Complex experimental research on internal tooth dosimetry for the Techa River region: A model for...

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Journal Radiation Measurements

Article ID RM_250

Title COMPLEX EXPERIMENTAL RESEARCH ON INTERNAL TOOTH

DOSIMETRY FOR TECHA RIVER REGION: VALIDATION OF

MODEL FOR 90SR IN THE TEETH FORMED BY TIME OF INTAKES

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COMPLEX EXPERIMENTAL RESEARCH ON INTERNAL TOOTH DOSIMETRY FOR THE TECHA RIVER REGION: A

MODEL FOR 90SR ACCUMULATION IN HUMAN TEETH FORMED BY TIME OF INTAKES

D.D. Tikunov*1, A.I. Ivannikov*, E.A. Shishkina†, D.V. Petin*, N.B. Borysheva*, S. Orlenko*, M. Nalapko*, V.A.

Shved†, V.G. Skvortsov*, V.F. Stepanenko*

* Medical Radiological Research Center, Korolyov str., 4, Obninsk 249036, Russia

† Urals Research Center for Radiation Medicine, Medgorodok, Chelyabinsk 454076, Russia

Abstract – Samples of calcified tooth tissues (enamel, root and crown dentine) collected from the Techa riverside

population exposed to radiation caused by radioactive releases from the nuclear weapon plant in South Ural were

investigated. Accumulated absorbed dose in the samples was measured using the EPR-spectroscopy method. Beta

activity of the samples containing radioactive 90Sr was measured by the method of low background anti-coincidence

thin scintillating detection. High correlation between absorbed dose and beta activity was observed for enamel and

root dentin but not for crown dentin. Otherwise, poor correlation was observed between absorbed doses as well as

between beta activities for different tooth tissues of the same tooth. The results of dose measurement by EPR-

spectroscopy are analysed with the use of Monte Carlo simulation of dose formation due to 90Sr incorporated in

tooth tissues taking into account biological elimination of 90Sr. Influence of 90Sr distribution in the tooth body on

absorbed dose is discussed.

Keywords: internal dosimetry, external dosimetry, EPR/ESR, tooth enamel.

INTRODUCTION

Vast territories of South Ural region were contaminated as a result of radioactive wastes released by the

Mayak Production Association into the Techa River in 1949-1956. During this period 76⋅106 m3 of liquid wastes

with total activity of about 1017 Bq were discharged into the river. The radioecological conditions and dosimetric

investigations in this region are described elsewhere (Marey, 1959; Stepanenko et al., 1990; Degteva et al., 1994;

Vorobiova et al., 1999; Tolstykh et al., 2003).

About 98% of total activity was released in the period from March 1950 to October 1951. The average

daily release during this period was 1.6 1014 Bq with the following contribution of different radionuclides to the total

1 Corresponding author. Tel/Fax: (095) 956-14-40, e-mail: [email protected]

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activity: 89Sr – 8.8%, 90Sr – 11.6%, 137Cs – 12.2%, 95Zr-95Nb – 13.6%, 103,106Ru – 25.9%, and rare-earth elements –

26.8% (Vorobiova et al., 1999). Exposure of the population (about 30,000 persons in the Techa riverside) was

caused mainly by long-lived isotopes of 90Sr (half-life 29 years) and 137Cs (half-life 30 years). Internal irradiation

was resulted primarily from bone seeking beta-emitting 90Sr. External irradiation was caused primarily by 137Cs

(Degteva et al., 1994). External doses estimated using the Techa River Dosimetry System 2000 (TRDS-2000)

(Degteva et al, 2000) expected to vary in the range from 0 to 3 Gy depending on residence history of individuals.

The maximal doses of external exposure were estimated for residents of the Upper Techa region who lived in

immediate proximity to the shoreline.

Estimated external doses could be validated by the method of EPR-dosimetry. This method gives

possibility to determine a dose accumulated in calcified tooth tissues (IAEA 2002). For the population of the Techa

riverside, a dose accumulated in tooth tissues is a result of both external and internal exposure. Therefore, in order to

obtain external dose from EPR dosimetry data, the total accumulated dose detected by EPR should be corrected on

internal dose contribution taking into account the presence of 90Sr in the tooth tissues.

Several pilot studies of the teeth collected from population of the Techa riverside were aimed to find out

the relationship between 90Sr content in calcified tissues and cumulative dose (Tolstykh et al., 2000; Romanyukha et

al., 1996, 2001; Wieser et al., 1996; Göksu et al., 2002). All these studies demonstrate significant influence of 90Sr

on cumulative dose in enamel that cannot be ignored in the framework of the Techa River dosimetry. In Tolstykh et

al. (2000) internal dose formation was investigated for enamel of teeth that crown formation had been completed by

the time of radioactive release in assumptions that the amount of 90Sr incorporated in the enamel is negligible and

the main source of internal dose should be the crown dentin; 90Sr was assumed to be uniformly distributed in the

whole dentin body (root and crown dentine). For Metlino residents (Upper Techa) the results of individual

measurements using EPR were found to be consistent with the doses calculated according to TRDS-2000 approach

(Degteva et al., in press). In Göksu et al. (2000) accumulated dose in tooth tissues was compared with 90Sr content in

the same tissues. High correlation was observed for absorbed dose in enamel, root and crown dentin with beta dose

rate from incorporated 90Sr. No correlation was observed for beta dose rate in enamel and dentine as well as between

absorbed dose in these tissues of the same tooth. For the population of the Techa region it was found that maximal

doses in tooth tissues measured by EPR are observed for tissues, whose period of formation coincided with maximal

releases of radioactivity (Romanyukha et al., 2000a; Romanyukha et al., 2001; Tolstykh et al., 2003). The process of

dose formation in dentine and enamel was analysed for dog's teeth after a short period of 52 hours after injection of

90Sr (Ignatiev et al., 1999; Shishkina et al., 2001). Good agreement was found for doses modelled using Monte-

Carlo method and experimental doses measured by EPR-spectroscopy.

