"Radiation & Risk", 1996, issue 7 Scientific Articles Retrospective estimation of individual accumulated doses for population of Bryansk region irradiated as a result of Chernobyl accident using tooth enamel EPR-spectroscopy technique Skvortsov V.G., Ivannikov A.I., Khamidova L.G., Fedosov I.M., Tikunov D.D., Stepanenko V.F., Parshkov E.M., Shakhtarin V.V., Proshin A.D.*, Rivkind N.B.** Medical Radiological Research Center of RAMS, Obninsk; * - Department of Health, Administration of Bryansk; ** - Bryansk Regional Medical and Diagnostic Center The standardized procedure that has been developed in MRRC of RAMS for estimating individual accumulated dose based on EPR-spectroscopy of tooth enamel samples was applied in a large-scale dosimetric study. Measurements were made of over 2000 individual accumulated doses in residents of the south- west areas of Bryansk region contaminated as a result of Cher- nobyl accident (with 137Cs soil contamination density between 5 and 30 Ci/km2) and in control territories. Teeth extracted by medical indications were collected from dentist outpatients. Doses were estimated using the calibration relationships re- sulted from exposure of enamel samples to -radiation from a collimated 137Cs source. The statistical analysis of the meas- urement results has shown that dose distribution is adequately described by the log-normal function for the population of dif- ferent territories. With allowance for a fixed error, the aver- age doses range from 3 to 5 cGy for the uncontaminated areas and about 12 cGy for the contaminated ones. Residents having accumulated doses much higher the average ones are referred to a risk group and have to be followed up and subjected to dosi- metric investigation. It has been established that there is a clear correlation between the average individual accumulated doses measured by EPR and the level of radioactive contamination of the area. The linear regression coefficient has been found to be 0.24 0.08 cGy (Ci/km2). The dependence of individual doses on age is given by a linear regression with the slope of 0.14 0.05 cGy/year. The indicated relationships are in agreement with re- sults obtained with the models estimating retrospectively the accumulated doses. Introduction individual doses will vary significantly and there will be noticeable deviations The Chernobyl accident has led to ra- from average values even within popula- dioactive contamination of extensive tion points. Therefore, estimation of areas in central Russia. Due to logisti- individual doses using calculation meth- cal reasons it was impossible to ensure ods based on the level of contamination individual dosimetric control of the is rather problematic. At the same time, population on a large scale. Originally, it is essential to know individual accu- in the contaminated areas only collec- mulated doses when forming groups of tive radiation doses and doses averaged enhanced risk for compulsory medical over a settlement were critically esti- surveillance and rehabilitation. mated (see for example [1]). Because of Recent studies [2-4] have demon- considerable heterogeneity in the con- strated that the method of electron par- tamination density and differences in amagnetic resonance (EPR) of tooth life patterns it should be expected that 80 "Radiation & Risk", 1996, issue 7 Scientific Articles enamel for evaluation of individual should be taken of features of formation doses holds much promise. The essence of of radiation-induced paramagnetic cen- the method is as follows. Under exposure tres in enamel. to ionizing radiation stable radiation The doses determined by tooth enamel induced paramagnetic centres are formed are stored in a computer database to- in enamel and they accumulate during the gether with data on a patient; the tooth whole life of the tooth enamel. These samples are also preserved. As the centres can be registered by EPR- method is updated the results obtained spectroscopy and based on their concen- with the standardized methodology will tration and using calibration curves one be refined. Some data and results of can determine the accumulated dose. statistical analysis were published ear- We used EPR-spectroscopy of tooth lier [8, 9]. This paper presents results enamel during examination of the popula- of a statistical analysis of available tion in south-west areas of the Bryansk data which allows the evaluation of the region affected by radioactive contami- contribution of different factors to nation at the density of 137Cs contamina- formation of individual doses measured tion of up to 40 Ci/km2. For comparison, by EPR-spectroscopy of tooth enamel and we surveyed the population of uncontami- some refined data and results of their nated (control) areas in Borovsk and analysis. Zhukovo districts of the Kaluga region. Methods and materials Altogether, 2000 samples of enamel of teeth extracted in dentist clinics on The methodology used for sample medical grounds were analysed. preparation