Assessment of performance parameters for EPR dosimetry with tooth enamel

Radiation Measurements 43 (2008) 731 – 736 www.elsevier.com/locate/radmeas

Assessment of performance parameters for EPR dosimetry with tooth enamel A. Wieser a,∗ , P. Fattibene b , E.A. Shishkina c , D.V. Ivanov d , V. De Coste b , A. Güttler a , S. Onori b a Helmholtz Center Munich, German Research Center for Environmental Health, Institute of Radiation Protection, Neuherberg, Germany b ISS-Istituto Superiore di Sanità, Department of Technology and Health, Rome, Italy c URCRM-Urals Research Center for Radiation Medicine, Chelyabinsk, Russia d IMP-Institute of Metal Physics, Russian Academy of Sciences, Ekaterinburg, Russia

Abstract In the framework of a comparison between three laboratories, electron paramagnetic resonance (EPR) signal-to-dose response curves were measured for sets of 30 tooth enamel samples and the variance of EPR measurements in dependence on absorbed dose was evaluated, in nine combinations of laboratory of sample preparation and EPR evaluation, respectively. As a test for benchmarking of EPR evaluation, the parameters ‘critical dose’ and ‘limit of detection’ were proposed as performance parameters following definitions from chemical-metrology, and a model function was suggested for analytical formulation of the dependence of the variance of EPR measurement on absorbed dose. First estimates of limits of detection by weighted and unweighted fitting resulted in the range 101–552 and 67–561 mGy, respectively, and were generally larger with weighted than with unweighted fitting. Indication was found for the influence of methodology of sample preparation and applied EPR measurement parameters on performance of EPR dosimetry with tooth enamel. © 2008 Elsevier Ltd. All rights reserved. Keywords: Tooth enamel; EPR; Retrospective; Dosimetry

1. Introduction Human teeth are one of the most important biological dosimeters for retrospective assessment of personal exposure to ionising radiation (ICRU, 2002). Exposure of enamel generates stable CO− 2 radicals from carbonate impurities in enamel, which can be measured by electron paramagnetic resonance (EPR) spectroscopy and used to determine the absorbed dose. The lifetime of the radiation induced radicals is considerably longer than human life expectancy. EPR measurement of tooth enamel provides the lifetime individual dose and its use for retrospective dosimetry is especially recommended in cases when exposure occurred several decades ago. In the last decade several international intercomparison programmes on EPR tooth dosimetry were performed and analysed with the objective to assess the respective actual stateof-the-art and to disseminate expertise between laboratories (e.g., Chumak et al., 1996; Wieser et al., 2000, 2006a; Hoshi ∗ Corresponding author.

E-mail addresses: [email protected], [email protected] (A. Wieser). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.032

et al., 2007; Ivannikov et al., 2007). Each of the intercomparison programmes were addressed to specific subissues of EPR dosimetry with tooth enamel and special criteria were defined to evaluate the performance of a system. At present standardised criteria for evaluation of the performance of an EPR dosimetry system at a specific laboratory have not been established. The lack of standardised performance criteria makes it difficult to compare EPR results of different laboratories and to pool results, e.g., for dosimetry in radio-epidemiological studies. The goal of the European research project Southern Urals Radiation Risk Research, SOUL (2005), is exploration and quantification of health risks due to chronic exposures to plutonium, strontium and external radiation. EPR measurements of tooth enamel is a key dosimetric element of the project aimed at quantifying external dose values for the Mayak workers and Techa River cohorts, in order to validate or improve external dosimetry systems that are based on occupational dose monitoring with film badges (Romanov et al., 2002; Wieser et al., 2006b) and model calculations (Degteva et al., 2000, 2005), respectively. In the framework of SOUL three laboratories are providing EPR measurements of tooth enamel that are complemented with results from the previous projects.

