RISK OF RADIOGENIC THYROID CANCER IN THE POPULATION OF
There are a large number of publications on radiation-induced thyroid
cancer after the
aforementioned studies devoted to analysis of thyroid cancer incidence after
Material and methods
Study area and population
The basic demographic characteristics of the studied population are presented in Table 1.
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
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
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 (
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
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 the calculations dynamics of thyroid cancer spontaneous rate of
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 (еi), 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
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 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.
age structure has drastically changed for cases among children and adolescents
Figure 5. The relationship of the distribution
function of spontaneous and observed cases
of thyroid cancer and age at exposure (the
Figure 6. The relationship of the distribution function
of spontaneous and observed cases
of thyroid cancer and age at exposure (the Oryol region).
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
The results of risk estimation for the population under consideration (age at exposure 0-17)
Mean dose (Gy) healthy/cases
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)
spontaneous incidence rate in the considered period differs from the incidence
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
Figure 8. Prediction of thyroid cancer
incidence among children and adolescents
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
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.
for reconstruction of thyroid doses from iodine radioisotopes in
residents of the
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.