and dose measurements is The method under discussion is cur- described in detail in [10]. Here we rently being actively developed and im- will discuss it only briefly. The enamel proved. Sources of systematic errors samples were produced by removing den- (relationship of radiation spectrum and tine from a crown of tooth using hard- radiation sensitivity of enamel, influ- alloy dental drills and were ground to ence of solar light, features of spectra pieces of 1-2 mm. The analysis sample analysis) are being identified and way weight was 50-150 mg. The EPR-spectra to eliminate them are sought [5, 6]. were recorded at room temperature with With EPR-spectroscopy of tooth enamel ESP-300E (Bruker, Germany) spectrometer different methodological approaches can using a standard rectangular resonator be used for sample preparation, measure- in X-range with microwave power of 10 ments of spectra and their mathematical mW. The time of recording of each spec- processing and interpretation of results trum with 16-time accumulation was 45 for determination of individual doses min. [7]. Each of these methods has its pecu- The background signal from the or- liarities which ultimately show them- ganic component of enamel was subtracted selves as a systematic error in the dose with a built-in computer using basic estimate. software. For modelling the subtracted We used a methodological approach background signal we used a spectrum of which was standardised in the experimen- enamel of childrens’ teeth selected by tal nuclear medicine laboratory of MRRC the criterion of “ minimum signal” . The of RAMS to analyse all collected tooth shape of the modelled signal was fitted samples. The special feature of this to the signal from organic components by method is that there is no additional changing its amplitude, field shift and irradiation of the samples and they can width using software with operator con- be subject to repeated analysis, as the trol. The subtraction resulted in the method is perfected and new factors to radiation-induced signal whose intensity be taken into account are identified. was determined by amplitude of low-field The systematic errors of the method make component. an equal contribution to all doses that Accumulated dose was evaluated by the are determined and can be allowed for in signal intensity normalized to a sample data analysis as EPR-dosimetry continues weight using the universal calibration to be developed. coefficient. This coefficient was taken The resulting accumulated doses to be the value of the average slope of should be considered as specific tooth calibration dependencies measured for enamel. Using these dose measures to several tens of enamel samples exposed estimate whole-body doses, account to a direct collimated beam of - 81 "Radiation & Risk", 1996, issue 7 Scientific Articles radiation of 60Co and 137Cs under electron 3. Additional input to the spread of equilibrium (the samples were placed doses can be attributed to heterogeneity between plates of tissue equivalent ma- of the contamination. This explains the terial of 1 g/cm2). For both types of difference in variance in dose distribu- sources the same values of the calibra- tion for the population of the control tion coefficient were derived. and contaminated areas. The trend has been towards an increase in the average Results and discussion doses measured in residents of different population points with an increase in The frequency distribution of indi- contamination level. Dependence of aver- vidual doses evaluated by tooth enamel age dose for some population points on without account taken of the tooth num- 137 Cs soil contamination level is shown ber (incisor, premolar, and molar tooth) in Figure 2. The figure indicates doses for separate areas and major population only for those population points in points were described by a lognormal which more than 30 measurements have function. The average dose for adults of been made. the control areas is 10-12 cGy (standard The data shown in Figure 2 can be ap- deviation of 7-8 cGy). In the areas con- proximated with a linear regression re- taminated after the Chernobyl accident lationship with a slope 0.240.08 the average doses for some population 2 cGy/(Ci/km ) and the intersection point points was 15-20 cGy (standard deviation with the ordinate axis of 11.32.0 cGy. of 8-12 cGy) and depended on 137Cs con- The value of the slope with an allowance tamination density. Histograms of dose for the adjustment coefficient for scat- distribution for some areas of the Bry- tered radiation (K 1.5) [5] is ansk region have been presented in an av 0.160.05 cGy/(Ci/km2) and close to the earlier work [9]. value of 0.126 cSv/(Ci/km2) derived by The observed wide spread in the dose calculations with allowance for 8 years values can be attributed to several rea- of external -radiation exposure of the sons: rural population in the contaminated 1. A major contribution is made by areas (Balonov M.I.