2.
PREDICTION OF RADIATIONINDUCED 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=OE.
The relative significance of exposure is described by EER  excess
relative risk.
ERR = EAR/E = (OE) /E. (2.1)
One of the key characteristics of the level of
radiationinduced 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 radiationinduced
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 BEIRV
[1] recommended by the ICRP is used for calculating thyroid cancer:
_{}, (2.4)
where F is the efficiency factor (for isotopes ^{125}I,
^{131}I 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 Ł 18 and G = 0.5 at g > 18. The latent period is
taken to be T_{L }= 5 years.
The calculation of radiationinduced 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 23 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 2829 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 34
times lower than those in children. As a consequence, the risk of
radiationinduced thyroid cancers is estimated to be 68 times higher in
children than in adults (for children the factor G=1 for adults G=0.5).
2.3.
Mathematical model for predicting radiationinduced 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, n_{i} is the number of patients with the
background ith disease, dn_{i} is the number of patients with
radiationinduced ith disease, m is the background death rate, h_{i} is the
survival rate for the ith disease, m_{i} is the death rate from the ith
disease, Q accounts for birth rate and migration process. The
background coefficients in equation (2.5) depend on time t and
age u. The radiationinduced coefficients are a function of
radiation dose and other parameters. If the number of diseases is k
(1 Ł I Ł 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 u_{m}, n(u,t)=0
at u > u_{m} (further in calculations u_{m }=
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)=n_{0}. 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 ith
background disease (number of cases per year) for a given age at the time
moment t>T_{L} was calculated as follows:
_{}, (2.8)
where m_{i}(u) is the coefficient of the ith
incidence rate. The incidence rates are shown in Table 2.1.
The incidence rate dn_{i} of radiationinduced diseases
at a given age at the time moment t was calculated by the
equation:
_{}. (2.9)
The cumulative number of background N_{i}
and radiationinduced dN_{i} diseases at the time moment t>T_{L} is found
as follows:
_{}, (2.10)
_{}. (2.11)
Corresponding lifetime risks are determined as N_{i}(u_{m})
and dN_{i}(u_{m}) (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 radiationinduced cases were calculated for each year.
2.4.
Information and reference software PUBRASS2002
For calculating and predicting background and
radiationinduced cancers in the residents of the Oryol oblast an information
and reference software program PUBRASS2002 (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 FORTRAN90, 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 PUBRASS2002
with the main menu.
Each item of the main menu contains a pulldown 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 PUBRASS2002
with pulldown 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 radiationinduced). 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 radiationinduced risk need to be
determined for a person who received a dose at the age of 30 years. In this
case the ageatexposure interval of 3030 should be specified.
2.5.
Prediction of radiationinduced thyroid cancers in the population
of the Oryol oblast
The section presents results of the calculations and
prediction of background and radiationinduced 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
shortlived iodine. Calculations were made separately for children (014 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 34 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 1015 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 radiationinduced thyroid cancers is
shown by the broken line. The lifetime number of radiationinduced 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 radiationinduced thyroid cancers.
The attributive risk accounts for the ratio of the
number of radiationinduced 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 radiationinduced 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 radiationinduced 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 radiationinduced 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 radiationinduced 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 radiationinduced. 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 2990). 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
radiationinduced 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 radiationinduced 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 radiationinduced thyroid cancers by 2002. In the most
heavily populated Oryol rayon the number of background cases is 141 cases and 1
case is radiationinduced.
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 radiationinduced 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 radiationinduced. 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
BEIRV and software PUBASS the background and radiationinduced 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 radiationinduced cases (cumulative attributive risk) in children
of the oblast, on average, will be 30% (each third case is radiationinduced).
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 19922002 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). 
2. Trapeznikov N.N., Aksel E.M. Incidence
of malignant neoplasms and deaths from them in the population of CIS countries
in 1996. 