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EPR dose reconstruction for bone-seeking 90Sr
Applied Radiation and Isotopes PERGAMON
Applied Radiation and Isotopes 51 (1999) 151±159
EPR dose reconstruction for bone-seeking
90
Sr
E.A. Ignatiev a, *, N.M. Lyubashevskii b, E.A. Shishkina c, A.A. Romanyukha a a
Institute of Metal Physics, Russian Academy of Sciences, 620219 Ekaterinburg, Russia Institute of Plants and Animals Ecology, Russian Academy of Sciences, 620219 Ekaterinburg, Russia c Urals Research Center for Radiation Medicine, Medgorodok, 454076 Chelyabinsk, Russia
b
Received 6 May 1998; received in revised form 22 June 1998
Abstract The results of the EPR dose reconstruction in calci®ed tissues of dog injected with 90 Sr are presented. It has been established that there is no essential dierence in the values of doses absorbed in tooth tissues of teeth in symmetric positions in the mouth, whereas a signi®cant dierence occurs in the values of absorbed doses in teeth in nonsymmetric positions. In the case of 90 Sr internal exposure the dose reconstruction in crown dentine plays an important role. It has been found that its quantity is close to the dose in diaphyseal cortical bone of the femur, dose at the endosteal bone surface and in femural fatty marrow. The fact that these values exceed doses absorbed in tooth enamel points out the predominant contribution of internal exposure. The highest absorbed doses have been observed in metaphyseal trabecular femur bones, tooth alveolar bone walls, and cortical and trabecular vertebra that can be considered as suitable candidates for biomarkers of internal 90 Sr exposure for post-mortal autopsy. The satisfactory correlation has been found between the doses reconstructed in calci®ed dog tissues and the doses measured by EPR in alanine dosimeters ®xed in (or nearby) the sites of autopsy of bones/teeth. The experiments provide support for the view that EPR retrospective dosimetry with calci®ed tissues for internal exposure is unique in providing useful information on the doses obtained. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: EPR dosimetry; Strontium-90; Internal exposure; Calci®ed tissues; Tooth tissues
1. Introduction Environmental contamination by strontium-90 (90 Sr) is often the consequence of radioactive accidents. It took place after the Chernobyl accident as well as after several accidents that occurred on Mayak Production Association at Southern Urals, Russia (Romanyukha et al., 1996a,b). As a result of the discharges of liquid radioactive wastes in 1949±1956 by the latter nuclear facility, the population of the Techa riverside was exposed to excessive ionizing radiation. During 1949± 1951 about 76 106 m3 of radioactive waste water containing a total activity of 1017 Bq was discharged into
* Corresponding author.
the Techa river. The contribution from 90 Sr to the total radioactivity was 11.62% and comprised the main source of internal exposure of the local population. Water consumption led to the considerable internal overexposure of the Techa river valley population (about 28 000 people) and to the excessive incidence of leukemia (Kossenko et al., 1992). Besides, as a consequence of the so-called Kyshtym accident (explosion of the radioactive waste facility of Mayak Production Association in 1957), a considerable area of Southern and Middle Urals was again contaminated by 90 Sr that caused a signi®cant internal overexposure of the local population. Therefore, dose reconstruction of internal exposure by 90 Sr is an important problem for epidemiological studies of the consequences of many radioactive accidents. At the present time the whole body counter measurements of body burden and
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the model calculations on the basis of metabolic pathways and typical diet of population are used for dose evaluation of 90 Sr internal exposure (Degteva et al., 1994). However, experimental variations in metabolism and radionuclide distribution among humans must be also considered (Lyubashevskii, 1980). The required physiological uptake data for bones and other key organs are usually available only from biodistribution studies on the relatively small number of humans or animals. For the whole body counter, a serious problem also arises in connection with the calibration of measurement results in body burden units. The method of electron paramagnetic resonance (EPR) serves to reconstruct the dose absorbed in teeth and bones. It is based on the EPR measurements of the concentration of stable radiation-induced radicals in hydroxyapatite that is the mineral component of all calci®ed tissues (teeth and bones). Hydroxyapatite constitutes 95±97% of tooth enamel, 70±75% of dentine and 60±70% of bones. Therefore, from the physical point of view, tooth enamel is the most suitable material for retrospective dosimetry. EPR dosimetry in tooth enamel has been already applied to reconstruct radioactive doses obtained by survivors after A-bombardment of Hiroshima and Nagasaki (Ikeya, 1993), Russian nuclear workers (Romanyukha et al., 1994), the victim of the accident at the accelerator in Maryland (Schauer et al., 1996) and some other radioactive accidents (Serezhenkov et al., 1992). Desrosiers et al. (1993) have demonstrated the use of EPR dosimetry with bone for reconstruction of the internal dose. However, in practice the EPR dose reconstruction is generally done with teeth extracted according to medical indications, and there are essential dierences in the metabolism of 90 Sr in tooth enamel and bones (Lyubashevskii et al., 1996). Some ways to solve this problem have already been discussed by Romanyukha et al. (1996a,b). In the case of application of EPR doses reconstruction with teeth to the population exposed internally due to radioactive waste accidents there is an additional problem to determine the relationship between the doses absorbed in tooth enamel and, e.g. in the bone tissue or bone marrow, as would usually be considered in practical dosimetry. However, there is still a lack of understanding on how to do this. There are also some other questions, e.g. concerning the variation of the absorbed doses from tooth to tooth, the contribution of the self-irradiation of the tissue in the reconstructed dose, and the time-dose evolution in the tooth containing 90 Sr. The answers to these and other similar questions can be obtained only by experimental investigations of the dose distribution in skeletons of animal injected by 90 Sr. Thus, our EPR experiments with bones and teeth of the dog injected by 90 Sr have been performed to achieve the following purposes:
1. Comparison of the doses absorbed by tooth tissues from symmetric and non-symmetric positions in the dog mouth; 2. Investigation of the contributions in the doses absorbed by tooth tissues, especially the contribution of tooth tissue self-irradiation; calibration of the doses reconstructed by tissue equivalent dosimeter (alanine); 3. Comparison of the doses absorbed in dierent tooth tissues with the doses absorbed in other bones of the dog skeleton.
