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Radiat Meas. Author manuscript; available in PMC 2017 May 01. Published in final edited form as: Radiat Meas. 2016 May 1; 88: 41–47. doi:10.1016/j.radmeas.2016.02.014.
Emergency EPR dosimetry technique using vacuum-stored dry nails S. Sholom1,* and S.W.S. McKeever1 1Radiation
Dosimetry Group, Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA
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Abstract Human finger- and toenails have been tested with an X-band EPR technique for different conditions of nail storage. The main radiation-induced signal at g=2.005 demonstrated good stability if the samples were stored in a vacuum at room temperature after nail harvesting and irradiation. On the basis of this phenomenon, a new protocol is proposed to use the nails as possible emergency EPR dosimeters. The dosimetry protocol was tested on laboratory-exposed samples and demonstrated the ability to recover doses in the region 0-10 Gy with an estimated uncertainty of approximately 0.3-0.4 Gy for doses in the range < 2 Gy, increasing to 0.6-0.7 Gy for doses in the range 5-10 Gy.
1. Introduction Author Manuscript Author Manuscript
Dosimetric EPR properties of finger- and toe-nails have been extensively studied for more than two decades (see e. g. Chandra and Symons, 1987; Symons et al., 1995; Trompier et al., 2014; He et al., 2014; Wang et al., 2015; and references therein), but the question - can nails be used for emergency/retrospective dose assessment? - remains still open. From an examination of Q-band EPR spectra, Trompier et al. (2014) came to a conclusion that only one component from irradiated nails (they called it the ‘RIS5’ EPR signal, where RIS stands for radiation-induced signal) can be used for dosimetric applications due to its stability during both thermal and water treatments. This component could be obtained and isolated through elimination of unstable EPR signals by humidification of the nails. Unfortunately, the intensity of the RIS5 signal observed in Trompier et al. (2014) was very low, and the corresponding EPR dosimetry technique was only able to reconstruct doses higher than 10 Gy. This is unacceptably high for possible applications of the protocol devised by Trompier et al. (2014) in emergency triage. Significant sensitivity increase can be obtained if one uses another EPR signal of nails, namely RIS2, according to the terminology of Trompier et al. (2014). (Hereafter we will call this signal simply RIS.) However, there are some challenges related to possible applications
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of this signal for emergency dose assessment. The main problem is the very high sensitivity of the RIS to water treatment of nails: soaking of the nail clippings for times of 10-15 min was enough to almost entirely remove this signal (Romanyukha et al., 2014; Marciniak et al., 2014). Thus, in order to use RIS for dose assessment it appears that the nail samples should be stored without any contact with water for all time between nail harvesting and EPR measurements. Another possible issue is the instability of all the main EPR signals from nails if the samples are stored under ambient conditions (Wang et al., 2015; Reyes et al., 2012; Black and Swarts, 2010). Due to the instability of these signals it is difficult to predict the time behavior of the cumulative EPR signal of nails at g-factor ~2.005. Greater complexity is introduced after exposure and clipping because the g=2.005 signal consists of multiple, strongly overlapping components. At a minimum, the RIS signal itself, a background signal (BG) and a mechanically induced singlet signal (MIS) all appear at the same g-factor. Furthermore, these signals may change their intensities in opposite directions with time. The next potential problem is a correct determination and separation of RIS, BG and MIS contributions to the cumulative EPR signal. This is especially a problem if the first EPR measurement is conducted a few days after the emergency exposure and sample clipping. All mentioned issues should be properly addressed before an EPR dosimetry technique with the RIS can be used for dose assessment.
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In the present study some innovations are introduced in order to improve the EPR dosimetry technique with nails. We focus on X-band EPR, since this is the most widely available EPR frequency range, and we propose storage of the samples in vacuum for all time between sample clipping and EPR measurements. For such storage, as we will show, the RIS intensity is stable for at least 7 days after exposure, and the evolutions of the BG and MIS signals with storage time are reproducible for different samples. We developed a procedure of isolation and separate observation of the RIS, MIS and BG signals using a combination of water soaking, irradiation and additional clipping for multiple nail samples. Finally, we developed a so-called vacuum-stored, dry-nail dose reconstruction protocol that allows the reconstruction of doses as low as 2 Gy up to at least 7 days after exposure.
2. Materials and Methods
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Twenty volunteers provided toe- and finger-nails for this study. Nails were collected during routine hygienic procedures and were stored at ambient conditions between clipping and submitting to the research laboratory. Before any measurement, the nail clippings were further cut (if necessary) to pieces of 4-5 mm length (in order to fit the size of an EPR sample tube) and soaked in water for one hour followed by drying in a vacuum oven also for one hour. This procedure removed all possible pre-experiment, radiation-induced and mechanically induced signals and minimized the background signal intensity. Three experiments were conducted with different sample aliquots. In Experiment #1, twenty seven sample aliquots were weighed from the nails of three individuals (nine aliquots from each individual) and subsequently divided into three sets (each set including three aliquots from each individual). Aliquots from the different sets were treated in different ways to obtain spectra either with dominating radiation-induced, or mechanically induced, or background signals. Aliquots from Set #1were exposed to 50 Gy to produce the radiation-
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induced signals. A high dose (50 Gy) was used so that the RIS signal dominated the spectrum and so that the shape for this signal could be clearly discerned. Aliquots from Set #2 received additional cuts, without any irradiation, to produce pieces of 1-2 mm length. This procedure induced a strong mechanically induced signal (MIS) with neither a RIS nor an increase in BG. No additional treatments were applied to the aliquots of Set #3. These were thus used to obtain the background (BG) signal only. Each of the three sets was additionally split into three subsets (of one aliquot each) and stored at three different values of humidity.
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The following cases were tested: (a) storage in a vacuum (zero humidity); (b) storage in ambient laboratory conditions (at 20°C and 62% of humidity); and (c) storage in water (i.e. 100% humidity). X-band EPR spectra were recorded at different times after either sample exposure (Set #1 aliquots), extra clipping (Set #2 aliquots), or 1-h soaking and 1-h drying (Set #3 aliquots) over a period of 7 days. In Experiment #2, thirty two aliquots were weighed from nails from eight individuals (four aliquots from each individual) and used to study the dose response. For this experiment, nail clippings of all aliquots were additionally cut to obtain pieces of 1-2 mm length. This simulated clipping during nail harvesting and generated mechanically induced signals. Aliquots of every individual were then exposed to doses of 0 Gy, 2 Gy, 5 Gy and 10 Gy to simulate the emergency exposure. We also studied how the dose response curves of the individual samples changed with time after exposure. All sample aliquots in this experiment were stored in a vacuum for all times, excepting the periods when the EPR spectra were recorded. (The EPR measurement procedure normally took approximately 5 minutes under ambient conditions.)
