2. PREDICTION OF RADIATION-INDUCED THYROID CANCERS AMONG RESIDENTS
OF THE ORYOL OBLAST BASED ON THE ICRP MODELS

 

2.1. Model of radiation risks for thyroid cancer

 

 

Let us first define the terminology used here before describing the model for the radiation risk. A risk of disease (death) is understood as a probability m of developing disease by an individual during a given time interval. The risk or probability of developing disease depends on age, sex, profession, lifestyle, place of residence, time and other factors. By way of an example, let us consider a group of N persons not exposed to radiation, followed up for a year with a view to determine how many cases occurred in this group. If during a year E of persons (expected number of cases) developed a disease in this group, then the risk over a year will be estimated as

m = E/N (the risk m is called spontaneous or background). Given N = 100 thousand people, then m is to the spontaneous incidence rate per 100 thousand persons. If the group was exposed to radiation, then the number of cases will change and be equal to O (observed number of cases). In absolute terms, the effect of exposure is characterized by the excess absolute risk EAR=O-E. The relative significance of exposure is described by EER - excess relative risk.

ERR = EAR/E = (O-E) /E.                                                                                                           (2.1)

One of the key characteristics of the level of radiation-induced diseases is the attributive risk ATR (sometimes called the probability of causation  POC or simply PC ) defined as:

.                                                                                                                (2.2)

The attributive risk is the ratio of radiation-induced diseases to the number of all diseases. The attributive risk is often expressed in percent. The excess absolute risk EAR is calculated as:

,                                                                                                                 (2.3)

where m is the background incidence rate.

In this work the model of excess absolute risk BEIR-V [1] recommended by the ICRP is used for calculating thyroid cancer:

,                                                                                 (2.4)

where F is the efficiency factor (for isotopes 125I, 131I F = 1/3, for other iodine isotopes F = 1); the sex factor

S = 2/3 for males and S = 4/3 for females; the age factor G = 1 at g £ 18 and G = 0.5 at g > 18. The latent period is taken to be TL = 5 years.

The calculation of radiation-induced risks requires a knowledge of the background incidence rates. We use the average Russian incidence rates for 1996 [2] given in Table 2.1 for the background rates. For comparison the table contains general cancer incidence rates. As can be seen, thyroid cancer is a fairly rare disease. Thyroid cancer makes, on average, only a few percent of all cancers. This section describes a model of radiation risks of thyroid cancer. This disease occurs 2-3 times more frequently in females than in males.  In the subsequent chapter there is a projection of radiation risks of this disease for residents of the Oryol oblast.

 

 

 

 

 

 

Table 2.1. Background incidence and death rates in 1996.

 

Age interval

Incidence rate per 100 thousand persons

Death rate per 1 thousand from all causes

All Cancer

Thyroid Caner

males

females

males

females

males

females

0 -   4

12

11

0.00

0.00

4.45

3.33

5 -   9

11

8

0.04

0.09

0.61

0.37

10 - 14

10

8

0.13

0.40

0.58

0.33

15 - 19

16

14

0.25

0.91

2.14

0.80

20 - 24

20

24

0.30

2.0

4.12

0.98

25 - 29

23

37

0.59

2.8

4.96

1.22

30 - 34

36

67

0.74

4.5

6.57

1.57

35 - 39

64

114

0.89

5.9

8.56

2.24

40 - 44

136

194

1.2

8.7

12.0

3.32

45 - 49

289

314

2.3

11.6

16.8

5.09

50 - 54

543

421

3.4

13.0

23.3

7.46

55 - 59

804

480

2.8

10.8

30.5

10.5

60 - 64

1175

632

3.6

10.7

41.3

15.9

65 - 69

1539

755

3.8

10.1

55.6

24.5

70 - 74

1974

944

5.4

9.5

71.0

39.0

>74

1814

856

3.6

7.1

138.0

106.

 

2.2. Demographic data and doses for the population of the Oryol oblast

 

The depositions from the Chernobyl accident resulted in radioactive contamination of the territories of the Bryansk, Kaluga, Lipetsk, Oryol, Ryazan and Tula oblasts. Starting from the moment of contamination the population of these territories was exposed to internal and external irradiation from a mix of a variety of fission products and activation products. The main exposure source were radioisotopes of iodine, cesium, strontium and plutonium. So far, mean thyroid doses have been calculated for residents of the indicated oblasts. Table 2.2 includes data on accumulated doses and populations of the rayons of the Oryol oblast. As of 1986 the general population of the oblast was 887 thousand people (of them 190 thousand children and 697 thousand adults).