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The objectives of the present work are the following: to investigate 90Sr distribution and absorbed dose

after 50 years of intakes in various calcified tooth tissues which were in the process of maturation by the time of

radioactive releases into the Techa River; to analyse the effect of 90Sr incorporated in tooth tissues on the absorbed

dose determined by EPR-spectroscopy. Teeth obtained from the residents of the Middle and Low Techa regions

were used for this study.

MATHERIALS AND METHODS

Sample collection and preparation

Teeth obtained from the Techa riverside donors were selected specifically for this study. Teeth were

extracted according to medical indications in the frame of creation of URCRM tooth sample bank. Part of samples

was collected from control uncontaminated localities of the South Ural region. Totally, 8 teeth of exposed people

(22 samples of tooth tissues) and 2 teeth from controls (6 samples of tooth tissues) were studied. For these teeth the

calcified tissues were at different formation stages of at the time of radioactive releases (1950-1951).

Crown dentin and enamel samples were prepared by sawing off the surface enamel layer of the tooth by a

diamond disk saw. Root dentin was prepared from the root sawed off from the crown. Dental cement was removed

from the root by dental drills. In order to eliminate effects of mechanically induced radicals due to mechanical

treatment, the sawing was performed at low linear speed under water-cooling. Dentine and enamel fragments were

crashed by nippers to chips of 0.5 – 2.0 mm in size. After mechanical treatment, the samples were etched 2-3 min in

20% acetic acid at room temperature, then rinsed up by distilled water, ethanol and dried.

EPR dosimetry

EPR spectra measurement and spectra processing

EPR spectra measurement was performed at room temperature with the use of ESP-300E (Bruker,

Germany) X-band spectrometer equipped with high sensitivity TM110 mode cavity ER-4108-TMH. The following

registration parameters were taken: microwave power 10 mW; modulation 0.3 mT at 100 kHz; sweep width 8 mT;

sweep time 165 s; receiver time constant 163 ms; number of accumulations 16. Fourth line of an Mn2+ containing

sample permanently mounted near the bottom of the cavity was used as a reference amplitude signal. Aliquots of

100 - 130 mg sample mass were used for measurements.

Spectra processing was performed with the use of the basic software of the built-in spectrometer computer

as described in Skvortsov et al. (1995). A dosimetric radiation-induced signal was obtained by subtraction of a

simulated native signal, which was obtained from a standard spectrum recorded for enamel of milky teeth. Shift,

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amplitude and width of the standard spectrum were changed using control knobs of the computer to fit a low-field

part of the recorded spectrum where the native signal is positioned. The same standard spectrum was used for

simulation of the native signal both in enamel and dentin. The intensity of dosimetric signal was determined as

amplitude of its maximum. This methodical approach was tested at 2-nd International Intercomparison on EPR tooth

dosimetry (Wieser et al., 2000), and an accuracy of dose determination in enamel was characterized by mean

standard deviation of experimental doses from nominal doses as 32 mGy in dose range up to 800 mGy.

Sample irradiation

Irradiation of dentin and enamel samples was performed by 60Co gamma-ray source. Samples at irradiation

were placed between two 4-mm polymethylmethacrylate plates. At such conditions of irradiation, dose absorbed in

enamel and dentin is very close to dose absorbed in soft biological tissue (Wieser et al., 2000; Ivannikov et al.,

2004). Therefore, dose values obtained with the use of tissue equivalent dosimeter (with standard uncertainty of 3%)

were interpreted as doses absorbed in the samples.

Dose determination in enamel

Absorbed dose in enamel was determined from the amplitude of radiation-induced EPR signal normalized

by sample mass and marker signal amplitude with the use of universal calibration coefficient obtained at irradiation

of pooled enamel by 60Co gamma-rays. Standard variation of individual enamel radiation sensitivity (the ratio of

EPR signal to dose absorbed in enamel) was assumed to be 10% (Tikunov et al., 1996; Wieser et al., 2001). This

variation gives appropriate relative contribution to the uncertainty of dose determination in enamel at the use of

universal calibration coefficient, which was taken into account at error estimation. For enamel grain size used,

contribution to the uncertainty of radiation-induced signal amplitude due to sample anisotropy was accepted as not

exceeding 5% (Iwasaki et al., 1995). The uncertainty of dose determination in enamel (Ue, in Gy) was estimated

using an expression analogous to that derived in Ivannikov et al. (2002), which comprises constant, variable

depending on sample quality and dose dependent contributions:

( ) ( )212

1 eeee DkUU ⋅+= , (1)

here: Ue1 = 40 mGy – constant contribution to the uncertainty resulting from an error in spectra processing caused

by variation in the native signal line shape of individual samples, anisotropy of the native signal and an error in the

intercept of calibration line; De – experimental dose; ke1 = 0.12 – relative contribution resulting from the variation of

individual radiation sensitivity (10%), sample anisotropy (5%) and uncertainty of the slope of calibration line (3%).

Dose determination in dentin

Since variation of radiation sensitivity of dentin was expected to be significant, dose determination was

performed with the use of additive dose method. Samples were irradiated in dose 5 Gy, 10 Gy and 20 Gy. After

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additional irradiation, the samples were aged for about 10 days before being measured to eliminate transient signals.

Dose in dentin was determined from an abscissa intersection of the regression line of the radiation-induced EPR

signal amplitude versus the irradiation dose. An uncertainty of dose determination in dentin was estimated taking

into account the uncertainties of regression analysis and uncertainty of reference dosimetry.