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Experiment #3 was a dose reconstruction test. 18 aliquots were prepared from nails of three individuals (aliquots ##1-6 from each individual) and treated in the following way. Aliquots ##1-4 were cut to pieces of 1-2 mm length to simulate clipping during nail harvesting followed by exposure to doses of 0 Gy, 2 Gy, 5 Gy and 10 Gy to simulate the emergency exposure; these aliquots were stored in vacuum and measured at 4 hours, 1 day, 3 days and 7 days after exposure to simulate the fading. Aliquots ##5-6 were also cut to 1-2 mm pieces followed by exposure to 10 Gy dose, but then these aliquots were soaked for 1 h in water (to remove all signals from previous mechanical stress and exposure), dried for 1 h in vacuum and used to regenerate the BG, MIS and RIS signals, which were used according to a dose reconstruction protocol described in Section 4 below. To regenerate the BG+MIS signals, an aliquot #5 was extra cut to approximately the same total cut length as that of corresponding aliquots ##1-4; to regenerate the BG+MIS+RIS signals, an aliquot #6 was cut in the similar way as the aliquot #5 followed by exposure to 10 Gy dose. Aliquots ##5-6 were stored in vacuum and also measured at 4 hours, 1 day, 3 days and 7 days after exposure. All samples for Experiments #1 and #3 were fingernails while in Experiment #2 six samples were fingernails and two more samples were toenails. (In fact, no differences were observed between the properties of finger- and toenail samples in subsequent experiments.)
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A Bel-Art Scienceware F42010 compact vacuum desiccator was used to store the samples in vacuum. By using a Robinair 15115 vacuum pump, a partial vacuum of 7.59 mm Hg can be obtained in this desiccator within a few minutes. The vacuum could be held for at least 48 hours without additional pumping. Maintaining a vacuum for longer periods required additional pumping. EPR measurements were conducted on a Bruker EMX spectrometer equipped with a Bruker ER 4119HS resonator. The spectral recording parameters were: microwave power 0.86 mW; field sweep 10 mT; modulation amplitude 0.4 mT; conversion time 20 ms; time constant 20 ms; spectral resolution 1024 data points; number of scans 8; total recording time 5 min (which included the sample loading and spectrometer tuning times). The 3rd line of a Mn2+:MgO reference sample was used to adjust spectra of the samples to the same g-factor values.
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A 250 mCi 90Sr/90Y beta source was used for irradiation of all samples. The source was calibrated against a National Institute of Standards and Technology (NIST) secondary standard 60Co source in terms of absorbed doses to water using Luxel Al2O3:C optically stimulated luminescence (OSL) dosimeters.
3. Results and Discussion 3.1 EPR spectra
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Examples of EPR signals observed in samples from Sets #1-#3 are shown in Fig. 1. Signals in Fig. 1(a) were recorded soon after corresponding treatments (exposure, cutting or soaking) while those in Fig. 1(b) were collected 5 days later (between the two measurements the samples were kept at ambient conditions). All signals demonstrated significant instability with time under such storage conditions.
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To deconvolute the spectra shown in Fig. 1 and similar ones, we used the method described by Sholom and Chumak (2003). We found that for most samples it was enough to use just two reference spectra to fit the experimental spectra. These reference spectra are shown in Fig. 2; they were obtained in the following ways. Ref1 is a result of subtraction of two spectra from a 50 Gy sample: first one recorded at 3 h after exposure and second one recorded after sample storage for 5 days at ambient conditions. Ref2 was obtained from another (unirradiated) sample and is a subtraction of two spectra: recorded immediately after cutting and after storage for 3 hours at ambient conditions.. The Ref1 signal is a singlet at g=2.005 that describes the RIS, BG and the stable component of MIS, while Ref2 is a doublet with the center at g=2.005 that corresponds to an unstable component of MIS, but may which also contribute to the radiation-induced signal. Both signals are very similar to those described in other publications (e.g. Black and Swarts, 2010). 3.2 Stability Fig. 3 shows the RIS changes in the sample following repeated 15-min soakings in water (i.e. storage at 100% humidity). The RIS intensity shown here and elsewhere in this paper was obtained using the Ref1 reference spectrum according to the method of Sholom and Chumak (2003). (All signals were normalized to sample weight and adjusted to the intensity
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of the 3rd line of a Mn2+:MgO reference sample.) It should be noted that for all tested samples we did not observe a stable radiation-induced signal. That is, we did not observe the X-band equivalent signal to the Q-band RIS5 signal observed by Trompier et al., 2014). After the 4th soaking (for a total soaking time of 60 min) the RIS intensity usually became the same as the BG intensity of the sample (dotted line in Fig. 3(a)). Experimental data points in Fig. 3(a) were fitted with an exponential decay function y = A1*exp(−x/t1) + y0 (the dashed line in Fig. 3(b)). The best fit corresponded to the following parameters: y0 = 0.041 arbitrary units, corresponding to the stable background signal; A1 = 0.96, corresponding to the signal amplitude (in arbitrary units); and t1 = 6.73 min, corresponding to the decay constant. These parameters can be used to estimate how the main radiationinduced signal is reduced with soaking time; the corresponding plot is shown in Fig. 3(b). According to this plot, 50 % of the initial RIS is left in nails after soaking for 5 min and only 26 % after soaking for 10 min, which confirms that possible post-irradiation contact of nail clips with water is one of the factors creating the most uncertainty in the EPR dosimetry using nails. We propose that such contact should be avoided all times, at least following nail harvesting and before EPR measurements. We then tested the stability of the EPR signals for two more storage conditions: in vacuum (0 % humidity) and at ambient laboratory conditions (62 % humidity, 20°C). RIS, MIS and BG signals from the corresponding sample aliquots are shown in Figs. 4-6, respectively. They were recorded at 1 h (15 min for MIS), 1 day and 5 days after the corresponding treatment. (Data for other times are also available, but not shown in Figs. 4-6 for clarity.)
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It can be seen in Fig. 4 that storage in a vacuum resulted in good stability of the RIS. Its intensity dropped only about 10% during the first day after exposure and then remained almost constant. For comparison, the RIS intensity in samples stored at 62% humidity dropped on about 58 % during the first day after exposure and 81 % over five days. We note that the RIS positive peak (at about g=2.007) in Fig. 4(a) showed no instability and was the same for all tested times. Thus, we believe that the 10 % reduction seen in the vacuumstored samples is related only to a contribution to the RIS negative peak in the signal (located at about g=2.0025) from other, unidentified, unstable radicals. Another observation can be seen in Fig. 5. Both unstable (doublet) and stable (singlet) components of the mechanically induced signals displayed similar behavior with time for both conditions of storage - the unstable component was almost gone 2-3 h after sample cutting - but the shape of the stable MIS was slightly different for samples that were kept in vacuum compared with those stored at 62% humidity.
