Assessment of Chernobyl Malignant Neoplasms In

9

ASSESSMENT OF CHERNOBYL MALIGNANT NEOPLASMS IN

EUROPEAN COUNTRIES

M.V. Malko

Joint Institute of Power and Nuclear Research – Sosny

National Academy of Sciences of Belarus

Krasin Str. 99, Minsk, 220109, Belarus.

Теl.: +375 (17) 299 45 89, Fax: +375 (17) 299 43 84

E-mail: ,

Key words: Chernobyl, radiation risk, malignant neoplasms, prognoses, European countries

Abstract. The present report estimates the expected increase in the incidence and mortality from malignant neoplasms in 1986-2056 in European countries resulting from the accident at the Chernobyl nuclear power plant in 1986. The estimation of the number of excess incident cancers is based on the absolute excess radiation risk determined for the population of Belarus. For all European countries combined, 92,600 excess thyroid cancers (90% CI from 44,000 to 141,200 cases), 130,400 solid cancers other than thyroid and non-melanoma skin cancers (42,900 to 217,900), and 12,900 leukaemia cases (2,800 to 23,000) are predicted during 1986-1956. This corresponds to a time-averaged relative increase of RR=1.050 for thyroid cancers (90% CI from 1.024 to 1.077), RR=1.001 for solid cancers other than thyroid and non-melanoma skin cancers (90% CI from 1.000 to 1.001), and RR=1.003 for leukaemia (90% CI from 1.001 to 1.005).

Approximately two thirds of all additional cancers will occur in Belarus, Ukraine, and Russia. Belarus will account for about 20% of all additional solid cancer and leukaemia cases. In Belarus alone, approximately 31,400 additional incident thyroid cancers (90% CI from 15,400 to 47,500), 28,300 solid cancers other than thyroid and non-melanoma skin cancers (90% CI from 11,800 to 44,800), and approximately 2,800 additional leukaemia cases (90% CI from 1,000 to 4,600) are expected. The corresponding time-averaged relative risks are RR=2.625 for thyroid cancers (90% CI from 1.797 to 3.460), RR=1.015 for solid cancers other than thyroid and non-melanoma skin cancers (90% CI from 1.008 to 1.023), and RR=1.047 for leukaemia (90% CI from 1.017 to 1.078).

The predicted numbers of excess fatal cancers in Europe, 1986-2056, are as follows: 26,300 thyroid cancers (90%CI from 12,500 to 40,100), 81,300 solid cancers other than thyroid and non-melanoma skin cancers (90%CI from 23,000 to 139,500), and 9,100 leukaemia cases (90%CI from 1,480 to 16,700). Again, approximately two thirds of the additional fatal cancers will occur in Belarus, Ukraine and the Russian Federation.

Introduction.

The Chernobyl accident caused radioactive contamination of many countries of the Northern Hemisphere including the USA, Japan, China, and India [1, 2]. Millions of people were exposed to radiation from the Chernobyl fallout. According to the present knowledge there is no threshold for the carcinogenic effect of ionising radiation [3]. Therefore any additional irradiation will induce additional cancers and leukaemia cases in irradiated populations, not only in the most contaminated regions of the former Soviet Union but all around the Northern Hemisphere. The first prognosis of the possible number of excess cancer cases in the Northern Hemisphere was already performed some months after the Chernobyl accident by Gofman [4]. It yielded 475,000 additional fatal solid cancers and 19,500 leukaemia cases for an infinite time period. A later study [5] yielded 17,400 fatal cancer and leukaemia cases within 50 years following the accident. Also subsequent studies [6-10] show large differences in the estimated numbers of excess cancers [9, 10].

The objective of this study is to analyse the reasons for the disagreements in the existing estimates and to present a more reliable prognosis based on the effects observed in Belarus. The reason for choosing Belarus as a reference country is that Belarus is the country with the highest average fallout from Chernobyl, and that Belarus, other than Ukraine and Russia, has a cancer register which was already established long before 1986.

Materials and methods

Methods

Data on deposition of Chernobyl radionuclides in affected countries of Europe, population numbers and territories of this countries, data on thyroid doses caused by the Chernobyl accident as well as other related information given in [1, 2] were used to forecast possible cancers and leukemias.