Detection of 90Sr concentration

Low background beta-counting measurement

Beta activity was measured by a nuclear information processor SNIP 204G (Silena, Italy) supplied with a

low-background beta counting set. It consists of the following components: plastic scintillation beta detector S2P;

plastic scintillation anti-coincidence detector S4P for the detection of coincidence gamma radiation; background led

shield with sample holder drawer; two analogue electronic units HPA/B, each accommodating the signal

amplification circuitry and the high voltage power supplier for the photo-multiplier.

The effectiveness of registration of the beta counter was determined using a set of 90Sr/90Y reference

sources with an overall activity ranged from 3.9 up to 82 Bq. The effectiveness of registration was found to be

0.350 ± 0.012 counts s-1 Bq-1.

The background count rate without sample was estimated as followed. The background measurement was

repeated 130 times by 10 min. The mean value of count rate was found to be 6.4 ± 0.1 min-1. For the average value

obtained after six time 10-min measurements, standard deviation of background count rate was estimated as 0.4

min-1, which corresponds to 0.02 Bq taking into account estimated value of effectiveness of registration. This value

was subtracted from measuring activity of the samples. Contribution of natural radionuclides and radionuclides from

global fallouts to the measured beta activity of tooth tissues was estimated using tooth samples of donors resident in

the control localities.

Determination of the total beta activity in tooth tissues

For activity measurement, tooth tissue samples were additionally powdered to have grain size of about

0.1 mm. The measurements were performed six times by 10 min and an average was taken.

In order to estimate the contribution of sample geometry and sample mass into the error of activity

determination, the activity measurements were performed for different aliquots of one of the dentin samples. For the

aliquots with the masses of 0.11, 0.15, 0.21, 0.25 and 0.30 g and experimental activities ranged from 11 to 27 Bq,

the count rate versus the sample mass was found to be linear within 3%.

An uncertainty of the specific activity determination (UN, in Bq g-1) was determined using the equation:

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( )

mAm

kAkUUU

mAAA

N

222

22

1

⋅+⋅++

= , (2)

here: UA1 = 0.02 Bq – constant contribution due to uncertainty of background count rate; UA2 – uncertainty of an

average of six measurements; A – measured activity (in Bq); kA = 0.05 – coefficient, contribution to which is given

by uncertainties of effectiveness of registration at calibration (3.5%) and linearity of mass response (3%); km =

0.01 g – uncertainty of sample mass; m – sample mass (in g).

Electron transport simulation

Absorbed doses in enamel and dentin were calculated by Monte Carlo simulation of electron transport

using the MCNP-4C code (Briemeister, 2000). The coupled electron-photon transport physics in the MCNP-4C code

take into account in a rather accurate way the diffusion and slowing down of all radiations in the electron-photon

cascade established in the media. The most important theory for the algorithms in MCNP is the Goudsmit-

Saunderson theory. A continuous-slowing-down model is used for electron transport that includes positrons, x-rays,

and bremsstrahlung, but does not include external or self-induced fields. The geometry of MCNP treats an arbitrary

three-dimensional configuration of user-defined materials in geometric cells bounded by first- and second-degree

surfaces.

The default mode of detailed physics for electron transport was used with the 1.0 keV energy cut-off. The

energy deposited in the probe cells was calculated using the pulse height tally, which records the energy deposited in

a cell by each source particle and its secondary particles. The calculated deposited energy was then normalized by

the cell mass to give the absorbed dose.

The initial approach to the problem of tooth-geometry description involved an extreme simplification of

tooth shape. Fig. 1a demonstrates simple geometric approximations for posterior teeth. The dimensions of the

dentin cylinder were associated with representative dimensions of crown dentin in various posterior tooth positions.

In this case a cylindrical shell (the lateral enamel) is an annual ring. The dentin and the lateral enamel form a single

whole cylinder. The cylinder head (masticatory enamel) and its base (root dentin) are prisms.

A more accurate model was constructed for the first bottom molar of permanent teeth. This geometric

model is presented in Fig. 1b. The shape of the crown, crown dentin, pulp, and the fragment of root dentin adjacent

to the cervical junction can be described as a rectangle with rounded corners. Actual values of the average geometric

parameters of teeth typical for Ural population were used in the refined tooth model. The thickness of the lateral

enamel layer increases from the cervical junction to the height of the crown. For the description of the lateral enamel

shape, the intersection of a truncated cone and a truncated pyramid turned small base down is used. The shape of

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the masticatory enamel arises from the intersection of a parallelepiped and a prism with an elliptical base. A

parabolic equation describes the upper surface of the masticatory enamel.

The absorbed dose was scored in the dentin and enamel separately. The different source tissues (enamel,

crown dentin including its primary and secondary fractions, root dentin) were examined as dose forming factors.

Uniform, isotropic sources of electron emission from 90Sr/90Y decay were assumed in the each specific source

volume. The density of each tooth tissue was considered as a constant. Element composition and density were

taken for moist weight according to Tolstykh et al. (2000). All results are based on a sample of 10M histories of

emitted beta particles.

RESULTS AND DISSCUSSION

Experimental estimation of absorbed dose and 90Sr concentration in tooth tissues

Results of absorbed dose and total beta activity determination in tooth tissues are presented in Table 1.

Personal code and sample numeration are given according to URCRM register. Samples 89 – 250 were collected in

the Techa region, samples 276 and 277 in control territories.

It should be noted that in contrast to tooth enamel, dosimetric properties of dentin are poorly investigated.

Based on the measurements performed in this work, it was found that radiation sensitivity of different dentin

samples varies with standard deviation of 35%. Average dose response of dentin is approximately five times lower

than that of enamel. Similar ratio was found for dog's teeth (Ignatiev et al., 1999). No significant difference was

observed for the average dose response of crown and root dentin.