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Finally, very similar changes were observed for the BG signal for both vacuum and ambient storage (compare Figs. 6(a) and 6(b)). The intensities of this signal increased slowly with time at a rate that was slightly higher in the case of ambient storage. 3.3 Dose Response Fig. 7 demonstrates the dose response curves obtained from Experiment #2 while Table 1 shows the corresponding experimental data; each data point was obtained as an average of three EPR spectra, with shaking of the sample inside of a sample tube between two
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consecutive measurements. Four aliquots of the sample were exposed to doses in the range 0-10 Gy to simulate the emergency exposure. They were then additionally cut to simulate nail harvesting and measured at different times after exposure and cutting to simulate possible emergency dose reconstruction. Measurements in Fig. 7 started at 4 h at which point the unstable component of the MIS (see Ref2 in Fig. 2) had almost decayed to zero. At all times, excepting the EPR measurements themselves, the samples were stored in vacuum.
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As is seen in Fig. 7, the dose response curves for this sample can be fitted by a linear function for doses below 10 Gy for all time intervals over a period of 7 days after exposure. A clear tendency of the y-axis intercept to increase with time (from 0.88 to 1.79 arbitrary units) is also seen in Fig. 7. This reflects the increase of the MIS + BG cumulative intensity with time after exposure and cutting. (From the data presented in Figs. 5-6 it is clear that this increase is mainly due to an increase of the BG with a storage time.) The dose response curves for the other tested samples demonstrated similar properties, with a small variability for the dose response slope and a significant variability for the dose response intercept.
4. A possible protocol for dose reconstruction using EPR of nails On the basis of the obtained results, the following protocol is proposed for emergency dose reconstruction with nails: Keep all collected nail samples in a vacuum for all times following nail harvesting (excepting during EPR measurements and sample treatment). A portable vacuum desiccator is appropriate for such storage, but possibilities of home vacuum sealers that are used to seal the food can also be tested.
2.
Make the first EPR measurement as soon as possible (but not earlier than 3 h after nail harvesting). Record the time tc between nail harvesting and first EPR measurement. Avoid any additional cuts at this step; for large nail clippings use a sample tube of 8 mm in diameter. Determine the intensity EPR(Dx) of the EPR signal around g=2.005 (normalized on the sample weight and adjusted to the intensity of the reference spectra from Mn2+:MgO). This intensity can be written as an expression: EPR(Dx) = RIS(Dx) + MIS(tc) + BG(tc), where RIS(Dx) is a RIS caused by emergency exposure to unknown dose Dx, and MIS(tc) and BG(tc) are the intensities of MIS and BG, respectively, at the time tc after nail cutting. All EPR measurements at this step and later should be repeated three times with shaking of the sample inside of the tube between consecutive measurements; average values should be used for dose assessment.
3.
Soak the sample in water for 1 h followed by drying in vacuum for another 1 h to remove all RIS and MIS and minimize the BG signal.
4.
Select two aliquots from the sample to obtain the sample-specific dose response curves. One aliquot should be exposed to 10 Gy, another should be kept unexposed.
5.
Make an extra cut for nail clippings of two aliquots for a total length approximately the same as the length of the original cut (during nail harvesting). This extra cut is assumed to generate the same MIS as was generated during nail harvesting.
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1.
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6.
Conduct EPR measurements for 0 Gy and 10 Gy aliquots at the time tc after second cutting of the nail. Determine the weight-normalized, Mn2+ -adjusted intensities EPR(0 Gy) and EPR(10 Gy). These can then be written as expressions: EPR(0 Gy) = MIS(tc) + BG(tc) and EPR(10 Gy) = RIS(10 Gy) + MIS(tc) + BG(tc) where RIS(10 Gy) is the RIS caused by exposure to 10 Gy.
7.
The emergency dose Dx (in Gy) can then be estimated according to the expression: Dx = 10 * (EPR(Dx) – EPR(0 Gy))/(EPR(10 Gy) – EPR(0 Gy)).
The protocol is summarized in Figure 8. We should note that step 5 above (extra cut) should be made as soon as possible after the initial cut to ensure, as much as possible, that the extra cut is made under the same conditions (i.e. ambient humidity and temperature) as the initial harvesting cut. Failure to ensure this will result in failure of the protocol.
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4.1 Test of the proposed protocol The results of the dose recovery test (Experiment #3) are shown in Table 2, using the protocol outlined above. Three values are reported in Table 2 for each nominal dose/fading time combination, each value corresponding to one of three different tested samples. It can be observed from the Table 2 that deviations in the reconstructed doses from corresponding nominal values increase with nominal dose increase from about 0.3-0.4 Gy for the nominal doses in the range 0-2 Gy to about 0.7 Gy for the nominal dose of 10 Gy, with an average value of about 0.5 Gy for the entire tested dose range.
5. Conclusion Author Manuscript
A new procedure is proposed for EPR dosimetry using finger- or toenails. The new procedure exploits the stability of the radiation-induced signal (RIS) at g=2.005 if the samples are kept in a vacuum after nail harvesting. Procedures were also developed to account for other EPR signals (mechanically induced, MIS, and background, BG), which overlap with RIS. The proposed dosimetry technique was tested in the laboratory to calculate the absorbed dose for laboratory-irradiated samples. It is demonstrated that the method has the ability to recover the doses in the region 0-10 Gy with a mean estimate of uncertainty of ~0.5 Gy over the entire dose range tested. The proposed technique doesn’t account for possible fading in vivo; this potential issue should be tested in separate experiments such as studying the fingernails of TBI (total body irradiation) patients. It is possible only to point out that storage in vacuum after clipping stops any further decay, and because measurement may not take place immediately after harvesting, arresting the fading after clipping is important.
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Acknowledgements The authors would like to acknowledge funding from the Pilot Project Program of the Dartmouth Physically Based Center for Medical Countermeasures Against Radiation, with NIH funding from the National Institute of Allergy and Infectious Diseases (U19-AI091173).