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In order to use these data in a joint analysis of reconstructed dose values a protocol for benchmarking of the data has to be implemented including the definition and the protocol for assessment of performance parameters for EPR measurements with tooth enamel. The benchmarking protocol should allow assessing objectively the application limit and the predicted uncertainty of the results provided by a specific EPR evaluation system. In the current study as a first test for benchmarking of EPR evaluation, the parameters ‘critical dose’ and ‘limit of detection’ following definitions from chemical-metrology were chosen as performance parameters and a model function was proposed for the analytical formulation of the dependence of variance of EPR measurement on absorbed dose. In the framework of an interlaboratory comparison each of the three laboratories prepared a set of 30 tooth enamel samples which were exposed to doses up to 1.5 Gy and were measured with EPR spectroscopy by each laboratory after mutual exchange of samples. From the obtained dose–response curves performance parameters were assessed and the dependence of variance on absorbed dose was evaluated for the nine combinations of laboratory of sample preparation and EPR evaluation, respectively. Effects of sample preparation and EPR spectrum recording on performance parameters were analysed. 2. Materials and methods Each laboratory prepared a pool of enamel powder from 30 molars. The samples prepared at laboratories 1 and 2 were provided by the URCRM sample bank and were donated by a Russian rural population, whereas the samples prepared at laboratory 3 were donated by the population of Cairo, Egypt. For sample preparation all laboratories removed mechanically the root from the tooth and the crown was cut into halves before dentine was separated from enamel with a chemical treatment of 5 M NaOH in an ultrasonic bath at room temperature at laboratory 1 and at 60–70 ◦ C at laboratories 2 and 3. At laboratory 1, before chemical treatment a pre-treatment was applied including mechanical removal of dentine and grinding of the samples to grain size in the 0.1–0.6 mm range. At laboratory 2, after chemical treatment the dentine remaining in the enamel chips were manually removed by dental drill and the tooth enamel was ground to size range 0.5–1 mm. At laboratory 3, before chemical treatment the tooth crown pieces were etched using a Titriplex(III) solution and after chemical treatment the dentine remaining in the enamel chips were manually removed. The enamel was then ground and sieved to a grain size range 0.1–0.6 mm and finally etched using acetic acid. The three pools of enamel powder were divided into sets of 30 aliquots of 100 mg. The respective laboratory of sample preparation irradiated groups of five aliquots in their own laboratory with doses of 0.1, 0.2, 0.5, 1.0 and 1.5 Gy, and kept one group unirradiated. The irradiations, performed to ensure secondary electron equilibrium, were delivered in units of absorbed dose to hydroxyapatite (enamel). The laboratories measured the EPR dosimetric signal-to-dose response curves for each of three sets of samples, at room

temperature with X-band CW-EPR spectrometers. Each sample measurement was repeated three times after shaking of the sample and reinsertion into the microwave cavity. At laboratory 1, measurements were performed with an ERS231 (produced in the former GDR) spectrometer equipped with standard microwave cavity ZSX18, using 13 mW incidence microwave power and 0.45 mT magnetic field modulation amplitude. At laboratory 2, measurements were done with a Bruker EleXsys spectrometer equipped with a spherical Super-High-Q cavity, using 2 mW incidence microwave power and 0.2 mT magnetic field modulation amplitude. A CaO(Mn) sample was used as a reference for magnetic field position. More details about the use of the reference sample can be found in Fattibene et al. (2007). At laboratory 3, samples were measured with a Bruker ECS106 spectrometer with a 4108TMH cavity, using 25 mW incidence microwave power and 0.15 mT magnetic field modulation amplitude. All laboratories applied spectrum deconvolution in order to extract the dosimetric EPR signal (due to CO− 2 radicals) from the recorded EPR spectrum. At laboratories 1 and 3, the software code DOSIMETRY (Koshta et al., 2000) was used for spectrum deconvolution. With this software the use of a reference sample is not required for adjustment of the magnetic field positions of experimental and fitted EPR spectra, the field shift between experimental and fitted spectrum is included here as an additional fitting parameter. Both simulated powder EPR spectra and the derivative of single Gaussian functions were used for spectrum deconvolution. The dosimetric CO− 2 EPR signal and one of two non-radiation induced native signal components were implemented as simulated powder spectra (Zdravkova et al., 2003). The second native isotropic signal component was obtained as the derivative of a Gaussian function with g-value 2.0046 and a line width of 0.6 mT. In addition four minor isotropic signals were included as derivatives of Gaussian functions, two for adjustment of spectrum bias (g-value/line width, mT: 2.0030/0.8 and 2.0090/0.5) and two for fine tuning of the fitted experimental EPR spectrum (g-value/line width, mT: 2.0006/0.1 and 2.0053/0.1). At laboratory 2, the software code EPRDOSE (Onori et al., 2000) was used for spectrum deconvolution involving simulated powder spectra with Gaussian line shape for the dosimetric CO− 2 EPR signal with axial symmetry, and two non-radiation induced native signal components, one of quasi-axial symmetry (g1,2,3 = 2.0055, 2.0023, 2.0055) with a line width 0.4 mT and one of isotropic symmetry g = 2.0053) with a line width 1.2 mT. This software uses the magnetic field position of the third and the fourth line of the Mn2+ EPR spectrum of a CaO(Mn) sample as reference for adjustment of the magnetic field positions of experimental and fitted EPR spectra. The performance parameters critical dose and limit of detection were chosen for benchmarking of EPR measurements. The terminology are similar to ‘critical level of concentration’ and ‘limit of detection of concentration’ used in chemicalmetrology (Currie, 2004). The concept of critical values and limit of detection is based on the principles of statistical hypothesis testing and follows published recommendations for the international chemical community (IUPAC, 1995) and the