The cumulative absorbed dose for Techa River residents detected by EPR represents the superposition of

three main types of exposure, viz.: background, external and internal irradiation. Contribution of the natural

background radiation to absorbed dose in enamel was assumed to be 1.0 mGy y-1 (Ivannikov et al., 1997) and it was

subtracted from cumulative dose in tooth enamel. It should be noted that cumulative doses detected in enamel of

Ural control teeth (276 and 277) do not contradict to this estimation. The contribution of background radiation to

absorbed dose in dentin was not investigated before. Therefore, for this study as a preliminary result the average

dose in dentine of control teeth detected by EPR have been used. The background dose level in the dentin of

investigated control samples was estimated as 0.77±0.15 Gy. This rough estimation was used at dose determination

in dentin.

The results of beta counting were converted into 90Sr concentration in the tooth tissues taken into account

the beta count rate detected for the tissues of control teeth. Only 2 control teeth (6 samples of tooth tissues) were

tested for determination of radionuclide contamination. Average total activity of all investigated tooth tissues from

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unexposed donors was estimated as 1.3±0.4 Bq g-1. This value was subtracted from the total beta activity measured

for exposed donors to determine 90Sr/90Y specific activity (N). Since the 90Sr/90Y is assumed to be in secular

equilibrium, the concentration of 90Sr is equal to 0.4985 N.

It should be noted, that the background doses cannot be estimated individually and the values used

represent the expected values that are more probable for the investigated samples. Therefore, after subtraction of

natural background dose, the experimental estimations and uncertainties of cumulative external and internal dose (as

far as its current 90Sr content) for the Techa River residents can be considered as mathematical expectation and

corresponding confidential bounds (Table 2).

Dependencies of accumulated dose on 90Sr concentration in tooth tissues together with linear regression

parameters are presented in Fig. 2. High correlations are observed for enamel and root dentin. The crown dentin

demonstrates a weak conformity between accumulated dose and 90Sr content. Five samples of crown dentin that

have the variation of 90Sr concentration over the range from 3 to 22 Bq g-1, demonstrate dose independence on the

90Sr activity because theirs cumulative doses are practically equal (about 2 Gy).

Dependence of 90Sr concentration on the time of intake

The both concentration of 90Sr in the tooth tissues and the radionuclide distribution in the tooth body should

strongly depend on mineralization status of tooth at the time of radioactive releases. The maturation processes in

root dentin, crown dentin and enamel are very different (Tolstykh et al., 2000; Romanyukha et al., 2001; Göksu et

al., 2002). In the case of coincidence of the periods of 90Sr intake and tissue mineralization, the concentration of

90Sr in enamel is high. Table 3 shows the years of birth for the Techa River residents for whom the highest

concentrations of 90Sr (and, accordingly, the highest cumulative doses) in tooth tissues at various positions should be

expected (based on the data of tooth mineralization presented in Logan and Kronfeld (1933); Lewis and Garn

(1960); Novik (1971) and Kolesov (1991)). It should be noted that individual variations in tooth-development times

are significant, and these calculations represent average values.

Fig. 3 demonstrates the concentration of 90Sr in the investigated samples depending on the mineralization

status. The results obtained generally correspond to the conception of the maximal radionuclide accumulation at the

time of tissue calcification at high rate. Although the reliable detected levels of 90Sr concentration were found in 4

out of 7 samples of enamel completed by 1950-1951. As seen in the Fig. 3, those were the teeth with crown dentin at

different stages of formation (regions 2 and 3 in Fig. 3a). The estimated 90Sr concentration in the enamel for such

teeth was found in the value interval from 0.09 up to 9.3 Bq g-1. From three teeth, which related to regions 4 and 5 in

Fig. 3a (formation of root dentin and beginning of secondary dentin formation), two enamel samples have

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insignificant content of 90Sr and one demonstrates a concentration level not exceeding 0.5 Bq g-1 (expected value is

0.2 Bq g-1).

Two teeth with different positions belonging to the same person (IDP 66951) demonstrate practically equal

enamel doses and 90Sr concentrations. However, for the same teeth the high differences between both crown and

root dentins were found. Both of these teeth can be related to the period of completion of primary crown dentin

mineralization and beginning of root dentin formation in terms of their mineralization status in 1950-1951 (see

Fig. 3). Earlier (Shishkina et al., 2001) the large individual variations in the tooth burden were described in a dog

experiment for a short period after intake. However, even 50 years after 90Sr intake we can observe a large

difference in radionuclide concentration between different teeth of the same person.

Preliminary estimate of internal dose fraction in enamel (Tolstykh et al., 2000) had been performed

assuming an absence of 90Sr in the enamel of teeth with crown matured by the time of radioactive releases.

However, the detection of 90Sr in the enamel of teeth which crown had been completed by 1950-1951 indicates that

enamel self-irradiation can take place even for such teeth and therefore it cannot be ignored in the internal enamel

dose estimations.

Comparing the 90Sr concentration in the different parts of dentin (crown and root zones) it is obvious that

the radionuclide is distributed non-uniformly in the whole dentine body. Moreover, the concentrations in the crown

and root dentin are not only unequal but also miscorrelated, because at the time of 90Sr intakes the dentin of different

teeth was at different stages of maturity (depending on donor’s age, tooth position and individual variations in the

rate of tissue mineralization). This agrees with the results of Göksu et al. (2002), who have no found correlation

between cumulative dose and activity in different tooth tissues. Therefore, obtained results allow verifying other

approximation, which was used in (Tolstykh et al., 2000) as a first rough approximation for mature teeth - uniform

radionuclide distribution in the whole dentin body. For teeth that were at the stage of mineralization at the time of

intakes, the assumption of equal 90Sr concentration in the crown and root dentin cannot be applied.