The estimation of radiation-induced malignant neoplasms was carried out in the present report for the period 1986-2056 by using the method developed in reports [9, 10]. This method is based on transfer of radiation risks established for the Belarusian population on other affected populations. The additive model of radiation risk can by used for this purpose. According to reports [9, 10] the number of radiation-induced malignant neoplasms in ith site in the irradiated population X in the period of time up to T years after irradiation, can be estimated by the following expression:

/ (1)

were and are collective doses irradiation of the population X and the population of Belarus in the period T, and -numbers of additional cancers in the site in these populations in this period.

In case of the absence of data on collective doses of irradiation the approximate value of can be assessed by using the relation:

. / (2)

Where and are time-averaged population densities of the country X and Belarus, and - total activities of the reference radionuclide deposited in the country X and Belarus.

Expression (1) was used in the present report for an assessment of radiation-induced thyroid cancers in affected countries of Europe. Numbers of radiation-induced solid cancers other than thyroid and non-melanoma skin cancers as well as numbers of additional leukemias were estimated on the basis of the expression (2). The isotope 137Cs is used as the reference radionuclide by this estimation. In both cases Belarus was chosen as a reference country. This means that the assessment of radiation-induced cancers in affected countries of Europe carried out in the present report is based simply on transferring of radiation risks of the Belarusian population on other populations of Europe.

Numbers of additional thyroid cancers, solid cancers other than thyroid and non-melanoma skin cancers as well as number of additional leukemias that are expected in Belarus during 1986-2056 were taken from reports [9, 10]. They are: 31,400 additional thyroid cancers, 28,300 additional solid cancers other than thyroid and non-melanoma skin cancers and 2,800 additional leukemia cases.

Time-averaged relative risks of additional cancers and confidence intervals of relative risks were also assessed in the report.

The relative risk was estimated as the ratio of the sum of additional cancers and spontaneous cancers to spontaneous cancers:

/ (3)

Here is the time-averaged relative risk of additional cancers in the population X, is the number of additional cancers and is the number of spontaneous cancers manifested in the period 1986-2056.in this population.

Spontaneous cancers used for estimation of relative risk were assessed in the report on the basis of the incidence of cancers observed in 2002 [11]. It was assumed that the same incidence as in 2002 was in any year of the period 1986-2056. This assumption causes some additional errors in estimation of relative risks but they are insignificant in comparison with other errors of the relative risk assessment.

According to reports [9, 10] the accuracy of estimations of additional thyroid cancers, solid cancers other than non-melanoma skin cancers as well as additional leukemias that can be expected in Belarus in 1986-2008 might be approximately 50%. This fact was considered in the present report by estimation of confidence intervals of relative risk that were evaluated on the basis of the Poisson distribution. The Poisson distribution was used because the possible variation of expected cancers was not considered in the present report.

In order to take into account the inaccuracy in estimation of additional cancers in Belarus chosen as the reference country by assessment of additional cancers in other affected countries the following simplified procedure was used. In case of each cancers and each country low, middle and high numbers of additional cancers were estimated by using expression (1) or expression (2). The middle numbers of additional cancers was evaluated on the basis of respective middle numbers of additional cancers expected in the period 1986-2056 in Belarus. For example, in case of solid cancers other than thyroid and non-melanoma skin cancers this retrospective middle number of additional cancers in this period in Belarus or is equal to 28,300 cases [9, 10]. The low and high numbers of additional solid cancers other than thyroid and non-melanoma skin cancers in the country X were then calculated by using as the numbers of additional solid cancers other than thyroid and non-melanoma skin cancers in Belarus values estimated with relations:

, / (4)
. / (5)

Inserting the values, and into the expression (2) together with values of populations densities of the country X and Belarus as well as amounts of the isotope 137Cs deposited in the country X and Belarus gives middle, low and high estimations of additional solid cancers other than thyroid and non-melanoma skin cancers expected for the country X in the period 1986-2056 respectively. They are:, and respectively/. The middle number of additional cancers is then used for an assessment of the relative risk. The low, , and the high, , numbers of additional solid cancers other than thyroid and non-melanoma cancers assessed for the country X are used for the assessment of low and high confidence intervals of the relative risk:

, / (6)
. / (7)

Here , - are low and upper limits of the relative risk of additional solid cancers expected in the country X in 1986-2056 assessed on the basis of the Poisson distribution for the low number () of additional solid cancers in this country. and are the lower and upper limits of the relative risk of additional solid cancers in the country X estimated with the high number () of additional cancers in this country respectively.