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References
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Black PJ, Swarts SG. Ex-vivo analysis of irradiated fingernails: chemical yields and properties of radiation-induced and mechanically induced radicals. Health Phys. 2010; 98:301–308. [PubMed: 20065698] Chandra H, Symons MC. Sulphur radicals formed by cutting a-keratin. Nature. 1987; 328:833–834. [PubMed: 2442616] He X, Swarts SG, Demidenko E, Flood AB, Grinberg O, Gui J, Mariani M, Marsh SD, Ruuge AE, Sidabras JW, Tipikin D, Wilcox DE, Swartz HM. Development and validation of an ex vivo electron paramagnetic resonance fingernail biodosimetric method. Radiat Prot Dosimetry. 2014; 159(1-4): 172–181. [PubMed: 24803513] Marciniak A, Ciesielski B, Prawdzik-Dampc A. The effect of dose and water treatment on EPR signals in irradiated fingernails. Radiat. Prot. Dosim. 2014; 162(1-2):6–9. Reyes RA, Trompier F, Romanyukha A. Study of the stability of EPR signals after irradiation of fingernail samples. Health Phys. 2012; 103:175–180. [PubMed: 22951476] Romanyukha A, Trompier F, Reyes RA, Christensen DM, Iddins CJ, Sugarman SL. Electron paramagnetic resonance radiation dose assessment in fingernails of the victim exposed to high dose as result of an accident. Radiat Environ. Biophys. 2014; 53:755–762. [PubMed: 24957016] Sholom SV, Chumak VV. Decomposition of spectra in EPR dosimetry using the matrix method. Radiat. Meas. 2003; 37:365–370. Symons M, Chandra H, Wyatt J. Electron paramagnetic resonance spectra of irradiated finger-nails: a possible measure of accidental exposure. Radiat Prot Dosim. 1995; 58:11–15. Trompier F, Romanyukha A, Reyes R, Vezin H, Queinnec F, Gourier D. State of the art in nail dosimetry: free radicals identification and reaction mechanisms. Radiat. Environ. Biophys. 2014; 53(2):291–303. [PubMed: 24469226] Wang L, Wang X, Zhang W, Zhang H, Ruan S, Jiao L. Determining Dosimetric Properties and Lowest Detectable Dose of Fingernail Clippings from their Electron Paramagnetic Resonance Signal. Health Phys. 2015; 109(1):10–14. [PubMed: 26011494]
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HIGHLIGHTS •
A dry storage technique for EPR dosimetry of finger- and toenails is introduced.
•
The main EPR signals from nails are isolated and their stability is determined.
•
Uncertainties are of the order of ~0.5 Gy for the dose range 0-10 Gy.
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Fig. 1.
EPR signals of nails recorded at different times after sample treatment (exposure, cutting or soaking for 1 h). Fig. 1(a): RIS – a signal from a sample exposed to 50 Gy and recorded 1 h after exposure (Set #1 aliquots); MIS – a signal recorded at 15 min after an additional cut and 0 Gy (Set #2 aliquots); BG – a signal after sample soaking for 1 h (Set #3 aliquots). Fig. 1(b) shows the signals of the same samples recorded 5 days later after storage at ambient conditions.
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Fig. 2.
Reference EPR spectra used for deconvolution of EPR spectra of nails.
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Fig. 3.
Influence of repeated 15-min soakings on the EPR signal for samples exposed to a dose of 50 Gy. Fig. 3(a) shows three curves: average normalized RIS intensity with error bars (solid line); average normalized BG intensity (dotted line); and fitting with an exponential decay function (dashed line) of the RIS decay curve. Fig. 3(b) is a calculation of the RIS intensity reduction using the exponential decay fit from plot a.
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Fig. 4.
RIS evolution with time after sample exposure for sample storage in vacuum (Fig. 4(a)) and at 62 % humidity (Fig. 4(b)). “Fd1h” means fading for 1 hour, etc.
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Fig. 5.
MIS evolution with time after sample cutting for sample storage in vacuum (Fig. 5(a)) and at 62 % humidity (Fig. 5(b)). Same notation as in Fig. 4.
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Fig. 6.
BG evolution with time after sample soaking for 1 h for sample storage in vacuum (Fig. 6(a)) and at 62 % humidity (Fig. 6(b)). Same notation as in Fig. 4.
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Fig. 7.
Dose response curves for vacuum-stored samples obtained at different times after exposure and cutting. Fd = fading time. Each data set is fitted with a linear function of the form y = mx + c, with m = slope and c = intercept.
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Fig. 8.
Summary of proposed protocol.
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Table 1
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EPR signals with corresponding uncertainties observed in samples exposed to different doses and measured at different times after exposure. Three EPR spectra were recorded for each dose/time; the shown values are averages for three consecutive measurements of the samples. Time after exposure Dose, Gy
4h
1d
2d
3d
5d
7d
EPR signal, arbitrary units (corresponding standard deviation, same arbitrary units)
0
0.87 (0.07)
1.22 (0.03)
1.31 (0.07)
1.41 (0.09)
1.59 (0.03)
1.78 (0.03)
2
1.15 (0.05)
1.37 (0.06)
1.66 (0.03)
1.66 (0.04)
1.93 (0.09)
2.03 (0.05)
5
1.69 (0.02)
1.99 (0.05)
2.02 (0.08)
2.28 (0.07)
2.35 (0.03)
2.56 (0.08)
10
2.35 (0.07)
2.75 (0.05)
2.90 (0.03)
2.90 (0.11)
3.01 (0.06)
3.14 (0.08)
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Table 2
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Results of dose reconstruction for samples exposure to 0 Gy, 2 Gy, 5 Gy and 10 Gy and measured at different times after exposure
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Nominal dose, Gy
Fading time, days
Reconstructed dose, Gy
Mean (3 samples), Gy (SD*, Gy)
0
0.125
−0.1; −0.1; −0.9
−0.38 (0.5)
0
1
−0.4; 0.3; 0.2
0.04 (0.3)
0
3
0.1; 0.8; −0.5
0.16 (0.5)
0
7
0.1; 0.2; 0.3
0.18 (0.2)
2
0.125
2.1; 2.5; 2.5
2.36 (0.3)
2
1
2.7; 2.1; 2.6
2.46 (0.3)
2
3
2.0; 2.2; 3.0
2.41 (0.5)
2
7
2.3; 2.4; 3.1
2.64 (0.4)
5
0.125
5.5; 3.8; 4.7
4.65 (0.8)
5
1
5.0; 4.2; 4.9
4.70 (0.5)
5
3
5.6; 4.2; 4.7
4.83 (0.6)
5
7
5.9; 4.3; 4.8
5.01 (0.7)
10
0.125
10.4; 10.0; 10.7
10.4 (0.5)
10
1
10.9; 9.6; 11.1
10.5 (0.8)
10
3
11.0; 10.3; 11.1
10.8 (0.9)
10
7
10.5; 9.6; 10.8
10.3 (0.6)
Mean (all Fd), Gy (SD, Gy)
0.0 (0.4)
2.5 (0.3)
4.8 (0.6)
10.5 (0.7)
*
SD is a standard deviation for values of reconstructed doses calculated using corresponding true (nominal) doses
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Radiat Meas. Author manuscript; available in PMC 2017 May 01. Published in final edited form as: Radiat Meas. 2016 May 1; 88: 41–47. doi:10.1016/j.radmeas.2016.02.014.