Verification of dose formation model in teeth

In order to analyse the dependencies of the cumulative dose on the specific activity, dose formation in tooth

tissues due to incorporated 90Sr should be considered. Dose formation may be described by a following equation

analogous to that presented by Shved and Shishkina (2000b):

∑ ∫=

=

n

ii

ij

T

j dttNNDD

1 0

)(& (3)

Dose formation in the tissue-target (j) is caused by radiation from tissue-sources (i). The enamel and crown dentin is

assumed as target tissues (detectors). Enamel, crown and root dentin are assumed to be a source of irradiation (n=3).

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The dose accumulation period, T, for the Techa River donors is assumed as a period from beginning of 90Sr intakes

to the year of tooth examination. The value of ijN

D

& is the dose rate coefficient (DRC) in the target tissue (j), due to

exposure by one of the possible source tissue (i) with specific activity of 90Sr/90Y 1 Bq g-1. Ni(t) is the specific

activity of 90Sr/90Y in the source tissue (i).

Model approximations for description of radionuclide concentration in the source tissues

The specific activity of the source tissue is a function of time that depends upon metabolism and

radioactive decay. The specific activity of 90Sr in tooth tissues of Techa River residents is, in general, described by

equation:

)()()( ttRtN ε⋅= , (4)

here: t is the time since 1950 y; R(t) is retention function; ε(t) is excretion function.

Because the time of intake was significantly less than the time of exposure (the experimental examination

of teeth were performed in 2000) the intake can be assumed as a singular event. Correspondingly, the retention

function can be approximated by a constant value:

0)(,0 =< tRt 0)(,0 NtRt =≥ )()( 0 tNtN ε⋅= , (5)

here: N0 is initial specific activity of 90Sr/90Y after the intake.

In the Tolstykh et al (2000) the excretion functions for dentin were assumed as an exponential decay:

tet ⋅+−= )()( ξλε , (6)

here: λ = 0.0238 is the constant of radioactive decay of 90Sr corresponding to its half-life period of 29 y and ξ is a

90Sr excretion rate. The excretion rate was estimated on the base of in vivo long-term monitoring of surface-beta

activity on front teeth (e.g., tooth beta counting, TBC) for Techa River residents (Tolstykh et al., 2000). The

measurements of teeth at high level of 90Sr accumulation in enamel (for age cohorts born in 1949-1951 which have

maximal TBC levels) were taken into account for excretion rate estimation. It was demonstrated that beta count rate

decreased with time and the rate of that decrease was higher than the radioactive decay by 1.9% per year. The TBC

method registers the surface-beta activity of anterior teeth. The thickness of enamel (0.3−0.5 mm) is shorter than the

average path length (2.1 mm) in enamel of beta particles emitted by 90Y (daughter of 90Sr). Therefore, mostly beta

particles originating in enamel, but also partially in the dentin layer (located near the junction of the enamel and

dentin), are registered by this method. Thus, the value of 1.9% per year can reflect the rate of biological elimination

of 90Sr from both enamel and dentin. Moreover, as was noted by Tolstykh et al (2000; 2002), tooth enamel

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undergoes attrition, and age decreasing of beta count (1.9% per year) can reflect the loss in amount of emitting stuff.

Therefore, the approach of biological elimination rate should be verified.

Model approximation for calculation of DRC

The DRC depends on tooth position (size and shape variation in denture). The initial approach to the

problem of tooth-geometry description involved an extreme simplification of tooth shape. Such a very simplified

model of teeth (Seltzer et al., 2001, Shved and Shishkina, 2000b) did not take into account the complicated

morphology of dental tissues and tissue alteration during the human life. For example, the part of dentin formed

after eruption is often called secondary dentin. The extension of secondary dentin results in gradual narrowing of the

pulp chamber. According to Samusev et al. (2002), this process reaches its maximum speed in 13-19 years of

individual. During this period the pulp cavity decreases twice in comparison with its initial size. Further on, the

process of secondary dentin formation continues at smaller intensity until pulp is alive. Therefore, the assumption of

permanent dentin volume during the period of dose accumulation should be considered as preliminary

approximation. The age dependence of tissue volume can result in age dependence of DRC. The next

approximation was in the uniform radionuclide distribution in the tissues. Special investigations of 90Sr distribution

in dental tissues of Techa River residents were performed in 1960−1970 with the use of autoradiography (Ivanov,

1968; Saurov et al., 1972) and later using photostimulable phosphor imaging (Romanyukha et al., 2002). Their

results demonstrate two different patterns of 90Sr deposition: (1) for persons who were adult in the period of intake,

90Sr is concentrated mainly on the surface of the pulp channel and in the root and (2) for persons with tooth

formation during the period of intake, 90Sr is concentrated mainly in the enamel and dentin layer near the junction of

enamel and dentin.

Accumulated dose in enamel due to incorporated 90Sr in tooth tissues

The thickness of tooth enamel can, on average, vary from 0.1 mm (near the tooth neck) to 2 mm

(masticatory enamel). The mean path length of 90Sr/90Y beta particles in the enamel is 0.6 mm and maximal path

length is 2.3 mm (Seltzer et al., 2001). Because of it, non-uniformity of 90Sr distribution inside the enamel has

practically no effect on the dose accumulation.

The predictions of DRC for enamel self-exposure computed based on the described tooth models of 1st low

molar were compared. The statistical computation uncertainty was not exceeding 0.2%. The computational

uncertainty due to the uncertainty in the description of chemical composition and in the discretization of the source

energy distribution does not exceed 3%. Variation of enamel DRC due to average enamel thickness for both models

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was 6%. According to the simple model the average value of DRC for enamel self-irradiation is 0.063±0.004 nGy g

s-1 Bq-1 and the average value estimated by refined model is 0.056±0.004 nGy g s-1 Bq-1. Therefore, the results

obtained were not in contradiction one for the other, and the cylindrical simplification can be used for estimation of

dose accumulation for enamel self-exposure with 13% of uncertainty. The results of the computation are presented

in Table 4.