 

Table 2.2. Populations of rayons of the Oryol oblast and the accumulated doses averaged over each rayon.

 

Administrative name

Population

Accumulated thyroid dose (adults), mGy

Accumulated thyroid dose (children), mGy

children

adults

total

BOLKHOVSKY

5339

19586

24925

17.1

71.4

VERKHOVSKY

5479

20103

25582

8.59

28.4

GLAZUNOVSKY

3728

13677

17405

14.3

49.5

DMITROVSKY

4262

15636

19898

21

84.3

DOLZHANSKY

3480

12768

16248

5.29

16.3

ZALEGOSHENSKY

4156

15248

19404

9.03

31

ZNAMENSKY

1438

5277

6715

8.97

27.3

KOLPNYANSKY

5014

18395

23409

7.73

24.6

KORSAKOVSKY

1129

4143

5272

10.5

36.7

KRASNOZORENSKY

2101

7707

9808

12.4

35.4

KROMSKY

5524

20266

25790

14.8

54.6

LIVENSKY

18031

66153

84184

5.8

21

MALOARKHANGELSKY

3476

12755

16231

22

66.1

MTSENSKY

14778

54219

68997

8.05

32.2

NOVODEREVENKOVSKY

3276

12021

15297

9.08

29.4

NOVOSILSKY

2661

9764

12425

10.5

36.7

ORLOVSKY

82741

303552

386293

9

40.6

POKROVSKY

4443

16303

20746

10.3

31.3

SVERDLOVSKY

4317

15841

20158

14.4

48

SOSKOVSKY

2027

7437

9464

12.8

37

TROSNYANSKY

3140

11521

14661

15.9

48.9

URITSKY

4219

15481

19700

10.8

38.3

KHOTYNETSKY

2896

10627

13523

6.73

21.6

SHABLYKINSKY

2426

8903

11329

10.5

34

TOTAL OBLAST

190095

697393

887488

13

38.7

As a result of the intense rainfall on 28-29 April 1986 the territory of the Oryol oblast was contaminated by radioactivity. The rayons worst affected were Bolkhovsky, Dmitrovsky, Kromsky and Maloarkhangelsky rayons. The accumulated doses in children of these rayons exceed 50 mGy and the doses in adults are up to 22 mGy. Figures 1.15 and 1.16 of chapter 1 present the maps of the Oryol oblast with mean accumulated doses (iodine) in mGy in children and adults of the studied rayons, respectively.

In adults the accumulated thyroid doses are about 3-4 times lower than those in children. As a consequence, the risk of radiation-induced thyroid cancers is estimated to be 6-8 times higher in children than in adults (for children the factor G=1 for adults G=0.5).

 

2.3. Mathematical model for predicting radiation-induced risks

 

In a general case, the dynamics of cancer incidence in the population with uniform doses is described by a system of differential equations with partial derivatives written as:

         (2.5)

Here n is the number of healthy individuals, ni is the number of patients with the background i-th disease, dni is the number of patients with radiation-induced i-th disease, m is the background death rate, hi is the survival rate for the i-th disease, mi is the death rate from the i-th disease, Q accounts for birth rate and migration process. The background coefficients in equation (2.5) depend on time t and age u. The radiation-induced coefficients are a function of radiation dose and other parameters. If the number of diseases is k (1 £ I £ k), then the total number of equations equals to 2k + 1. Taking into account the dependence of the equation parameters on sex, the number of equations is doubled.

If the dose is not uniform over the population, for each dose interval a system of equations similar to system (2.5) is written. At the initial time moment the distribution of population by age n(u,o) is specified. Assuming the maximum age um, n(u,t)=0 at u > um (further in calculations um = 90 years).

Considering the uncertainty in the demographic and epidemiological data over the years since the accident and in projections, the prognostic model was based on the following assumptions. It is assumed that the accumulated radiation dose (iodine) was received only by the population living in the Oryol oblast in 1986. Thus, at a starting time moment the distribution n(u,s,0) of the population of each rayon by age u and sex s are considered to be known. As n(u,s,0) we take the age distribution of the population of the whole Oryol oblast normalized to the number of residents in a particular rayon. The changes in population as a result of background deaths from all causes at t>0 (with allowance for sex) is described by the equation:

,                                                  (2.6)

where m(u,s) is the death factor dependent only on age and sex. For brevity the sex parameter s is omitted. In the calculations the mean Russian death rates for 1996 shown in Table 2.1 are used.

To elucidate the influence of uncertainties in demographic data on prediction results we used “standardized” age distribution of population derived from the solution of the following equation:

                                                                                                    (2.7)

at the initial condition n(0)=n0. This distribution (for each sex) was normalized to the number of residents of a given rayon. Figure 2.1 presents both age distributions of the population for the whole Oryol oblast.