Emergency EPR dosimetry technique using vacuum-stored dry nails S. Sholom1,* and S.W.S. McKeever1 1Radiation
Dosimetry Group, Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA
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Abstract Human finger- and toenails have been tested with an X-band EPR technique for different conditions of nail storage. The main radiation-induced signal at g=2.005 demonstrated good stability if the samples were stored in a vacuum at room temperature after nail harvesting and irradiation. On the basis of this phenomenon, a new protocol is proposed to use the nails as possible emergency EPR dosimeters. The dosimetry protocol was tested on laboratory-exposed samples and demonstrated the ability to recover doses in the region 0-10 Gy with an estimated uncertainty of approximately 0.3-0.4 Gy for doses in the range < 2 Gy, increasing to 0.6-0.7 Gy for doses in the range 5-10 Gy.
1. Introduction Author Manuscript Author Manuscript
Dosimetric EPR properties of finger- and toe-nails have been extensively studied for more than two decades (see e. g. Chandra and Symons, 1987; Symons et al., 1995; Trompier et al., 2014; He et al., 2014; Wang et al., 2015; and references therein), but the question - can nails be used for emergency/retrospective dose assessment? - remains still open. From an examination of Q-band EPR spectra, Trompier et al. (2014) came to a conclusion that only one component from irradiated nails (they called it the ‘RIS5’ EPR signal, where RIS stands for radiation-induced signal) can be used for dosimetric applications due to its stability during both thermal and water treatments. This component could be obtained and isolated through elimination of unstable EPR signals by humidification of the nails. Unfortunately, the intensity of the RIS5 signal observed in Trompier et al. (2014) was very low, and the corresponding EPR dosimetry technique was only able to reconstruct doses higher than 10 Gy. This is unacceptably high for possible applications of the protocol devised by Trompier et al. (2014) in emergency triage. Significant sensitivity increase can be obtained if one uses another EPR signal of nails, namely RIS2, according to the terminology of Trompier et al. (2014). (Hereafter we will call this signal simply RIS.) However, there are some challenges related to possible applications
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of this signal for emergency dose assessment. The main problem is the very high sensitivity of the RIS to water treatment of nails: soaking of the nail clippings for times of 10-15 min was enough to almost entirely remove this signal (Romanyukha et al., 2014; Marciniak et al., 2014). Thus, in order to use RIS for dose assessment it appears that the nail samples should be stored without any contact with water for all time between nail harvesting and EPR measurements. Another possible issue is the instability of all the main EPR signals from nails if the samples are stored under ambient conditions (Wang et al., 2015; Reyes et al., 2012; Black and Swarts, 2010). Due to the instability of these signals it is difficult to predict the time behavior of the cumulative EPR signal of nails at g-factor ~2.005. Greater complexity is introduced after exposure and clipping because the g=2.005 signal consists of multiple, strongly overlapping components. At a minimum, the RIS signal itself, a background signal (BG) and a mechanically induced singlet signal (MIS) all appear at the same g-factor. Furthermore, these signals may change their intensities in opposite directions with time. The next potential problem is a correct determination and separation of RIS, BG and MIS contributions to the cumulative EPR signal. This is especially a problem if the first EPR measurement is conducted a few days after the emergency exposure and sample clipping. All mentioned issues should be properly addressed before an EPR dosimetry technique with the RIS can be used for dose assessment.
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In the present study some innovations are introduced in order to improve the EPR dosimetry technique with nails. We focus on X-band EPR, since this is the most widely available EPR frequency range, and we propose storage of the samples in vacuum for all time between sample clipping and EPR measurements. For such storage, as we will show, the RIS intensity is stable for at least 7 days after exposure, and the evolutions of the BG and MIS signals with storage time are reproducible for different samples. We developed a procedure of isolation and separate observation of the RIS, MIS and BG signals using a combination of water soaking, irradiation and additional clipping for multiple nail samples. Finally, we developed a so-called vacuum-stored, dry-nail dose reconstruction protocol that allows the reconstruction of doses as low as 2 Gy up to at least 7 days after exposure.
2. Materials and Methods
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Twenty volunteers provided toe- and finger-nails for this study. Nails were collected during routine hygienic procedures and were stored at ambient conditions between clipping and submitting to the research laboratory. Before any measurement, the nail clippings were further cut (if necessary) to pieces of 4-5 mm length (in order to fit the size of an EPR sample tube) and soaked in water for one hour followed by drying in a vacuum oven also for one hour. This procedure removed all possible pre-experiment, radiation-induced and mechanically induced signals and minimized the background signal intensity. Three experiments were conducted with different sample aliquots. In Experiment #1, twenty seven sample aliquots were weighed from the nails of three individuals (nine aliquots from each individual) and subsequently divided into three sets (each set including three aliquots from each individual). Aliquots from the different sets were treated in different ways to obtain spectra either with dominating radiation-induced, or mechanically induced, or background signals. Aliquots from Set #1were exposed to 50 Gy to produce the radiation-
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induced signals. A high dose (50 Gy) was used so that the RIS signal dominated the spectrum and so that the shape for this signal could be clearly discerned. Aliquots from Set #2 received additional cuts, without any irradiation, to produce pieces of 1-2 mm length. This procedure induced a strong mechanically induced signal (MIS) with neither a RIS nor an increase in BG. No additional treatments were applied to the aliquots of Set #3. These were thus used to obtain the background (BG) signal only. Each of the three sets was additionally split into three subsets (of one aliquot each) and stored at three different values of humidity.
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The following cases were tested: (a) storage in a vacuum (zero humidity); (b) storage in ambient laboratory conditions (at 20°C and 62% of humidity); and (c) storage in water (i.e. 100% humidity). X-band EPR spectra were recorded at different times after either sample exposure (Set #1 aliquots), extra clipping (Set #2 aliquots), or 1-h soaking and 1-h drying (Set #3 aliquots) over a period of 7 days. In Experiment #2, thirty two aliquots were weighed from nails from eight individuals (four aliquots from each individual) and used to study the dose response. For this experiment, nail clippings of all aliquots were additionally cut to obtain pieces of 1-2 mm length. This simulated clipping during nail harvesting and generated mechanically induced signals. Aliquots of every individual were then exposed to doses of 0 Gy, 2 Gy, 5 Gy and 10 Gy to simulate the emergency exposure. We also studied how the dose response curves of the individual samples changed with time after exposure. All sample aliquots in this experiment were stored in a vacuum for all times, excepting the periods when the EPR spectra were recorded. (The EPR measurement procedure normally took approximately 5 minutes under ambient conditions.)