The values and are used in the present report as the lower and upper limits of the time-averaged relative risk of additional cancers that can manifest in the country X in 1986-2056 as a result of the accident at the Chernobyl nuclear power plant. These values are given in tables of the present report containing middle numbers of additional thyroid cancers, solid cancers other than thyroid and non-melanoma cancers as well as additional leukemias assessed in the present report.

Thyroid doses

Collective thyroid doses used in the present report for an assessment of additional thyroid cancers in affected countries of Europe were estimated on the basis of data given in UNSCEAR 1988 Report [1]. It presents mean thyroid doses for infants and adults (see Table 17 on pages 361-362 [1]). They were calculated by experts of the UNSCEAR by using thyroid dose equivalents and integrated measured or inferred concentrations of 131I in food consumed in the period from 26 April 1986 to 30 April 1987. Contribution of other pathways to thyroid doses such as inhalation or irradiation of the thyroid gland from other radionuclides was also taken into account by the UNSCEAR experts. For estimation of integrated measured or inferred concentrations of 131I experts of the UNSCEAR used data on food contamination supplied by national organizations of the affected countries.

A comparison of thyroid doses estimated by expert of the UNSCEAR with data established by experts of the affected countries shows quite good agreement in them that is an indication of validity of the method used by UNSCEAR experts.

Mean thyroid doses and other related values for infants and adults of Belarus (Region 1 of the USSR), for infants and adults of high contaminated areas of Ukraine (Regions 2 of the USSR) and low contaminated areas of Ukraine (Regions 3 of the USSR) as well as for infants and adults of Russia (Regions 4 of the USSR) were also estimated in the report [1]. However, our analysis demonstrated that these data of the UNSCEAR experts [1], especially data for Belarus, demonstrate significant incorrectness that is outside of the range of incorrectness of data estimated for other affected countries of the world. This does not allow using estimates of UNSCEAR experts carried out for Belarus as well as for Russia and Ukraine for assessment of additional thyroid cancers. A simplified method of the collective thyroid dose of the Belarusian population was developed in the present report. It is described below.

Thyroid cancer in children is a very rare disease. According to data of Demidchic et al. [12], only 21 cases of thyroid cancer were registered among Belarusian children (less than 15 years old at the time of diagnosis) in 1966-1985 or one case annually. The observed number of thyroid cancers in children corresponds to the number of person-years accumulated in the period 1966-1985 that is equal to 4.74·107 PY. The last value was assessed on the basis of demographic data given in handbooks [13, 14]. Dividing the number of thyroid cancers observed among children by this number of person-years gives the incidence rate of this cancer in children of Belarus equal to 0.443 cases per million persons-years. Table 1 shows that this value is comparable with incidence rates of thyroid cancers in children of other European countries.

Table 1. Time-averaged crude and standardized (World standard) rates of the incidence of thyroid cancers in children.

Country / Time Period / Crude
rate, 106 a-1 / Standardized rate, 106 a-1 / Sources
UK, England and Wales / 1981-1990 / 0.6 / 0.5 / [15]
UK, England and Scottish
Cancer Register / 1981-1990 / 0.6 / 0.5 / [15]
Poland / 1980-1989 / 0.5 / 0.5 / [15]
Slovakia / 1980-1989 / 0.7 / 0.6 / [15]
Hungary / 1985-1990 / 0.3 / 0.3 / [15]
Ukraine / Before Chernobyl accident / 0.5 / - / [16]
Belarus / 1966-1985 / 0.44 / - / This
report

A very good agreement of the incidence rate of thyroid cancers in children of Belarus established for the time before the accident at the Chernobyl NPP with corresponding incidence rates of childhood thyroid cancers observed in other countries of Europe before and after the accident allows us to use the value 0.443 cases per million person-years as the incidence rate of spontaneous thyroid cancers in Belarusian children. According to Demidchic et al. [17] 674 thyroid cancers were registered in children of Belarus (younger than 15 years at diagnosis) in 1990-2000. Assessment by using the spontaneous incidence rate of thyroid cancers equal to 0.443 cases per million person-years allows us to estimate the number of spontaneous thyroid cancers in children of Belarus in 1990-2000 equal to 11 cases. Subtraction of the last value from the total number of thyroid cancers (674 cases) registered in the children of Belarus in this period gives 663 additional cases of childhood thyroid cancers in Belarus for 1990-2000.