 

 

Fig. 2.1. Age distribution of the population of the Oryol oblast.

The solid line is the standardized distribution calculated with equation (2.7).

 

 

The incidence rate for the i-th background disease (number of cases per year) for a given age at the time moment t>TL was calculated as follows:

,                                                                                             (2.8)

where mi(u) is the coefficient of the i-th incidence rate. The incidence rates are shown in Table 2.1.

The incidence rate dni of radiation-induced diseases at a given age at the time moment t was calculated by the equation:

.                                                                    (2.9)

The cumulative number of background Ni and radiation-induced dNi diseases at the time moment t>TL is found as follows:

,                                                                                             (2.10)

.                                                                                        (2.11)

Corresponding lifetime risks are determined as Ni(um) and dNi(um) (i.e. the number of cases over the whole time of the cohort existence).

Equation (2.6) was solved by the numerical method with the step of time and age integration of 1 year. Accordingly, the number of background and radiation-induced cases were calculated for each year.

 

 

 

2.4. Information and reference software PUBRASS-2002

 

For calculating and predicting background and radiation-induced cancers in the residents of the Oryol oblast an information and reference software program PUBRASS-2002 (Public Risk ASSessment) has been developed. The size of this software is 1.8 Mb (execution module) and 0.5 Mb are the service files. The software is based on a mathematical model for predicting cancer risks described in the previous section. The software is written in the algorithmic language FORTRAN-90, the environment is Fortran Power Station 4.0. Figure 2.2 shows a part of the main window of the PUBRASS software with the main menu of 4 items (RISKS, CALCULATION RESULTS, INPUT DATA AND REFERENCES).

 

 

Fig. 2.2. Fragment of the display window of software PUBRASS-2002
with the main menu.

 

 

Each item of the main menu contains a pull-down menu, as shown in Fig. 2.3. When the first item of the menu is activated, a dialogue window shows up and a user can select a rayon of the Oryol oblast or the whole oblast, type of cancer, age distribution, sex and age interval at the time of exposure. Among other things, a button “REFERENCES” is available in the dialogue window for obtaining explanatory information. The dialogue window is shown in Fig. 2.4.

 

 

Fig. 2.3. Fragment of the main window of software PUBRASS-2002
with pull-down menus.

 

Fig. 2.4. Dialogue window for input of source data for calculating risks
for residents of the Oryol oblast.

 

 

Results of the calculation and the prediction of cancer risks are presented as time functions of risks and maps of the Oryol oblast with indication of cumulative risks (lifetime and current year values). Figure 2.5 presents a fragment of the screen display with the results of predicted incidence (number of persons) plotted. The plot is accompanied by brief information about the time dependence of risk. The second item of the menu “RESULTS OF CALCULATION” provides an opportunity to look at risks of interest. Activating the submenu “MAPPED RISKS” the user can select a map with risks of interest (background and radiation-induced). This window is shown in Fig. 2.6.

 

 

 

Fig. 2.5. Part of screen with the plot of predicted cases.

The third item of the main menu “INPUT DATA” makes it possible to look at demographic and epidemiological data used in calculations. Demographic data and information about accumulated doses (cesium and iodine) are also presented as maps.

The forth item of the main menu “REFERENCES” provides an opportunity to read a detailed description of software, its developer etc. It also contains a list of opened windows. Moreover, displayed information can be copied to the exchange buffer. For doing this, after activation of the item “HIGHLIGHT GRAPHICS” a part of the screen (plot of map) should be highlighted with a cursor. After copying the buffer content can be transferred to another document or graphic editor (the figure copied to the buffer has the format “bmp”).

 

 

 

Fig. 2.6. Dialogue window to select risk maps for residents of the Oryol oblast.

 

 

The software PUBRASS can be used for calculating individual risks. Suppose a background and radiation-induced risk need to be determined for a person who received a dose at the age of 30 years. In this case the age-at-exposure interval of 30-30 should be specified.

 

2.5. Prediction of radiation-induced thyroid cancers in the population
of the Oryol oblast

 

The section presents results of the calculations and prediction of background and radiation-induced thyroid cancers among residents of the Oryol oblast. All calculations were made using the software PUBRASS. We would like to stress again that all risks were calculated for people living in 1986 in the Oryol oblast. Those born after 1986 are not included in the projections. It was assumed that accumulated doses were received on the very same year.  This must be true for the short-lived iodine. Calculations were made separately for children (0-14 years old in 1986) and adults (15 years of age and older in 1986). The changes in the whole exposed population over time are shown in Fig. 2.7 by a solid curve, and the dash line shows the number of exposed people who were under age 15 in 1986. The general population declines with time almost linearly, while the number of people in the age group less than 15 years of age starts decreasing significantly only 30 years after the accident.