A. Wieser et al. / Radiation Measurements 43 (2008) 731 – 736

0

DC

1.2

Absorbed dose D DD

0.5

1.0 1.645 σ0

EPR amplitude, a.u.

1.645 σD

Relative frequency

0.4 0.3

733

σ0

0.2

0.8 0.6 90% Prediction bands

AC 0.4 0.2

σD

0.1 β=0.05

α=0.05

0.0 A0 Ac

AD

EPR signal amplitude A Fig. 1. Illustration of the definition of critical EPR signal amplitude (AC ), critical dose (DC ), detection limit of EPR signal amplitude (AD ) and absorbed dose (DD ). Critical values and detection limits are determined from the standard deviation (0 ) of EPR signal amplitude of unexposed samples with mean amplitude (A0 ) and the standard deviation (D ) of EPR signal amplitude of samples irradiated with dose DD .

international metrological community (ISO, 1997). The definitions of critical dose and limit of detection for EPR measurements are illustrated in Fig. 1. The definition of critical dose, DC , follows from the hypothesis test for 95% probability of an unirradiated sample, and hence allowing for a false positive error rate  of 5% to indicate exposure of the sample by EPR measurement. That is, within the distribution of measured EPR signal amplitudes with average amplitude, A0 , from unexposed samples is probability  of 5% that amplitude is larger than the critical amplitude, AC . The critical amplitude is the decision limit at which it may be decided whether a sample was exposed or not. The dose value that is obtained with the EPR signal-to-dose response curve from the critical amplitude, AC , is called the critical dose, DC . The definition of the limit of detection, DD , follows from the hypothesis test for 95% probability that the sample was exposed, and hence allowing for a false negative error rate  of 5% to indicate unexposure of the sample by EPR measurement. That is, within the distribution of measured EPR signal amplitudes with average amplitude, AD , from samples exposed with the dose, DD , is probability  of 5% that amplitude is lower than the critical amplitude, AC . In this study, AC , DC and DD were evaluated in a calibration design from 90% prediction bands of a linear least-squares fit of the EPR signal-to-dose response curves as illustrated in Fig. 2. Prediction bands, AC , DC and DD were calculated following the procedures provided in Zorn et al. (1997). According to this, AC is equal to the amplitude arising from upper 90% prediction curve at zero applied dose, i.e., no additionally applied dose in laboratory. DC is equal to the dose resulting from the regression curve at the amplitude AC , and DD is equal to the dose following from lower 90% prediction curve at amplitude AC . It has to be mentioned that in practice tooth enamel

0.0 0

DC 100

DD

200

300

Absorbed dose, mGy Fig. 2. Illustration of the assessment of critical dose (DC ) and limit of detection (DD ) from critical EPR signal amplitude (AC ) and 90% prediction bands of a linear least-squares fit of the EPR signal-to-dose response curve.

samples which are claimed to be unexposed are not totally free of exposure due the small exposures arising from natural background radiation. Therefore, the terms ‘critical dose’ and ‘limit of detection’ underlie the meaning of critical dose and limit of detection above background dose, respectively. The performance parameters were evaluated from both, leastsquares fitting without weights and with inverse squares of variance of EPR measurements as weights. The evaluation of the limit of detection from weighted fitting requires for calculation of prediction bands an analytical model function of the variance in dependence on absorbed dose that was proposed on basis of the results from this study. The parameters of the model function for different data sets were evaluated using non-linear curve fitting routine of Origin v.7.5, OriginLab data analysis software. 3. Results and discussion The amplitudes of the dosimetric EPR signal from tooth enamel were measured in sets of 30 samples in nine combinations of laboratory of sample preparation and EPR evaluation. The linear EPR signal-to-dose response curves were evaluated from least-squares fitting of the average amplitudes of the groups of five samples exposed at the same dose and the variance of the amplitudes of the five samples was evaluated. The variance was found to be dependent on the level of absorbed dose, but in different extent depending on the combination of laboratory of sample preparation and EPR evaluation. There was the tendency of lower variance in unexposed than in exposed samples, at five of the nine combinations. Furthermore, there was the tendency of increasing variance with exposure from 0.1–1.5 Gy, for all except of one combination. The standard deviation of EPR measurements of tooth enamel in dependence on absorbed dose for samples prepared and evaluated at laboratory 3 is shown in Fig. 3. Eqs. (1)–(4) were proposed in this study as a first approach for fitting of the dependence of