2. Experimental Strontium-90 (t 1/2 = 28 yr) was obtained as 90 SrCl2 solution. The radioactive 90 Sr was diluted with citric acid±sodium citrate buer solution to form a boneseeking radiopharmaceutical. A normal dog with a weight of 8.1 kg and age of 3.5 yr was used for this study. The 90 Sr radiopharmaceutical was injected intravenously at a dosage of 3 mL of 88.8 MBq solution (10.96 MBq kg ÿ 1) to this dog via the cephalic vein over a 1.5-min period. The dog was sacri®ced 52 h after the injection; then dierent bones and teeth were removed for study. During this 52 h the probes of blood and saliva were taken. All experiments with the dog were carried out in compliance with Russian national regulation relating to the conduct of animal experimentation. Two types of EPR experiments were conducted. The ®rst was an EPR dose reconstruction with dierent calci®ed tissues of dog, i.e. enamel and dentine of dierent teeth, alveolar bone of jaw, metaphysis and diaphysis of femur, metaphyseal bone of vertebra. The second is the dose assessment by alanine dosimeters placed in the same positions (or nearby) in the bone or teeth where the EPR probes of calci®ed tissues were taken (Figs. 1 and 2). The EPR measurements of exposed alanine pellets allowed determination of tissue
Fig. 1. Position of the alanine dosimeter No. 6 in the dog's jaw.
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Fig. 2. Positions of the alanine dosimeters and bone's autopsy for EPR analysis in diaphiseal cortical bone of right back leg of the dog.
equivalent dose (Regulla and Dener, 1982), which is important for estimation of dose contribution of internal irradiation. The dosimeters used were produced at the Institute for Radiation Protection of GSF National Research Center, Munich. In the ®rst type of EPR experiments the spectra were recorded at room temperature with the ERS-231 spectrometer (produced in the former GDR) operating in the X-band. For the mathematical treatment of spectral ®les this spectrometer was connected with an IBM/PC. The experimental parameters of the spectra recording were the following: 100 kHz modulation frequency, 0.45 mT modulation amplitude, 10 and 5 mT ®eldsweep (Fig. 3). The peak-to-peak amplitude of the g _ = 2.0018 line was taken as a relative measure of the concentration of the radiation-induced radicals. The precision of the individual EPR measurement of the amplitude was 210%. The EPR spectra were recorded at 12 and 2 mW. In order to subtract the broad back-
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ground signal from the total spectrum the method of selective saturation described by Ignatiev et al. (1996) was used. No chemical treatment was applied for the preparation of the EPR samples from teeth and bones to avoid radioactive contamination; only the careful mechanical separation of tooth enamel and dentine was performed. Before measurements the bone, enamel and dentine were dried, crushed and sieved to 0.2 mm grain size. Notations and of positions of the studied teeth in the dog's jaws are given on Fig. 4(a) and (b). The complete list of investigated samples prepared from the teeth are given in Tables 1 and 2 and the list of bone samples is given in the Table 3, namely, it was diaphyseal cortical femur bone of right back leg; metaphyseal cortical and trabecular femur bone of right back leg; then two samples were prepared from sinciput bone adjacent to interparietal suture (2 mm length and 1 and 3 mm widths respectively); and cortical and trabecular bone of vertebra. The dose reconstruction was performed by the additive dose method with the unweighted linear leastsquare-®t employing the Levenberg±Marquart algorithm. Five to six measurements, one original and four to ®ve after additional irradiation in steps of 5±60 Gy each (depending on the value of the reconstructed dose) were used for the linear ®t. Due to the signal anisotropy the EPR spectra were recorded ®ve to seven times, with the sample being re-shaken before and after every additional irradiation (Fig. 5). For exposure-to-absorbed dose conversion according to the mass energy-absorption the coecients for gammaradiation, as calculated by Schauer et al. (1996) were
Fig. 3. EPR spectra of tooth tissues of upper left PM4 tooth and adjacent jaw cortical bone. Enamel, bone, root dentine and crown dentine spectra are arranged from upper to lower parts.
0.242 0.02 0.392 0.03 0.662 0.05 0.752 0.05 Ð 0.1920.02 0.3920.03 0.6820.05 Ð Ð dose dose dose dose dose
rate rate rate rate rate
in in in in in
enamel, Gy per day crown dentine, Gy per day root dentine, Gy per day alveolar bone adjacent to root, Gy per day partition between roots, Gy per day Daily Daily Daily Daily Daily
0.33 20.02 0.44 20.03 1.08 20.07 1.07 20.07 Ð
Lower right M1 (extraction in 174 days) Upper left PM4 (extraction in 141 days) Upper right PM4 (extraction in 76 days) Tissue
used, namely: 8.72 10 ÿ 3 Gy R ÿ 1 for tooth enamel, 9.03 10 ÿ 3 Gy R ÿ 1 for bone and dentine. The irradiations were made with a medical 60 Co source at Ekaterinburg Regional Cancer Hospital with an uncertainty of 5% at 95% con®dence level. In the second series of EPR experiments; 10 alanine pellets were ®xed inside or nearby the dierent calci®ed tissues, parts of which were used for dose reconstruction. Dosimeter number 1 was inserted in place of the root of lower right eighth tooth (PM4) and was covered by piece of overroot slice of this tooth (Fig. 1). Five alanine dosimeters (numbers 2±6) were ®xed with help of wax paper and scotch tape on the surface of the jaw bone adjacent to the roots of the teeth investigated. Dosimeters numbered 2±5 were ®xed on the lingual surface of the jaw and dosimeter number 6 was ®xed on the labial surface of the jaw. Dosimeter 2 was close to the upper left ninth tooth (M1), number 3 was
Table 1 Results of dose rate reconstruction for upper eighth and lower ninth tooth positions
Fig. 4. Notations and positions of the teeth studied with EPR on schematic design of the dog's jaws. (a) upper jaw; (b) lower jaw.
0.1920.02 0.3820.03 0.6720.05 0.6020.06 1.2220.8
E.A. Ignatiev et al. / Applied Radiation and Isotopes 51 (1999) 151±159 Lower left M1 (extraction in 208 days)
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Table 2 Results of dose rate reconstruction for roots of PM2's and PM3's extracted at 234 days after injection Tooth position
Upper right PM2
Upper left PM2
Lower right PM3
Lower left PM3
Daily dose rate in root dentine, Gy per day
0.9320.06
1.282 0.09
0.9720.08
1.002 0.07
Table 3 Results of dose rate reconstruction for dierent bone tissues Tissue
Daily dose rate, Gy per day
Sinciput bone adjacent to junction (1 mm width) Sinciput bone adjacent to junction (3 mm width) Diaphiseal cortical bone of right back leg
0.73 20.07 0.68 20.07 0.52 20.05
Fig. 5. Dose reconstruction for tooth tissues of upper left PM4: (w) enamel, () crown dentine, (q) root dentine, (Q) adjacent jaw bone.