The assumption of biological elimination of 90Sr from enamel can be tested using samples for which

enamel self-exposure should be the main factor of dose accumulation. Only one sample (IDT 200 with very large

90Sr concentration in enamel) is suitable for such test. High 90Sr concentration in the enamel of this tooth should

result in a high internal dose in the enamel in such a way that the external dose fraction and uncertainty of

background dose estimation are significantly smaller than the detected value of the total accumulated dose.

According to Eqs 3-6, the computation of internal enamel dose based on measured current 90Sr contents due to self-

irradiation was performed for this tooth (Eq. 7). In this case, the current concentration of 90Sr/90Y is N = 2.006 NSr.

( )1)(0 −+

= ⋅+ TeN

NDD ξλ

ξλ& , (7)

The accumulated dose during T = 50 y computed under an assumption of biological elimination (ξ=1.9) is

equal to 12±2 Gy. If the radionuclide concentration decreased only due to radioactive decay (ξ=0), the enamel dose

should be equal to 6.7±1.2 Gy. According to EPR measurement, the dose was found as 7.9±0.8 Gy. It should be

taken into account that the computation was made only for enamel self-irradiation. Some additional influence of

dentin should be present in the total internal enamel dose. The comparison of computations and experimental data

demonstrates that the approximation of biological elimination for enamel results in overestimation of internal dose,

whereas the underestimation was expected. Therefore, the assumption about the absence of biological elimination of

90Sr from enamel hydroxyapatite seems more correct.

Figure 4 demonstrates the comparison of EPR detected dose and dose due to self-exposure for the enamel

of investigated teeth computed under assumption of ξ=0. Teeth were divided on two groups. The first group

comprised the teeth with crowns matured by 1950, so accumulation of radionuclides should be mostly in the

secondary dentin. The second group was the teeth that have been formed in the time of 90Sr intakes, so radionuclides

can be located inside the primary dentin. Regression line that was plotted for the second group of teeth is practically

parallel to the line of data overlapping. The difference between computed enamel self-exposure and experimental

results amounted to 1.0 ± 0.2 Gy. This value can be conditioned by additional exposure due to 90Sr in the crown and

root dentin as well as by external exposure. It was surprising that for the second group the individual variation in

90Sr concentration in the dentin (from 3 Bq g-1 to 44 Bq g-1) produced approximately the same variation of

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accumulated dose in tooth enamel. Such effect can be possible only due to non-uniform radionuclide distribution in

the dentin body.

The variations in 90Sr localization in the crown dentin can significantly influence on dose formation in

enamel. The enamel DRC of dentin irradiation computed at different assumptions of source localization for 1st low

molar is shown in Table 5. These computations assumed the uniform distribution of 90Sr in the primary dentin (zone

1 in Fig. 1b), secondary dentin (zone 2 in Fig. 1b) and whole dentin (zone 1 + zone 2). Based on Table 5, it has been

shown that variations in 90Sr distribution can result in the difference of enamel DRC by about an order of magnitude.

Therefore the enamel dose due to dentin irradiation cannot be calculated based only on the knowledge about the

average radionuclide contents. For the teeth, which have been formed in the time of intake, the approximation of

uniformity does not work.

For Techa River residents the variations in radionuclide distribution is more expectable for teeth with

crowns in the process of formation in 1950-1951. Most of investigated teeth exactly correspond to this condition.

Therefore, it seems reasonable that the external dose validation were done only using EPR measurements of teeth

that had been completed toward the 1950 (Degteva et al., in press).

The influence of root dentin on the enamel dose formation was estimated as 0.0002 nGy g Bq-1 s-1,

independent on tooth position.

Accumulated dose in crown dentin due to incorporated 90Sr in tooth tissues

The necessity of additional information about 90Sr distribution is vital for crown dentin too. According to

Table 5, the dose computed under the assumptions of uniform contamination of primary and secondary dentins can

be differing by about factor 3. Non-uniform 90Sr distribution inside the primary and secondary dentin can result in

yet more difference. This fact can only partially explain the discrepancy between the results obtained for crown

dentin by EPR measurements and beta count for teeth with IDTs 204, 116, 92 and 89. As can be seen from Table 2,

these samples have similar EPR doses (about 2 Gy), but their average specific activity of 90Sr varies in the range

from 3 to 21.8 Bq g-1 (the difference is about factor 7). The other source of the found discrepancy can be the

permanent extension of dentin volume. In contrast to bone, wherein osteoblasts transform into osteocytes that

become embedded within the mineralised matrix where they implement the bone resorption, the continuous

secretion of dentin matrix proteins is associated with the progressive lengthening of the odontoblast cell process and

retraction of the odontoblasts toward the dental pulp. (Papagerakis et al., 2002). At elderly age the continuous

process of dentin formation can result in full or partial obliteration of pulp chamber (zone 3 in Fig 1b). The new

formed hydroxyapatite is, so to speak, a dilution of exposed crystals. Correspondingly, the parameter

ND& in Eq. 3

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should depend on time after intake. Therefore, the formation of EPR centres should depend on time due to

radionuclide disintegration, biological elimination and hydroxyapatite extension. Thus, the obtained results

demonstrate that the dose accumulation in the crown dentin strongly depends on tooth physiology and mineral

exchange that is especially critical for teeth under formation at the time of radionuclide intakes. Another source of

discrepancy can be due to sample loss, which is unavoidable in the process of sample preparation. This fact can be

reflected in a certain disproportion between the detected dose and activity (due to non-uniformity of radionuclide

distribution).

The dentin DRC due to enamel irradiation is equal to 0.009 nGy g Bq-1 s-1. Root dentine adjacent to tooth

neck can induce the dose in the dentine that on the average was equal to 0.005 nGy g Bq-1 s-1. However, the main

dose forming factor in the dentin is self-exposure.