 

 

Fig. 2.7. Changes in the exposed population of the Oryol oblast over time.

The broken line - population under age 15 in 1986.

 

 

As was mentioned, thyroid cancer is a rare disease. The mean Russian incidence rate in 1996 is 3-4 cases a year per 100 thousand people. For children this rate is less than 0.5 cases each year are 100,000 children. In the Oryol oblast the same year the crude incidence rate was 14 cases a year per 100 thousand people.

 

2.5.1. Incidence in children

 

Figure 2.8 shows the changes with time of background (spontaneous) thyroid cancers in the population of the Oryol oblast in those less than 15 years of age (children) in 1986.

 

Fig. 2.8. Time changes in the number of background (spontaneous) thyroid cancers in the population
of the Oryol oblast among those less than 15 years of age (children) in 1986.

 

 

 

As can be seen from the figure, some 3 cases of background diseases are predicted to occur in the current year in this age group (this group includes people from 16 to 30 years old in the current year 2002). Since the group consists of children, for whom the background incidence is low, the number of cases in the first 10-15 years is low. Then the group ages and the incidence increases over time. Finally, the size of the group decreases rapidly as a result of mortality and the number of cases decreases accordingly.

Figure 2.9 shows the time dependence of the cumulative (accumulated from 1992) number of thyroid cancer cases among persons who were under 15 years of age in 1986. It can be seen that the total number of thyroid cancer cases over the whole time of the existence of this group will be 500 cases. In the same figure the cumulative number of radiation-induced thyroid cancers is shown by the broken line. The lifetime number of radiation-induced cancers in this group is predicted to be 37 cases. Accordingly, the lifetime attributive risk will be about 7%.

 

Fig. 2.9. The same as in Fig. 2.8, but for cumulative number of cases starting from
1992 (excluding the latent period of 5 years). The broken line shows the cumulative
number of radiation-induced thyroid cancers.

 

 

The attributive risk accounts for the ratio of the number of radiation-induced cancers to the entire number of cases as percentage and is independent of parameters such as background incidence rate and size of studied population group. The time dependence of the attributive risk for residents of the Oryol oblast (children) is presented in Fig. 2.10. In the first years after the latent period, as follows from the figure, high values of attributive risk above 40% are observed. This value suggests that about half of all cases are radiation induced. In 2002 the attributive risk is about 18% (of 5 cases one is radiation induced). Starting from 2015 the attributive risk varies between  3% and 6%.

Figures 2.11 and 2.12 show maps of the cumulative numbers of background and radiation-induced thyroid cancers among residents of the Oryol oblast as of 2002. In the Dmitrovsky rayon which was the worst contaminated (accumulated dose 84 mGy), according to estimates, as of 2002 there will be 0.3 background cases and 0.3 radiation-induced cases. In the most heavily populated Oryol rayon (dose of 40.6 mGy) the number of background cases is 5.8 and the number of calculated radiation-induced cases is 2.8.

 

 

 

 

 

 

 

 

Fig. 2.10. The time change of the attributive risk of thyroid cancer for residents
of the Oryol oblast under age 15 in 1986 (children).

 

 

 

 

Fig. 2.11. Map of cumulative background (spontaneous) thyroid cancer cases in the rayons
of the Oryol oblast as of 2002 (children).

 

 

Fig. 2.12. Map of cumulative radiation-induced thyroid cancers among children in the rayons
of the Oryol oblast as of 2002.

 

 

The cumulative attributive risks of thyroid cancer as of 2002 in persons under age 15 in 1986 appear to be quite high. On the average, in the Oryol oblast the attributive risk is about 30%. Thus, between 1992 and 2002 one out of every three cases is radiation-induced. Figure 2.13 presents a map with the values of cumulative attributive risk of thyroid cancer in the population of the Oryol oblast. For the Dmitrovsky rayon the attributive risk is as high as 50%, which means that of 5 out of 10 cases are radiation induced. The lowest attributive risk of thyroid cancer of 16% occurs in the residents of the Dolzhansky rayon (the accumulated dose is 16 mGy).

 

 

Fig. 2.13. Map of the cumulative attributive risk of thyroid cancer for the child populations
of different rayons of the Oryol oblast as of 2002.

