Chapter 5

 

RISK OF RADIOGENIC THYROID CANCER IN THE POPULATION OF THE BRYANSK AND ORYOL REGIONS OF RUSSIA AFTER THE CHERNOBYL ACCIDENT (1991-1998 DATA)

 

 

Introduction

 

After the Chernobyl accident a definite increase of thyroid cancer incidence occurred in the contaminated areas of Russia, Ukraine and Belarus. One of the potential causes of the increase is exposure of the thyroid gland to incorporated iodine-131 (131I). This is a particular concern for those who were children and adolescents at the time of the exposure, as the risk of developing thyroid cancer (as well as the dose itself) is strongly dependent on age at time of exposure.

There are a large number of publications on radiation-induced thyroid cancer after the Chernobyl accident. Results of these studies have been described in Radiation and Thyroid Cancer (1999). This work focuses primarily on the descriptive approaches that are limited by analysis of incidence rate and standardized incidence ratio (SIR).

Among the aforementioned studies devoted to analysis of thyroid cancer incidence after the Chernobyl accident we refer, for example, to Ivanov et al., (1999), Heidenreich et al., (1999), Jacob et al., (1999), and Methodology for reconstruction of thyroid doses (2000).

 

Material and methods

 

Study area and population

 

The basic demographic characteristics of the studied population are presented in Table 1.

Gender and age distribution in the territorial units were calculated under the assumption that this distribution is identical with the distribution of a rayon, in which this population point is located. A total of 3,085 and 2,916 population points were considered in the Bryansk and Oryol regions, respectively, the population of which accounts for 99.9% of the whole population of these regions.

 

Thyroid doses in the Bryansk region

 

The personified mean thyroid doses for the population of the Oryol region were calculated based on the Methodology for reconstruction of thyroid dose from iodine radioisotopes in residents of the Russian Federation exposed to radioactive contamination as a result of the Chernobyl accident in 1986, the latest revision of which was issued on May 31, 2000.

Results of the calculation are shown in Figures 1 and 2 as maps of the thyroid dose distribution among children and adolescents in the Bryansk and Oryol regions.

The thyroid dose was determined using the dependence of dose on age at exposure and residence address at exposure time (the name of the population point). The basic dosimetric data for the studied cohort is shown in Table 1.

 

Figure 1. Geographical pattern of thyroid doses for children and adolescents
aged 0-17 years at exposure (
Bryansk region).

 

 

Figure 2. Geographical pattern of thyroid doses for children and adolescents
aged 0-17 years at exposure (Oryol region).

 

 

 

Registration of thyroid cancer cases

 

These are official data of the oncological dispensaries in the Bryansk and Oryol regions. They had the responsibility for registration of cancer patients in accordance with regulations of the Ministry of Health of Russia.

Information used in the analysis of dose-response includes date of birth, gender, address of residence at exposure, date of diagnosis or surgery and thyroid dose from incorporated iodine radioisotopes.

 

Background incidence rate of thyroid cancer

 

In Russia annual medical check-ups were started shortly after the accident to survey children for thyroid disease. Examination included palpation, ultrasound examination and thyroid hormone testing.

In the calculations dynamics of thyroid cancer spontaneous rate of Russia from 1989 to 1998 was used.

Risk analysis

 

As the background rate varies with time, the non-stationary Poisson series of events to model incidence was used in the risk analysis.

The likelihood function for the model under consideration is:

,

n is the number of cases; N is the number of healthy persons; parameter li for a person i is a function of age at exposure (i), time since exposure (ti) and absorbed dose (di); ti is the time interval from the accident time to detection of the case, for healthy persons is time interval from the accident time to the end of 1998.

Excess relative risk per 1 Gy (b) was determined under assumption of linear dependence of the thyroid cancer incidence rate on dose:

,

where is the spontaneous incidence rate of the thyroid cancer in Russia for attained age (e+t) at the time t, for person i; f is the factor taking into account the difference between incidence in the considered region and Russia as a whole (function of sex). This difference can be attributed to both the differences in screening effect levels for the population in general and to the difference in actual incidence levels in the study area. It is assumed that the shape of the incidence age distribution in Russia and in the region under study is identical.

di is the absorbed dose in the thyroid gland for the i-th person; ERR1Sv is excess relative risk per unit dose. This value is a function of age at exposure.

The obtained model parameters ERR1Sv and f are used for prediction of the cumulative mortality in the next 5-10 years.

 

Results

 

Results of estimation of the SIR for thyroid cancers in the studied groups are presented in the Figures 3 and 4.

Figure 3. The dynamics of SIR for males 0-29 years old.

 

Figure 4. The dynamics of SIR for females 0-29 years old.

 

 

The above distributions show a common trend. Up to 1986 the SIR is close to 1, i.e. the incidence rate in these regions is close to the general Russian value. In 1986 the SIR increased and remained nearly constant until the spring of 1990. The time interval from 1981 to 1990 is the period of appearance of spontaneous cancers (with allowance for the latent period of 5 years from 1986 to 1990). The increase in incidence relative to the control in this time interval can be explained only by the screening effect, better registration of diseases due to wider coverage by specialized medical examination, higher attendance of patients and higher quality of check-ups.

Increase in the incidence rate in the age group 0-29 years after 1990 can be explained by two reasons, the above mentioned screening effect and the induction of radiogenic cancers. As was pointed out above, this age group has a higher radiation sensitivity and thyroid doses from incorporated 131I.