CONCLUSIONS

Determination of 90Sr concentration in the tooth tissues allows detection of 90Sr in the enamel samples

completed by the time of radionuclide intakes. Therefore, in spite of the non-organic basis of tooth enamel (98% of

hydroxyapatite) the slow metabolism process (mostly due to ion exchange with saliva) results in the incorporation of

radionuclides in the tissue of completed teeth. Therefore the enamel self-exposure can not be ignored as a factor of

internal dose accumulation in the tooth enamel.

High individual variation between 90Sr tissue concentrations in the different teeth of the same donor was

found. Therefore, the precise estimation of internal dose in the tooth tissues required the individual measurements of

90Sr concentrations for each investigated tooth.

Teeth that were been formed in the time of radioactive releases into the Techa River demonstrate no

correlation between 90Sr contents in their different tissues as well as between doses accumulated in them. Therefore,

the assumption of a uniform (or proportional) tissue distribution of 90Sr inside the tooth cannot be applied to age

groups under study.

A preliminary test of one tooth does not allow making assumptions about the biological elimination of

radionuclides in the enamel. This fact casts doubt on the assumption of biological elimination in the enamel.

However, the final conclusions about enamel excretion rate can be made after a number of analogical tests are

performed.

Verification of simple tooth model describing the dose formation in the enamel and crown dentin

demonstrates its applicability for estimation of enamel self-exposure. However, the dentin as a source of irradiation

should be described by a more complicated geometry. Moreover, to enable internal dose estimation for teeth with

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crowns that were been formed mostly in the time of radionuclide intakes, additional information is needed on

radionuclide distribution in the dentin tissues.

There is a poor correlation between cumulative dose in the dentine and 90Sr concentration in the tissue.

Sensitivity of dose accumulation in dentine to radionuclide distribution and its permanent growth during the lifetime

make this tissue of small use for dosimetric aims, at least for the investigated age group of donors.

Acknowledgment – The studies described here were made possible in part by support of U.S. Department of

Energy’s Office of Health Studies and the Federal Department of the Ministry of Health of the Russian Federation.

The authors also thank Dr. M.O. Degteva at URCRM for her help and fruitful discussions.

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Figure 1. Schematic model of modelled geometries for Monte Carlo simulation of electron transport in the tooth

tissues: a) simple cylindrical model, b) refined model describing more accurately the dentin fractions and variations

in enamel thickness.

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Concentration of 90Sr (X), Bq g-1

0 5 10 15 20 25 30

Acc

umul

ated

dose

(Y),

Gy

0

2

4

6

8

10

12Y = 0.6(±0.2) + 0.34(±0.02) * XR = 0.98 p < 0.0001

A


0 10 20 30 40 50

Acc

umul

ated

dose

(Y),

Gy

0

5

10

15

20

25y = -0.6(±1.5) + 0.38(±0.08) * XR = 0.83 p < 0.0045

B


0 20 40 60 80

Acc

umul

ated

dose

(Y),

Gy

0

10

20

30

C

Y = -0.1(±0.1) + 0.36(±0.03) * XR = 0.97 p < 0.0001

Fig. 2. Dependencies of experimental accumulated dose in tooth tissues versus their concentration of 90Sr:

a - enamel, b - crown dentin, c - root dentin.

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Fig. 3. Concentration of 90Sr in the tooth tissues depending on the stage of tooth mineralization in 1950-1951.

According to Table 3, the crosshatched region coincides with crown dentin formation; white region corresponds to

root formation; 1 – enamel formation; 2 – formation of the primary crown dentin; 3 – completion of primary crown

dentin mineralization and beginning of root dentin formation; 4 - root dentin and secondary crown dentin formation;

5 – completion of root dentin formation.

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Cumulative dose (EPR), Gy

0 2 4 6 8Fr

actio

nof

enam

eldo

sefo

rmed

due

tose

lf-ex

posu

re,G

y

0

2

4

6

8

line of data overlapping

)2.0(0.1DD EPRosureexpself ±−=−

95% confidential intervalregression line

Figure 4. Comparison of EPR detected dose and dose due to self-exposure for the enamel. White symbols are teeth

with crowns completed by 1950. Black symbols are teeth with crowns that were been formed in 1950. The

regression line has been plotted only for black points.

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Table 1. Results of determination of absorbed doses by EPR spectroscopy and total beta activities in calcified tooth

tissues for persons resided in the Techa region.