®xed close to upper right ninth tooth (M1), number 4 was close to upper left eighth tooth (PM4) and dosimeters 5 and 6 were near to lower left ninth tooth (M1). Another group of alanine dosimeters was put inside femur bones of right back leg of the dog. Dosimeter 10 was placed inside the diaphysis of femur as shown in Fig. 2. Dosimeters 7, 8 and 9 were placed inside the proximal and distal femur bones in the drilled holes (Fig. 2, Table 4) and were covered with the bone fragments. After 43 days all alanine dosimeters were removed for the EPR analysis. The EPR measurements with the dose absorbed in alanine pellets were done with EPR spectroanalyzer EMS-104 (Bruker, Germany) operating in the X-band at GSFNational Research Center, Munich. The standard procedure developed at GSF by Regulla and Dener
Table 4 Results of dose rate measurements by alanine dosimeters. Comparison with reconstructed dose rates in calci®ed tissues adjacent to the positions of alanine dosimeters Dosimeter No.
Position of dosimeter
Dose rate measured by alanine dosimeter (Gy per day)
Reconstructed dose rate in adjacent calci®ed tissues (Gy per day)
1 2 3 4 5 6 7 8 9 10
root pit jaw bone jaw bone jaw bone jaw bone jaw bone proximal epiphyseal femur proximal epiphyseal femur distal epiphyseal femur diaphyseal femural bone
0.42 0.31 0.32 0.26 0.21 0.29 0.77 1.24 1.05 0.49
0.82 20.09 1.07 20.07 0.68 20.05 1.07 20.07 0.60 20.06 0.60 20.06 ± ± ± 0.52 20.05
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(1982) for such measurements was used. The abovementioned measurements with alanine were made independently on EPR dose reconstruction measurements at Ekaterinburg, without providing information on values of doses reconstructed in calci®ed tissues. Dierent samples of calci®ed tissues were extracted at dierent times after sacri®ce, and the corresponding EPR measurements were also performed at dierent times. Therefore, the value of daily dose rate was used for characterization both of doses absorbed by calci®ed tissues and measured by alanine dosimeters.
given in Table 3. EPR investigations of vertebra dose showed a saturation eect. Such eects were observed at dose rates greater than 1.5 Gy per day over 234 days after injection.
3. Results
1. The dose per day in the root pit of the eighth lower right tooth (PM4) is a factor of 2 lower than the dose rates absorbed in adjoining roots. Such result con®rms our above-mentioned suggestion concerning the essential contribution from alveolar bone walls to exposure of the root dentine; 2. Dosimeters 2±6 indicate the dose rates on the surfaces of cortical jaw bone. The average dose rate measured on the surface of jaw bone is a factor of 3 lower than the dose rate reconstructed in this bone (Table 1); 3. Dosimeters 7±9 indicate the dose rates inside the physeal proximal and distal femur bones (red marrow and trabecular bone); 4. The dose rate measured in the femur diaphyseal (dose in fatty marrow and on the endostal bone surface) by dosimeter 10 is slightly below the reconstructed dose rate in the cortical diaphyseal bone (0.49 Gy per day and 0.52 Gy per day, respectively).
76 to 208 days after sacri®ce; the four teeth of the symmetric positions were extracted. Due to the dierent sizes of upper and lower jaws of the dog (Fig. 4(a) and (b)) the upper PM4's are symmetrical to the lower M1's. Doses per day absorbed by tooth enamel, dentine (both crown and root) and adjacent alveolar bone were reconstructed (Table 1). At 234 days after sacri®ce the samples from next four symmetric teeth Ð upper six (PM2) and lower seven (PM3) were studied. For this set of teeth only samples from root dentine were prepared due to the relatively small quantity of tooth enamel on these teeth. The results of EPR reconstruction are given in Table 2 in terms of daily dose rate. To investigate the dose contribution of self-irradiation the upper left PM4 was used. This tooth was extracted 141 days after sacri®ce. Then it was cut into two halves in the crosswise direction of the jaw. One half of this tooth was used for the preparation of tooth enamel and dentine samples for the EPR measurements on the day of tooth extraction. The second half was stored in a thick-walled glass vessel for 86 days for subsequent EPR measurements. During storage the dose absorbed in tooth enamel increased in a factor of (1.612 0.10); the dose absorbed in crown dentine increased in a factor of (1.68 2 0.10), and the dose absorbed in root dentine remained essentially unchanged (1.17 20.08). The doses absorbed in the samples prepared from the following bones were reconstructed: . diaphyseal cortical femur bone of right back leg (Fig. 2); . metaphyseal cortical and trabecular femur bone of right back leg; two samples were also prepared from sinciput bone adjacent to interparietal suture (2 mm length and 1 and 3 mm widths respectively); . cortical and trabecular bone of vertebra extracted 234 days after injection. The results of dose reconstruction in bone samples are
Another important source of information about the contribution of the self-irradiation due to 90 Sr deposition is a comparison of the reconstructed doses in calci®ed tissues and the doses measured nearby with the help of alanine dosimeters. Table 4 presents dose rate results of the measurements with alanine dosimeters and reconstructed doses obtained simultaneously. These results can be summarized a follows:
4. Discussion The shape of the experimental time dependencies of Sr concentrations in the blood and saliva has indicated that the processes of transfer and deposition of 90 Sr in calci®ed tissues peak at the time of sacri®ce. Consequently, the contribution of 90 Sr contained in saliva is much less for the doses absorbed by calci®ed tissues than the contribution of 90 Sr deposited in calci®ed tissues. Therefore, the whole dog skeleton and its separate parts can be considered as phantoms of special types for investigation of dose distribution due to internal exposure by 90 Sr. As seen from Table 1, the doses absorbed in tooth enamel samples prepared from teeth in three symmetric positions are closely related. A similar situation takes place with regard to the both crown and root dentine samples of the same three teeth. However, for the upper left PM4 the considerably higher doses absorbed in all investigated samples (about 60% over) have been 90
