the Russian Federation, and Ukraine. In addition, it presents an evaluation of key publications in the peer-reviewed scientific literature up to December 2016, and an assessment of the fraction of the incidence of thyroid cancer attributable to the radiation exposure caused by the accident. The evaluation used (a) the epidemiological studies and analyses of radiation risk; (b) the molecular biology and pathological studies; and (c) Dosimetric studies.

A compilation of incidence rates of thyroid cancer between 1982 and 2015 among those exposed under the age of 18 in Belarus, the Russian Federation (the Bryansk, Kaluga, and Orel and Tula oblasts), and Ukraine is presented in the form of an annexure. Both the total number of cases and crude incidence rate per 105 person-years basically increased monotonically during the last decade (2006–2015). The total number of cases of thyroid cancer registered in the period 1991–2015 in males and females who were under 18 in 1986, for the whole of Belarus and Ukraine and for the four most contaminated regions of the Russian Federation, exceeded 19,000. This number is 2.8 times higher than the number of thyroid cancer cases registered in the same cohort in the period 1991–2005. On average, the registered numbers of thyroid cancer for females were about four times higher than for males.

The observed increase in the incidence of thyroid cancer was influenced by various factors as follows: an increased spontaneous incidence rate with adulthood, radiation exposure, and improvement in diagnostic methods. Discerning the effect of exposure to ionizing radiation contributing to this complicated situation requires both careful epidemiological analysis and basic research into the processes of molecular biology. The sharp increase in the incidence rate of thyroid cancer among children from about 5 years after the accident is represented graphically. The incidence rate of thyroid cancer among Belarusian children up to 10 years old at the time of the diagnosis was higher in the period 1991–1995 by about one order of magnitude compared with the incidence rate in other 5-year periods. Both before this period (i.e., before the minimum latent period of 4–5 years) and after this period (i.e., when the cohorts did not include those who were children in 1986), an increase in the incidence rate has not been observed. The crude annual incidence rate per million are as follows: 4.5 is for females and 4 for males (1986–1990); 33 for females and 18 for males (1991–1995); 4 for females and 3.5 for males (1996–2000); 3 for females and 2.5 for males (2001–2005); and <2 for females and <1 for males in both 2006–2010 and 2011–2015 blocks.

A similar, though less pronounced phenomenon can be observed in cohorts of Belarusian adolescents (aged 10–19 years at diagnosis). The incidence rate of thyroid cancer began to increase during the period 1991–1995, reached a peak during the period 1996–2000, and decreased from the period 2001–2005. After 2005, the incidence rate remained higher than that before the accident. This apparently reflects the effect of the screening regime. Nevertheless, the incidence rate was significantly lower than in the period 1996–2000 (10–15 years after the accident).

An increase in the incidence rates of thyroid cancer with time among those exposed as children and adolescents (<18 years) in Belarus at the time of the accident is also reported. There is no evidence for a decrease in the excess incidence of thyroid cancer up to 2015. Part of the increase is related to the normal age pattern of spontaneous disease occurrence, whereas another part can be deemed attributable to the radiation exposure from the accident. The crude annual incidence rate per 105 person years are as follows: 0.64 is for females and 0.45 for males (1986–1990); 5.72 for females and 2.75 for males (1991–1995); 8.71 for females and 3.82 for males (1996–2000); 11.88 for females and 3.44 for males (2001–2005); 16.64 for females and 4.10 for males in 2006–2010; and 22.38 for females and 5.34 for males in 2011–2015 blocks.

An assessment of the fraction of thyroid cancer incidence deemed attributable to radiation due to the accident has been made based on estimations of average doses to the thyroid, and preconditions that (a) the dose–response relationship in the dose range covering the bulk of the doses to the thyroid of the population group of interest is linear and (b) the estimates of the excess relative rate (ERR) per unit dose derived from the cohort studies are applicable to the population.

Cohort studies of thyroid cancer among Belarusians and Ukrainians, who were children or adolescents at the time of the accident, indicate an ERR per unit dose to the thyroid of about 2 Gy−1 for the period 2001–2008, and a higher value for earlier times after the minimum latent period of a few years. This is consistent with the result of about 6–8 Gy−1 in a case–control study of cases in the period 1992–1998.

The average dose to the thyroid of evacuated children and adolescents and of nonevacuated children and adolescents (at the time of the accident) in the contaminated areas of the former USSR was estimated to about 900 and 170 mGy, respectively. If it were assumed that for those having been children or adolescents at the time of the accident, the ERR per unit dose was 2 Gy−1 for the period 2001–2008, then the ERR for the evacuated and nonevacuated would be estimated to be slightly less than 2 and about 0.3, respectively.

Based on the estimates given above, the fraction of the thyroid cancer cases in the period 2001–2008 attributable to the radiation exposure caused by the accident is assessed to be about 0.6 and 0.25 for the evacuated and nonevacuated children and adolescents, respectively. There is no clear evidence for a value of the ERR per unit dose during other periods. However, there are indications that it was higher in earlier periods and will be slightly lower at later periods. Thus, the estimates of the attributable fraction of 0.6 and 0.25 may be considered to be rough estimates for the whole period 1991–2015. The uncertainty in the estimated attributable fraction of 0.25 ranges at least from 0.07 to 0.5, which is consistent with that obtained in a previous assessment of the committee.

The committee concluded that despite the efforts made during the past decade to better understand the risk of radiation-induced thyroid cancer, there are still open questions that require continued follow-up of the health status of the affected populations as well as basic scientific research on the underlying processes of cancer development. Key scientific questions to be resolved through future research include the continuing need (a) to quantify the risk of thyroid cancer after exposure of children to 131I with doses to the thyroid below 500 mGy and for adults at even higher doses; (b) to investigate how long any radiation-induced risk of thyroid cancer will persist and how it will reduce with time; (c) to improve the understanding of the effects of confounding factors such as the influence of iodine deficiency on the risk of thyroid cancer; and (d) to identify biomarkers for radiation-induced thyroid cancer.

Full pdf version of the paper is available on the UNSCEAR website at www.unscear.org.






 

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