To answer the question about the influence of the radiation factor on thyroid incidence growth, one should first analyze the dose-effect dependence. For solving this problem, knowledge of the dose distribution among exposed persons is required. To reconstruct radiation doses, results of directly measured activities of radionuclides are required. These data are often unavailable. However, some characteristics of thyroid exposure allow a better understanding of the dose effect.

The risk of induction of radiogenic cancers at the same dose and dose rate is known to depend on age at exposure. For malignant neoplasms of most localization, the decrease in age at exposure leads to an increase in the risk of cancer. This equally applies to radiogenic thyroid cancer. The above risk dependence will be better defined in case of thyroid exposure to incorporated 131I, as, in this case, the thyroid exposure dose will depend on the age at exposure. So, induction of radiogenic cancers should modify the shape of age distribution of cancers.

Figures 5 and 6 presents the distribution function of the observed and spontaneous cases in the population under consideration depending on age at exposure.

The age structure has drastically changed for cases among children and adolescents in the Bryansk region. The curve shape is indicative of a considerable increase in incidence for younger ages, as compared to Russia in general. These features of the distribution may be useful for the analysis of thyroid cancer incidence, because absorbed doses are not considered for this purpose. The distributions presented in the figures confirm the conclusions of the regression analysis that there is statistically significant risk for the Bryansk region, but no statistically significant risk for the Oryol region.

 

Figure 5. The relationship of the distribution function of spontaneous and observed cases
of thyroid cancer and age at exposure (the
Bryansk region).

 

Figure 6. The relationship of the distribution function of spontaneous and observed cases
of thyroid cancer and age at exposure (the Oryol region).

 

 

Table 1 illustrates the values of the risk coefficients (ERR1Gy) and f coefficient taking into account the difference between incidence in the considered region and Russia as a whole (function of sex) with 95% confidence intervals. The risks for children and adolescents of the Bryansk region appear to be significant.

 

Table 1

The results of risk estimation for the population under consideration (age at exposure 0-17)

 

Region

Bryansk

Oryol

Sex

Both

Both

Size

374 447

207 592

Mean dose (Gy) healthy/cases

0.071/0.15

0.013/0.009

ERR/Gy (95% CI)

11.8 (7.2, 16.6)

6.4 (-20.2, 30.1)

Coefficient f (95% CI)

3.5 (2.8, 4.2)

5.3 (4.0, 6.6)

 

 

The spontaneous incidence rate in the considered period differs from the incidence of Russia as a whole by 3-5 times. As stated above, the difference is conditioned by regional variation in incidence and screening effect of thyroid cancer cases.

The results of thyroid cancer prediction are presented in Figures 7 and 8. The result of prediction using the parameters of the model is in agreement with the observed incidence.

Figure 7. Prediction of thyroid cancer incidence among children and adolescents
after the
Chernobyl accident in the Bryansk region.

 

Figure 8. Prediction of thyroid cancer incidence among children and adolescents
after the
Chernobyl accident in the Oryol region.

 

 

 

Conclusion

 

The excess relative risk ERR1Gy per unit dose 1 Gy among children and adolescents at the Chernobyl accident (age 0-17 years) of the Bryansk region was found for the observation period 1991-1998 to be 11.8 with 95% CI (7.2, 16.6) for both sexes. The risk for the children and adolescents of the Oryol region has not been confirmed. The spontaneous incidence rate in the region under consideration is about 3.5-5 times higher than in Russia as a whole. This excess is attributed to the differences in registration of diseases and regional differences in spontaneous level.

 

 

 

References

 

Heidenreich W.F., Kenigsberg Y., Jacob P., Buglova E., Gulko G., Paretzke H.G., Demidchik E.P., Golovneva A. Time trends of thyroid cancer incidence in Belarus after Chernobyl accident//Radiat. Res. - 1999. - V. 151. - P. 617-625.

Ivanov V.K., Gorski A.I., Pitkevitch V.A., Tsyb A.F. Risk of radiogenic thyroid cancer in Russia following the Chernobyl accident Radiation and thyroid cancer. In: Thomas G., Karaoglou A., Willliams E.D., editors. Radiation and Thyroid Cancer. Proceeding of an International Seminar on Radiation and Thyroid Cancer. - Brussels-Luxembourg: World Scientific Publishing, 1999. - P. 89-96.

Jacob P., Kenigsberg Y., Zvonova I., Gulko G., Buglova E., Heidenreich W.F., Golovneva A., Bratilova A.A., Drozdovitch V., Kruk J., Pochtennaja G.T., Balonov M., Demidchik E.P., Paretzke H.G. Childhood exposure due to the Chernobyl accident and thyroid cancer risk in contaminated areas of Belarus and Russia//British J. of Cancer. - 1999. - V. 80, N 9. - P. 1461-1469.

Methodology for reconstruction of thyroid doses from iodine radioisotopes in residents of the Russian Federation exposed to radioactive contamination as a result of the Chernobyl accident in 1986 (in Russian). Guidelines MU-2.6.1-00b, 2000.

Radiation and Thyroid Cancer. Thomas G., Karaoglou A., Willliams E.D., editors. Proceeding of an International Seminar on Radiation and Thyroid Cancer. - Brussels-Luxembourg: World Scientific Publishing, 1999.