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obtained in comparison with other three symmetric teeth. Similar results took place for the PM2's and PM3's teeth (Table 2), i.e. for the tooth from the upper left position the considerably higher absorbed dose rate was found. Detailed visual investigation has shown the sites of serious trauma on the left side of the upper dog jaw. It is well known (see, for example, ICRP, 1973, Publication 20, or Starichenko et al., 1993) that the bone repair trauma is usually accompanied by prolonged remodeling with expanded blood supply, increased bone surface and other processes providing more intensive deposition of radionuclides. The dierence between the doses absorbed in root dentine of non-symmetric teeth (about 30%, compare data of Tables 1 and 2) is also essential. This fact may be explained by dierent blood supply of teeth in dierent positions and it needs to be ascertained in the course of future investigations. Comparison of the reconstructed doses for dierent tooth tissues showed a considerable dierence between the doses absorbed by tooth enamel, crown and root dentine. The mean ratio of these doses is 1:1.6:2.8 for enamel, crown and root dentine, respectively. These results generally agree with those obtained by Romanyukha et al. (1996a) for the teeth of Techa riverside residents who had consumed water polluted by 90 Sr (in the latter case the ratio of the absorbed doses in enamel and root dentine was 1:5). The observed dierence between the absorbed dose ratios for tooth enamel and root dentine of dog and humans can be explained by the dierence in the thickness of the tooth enamel layer. It was 0.3±0.5 mm for dog teeth and 1±2 mm for human teeth that is critical at limited depth penetration of b-particles emitting by 90 Sr and 90 Y. The evidence of the dose excess in dentine in comparison with tooth enamel can be used to distinguish the eect of internal exposure of 90 Sr from external exposure according to EPR dose reconstruction. Another important result is a signi®cant dierence between the doses absorbed by crown and root dentine. It can be explained by a better contact of root dentine with blood vessels, by root dentine next to the bone jaw, as well as by root partitions which have relatively high level of 90 Sr deposition. Similar results have been also obtained by chemical analysis of the teeth of Techa riverside residents for whom 90 Sr concentration in root dentine was higher in comparison with crown dentine as measured by Wieser et al. (1996a). Investigation of the contribution of self-irradiation in the absorbed doses carried out for two halves of the upper left PM4 has revealed the following ®ndings: During 86 days storage of its one half the dose absorbed in tooth enamel increased in a factor of (1.612 0.10); the dose absorbed in its crown dentine increased in a factor of (1.68 2 0.10), and the dose absorbed in root dentine remained essentially
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unchanged (1.17 2 0.08). On the other hand, if this tooth had still remained in the jaw, the doses in all tooth tissues would have to increase by a factor of 141/86 = 1.64. Consequently, for tooth enamel and crown dentine the absorbed doses are determined mostly by self-irradiation from internal 90 Sr. The anomaly observed for root dentine may be due to contribution of the absorbed dose from the tooth alveolar bone walls, where a very high daily absorbed dose rate was reconstructed (Table 1). Another possible reason of small changing of the dose in root dentine is the in¯uence of the above-mentioned old bone trauma of the left upper jaw. Comparison of the absorbed dose rates in calci®ed tissues under investigation (Tables 1 and 3) allows the distinction of the following four groups of tissues: 1. 0.2±0.3 Gy per day: tooth enamel; 2. 0.4±0.5 Gy per day: crown dentine, diaphyseal cortical femur bone, femur yellow marrow, femur endostal bone surface; 3. 0.7±1.0 Gy per day: root dentine, cortical jaw bone, sinciput bones, sinciput bone adjacent to interparietale suture; 4. higher than 1.0 Gy per day: tooth alveolar bone walls, proximal and distal epiphysial femur bone, cortical and trabecular bone of vertebra. The proposed classi®cation underlines a special importance of dose reconstruction in the crown dentine for the case of 90 Sr internal exposure, since it is close to the absorbed doses in diaphyseal cortical femur bones, in cortical bones, as well as on the endostal bone surface and in femur yellow marrow measured by alanine dosimeters. Moreover, the lowest dose rate scattering (in the limits of 25%) among dierent teeth (see Table 1) has been found for the crown dentine. Thus, contrary to the dose absorbed in tooth enamel, the value of the dose absorbed in the crown dentine can easily be interpreted. The dose absorbed in root dentine is close to the dose absorbed by trabecular bone and cortical jaw bone. The observed dose saturation in vertebra at the dose higher than 350 Gy disagrees with the published results on the dose saturation at external g-exposure at 100 kGy (see, for example, Ikeya, 1993). Possible explanation of this discrepancy may be connected with a considerable dose gradient at internal b-exposure due to limited depth penetration (1±3 mm) and essential variation of 90 Sr deposition in the bone tissue. Direct comparison of the doses measured by alanine dosimeters and reconstructed doses in adjacent calci®ed tissues (Table 4) permits the selection of three dierent groups of the dosimetry results:
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1. Dose measured by alanine dosimeter is approximately equal to that reconstructed in the adjacent calci®ed tissue (dosimeter No. 10); 2. Dose measured by alanine dosimeter is about half of the reconstructed one (dosimeters No. 1, 3, 6); 3. Dose measured by alanine dosimeter is more than factor of 2 less than the reconstructed one (dosimeters No. 2 and 5). It is reasonable to assume that in the ®rst selected group of dosimeters there are exactly the same geometries of the exposure of dosimeter and bone sample, that in the second group there are hemispherical geometries of the dosimeter's exposure because it was ®xed on the bone surface, and for the third group of two dosimeters the considerable dierence in doses can probably be explained by the bad contact between dosimeter and bone, which is very important for b-exposure. The latter assumption allows to us to estimate the mean dose deviation as measured by alanine dosimeter and reconstructed in calci®ed tissues on the base comparison of the resulting doses measured by dosimeters of ®rst two selected groups (1, 3, 6, 10) and the respective reconstructed doses. For the second group of dosimeter the factor 2 was used for the comparison with results of EPR reconstruction. The simple calculation shows that dose measured by alanine is factor of 1.01 2 0.15 less than reconstructed one. Thus, we can conclude that EPR dose reconstruction in calci®ed tissues gives a dose which is very close to the tissue equivalent value. 5. Conclusions The reconstructed doses at real accidents (e.g. Techa riverside) have been de®ned with the help of tooth reconstruction dosimetry. The results allow us to estimate the important role of the EPR method in the dose reconstruction of calci®ed tissues for internal exposure. One of the most interesting and unexpected results is the lack of correlation between the doses reconstructed in tooth enamel with EPR and 90 Sr chemical content in the samples and in root dentine for 14 Techa riverside residents (Wieser et al., 1996b). On the other hand, a close correlation between the doses measured by alanine dosimeters characterizing 90 Sr content and the doses absorbed in the same bone tissue has been found for the dog (Table 4). There are two essential distinguishing features concerning conditions of exposure and the dose reconstruction for dog and Techa riverside residences, namely the regime of 90 Sr ingestion and the period of time between the dose reconstruction performance and 90 Sr ingestion. As dierentiated from dog, the population of Techa river-