A. Wieser et al. / Radiation Measurements 43 (2008) 731 – 736

0.20

56

0.15

42

28

0.10

14

0.05

0.00 0

200

400

σ measured dose, mGy

σ EPR amplitude, a.u.

734

0 600 800 1000 1200 1400 1600 Absorbed dose, mGy

Fig. 3. Standard deviation of EPR measurements of tooth enamel in dependence on absorbed dose for samples prepared and evaluated at laboratory 3. The standard deviation of the amplitude was converted into absorbed dose units using the parameters from the fit of the signal-to-dose response curve. Shown are the values determined from EPR measurements (closed circles) and calculated with the proposed model function (solid line) using the function parameters given in Table 1.

variance of EPR signal amplitude on absorbed dose. 2tot = 2blank + 2dosed ,

(1)

2dosed = [2const + 2rel ] · (D − DC ),

(2)

(D − DC ) = 1 − exp(−D/DC ),

(3)

DC = 1.645 · sqr(2blank )/b.

(4)

The total variance, 2tot , (Eq. (1)) is calculated as the sum of the variance for unexposed, 2blank , and exposed, 2dosed , samples. The variance for exposed samples (Eq. (2)) is calculated as the sum of a constant component, 2const , and a relative component, 2rel , that is dependent on absorbed dose. The variance for exposed samples contributes only to the absorbed dose, D, larger than the critical dose, DC , modulated due to the step function (D − DC ) (Eq. (3)). The critical dose is calculated in this approach with equation Eq. (4) with use of the slope (b) from least-squares fitting of the EPR signal-to-dose response curve. The different components of variance were assessed by leastsquares fitting of the evaluated variance to the above model function. For this purpose, the variance determined for EPR signal amplitude was converted to dose units with use of the slope (b) from least-squares fitting of the EPR signal-to-dose response curve. The results for the nine combinations of laboratory of sample preparation and EPR evaluation are presented in Table 1. The standard deviation of fitting results for the various components of variance was found satisfactory except for three combinations (1/1, 1/2, 2/2). In these cases the variance of EPR measurements for samples with different exposure was found to be very heterogeneous. This may indicate contributions from other not yet known source of uncertainty which require further investigation. The performance parameters critical dose and limit of detection were calculated from least-squares fitting of EPR

Table 1 Results of fit parameters from model of dose dependent variance of EPR measurements for the nine combinations of laboratory of sample preparation and EPR evaluation Lab of sample prep./EPR evaluation

Fit parameter

blank , mGy 1/1 1/2 1/3 2/1 2/2 2/3 3/1 3/2 3/3

(SD)

rel , % (SD)

(SD)

66 46 112 60 22 30 35 36 10

11 7 9 4 0 5 7 5 2

51 0 83 90 50 43 70 30 37

(72) (26) (52) (33) (37) (19) (42) (14) (2)

const , mGy

(5) (2) (7) (6) (> 1000) (1) (4) (1) (0.2)

(141) (> 10000) (120) (33) (28) (19) (34) (23) (1)

Table 2 Critical dose and limit of detection calculated from least-squares fitting of EPR signal-to-dose response curves with variance of EPR measurements as weights and without weights (in paranthesis) for the nine combinations of laboratory of sample preparation and EPR evaluation Lab of sample

Critical dose/limit of detection, mGy

preparation

Lab of EPR evaluation 1

2

3

1

157/368 (93/186)

104/211 (103/206)

242/552 (121/241)

2

165/455 (281/561)

56/188 (89/177)

63/171 (45/90)

3

77/247 (80/160)

88/201 (61/122)