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Experiment #3 was a dose reconstruction test. 18 aliquots were prepared from nails of three individuals (aliquots ##1-6 from each individual) and treated in the following way. Aliquots ##1-4 were cut to pieces of 1-2 mm length to simulate clipping during nail harvesting followed by exposure to doses of 0 Gy, 2 Gy, 5 Gy and 10 Gy to simulate the emergency exposure; these aliquots were stored in vacuum and measured at 4 hours, 1 day, 3 days and 7 days after exposure to simulate the fading. Aliquots ##5-6 were also cut to 1-2 mm pieces followed by exposure to 10 Gy dose, but then these aliquots were soaked for 1 h in water (to remove all signals from previous mechanical stress and exposure), dried for 1 h in vacuum and used to regenerate the BG, MIS and RIS signals, which were used according to a dose reconstruction protocol described in Section 4 below. To regenerate the BG+MIS signals, an aliquot #5 was extra cut to approximately the same total cut length as that of corresponding aliquots ##1-4; to regenerate the BG+MIS+RIS signals, an aliquot #6 was cut in the similar way as the aliquot #5 followed by exposure to 10 Gy dose. Aliquots ##5-6 were stored in vacuum and also measured at 4 hours, 1 day, 3 days and 7 days after exposure. All samples for Experiments #1 and #3 were fingernails while in Experiment #2 six samples were fingernails and two more samples were toenails. (In fact, no differences were observed between the properties of finger- and toenail samples in subsequent experiments.)
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A Bel-Art Scienceware F42010 compact vacuum desiccator was used to store the samples in vacuum. By using a Robinair 15115 vacuum pump, a partial vacuum of 7.59 mm Hg can be obtained in this desiccator within a few minutes. The vacuum could be held for at least 48 hours without additional pumping. Maintaining a vacuum for longer periods required additional pumping. EPR measurements were conducted on a Bruker EMX spectrometer equipped with a Bruker ER 4119HS resonator. The spectral recording parameters were: microwave power 0.86 mW; field sweep 10 mT; modulation amplitude 0.4 mT; conversion time 20 ms; time constant 20 ms; spectral resolution 1024 data points; number of scans 8; total recording time 5 min (which included the sample loading and spectrometer tuning times). The 3rd line of a Mn2+:MgO reference sample was used to adjust spectra of the samples to the same g-factor values.
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A 250 mCi 90Sr/90Y beta source was used for irradiation of all samples. The source was calibrated against a National Institute of Standards and Technology (NIST) secondary standard 60Co source in terms of absorbed doses to water using Luxel Al2O3:C optically stimulated luminescence (OSL) dosimeters.
3. Results and Discussion 3.1 EPR spectra
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Examples of EPR signals observed in samples from Sets #1-#3 are shown in Fig. 1. Signals in Fig. 1(a) were recorded soon after corresponding treatments (exposure, cutting or soaking) while those in Fig. 1(b) were collected 5 days later (between the two measurements the samples were kept at ambient conditions). All signals demonstrated significant instability with time under such storage conditions.
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To deconvolute the spectra shown in Fig. 1 and similar ones, we used the method described by Sholom and Chumak (2003). We found that for most samples it was enough to use just two reference spectra to fit the experimental spectra. These reference spectra are shown in Fig. 2; they were obtained in the following ways. Ref1 is a result of subtraction of two spectra from a 50 Gy sample: first one recorded at 3 h after exposure and second one recorded after sample storage for 5 days at ambient conditions. Ref2 was obtained from another (unirradiated) sample and is a subtraction of two spectra: recorded immediately after cutting and after storage for 3 hours at ambient conditions.. The Ref1 signal is a singlet at g=2.005 that describes the RIS, BG and the stable component of MIS, while Ref2 is a doublet with the center at g=2.005 that corresponds to an unstable component of MIS, but may which also contribute to the radiation-induced signal. Both signals are very similar to those described in other publications (e.g. Black and Swarts, 2010). 3.2 Stability Fig. 3 shows the RIS changes in the sample following repeated 15-min soakings in water (i.e. storage at 100% humidity). The RIS intensity shown here and elsewhere in this paper was obtained using the Ref1 reference spectrum according to the method of Sholom and Chumak (2003). (All signals were normalized to sample weight and adjusted to the intensity
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of the 3rd line of a Mn2+:MgO reference sample.) It should be noted that for all tested samples we did not observe a stable radiation-induced signal. That is, we did not observe the X-band equivalent signal to the Q-band RIS5 signal observed by Trompier et al., 2014). After the 4th soaking (for a total soaking time of 60 min) the RIS intensity usually became the same as the BG intensity of the sample (dotted line in Fig. 3(a)). Experimental data points in Fig. 3(a) were fitted with an exponential decay function y = A1*exp(−x/t1) + y0 (the dashed line in Fig. 3(b)). The best fit corresponded to the following parameters: y0 = 0.041 arbitrary units, corresponding to the stable background signal; A1 = 0.96, corresponding to the signal amplitude (in arbitrary units); and t1 = 6.73 min, corresponding to the decay constant. These parameters can be used to estimate how the main radiationinduced signal is reduced with soaking time; the corresponding plot is shown in Fig. 3(b). According to this plot, 50 % of the initial RIS is left in nails after soaking for 5 min and only 26 % after soaking for 10 min, which confirms that possible post-irradiation contact of nail clips with water is one of the factors creating the most uncertainty in the EPR dosimetry using nails. We propose that such contact should be avoided all times, at least following nail harvesting and before EPR measurements. We then tested the stability of the EPR signals for two more storage conditions: in vacuum (0 % humidity) and at ambient laboratory conditions (62 % humidity, 20°C). RIS, MIS and BG signals from the corresponding sample aliquots are shown in Figs. 4-6, respectively. They were recorded at 1 h (15 min for MIS), 1 day and 5 days after the corresponding treatment. (Data for other times are also available, but not shown in Figs. 4-6 for clarity.)
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It can be seen in Fig. 4 that storage in a vacuum resulted in good stability of the RIS. Its intensity dropped only about 10% during the first day after exposure and then remained almost constant. For comparison, the RIS intensity in samples stored at 62% humidity dropped on about 58 % during the first day after exposure and 81 % over five days. We note that the RIS positive peak (at about g=2.007) in Fig. 4(a) showed no instability and was the same for all tested times. Thus, we believe that the 10 % reduction seen in the vacuumstored samples is related only to a contribution to the RIS negative peak in the signal (located at about g=2.0025) from other, unidentified, unstable radicals. Another observation can be seen in Fig. 5. Both unstable (doublet) and stable (singlet) components of the mechanically induced signals displayed similar behavior with time for both conditions of storage - the unstable component was almost gone 2-3 h after sample cutting - but the shape of the stable MIS was slightly different for samples that were kept in vacuum compared with those stored at 62% humidity.