side had consumed 90 Sr over a period of years, and EPR measurements were performed after the initial ingestion 40 years ago, when the main consumption of 90 Sr had taken place. Consequently, the 90 Sr content and the dose reconstructed by EPR in the corresponding calci®ed tissue dier from each other and can be considered as complementary parameters. The former parameter overlooks the simply accumulated dose and the latter parameter neglects both anatomical regime of injection and metabolism of 90 Sr. In other words, the EPR measurements of the samples of calci®ed tissues oer a means of reconstruction the doses obtained individually taking into account the real regime of injection and peculiarities of metabolism. Therefore, a knowledge of both parameters (the reconstructed dose and 90 Sr content) as well as the model of metabolism gives a principal possibility to reconstruct not only the accumulated dose but also the regime of its injection. There are also some other important technical conclusions concerning the method of EPR retrospective dosimetry with calci®ed tissues: 1. There is essentially no scattering of the doses values absorbed in tissues of teeth in symmetric positions in the mouth. However, the bone trauma can in¯uence the dose absorbed due to internal exposure; 2. There is an appreciable dierence between the doses values absorbed in tissues of teeth in non-symmetric positions in the dog mouth. Maximal dierence (up to 33%) was observed for root dentine; 3. In the case of 90 Sr internal exposure the dose reconstruction in crown dentine plays an important role because its quantity is close to the doses absorbed in diaphyseal cortical femur bone, dose on the endostal bone surface and femur yellow marrow. The greater dose absorbed in crown dentine when compared with the dose absorbed in tooth enamel points out the predominant dose contribution of internal exposure. Moreover, the dose absorbed in crown dentine has the greatest reproducibility for dierent teeth in comparison with the doses absorbed in other tooth tissues. The dose absorbed in root dentine is close to the dose absorbed in trabecular bones and cortical jaw bones; 4. The highest absorbed doses have been found in metaphysis trabecular femur bones, tooth alveolar bone walls, cortical and trabecular vertebra. Therefore, these tissues are promising candidates as biomarkers of internal 90 Sr exposure for post-mortal autopsy; 5. Direct comparison of the doses measured by alanine dosimeters and reconstructed doses in adjacent calci®ed tissues have shown that reconstructed doses are close to tissue equivalence; 6. The conclusions have been formulated based on the experiments with dog sacri®ced 52 h after injection
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by 90 Sr. Taking into account the in¯uence of 90 Sr metabolism on the dose distribution in the dog skeleton, it is advantageous to extend the time before sacri®ce. Finally, the experiments give us unique information by means of EPR retrospective dosimetry with calci®ed tissues for internal exposure, since such information can not be derived directly from whole-body counter or chemical analysis. Acknowledgements The authors thank Dr. D. Regulla and Dr. A. Wieser for providing alanine dosimeters and associated dose measurements. We also thank Dr. G. Kenner for editing the manuscript and helpful discussions. AAR would like also to express his gratitude to Dr. M.F. Desrosiers for fruitful discussion of these results. References Degteva, M.O., Kozheurov, V.P., Vorobiova, M.I., 1994. General approach to dose reconstruction in the population exposed as a result of the release of radioactive wastes into Techa river. Sci. Total Environ. 142, 49±61. Desrosiers, N., Avila, M.J., Schauer, D.A., Coursey, B.M., Parks, N.J., 1993. Experimental validation of radiopharmaceutical absorbed dose to mineralized bone tissue. Appl. Radiat. lsot. 44, 459±463. ICRP, 1973. Publication 20, Alkaline Earth Metabolism in Adult Man. Pergamon Press, Oxford. Ignatiev, E.A., Romanyukha, A.A., Koshta, A.A., Wieser, A., 1996. Selective saturation method for EPR dosimetry with tooth enamel. Appl. Radiat. Isot. 47, 333±337. Ikeya, M., 1993. In: New Application of Electron Spin Resonance. Dating, Dosimetry and Microscopy. World Scienti®c, Singapore. Kossenko, M.M., Degteva, M.O., Petrushova, N.A., 1992. Leukemia risk estimate to those exposed as a result of nuclear incidents in the Southern Urals. PSR Q. 2, 187± 197.
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Lyubashevskii, N.M., 1980. The Metabolism of Radioisotopes in Vertebrates Skeleton. Nauka, Moscow (in Russian). Lyubashevskii, N.M., Starichenko, V.I., Golubev, A.G., Shishkina, E.A., 1996. The theory of individual variability of osteotropic radionuclides metabolism. In: Proceedings of 1996 International Congress on Radiation Protection, IRPA9, vol. 3, pp. 128±130. Regulla, D.F., Dener, U., 1982. Dosimetry by ESR spectroscopy of alanine. Appl. Radiat. Isot. 33, 1101±1114. Romanyukha, A.A., Ignatiev, E.A., Degteva, M.O., Kozheurov, V.P., Wieser, A., Jacob, P., 1996a. Radiation doses from Ural region. Nature 381, 199±200. Romanyukha, A.A., Degteva, M.O., Kozheurov, V.P., Wieser, A., Ignatiev, E.A., Vorobiova, M.I., Jacob, P., 1996b. Pilot study of the population of the Ural region with EPR tooth dosimetry. Radiat. Environ. Biophys. 35, 305±310. Romanyukha, A.A., Regulla, D., Vasilenko, E., Wieser, A., 1994. South Ural nuclear workers: comparison of individual doses from retrospective EPR dosimetry and operational personal monitoring. Appl. Radiat. Isot. 45, 1195±1199. Schauer, D.A., Desrosiers, M.F., Kuppusamy, P., Zweier, J.L., 1996. Radiation dosimetry of an accidental overexposure using EPR spectrometry and imaging of human bone. Appl. Radiat. Isot. 47, 1345±1350. Serezhenkov, V.A., Domracheva, E.V., Klevezal, G.A., Kulikov, S.M., Kuznetsov, S.A., Mordvincev, P.I., Sukhovskaya, L.I., Schklovsky-Kordi, N.E., Vanin, A.F., Voevodskaya, N.V., Vorobiev, A.I., 1992. Radiation dosimetry for residents of the Chernobyl region: a comparison of cytogenetic and electron spin resonance methods. Radiat. Prot. Dosim. 42, 33±36. Starichenko, V.I., Lyubashevskii, N.M., Popov, B.V., 1993. Individual Variability of Metabolism of Osteotropic Toxical Substances. Nauka, Moscow (in Russian). Wieser, A., Romanyukha, A.A., Bunzl, K., Kracke, W., 1996a, private communication. Wieser, A., Romanyukha, A.A., Bunzl, K., Kracke, W., Degteva, M.O., Kozheurov, V.P., 1996b. The absorbed dose in tooth enamel by 90 Sr body burden. In: Proceedings of 1996 International Congress on Radiation Protection, IRPA9, vol. 1, pp. 425±426.