2.5.2. Incidence of adults

 

As follows from the model, the radiation risk of thyroid cancer for adults exposed at the age older than 18 years this risk is half that in children. Since the accumulated doses in adults are lower than those in children (see table 2.2), the attributive risk for this group will be much lower.

Figure 2.14 shows time trend in the number of background cases of thyroid cancer among the Oryol oblast residents more than 14 years of age in 1986 (the size of this group was about 700 thousand people).

 

 

Fig. 2.14. Time trend in the number of background (spontaneous) cases of adult thyroid cancer among
the Oryol oblast residents more than 14 years of age in 1986.

 

 

In this age group, as can be seen from the figure, about 30 cases of background cancers are predicted (in the current year 2002 this group includes people between the ages of 29-90). Due to aging the size of the group is declining rapidly because of deaths (see Fig 2.7) and the number of cases is decreasing as well.

Figure 2.15 shows the time dependence of the cumulative (accumulated from 1992) number of thyroid cancers in people who were older than 15 years in 1986. As follows from the figure, over the entire time of the existence of this group the total number of thyroid cancers, by estimates, will be more than 1100 cases. The lifetime number of radiation-induced cancers in this group is predicted to be about 8 cases. Accordingly, the lifetime attributive risk is less than 1%.

The time dependence of attributive risk for the adult population of the Oryol oblast is shown in Fig. 2.16. As can be seen from the figure, the attributive risk does not exceed 1.5 %. In the current year 2002 the attributive risk is about 0.7% (of 100 cases less than one is radiation induced).


 

 

Fig. 2.15. The same as in Fig. 2.14, but for cumulative number of cases starting
from 1992 (excluding the latent period of 5 years).

 

 

 

 

Fig. 2.16. The time trend of the attributive risk of thyroid cancer among
the Oryol oblast residents who were more than 14 years of age in 1986.

 

 

Figures 2.17 and 2.18 show the cumulative number of background and radiation-induced thyroid cancers in the population of the Oryol oblast in 2002. In the Dmitrovky rayon where (the average accumulated dose is 22 mGy), according to the projection, there will be more than 7 background cases and 0.1 case of radiation-induced thyroid cancers by 2002. In the most heavily populated Oryol rayon the number of background cases is 141 cases and 1 case is radiation-induced.

 

 

 

 

 

 

 

Fig. 2.17. Map of the cumulative number of background (spontaneous) thyroid cancer cases among adults
in the rayons of the Oryol oblast as of 2002.

 

 

 

 

Fig. 2.18. Map of the cumulative number of radiation-induced thyroid cancers among adults
in the rayons of the Oryol oblast as of 2002.

 

 

 

 

Fig. 2.19. Map of the cumulative attributive risk of thyroid cancer for adult residents
of different rayons of the Oryol oblast as of 2002.

 

 

As of 2002 the cumulative attributive risk of thyroid cancer in those older 14 years in 1986 is low. On the average, in the Oryol oblast the attributive risk is 0.8%. Thus, in the period from 1992 to 2002, of 100 cases less than one is radiation-induced. Figure 2.19 presents a map with the values of cumulative attributive risk of thyroid cancer in the populations of different rayons of the Oryol oblast. In the Dmitrovsky rayon the attributive risk is 1.7% - of 100 cases 2 are radiation induced. The lowest attributive risk of thyroid cancer of 0.4% is observed in residents of the Dolzhansky rayon.

 

Conclusion

 

In this chapter, based on the model of radiation risks BEIR-V and software PUBASS the background and radiation-induced incidence of thyroid cancer in children and adults of the Oryol oblast is predicted. The prognostic estimates lead us to make the following conclusions.

The projection shows that between 1992 and 2002 the percentage of radiation-induced cases (cumulative attributive risk) in children of the oblast, on average, will be 30% (each third case is radiation-induced). The highest attributive risk of about 50% has been derived for the population of the Dmitrovsky rayon (accumulated dose 84 mGy). During the indicated time period a total of 13 background cases of thyroid cancer is predicted to have occurred among those under age 15 (children ) in 1986 and 6.4 cases are radiation induced.

For the adult population (age more than 14 years in 1986) of the Oryol oblast the attributive risk in 1992-2002 was estimated to be 0.8% or less than one in 100 cases. The highest attributive risk occurs in the Dmitrovsky rayon - 1.7%.

References

 

1.     Health effects of exposure to low levels of ionizing radiation (BEIR V). - Washington, D.C.: National Academy Press, 1990.

2.     Trapeznikov N.N., Aksel E.M. Incidence of malignant neoplasms and deaths from them in the population of CIS countries in 1996. - Moscow: ONTs RAMS, 1997. - 302 p.