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Finally, very similar changes were observed for the BG signal for both vacuum and ambient storage (compare Figs. 6(a) and 6(b)). The intensities of this signal increased slowly with time at a rate that was slightly higher in the case of ambient storage. 3.3 Dose Response Fig. 7 demonstrates the dose response curves obtained from Experiment #2 while Table 1 shows the corresponding experimental data; each data point was obtained as an average of three EPR spectra, with shaking of the sample inside of a sample tube between two
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consecutive measurements. Four aliquots of the sample were exposed to doses in the range 0-10 Gy to simulate the emergency exposure. They were then additionally cut to simulate nail harvesting and measured at different times after exposure and cutting to simulate possible emergency dose reconstruction. Measurements in Fig. 7 started at 4 h at which point the unstable component of the MIS (see Ref2 in Fig. 2) had almost decayed to zero. At all times, excepting the EPR measurements themselves, the samples were stored in vacuum.
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As is seen in Fig. 7, the dose response curves for this sample can be fitted by a linear function for doses below 10 Gy for all time intervals over a period of 7 days after exposure. A clear tendency of the y-axis intercept to increase with time (from 0.88 to 1.79 arbitrary units) is also seen in Fig. 7. This reflects the increase of the MIS + BG cumulative intensity with time after exposure and cutting. (From the data presented in Figs. 5-6 it is clear that this increase is mainly due to an increase of the BG with a storage time.) The dose response curves for the other tested samples demonstrated similar properties, with a small variability for the dose response slope and a significant variability for the dose response intercept.
4. A possible protocol for dose reconstruction using EPR of nails On the basis of the obtained results, the following protocol is proposed for emergency dose reconstruction with nails: Keep all collected nail samples in a vacuum for all times following nail harvesting (excepting during EPR measurements and sample treatment). A portable vacuum desiccator is appropriate for such storage, but possibilities of home vacuum sealers that are used to seal the food can also be tested.
2.
Make the first EPR measurement as soon as possible (but not earlier than 3 h after nail harvesting). Record the time tc between nail harvesting and first EPR measurement. Avoid any additional cuts at this step; for large nail clippings use a sample tube of 8 mm in diameter. Determine the intensity EPR(Dx) of the EPR signal around g=2.005 (normalized on the sample weight and adjusted to the intensity of the reference spectra from Mn2+:MgO). This intensity can be written as an expression: EPR(Dx) = RIS(Dx) + MIS(tc) + BG(tc), where RIS(Dx) is a RIS caused by emergency exposure to unknown dose Dx, and MIS(tc) and BG(tc) are the intensities of MIS and BG, respectively, at the time tc after nail cutting. All EPR measurements at this step and later should be repeated three times with shaking of the sample inside of the tube between consecutive measurements; average values should be used for dose assessment.
3.
Soak the sample in water for 1 h followed by drying in vacuum for another 1 h to remove all RIS and MIS and minimize the BG signal.
4.
Select two aliquots from the sample to obtain the sample-specific dose response curves. One aliquot should be exposed to 10 Gy, another should be kept unexposed.
5.
Make an extra cut for nail clippings of two aliquots for a total length approximately the same as the length of the original cut (during nail harvesting). This extra cut is assumed to generate the same MIS as was generated during nail harvesting.
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1.
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6.
Conduct EPR measurements for 0 Gy and 10 Gy aliquots at the time tc after second cutting of the nail. Determine the weight-normalized, Mn2+ -adjusted intensities EPR(0 Gy) and EPR(10 Gy). These can then be written as expressions: EPR(0 Gy) = MIS(tc) + BG(tc) and EPR(10 Gy) = RIS(10 Gy) + MIS(tc) + BG(tc) where RIS(10 Gy) is the RIS caused by exposure to 10 Gy.
7.
The emergency dose Dx (in Gy) can then be estimated according to the expression: Dx = 10 * (EPR(Dx) – EPR(0 Gy))/(EPR(10 Gy) – EPR(0 Gy)).
The protocol is summarized in Figure 8. We should note that step 5 above (extra cut) should be made as soon as possible after the initial cut to ensure, as much as possible, that the extra cut is made under the same conditions (i.e. ambient humidity and temperature) as the initial harvesting cut. Failure to ensure this will result in failure of the protocol.
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4.1 Test of the proposed protocol The results of the dose recovery test (Experiment #3) are shown in Table 2, using the protocol outlined above. Three values are reported in Table 2 for each nominal dose/fading time combination, each value corresponding to one of three different tested samples. It can be observed from the Table 2 that deviations in the reconstructed doses from corresponding nominal values increase with nominal dose increase from about 0.3-0.4 Gy for the nominal doses in the range 0-2 Gy to about 0.7 Gy for the nominal dose of 10 Gy, with an average value of about 0.5 Gy for the entire tested dose range.
5. Conclusion Author Manuscript
A new procedure is proposed for EPR dosimetry using finger- or toenails. The new procedure exploits the stability of the radiation-induced signal (RIS) at g=2.005 if the samples are kept in a vacuum after nail harvesting. Procedures were also developed to account for other EPR signals (mechanically induced, MIS, and background, BG), which overlap with RIS. The proposed dosimetry technique was tested in the laboratory to calculate the absorbed dose for laboratory-irradiated samples. It is demonstrated that the method has the ability to recover the doses in the region 0-10 Gy with a mean estimate of uncertainty of ~0.5 Gy over the entire dose range tested. The proposed technique doesn’t account for possible fading in vivo; this potential issue should be tested in separate experiments such as studying the fingernails of TBI (total body irradiation) patients. It is possible only to point out that storage in vacuum after clipping stops any further decay, and because measurement may not take place immediately after harvesting, arresting the fading after clipping is important.
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Acknowledgements The authors would like to acknowledge funding from the Pilot Project Program of the Dartmouth Physically Based Center for Medical Countermeasures Against Radiation, with NIH funding from the National Institute of Allergy and Infectious Diseases (U19-AI091173).