Applied Radiation and Isotopes 51 (1999) 151±159
EPR dose reconstruction for bone-seeking
90
Sr
E.A. Ignatiev a, *, N.M. Lyubashevskii b, E.A. Shishkina c, A.A. Romanyukha a a
Institute of Metal Physics, Russian Academy of Sciences, 620219 Ekaterinburg, Russia Institute of Plants and Animals Ecology, Russian Academy of Sciences, 620219 Ekaterinburg, Russia c Urals Research Center for Radiation Medicine, Medgorodok, 454076 Chelyabinsk, Russia
b
Received 6 May 1998; received in revised form 22 June 1998
Abstract The results of the EPR dose reconstruction in calci®ed tissues of dog injected with 90 Sr are presented. It has been established that there is no essential dierence in the values of doses absorbed in tooth tissues of teeth in symmetric positions in the mouth, whereas a signi®cant dierence occurs in the values of absorbed doses in teeth in nonsymmetric positions. In the case of 90 Sr internal exposure the dose reconstruction in crown dentine plays an important role. It has been found that its quantity is close to the dose in diaphyseal cortical bone of the femur, dose at the endosteal bone surface and in femural fatty marrow. The fact that these values exceed doses absorbed in tooth enamel points out the predominant contribution of internal exposure. The highest absorbed doses have been observed in metaphyseal trabecular femur bones, tooth alveolar bone walls, and cortical and trabecular vertebra that can be considered as suitable candidates for biomarkers of internal 90 Sr exposure for post-mortal autopsy. The satisfactory correlation has been found between the doses reconstructed in calci®ed dog tissues and the doses measured by EPR in alanine dosimeters ®xed in (or nearby) the sites of autopsy of bones/teeth. The experiments provide support for the view that EPR retrospective dosimetry with calci®ed tissues for internal exposure is unique in providing useful information on the doses obtained. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: EPR dosimetry; Strontium-90; Internal exposure; Calci®ed tissues; Tooth tissues
1. Introduction Environmental contamination by strontium-90 (90 Sr) is often the consequence of radioactive accidents. It took place after the Chernobyl accident as well as after several accidents that occurred on Mayak Production Association at Southern Urals, Russia (Romanyukha et al., 1996a,b). As a result of the discharges of liquid radioactive wastes in 1949±1956 by the latter nuclear facility, the population of the Techa riverside was exposed to excessive ionizing radiation. During 1949± 1951 about 76 106 m3 of radioactive waste water containing a total activity of 1017 Bq was discharged into
* Corresponding author.
the Techa river. The contribution from 90 Sr to the total radioactivity was 11.62% and comprised the main source of internal exposure of the local population. Water consumption led to the considerable internal overexposure of the Techa river valley population (about 28 000 people) and to the excessive incidence of leukemia (Kossenko et al., 1992). Besides, as a consequence of the so-called Kyshtym accident (explosion of the radioactive waste facility of Mayak Production Association in 1957), a considerable area of Southern and Middle Urals was again contaminated by 90 Sr that caused a signi®cant internal overexposure of the local population. Therefore, dose reconstruction of internal exposure by 90 Sr is an important problem for epidemiological studies of the consequences of many radioactive accidents. At the present time the whole body counter measurements of body burden and
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 3 3 - X
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the model calculations on the basis of metabolic pathways and typical diet of population are used for dose evaluation of 90 Sr internal exposure (Degteva et al., 1994). However, experimental variations in metabolism and radionuclide distribution among humans must be also considered (Lyubashevskii, 1980). The required physiological uptake data for bones and other key organs are usually available only from biodistribution studies on the relatively small number of humans or animals. For the whole body counter, a serious problem also arises in connection with the calibration of measurement results in body burden units. The method of electron paramagnetic resonance (EPR) serves to reconstruct the dose absorbed in teeth and bones. It is based on the EPR measurements of the concentration of stable radiation-induced radicals in hydroxyapatite that is the mineral component of all calci®ed tissues (teeth and bones). Hydroxyapatite constitutes 95±97% of tooth enamel, 70±75% of dentine and 60±70% of bones. Therefore, from the physical point of view, tooth enamel is the most suitable material for retrospective dosimetry. EPR dosimetry in tooth enamel has been already applied to reconstruct radioactive doses obtained by survivors after A-bombardment of Hiroshima and Nagasaki (Ikeya, 1993), Russian nuclear workers (Romanyukha et al., 1994), the victim of the accident at the accelerator in Maryland (Schauer et al., 1996) and some other radioactive accidents (Serezhenkov et al., 1992). Desrosiers et al. (1993) have demonstrated the use of EPR dosimetry with bone for reconstruction of the internal dose. However, in practice the EPR dose reconstruction is generally done with teeth extracted according to medical indications, and there are essential dierences in the metabolism of 90 Sr in tooth enamel and bones (Lyubashevskii et al., 1996). Some ways to solve this problem have already been discussed by Romanyukha et al. (1996a,b). In the case of application of EPR doses reconstruction with teeth to the population exposed internally due to radioactive waste accidents there is an additional problem to determine the relationship between the doses absorbed in tooth enamel and, e.g. in the bone tissue or bone marrow, as would usually be considered in practical dosimetry. However, there is still a lack of understanding on how to do this. There are also some other questions, e.g. concerning the variation of the absorbed doses from tooth to tooth, the contribution of the self-irradiation of the tissue in the reconstructed dose, and the time-dose evolution in the tooth containing 90 Sr. The answers to these and other similar questions can be obtained only by experimental investigations of the dose distribution in skeletons of animal injected by 90 Sr. Thus, our EPR experiments with bones and teeth of the dog injected by 90 Sr have been performed to achieve the following purposes:
1. Comparison of the doses absorbed by tooth tissues from symmetric and non-symmetric positions in the dog mouth; 2. Investigation of the contributions in the doses absorbed by tooth tissues, especially the contribution of tooth tissue self-irradiation; calibration of the doses reconstructed by tissue equivalent dosimeter (alanine); 3. Comparison of the doses absorbed in dierent tooth tissues with the doses absorbed in other bones of the dog skeleton.