Na IDPb Birth Location IDTb TPc Tissue type m (g)d DEPR (Gy)e A (Bq g-1)f

enamel 0.11 3.6±0.4 16.9±2.9

crown dentin 0.30 19.6±4.0 89.9±13.6 1 91 4U

root dentin 0.36 5.6±1.1 21.5±3.3

enamel 0.23 3.3±0.3 15.6±2.5

crown dentin 0.35 3.0±0.6 7.2±1.2 2

66951 1943 Ibragimovo,

54 km

92 7L

root dentin 0.45 2.3±0.5 1.2±0.3

enamel 0.22 1.0±0.1 3.2±0.7

crown dentin 0.15 3.0±0.6 44.9±7.0 3 143626 1942 Muslumovo,

78 km 204 7L

root dentin 0.43 1.7±0.3 4.1±0.7

enamel 0.09 0.8±0.1 1.2±0.6

crown dentin 0.16 3.0±0.6 17.0±2.8 4 66612 1940 Ibragimovo,

54 km 89 5U

root dentin 0.16 21.0±4.0 116.9±17.8

enamel 0.22 0.11±0.04 1.7±0.4

crown dentin 0.16 0.8±0.2 1.0±0.4 5 196166 1935 Kurmanovo,

88 km 250 7U

root dentin 0.38 0.7±0.2 0.3±0.2

enamel 0.15 1.23±0.1 2.2±0.6


78 km 116 6U

root dentin 0.23 28.4±5.6 147.3±22.3

enamel 0.08 0.3±0.04 0.9±0.6


78 km 199 4L

root dentin 0.28 1.6±0.3 28.8±4.5

enamel 0.21 8.0±0.8 46.3±7.1

crown dentin 0g - -8 147073 1949 Muslumovo,

78 km 200 7U

root dentin 0g - -

enamel 0.22 0.1±0.04 0.6±0.3

crown dentin 0.18 1.5±0.3 1.2±0.4 9 276 4U

root dentin 0.28 1.6±0.3 2.6±0.5

enamel 0.06 0.1±0.04 1.9±1.0

crown dentin 0.30 0.5±0.1 0.1±0.2 10

209147 1935 Unpolluted

territory

277 7L

root dentin 0.19 0.5±0.1 0.2±-0.2

a - number of measurement; b - IDP and IDT –personal identification number and identification number of tooth

respectively according to register of URCRM; c - tooth position in the jaw (U – upper jaw, L – lower jaw);

d - sample mass used for beta activity determination; e – total accumulated dose determined by EPR;

f - total specific beta activity; g - the amount of dentin was insufficient for reliable determination of accumulated

dose and beta activity.

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Table 2. Results of determination of absorbed doses related to Techa River releases by EPR spectroscopy and 90Sr

concentrations in calcified tooth tissues for exposed persons.

IDPa IDTa Tissue type D, (Gy)b ASr,(Bq g-1)c

enamel 3.5 (3.1-3.9) 7.8 (6.3-9.3)

crown dentin 18.8 (14.8-22.8) 44.3 (37.5-51.1) 91

root dentin 4.8 (3.7-5.9) 10.1 (8.4-11.8)

enamel 3.2 (2.9-3.5) 7.2 (5.9-8.4)

crown dentin 2.2 (1.6-2.8) 3.0 (2.3-3.6)

66951

92

root dentin 1.5 (1.0-2.1) 0.0 (0.0-0.2)

enamel 0.9 (0.8-1.0) 1.0 (0.5-1.4)

crown dentin 2.2 (1.6-2.8) 21.8 (18.3-25.3) 143626 204

root dentin 0.9 (0.6-1.3) 1.4 (1.0-1.8)

enamel 0.7 (0.6-0.8) 0.0 (0.0-0.3)

crown dentin 2.2 (1.6-2.8) 7.9 (6.4-9.3) 66612 89

root dentin 20.2 (16.2-24.2) 57.8 (48.9-66.7)

enamel 0.05 (0.00-0.10) 0.2 (0.0-0.5)

crown dentin 0.03 (0.00-0.28) 0.0 (0.0-0.1) 196166 250

root dentin 0.00 (0.00-0.18) 0.0 (<0.05)

enamel 1.2 (1.1-1.3) 0.45 (0.09-0.81)

crown dentin 2.1 (1.5-2.7) 10.6 (8.6-12.5) 87895 116

root dentin 27.6 (22.0-33.2) 73.0 (61.8-84.2)

enamel 0.24 (0.18-0.30) 0.0 (0.0-0.2)

crown dentin 2.0 (1.4-2.6) 3.0 (2.3-3.0) 138861 199

root dentin 0.8 (0.5-1.2) 13.8 (11.5-16.0)

enamel 7.9 (7.1-8.7) 22.5 (18.9-26.1)

crown dentin - - 147073 200

root dentin - -

a - IDP and IDT - personal identification number and identification number of tooth respectively according to

register of URCRM; b - accumulated man-caused dose (confidential bounds) determined on the base of EPR

measurements; c - current concentration (confidential bounds) of 90Sr.

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Table 3. Birth years of donors whose teeth underwent mineralization in 1950-1951 (compiled basing on Logan and

Kronfeld (1933); Lewis and Garn (1960); Novik (1971) and Kolesov (1991)).

Tooth position Type of tissue

1st

incisor

2nd

incisor Canine

1st

premolar

2nd

premolar

1st

molar

2nd

molar

3rd

molar

Tooth enamel and primary

crown dentin

1949−

1951

1949−

1951

1949−

1951

1947−

1948

1946−

1948

1950−

1951

1946−

1948

1939−

1943

Crown dentin (mostly

primary)

1946−

1951

1946−

1951

1944−

1951

1943−

1948

1942−

1948

1946−

1951

1942−

1948

1937−

1943

Root dentin and

particularly secondary

crown dentin

1941−

1946

1940−

1946

1938−

1944

1939−

1943

1938−

1942

1939−

1946

1935−

1942

1928−

1937

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Table 4. Dose rate coefficients for self-exposure of enamel, depending on tooth position computed based on simple

cylindrical model.

Tooth position Enamel DRC due to self-exposure,

nGy g s-1 Bq-1

Upper jaw

4 (1st premolar) 0.045

5 (2nd premolar) 0.044

6 (1st molar) 0.054

7 (2nd molar) 0.049

8 (3rd molar) 0.048

Lower jaw

4 (1st premolar) 0.045

5 (2nd premolar) 0.047

6 (1st molar) 0.063

7 (2nd molar) 0.049

8 (3rd molar) 0.049

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Table 5. Dose rate coefficients for enamel and dentin computed for 1st low molar based on refined tooth model.

Whole dentine and its separated fractions were assumed as the source of exposure.

Target tissues DRC for source tissuesa, nGy g Bq-1 s-1

Whole crown dentin Primary crown dentin Secondary crown dentin

Enamel DRC 0.0086 0.010 0.0018

Dentin DRC 0.068 0.073 0.029

a - dose rate normalized on average 90Sr/90Y concentration in whole dentin.

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Complex experimental research on internal tooth dosimetry for the Techa River region: A model for...

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