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References
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Black PJ, Swarts SG. Ex-vivo analysis of irradiated fingernails: chemical yields and properties of radiation-induced and mechanically induced radicals. Health Phys. 2010; 98:301–308. [PubMed: 20065698] Chandra H, Symons MC. Sulphur radicals formed by cutting a-keratin. Nature. 1987; 328:833–834. [PubMed: 2442616] He X, Swarts SG, Demidenko E, Flood AB, Grinberg O, Gui J, Mariani M, Marsh SD, Ruuge AE, Sidabras JW, Tipikin D, Wilcox DE, Swartz HM. Development and validation of an ex vivo electron paramagnetic resonance fingernail biodosimetric method. Radiat Prot Dosimetry. 2014; 159(1-4): 172–181. [PubMed: 24803513] Marciniak A, Ciesielski B, Prawdzik-Dampc A. The effect of dose and water treatment on EPR signals in irradiated fingernails. Radiat. Prot. Dosim. 2014; 162(1-2):6–9. Reyes RA, Trompier F, Romanyukha A. Study of the stability of EPR signals after irradiation of fingernail samples. Health Phys. 2012; 103:175–180. [PubMed: 22951476] Romanyukha A, Trompier F, Reyes RA, Christensen DM, Iddins CJ, Sugarman SL. Electron paramagnetic resonance radiation dose assessment in fingernails of the victim exposed to high dose as result of an accident. Radiat Environ. Biophys. 2014; 53:755–762. [PubMed: 24957016] Sholom SV, Chumak VV. Decomposition of spectra in EPR dosimetry using the matrix method. Radiat. Meas. 2003; 37:365–370. Symons M, Chandra H, Wyatt J. Electron paramagnetic resonance spectra of irradiated finger-nails: a possible measure of accidental exposure. Radiat Prot Dosim. 1995; 58:11–15. Trompier F, Romanyukha A, Reyes R, Vezin H, Queinnec F, Gourier D. State of the art in nail dosimetry: free radicals identification and reaction mechanisms. Radiat. Environ. Biophys. 2014; 53(2):291–303. [PubMed: 24469226] Wang L, Wang X, Zhang W, Zhang H, Ruan S, Jiao L. Determining Dosimetric Properties and Lowest Detectable Dose of Fingernail Clippings from their Electron Paramagnetic Resonance Signal. Health Phys. 2015; 109(1):10–14. [PubMed: 26011494]
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HIGHLIGHTS •
A dry storage technique for EPR dosimetry of finger- and toenails is introduced.
•
The main EPR signals from nails are isolated and their stability is determined.
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Uncertainties are of the order of ~0.5 Gy for the dose range 0-10 Gy.
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Fig. 1.
EPR signals of nails recorded at different times after sample treatment (exposure, cutting or soaking for 1 h). Fig. 1(a): RIS – a signal from a sample exposed to 50 Gy and recorded 1 h after exposure (Set #1 aliquots); MIS – a signal recorded at 15 min after an additional cut and 0 Gy (Set #2 aliquots); BG – a signal after sample soaking for 1 h (Set #3 aliquots). Fig. 1(b) shows the signals of the same samples recorded 5 days later after storage at ambient conditions.
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Fig. 2.
Reference EPR spectra used for deconvolution of EPR spectra of nails.
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Fig. 3.
Influence of repeated 15-min soakings on the EPR signal for samples exposed to a dose of 50 Gy. Fig. 3(a) shows three curves: average normalized RIS intensity with error bars (solid line); average normalized BG intensity (dotted line); and fitting with an exponential decay function (dashed line) of the RIS decay curve. Fig. 3(b) is a calculation of the RIS intensity reduction using the exponential decay fit from plot a.
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Fig. 4.
RIS evolution with time after sample exposure for sample storage in vacuum (Fig. 4(a)) and at 62 % humidity (Fig. 4(b)). “Fd1h” means fading for 1 hour, etc.
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Fig. 5.
MIS evolution with time after sample cutting for sample storage in vacuum (Fig. 5(a)) and at 62 % humidity (Fig. 5(b)). Same notation as in Fig. 4.
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Fig. 6.
BG evolution with time after sample soaking for 1 h for sample storage in vacuum (Fig. 6(a)) and at 62 % humidity (Fig. 6(b)). Same notation as in Fig. 4.
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Fig. 7.
Dose response curves for vacuum-stored samples obtained at different times after exposure and cutting. Fd = fading time. Each data set is fitted with a linear function of the form y = mx + c, with m = slope and c = intercept.
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Fig. 8.
Summary of proposed protocol.
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Table 1
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EPR signals with corresponding uncertainties observed in samples exposed to different doses and measured at different times after exposure. Three EPR spectra were recorded for each dose/time; the shown values are averages for three consecutive measurements of the samples. Time after exposure Dose, Gy
4h
1d
2d
3d
5d
7d
EPR signal, arbitrary units (corresponding standard deviation, same arbitrary units)
0
0.87 (0.07)
1.22 (0.03)
1.31 (0.07)
1.41 (0.09)
1.59 (0.03)
1.78 (0.03)
2
1.15 (0.05)
1.37 (0.06)
1.66 (0.03)
1.66 (0.04)
1.93 (0.09)
2.03 (0.05)
5
1.69 (0.02)
1.99 (0.05)
2.02 (0.08)
2.28 (0.07)
2.35 (0.03)
2.56 (0.08)
10
2.35 (0.07)
2.75 (0.05)
2.90 (0.03)
2.90 (0.11)
3.01 (0.06)
3.14 (0.08)
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Table 2
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Results of dose reconstruction for samples exposure to 0 Gy, 2 Gy, 5 Gy and 10 Gy and measured at different times after exposure
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Nominal dose, Gy
Fading time, days
Reconstructed dose, Gy
Mean (3 samples), Gy (SD*, Gy)
0
0.125
−0.1; −0.1; −0.9
−0.38 (0.5)
0
1
−0.4; 0.3; 0.2
0.04 (0.3)
0
3
0.1; 0.8; −0.5
0.16 (0.5)
0
7
0.1; 0.2; 0.3
0.18 (0.2)
2
0.125
2.1; 2.5; 2.5
2.36 (0.3)
2
1
2.7; 2.1; 2.6
2.46 (0.3)
2
3
2.0; 2.2; 3.0
2.41 (0.5)
2
7
2.3; 2.4; 3.1
2.64 (0.4)
5
0.125
5.5; 3.8; 4.7
4.65 (0.8)
5
1
5.0; 4.2; 4.9
4.70 (0.5)
5
3
5.6; 4.2; 4.7
4.83 (0.6)
5
7
5.9; 4.3; 4.8
5.01 (0.7)
10
0.125
10.4; 10.0; 10.7
10.4 (0.5)
10
1
10.9; 9.6; 11.1
10.5 (0.8)
10
3
11.0; 10.3; 11.1
10.8 (0.9)
10
7
10.5; 9.6; 10.8
10.3 (0.6)
Mean (all Fd), Gy (SD, Gy)
0.0 (0.4)
2.5 (0.3)
4.8 (0.6)
10.5 (0.7)
*
SD is a standard deviation for values of reconstructed doses calculated using corresponding true (nominal) doses
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