2. Experimental Strontium-90 (t 1/2 = 28 yr) was obtained as 90 SrCl2 solution. The radioactive 90 Sr was diluted with citric acid±sodium citrate buer solution to form a boneseeking radiopharmaceutical. A normal dog with a weight of 8.1 kg and age of 3.5 yr was used for this study. The 90 Sr radiopharmaceutical was injected intravenously at a dosage of 3 mL of 88.8 MBq solution (10.96 MBq kg ÿ 1) to this dog via the cephalic vein over a 1.5-min period. The dog was sacri®ced 52 h after the injection; then dierent bones and teeth were removed for study. During this 52 h the probes of blood and saliva were taken. All experiments with the dog were carried out in compliance with Russian national regulation relating to the conduct of animal experimentation. Two types of EPR experiments were conducted. The ®rst was an EPR dose reconstruction with dierent calci®ed tissues of dog, i.e. enamel and dentine of dierent teeth, alveolar bone of jaw, metaphysis and diaphysis of femur, metaphyseal bone of vertebra. The second is the dose assessment by alanine dosimeters placed in the same positions (or nearby) in the bone or teeth where the EPR probes of calci®ed tissues were taken (Figs. 1 and 2). The EPR measurements of exposed alanine pellets allowed determination of tissue
Fig. 1. Position of the alanine dosimeter No. 6 in the dog's jaw.
E.A. Ignatiev et al. / Applied Radiation and Isotopes 51 (1999) 151±159
Fig. 2. Positions of the alanine dosimeters and bone's autopsy for EPR analysis in diaphiseal cortical bone of right back leg of the dog.
equivalent dose (Regulla and Dener, 1982), which is important for estimation of dose contribution of internal irradiation. The dosimeters used were produced at the Institute for Radiation Protection of GSF National Research Center, Munich. In the ®rst type of EPR experiments the spectra were recorded at room temperature with the ERS-231 spectrometer (produced in the former GDR) operating in the X-band. For the mathematical treatment of spectral ®les this spectrometer was connected with an IBM/PC. The experimental parameters of the spectra recording were the following: 100 kHz modulation frequency, 0.45 mT modulation amplitude, 10 and 5 mT ®eldsweep (Fig. 3). The peak-to-peak amplitude of the g _ = 2.0018 line was taken as a relative measure of the concentration of the radiation-induced radicals. The precision of the individual EPR measurement of the amplitude was 210%. The EPR spectra were recorded at 12 and 2 mW. In order to subtract the broad back-
153
ground signal from the total spectrum the method of selective saturation described by Ignatiev et al. (1996) was used. No chemical treatment was applied for the preparation of the EPR samples from teeth and bones to avoid radioactive contamination; only the careful mechanical separation of tooth enamel and dentine was performed. Before measurements the bone, enamel and dentine were dried, crushed and sieved to 0.2 mm grain size. Notations and of positions of the studied teeth in the dog's jaws are given on Fig. 4(a) and (b). The complete list of investigated samples prepared from the teeth are given in Tables 1 and 2 and the list of bone samples is given in the Table 3, namely, it was diaphyseal cortical femur bone of right back leg; metaphyseal cortical and trabecular femur bone of right back leg; then two samples were prepared from sinciput bone adjacent to interparietal suture (2 mm length and 1 and 3 mm widths respectively); and cortical and trabecular bone of vertebra. The dose reconstruction was performed by the additive dose method with the unweighted linear leastsquare-®t employing the Levenberg±Marquart algorithm. Five to six measurements, one original and four to ®ve after additional irradiation in steps of 5±60 Gy each (depending on the value of the reconstructed dose) were used for the linear ®t. Due to the signal anisotropy the EPR spectra were recorded ®ve to seven times, with the sample being re-shaken before and after every additional irradiation (Fig. 5). For exposure-to-absorbed dose conversion according to the mass energy-absorption the coecients for gammaradiation, as calculated by Schauer et al. (1996) were
Fig. 3. EPR spectra of tooth tissues of upper left PM4 tooth and adjacent jaw cortical bone. Enamel, bone, root dentine and crown dentine spectra are arranged from upper to lower parts.
0.242 0.02 0.392 0.03 0.662 0.05 0.752 0.05 Ð 0.1920.02 0.3920.03 0.6820.05 Ð Ð dose dose dose dose dose
rate rate rate rate rate
in in in in in
enamel, Gy per day crown dentine, Gy per day root dentine, Gy per day alveolar bone adjacent to root, Gy per day partition between roots, Gy per day Daily Daily Daily Daily Daily
0.33 20.02 0.44 20.03 1.08 20.07 1.07 20.07 Ð
Lower right M1 (extraction in 174 days) Upper left PM4 (extraction in 141 days) Upper right PM4 (extraction in 76 days) Tissue
used, namely: 8.72 10 ÿ 3 Gy R ÿ 1 for tooth enamel, 9.03 10 ÿ 3 Gy R ÿ 1 for bone and dentine. The irradiations were made with a medical 60 Co source at Ekaterinburg Regional Cancer Hospital with an uncertainty of 5% at 95% con®dence level. In the second series of EPR experiments; 10 alanine pellets were ®xed inside or nearby the dierent calci®ed tissues, parts of which were used for dose reconstruction. Dosimeter number 1 was inserted in place of the root of lower right eighth tooth (PM4) and was covered by piece of overroot slice of this tooth (Fig. 1). Five alanine dosimeters (numbers 2±6) were ®xed with help of wax paper and scotch tape on the surface of the jaw bone adjacent to the roots of the teeth investigated. Dosimeters numbered 2±5 were ®xed on the lingual surface of the jaw and dosimeter number 6 was ®xed on the labial surface of the jaw. Dosimeter 2 was close to the upper left ninth tooth (M1), number 3 was
Table 1 Results of dose rate reconstruction for upper eighth and lower ninth tooth positions
Fig. 4. Notations and positions of the teeth studied with EPR on schematic design of the dog's jaws. (a) upper jaw; (b) lower jaw.