22/101 (33/67)

signal-to-dose response curves with variance of EPR measurements, calculated with parameters from Table 1, weighted and unweighted for the nine combinations of laboratory of sample preparation and EPR evaluation. The results are presented in Table 2. A large variation was found in the assessed performance parameters for the different combinations. The critical doses estimated by weighted and unweighted fitting were in the range 22–242 and 33–281 mGy, respectively. The limits of detection estimated by weighted and unweighted fitting were in the range 101–552 and 67–561 mGy, respectively. In general, except in one case (2/1), larger limits of detection were obtained by weighted than by unweighted fitting. Also, the estimated critical doses were found to be larger by weighted than by unweighted fitting, except the cases for which much lower variance was found in unexposed than in exposed samples. In comparing samples it was found that lowest limits of detection were achieved with measurements of samples prepared at laboratory 3 (weighted-fit: 101–247 mGy, unweighted fit: 67–160 mGy). Limits of detection were found to be about doubled with the measurements of samples prepared at laboratory 1 (weighted-fit: 211–552 mGy, unweighted fit: 186–241 mGy).

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This indicates a strong influence of the methodology of sample preparation on the performance of EPR dosimetry. Comparing EPR evaluation procedures, it was found that the variability of assessed values for the limit of detection was largest with measurements of laboratory 3 (weighted-fit: 101–552 mGy, unweighted fit: 67–241 mGy) and lowest with the measurements of laboratory 2 (weighted-fit: 188–211 mGy, unweighted fit: 122–206 mGy). This may indicate an influence of EPR measurement parameters on the performance of dosimetry. Measurements at laboratory 2 were done with SHQ microwave cavity with an incident microwave power of 2 mW (equivalent to ca. 12 mW with TMH cavity) while measurements at laboratory 3 were done with TMH cavitiy with incident microwave power of 25 mW. It has been shown by other authors (Ignatiev et al., 1996) that with increasing microwave power the intensity of native EPR signals of tooth enamel diminishes and the intensity of the dosimetric signal increases, resulting in the improvement in detectability of the dosimetric signal. However, high microwave power can generate spurious EPR signals in contaminated samples that prevent the accurate measurement of the dosimetric signal and can only be applied with very pure samples. The use of high microwave power with samples with different degrees of purity could be a source of the large variability that was found with measurements at laboratory 3 but not with the measurements of laboratory 2 at low power. It might also suggest that the limit of detection with the measurements at laboratory 2 could be improved for pure samples by the use of higher microwave power.

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measurement parameters on the performance of EPR dosimetry with tooth enamel. Results of the current study may suggest that the limit of detection may be improve by using purer tooth enamel samples in combination with applying higher microwave power in EPR measurements. In this study ‘critical dose’ and ‘limit of detection’ were assessed from calibration curves in order to demonstrate the capability for benchmarking of EPR dosimetry with tooth enamel. Other authors (Güttler and Wieser, 2008) assessed performance parameters following the same definitions from the dose–response curves of single samples, and compared the results with those obtained from the calibration curves of homogenised samples from the same sample pool. They concluded that performance parameters assessed from calibration curves will provide average values, and due to variability of sample properties performance of a specific single sample can be better or worse. Therefore, performance parameters obtained from calibration curves as done in this study will not provide direct information on the performance of a specific tooth enamel sample in practice. Hence, the methodology for assessment of variability of performance parameters of individual enamel samples needs to be developed. Acknowledgements This work was supported by the EC project SOUL under the contract FIP6R-516478. References

4. Conclusions In the framework of an inter-laboratory comparison three laboratories prepared sets of 30 tooth enamel samples and exposed them to doses up to 1.5 Gy. EPR signal-to-dose response curves were measured and the variance of EPR measurements was evaluated after mutual exchange of samples, in nine combinations of laboratory of sample preparation and EPR evaluation, respectively. As a test for benchmarking of EPR dosimetry with tooth enamel, the parameters ‘critical dose’ and ‘limit of detection’ following definitions from chemical-metrology were proposed as performance parameters and a model function was suggested for analytical formulation of the dependence of variance of EPR measurement on absorbed dose. In three of the nine cases variance of EPR measurements could not be fitted satisfactorily with the model function which requires further investigation to identify contributions of other not yet known sources of uncertainty. Large variation was found in the assessed performance parameters for different combinations of laboratory sample preparation and EPR evaluation, respectively. First estimates of the limits of detection by weighted and unweighted fitting resulted in the range 101–552 and 67–561 mGy, respectively, and were generally larger with weighted than with unweighted fitting. Weighted fitting will be required in cases with obvious dependence of variance on absorbed dose. In these cases unweighted fitting will generally lead to underestimation of the performance parameters. Indication was found for an influence of methodology of sample preparation and applied EPR

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