0.1920.02 0.3820.03 0.6720.05 0.6020.06 1.2220.8
E.A. Ignatiev et al. / Applied Radiation and Isotopes 51 (1999) 151±159 Lower left M1 (extraction in 208 days)
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Table 2 Results of dose rate reconstruction for roots of PM2's and PM3's extracted at 234 days after injection Tooth position
Upper right PM2
Upper left PM2
Lower right PM3
Lower left PM3
Daily dose rate in root dentine, Gy per day
0.9320.06
1.282 0.09
0.9720.08
1.002 0.07
Table 3 Results of dose rate reconstruction for dierent bone tissues Tissue
Daily dose rate, Gy per day
Sinciput bone adjacent to junction (1 mm width) Sinciput bone adjacent to junction (3 mm width) Diaphiseal cortical bone of right back leg
0.73 20.07 0.68 20.07 0.52 20.05
Fig. 5. Dose reconstruction for tooth tissues of upper left PM4: (w) enamel, () crown dentine, (q) root dentine, (Q) adjacent jaw bone.
®xed close to upper right ninth tooth (M1), number 4 was close to upper left eighth tooth (PM4) and dosimeters 5 and 6 were near to lower left ninth tooth (M1). Another group of alanine dosimeters was put inside femur bones of right back leg of the dog. Dosimeter 10 was placed inside the diaphysis of femur as shown in Fig. 2. Dosimeters 7, 8 and 9 were placed inside the proximal and distal femur bones in the drilled holes (Fig. 2, Table 4) and were covered with the bone fragments. After 43 days all alanine dosimeters were removed for the EPR analysis. The EPR measurements with the dose absorbed in alanine pellets were done with EPR spectroanalyzer EMS-104 (Bruker, Germany) operating in the X-band at GSFNational Research Center, Munich. The standard procedure developed at GSF by Regulla and Dener
Table 4 Results of dose rate measurements by alanine dosimeters. Comparison with reconstructed dose rates in calci®ed tissues adjacent to the positions of alanine dosimeters Dosimeter No.
Position of dosimeter
Dose rate measured by alanine dosimeter (Gy per day)
Reconstructed dose rate in adjacent calci®ed tissues (Gy per day)
1 2 3 4 5 6 7 8 9 10
root pit jaw bone jaw bone jaw bone jaw bone jaw bone proximal epiphyseal femur proximal epiphyseal femur distal epiphyseal femur diaphyseal femural bone
0.42 0.31 0.32 0.26 0.21 0.29 0.77 1.24 1.05 0.49
0.82 20.09 1.07 20.07 0.68 20.05 1.07 20.07 0.60 20.06 0.60 20.06 ± ± ± 0.52 20.05
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(1982) for such measurements was used. The abovementioned measurements with alanine were made independently on EPR dose reconstruction measurements at Ekaterinburg, without providing information on values of doses reconstructed in calci®ed tissues. Dierent samples of calci®ed tissues were extracted at dierent times after sacri®ce, and the corresponding EPR measurements were also performed at dierent times. Therefore, the value of daily dose rate was used for characterization both of doses absorbed by calci®ed tissues and measured by alanine dosimeters.
given in Table 3. EPR investigations of vertebra dose showed a saturation eect. Such eects were observed at dose rates greater than 1.5 Gy per day over 234 days after injection.
3. Results
1. The dose per day in the root pit of the eighth lower right tooth (PM4) is a factor of 2 lower than the dose rates absorbed in adjoining roots. Such result con®rms our above-mentioned suggestion concerning the essential contribution from alveolar bone walls to exposure of the root dentine; 2. Dosimeters 2±6 indicate the dose rates on the surfaces of cortical jaw bone. The average dose rate measured on the surface of jaw bone is a factor of 3 lower than the dose rate reconstructed in this bone (Table 1); 3. Dosimeters 7±9 indicate the dose rates inside the physeal proximal and distal femur bones (red marrow and trabecular bone); 4. The dose rate measured in the femur diaphyseal (dose in fatty marrow and on the endostal bone surface) by dosimeter 10 is slightly below the reconstructed dose rate in the cortical diaphyseal bone (0.49 Gy per day and 0.52 Gy per day, respectively).
76 to 208 days after sacri®ce; the four teeth of the symmetric positions were extracted. Due to the dierent sizes of upper and lower jaws of the dog (Fig. 4(a) and (b)) the upper PM4's are symmetrical to the lower M1's. Doses per day absorbed by tooth enamel, dentine (both crown and root) and adjacent alveolar bone were reconstructed (Table 1). At 234 days after sacri®ce the samples from next four symmetric teeth Ð upper six (PM2) and lower seven (PM3) were studied. For this set of teeth only samples from root dentine were prepared due to the relatively small quantity of tooth enamel on these teeth. The results of EPR reconstruction are given in Table 2 in terms of daily dose rate. To investigate the dose contribution of self-irradiation the upper left PM4 was used. This tooth was extracted 141 days after sacri®ce. Then it was cut into two halves in the crosswise direction of the jaw. One half of this tooth was used for the preparation of tooth enamel and dentine samples for the EPR measurements on the day of tooth extraction. The second half was stored in a thick-walled glass vessel for 86 days for subsequent EPR measurements. During storage the dose absorbed in tooth enamel increased in a factor of (1.612 0.10); the dose absorbed in crown dentine increased in a factor of (1.68 2 0.10), and the dose absorbed in root dentine remained essentially unchanged (1.17 20.08). The doses absorbed in the samples prepared from the following bones were reconstructed: . diaphyseal cortical femur bone of right back leg (Fig. 2); . metaphyseal cortical and trabecular femur bone of right back leg; two samples were also prepared from sinciput bone adjacent to interparietal suture (2 mm length and 1 and 3 mm widths respectively); . cortical and trabecular bone of vertebra extracted 234 days after injection. The results of dose reconstruction in bone samples are
Another important source of information about the contribution of the self-irradiation due to 90 Sr deposition is a comparison of the reconstructed doses in calci®ed tissues and the doses measured nearby with the help of alanine dosimeters. Table 4 presents dose rate results of the measurements with alanine dosimeters and reconstructed doses obtained simultaneously. These results can be summarized a follows:
4. Discussion The shape of the experimental time dependencies of Sr concentrations in the blood and saliva has indicated that the processes of transfer and deposition of 90 Sr in calci®ed tissues peak at the time of sacri®ce. Consequently, the contribution of 90 Sr contained in saliva is much less for the doses absorbed by calci®ed tissues than the contribution of 90 Sr deposited in calci®ed tissues. Therefore, the whole dog skeleton and its separate parts can be considered as phantoms of special types for investigation of dose distribution due to internal exposure by 90 Sr. As seen from Table 1, the doses absorbed in tooth enamel samples prepared from teeth in three symmetric positions are closely related. A similar situation takes place with regard to the both crown and root dentine samples of the same three teeth. However, for the upper left PM4 the considerably higher doses absorbed in all investigated samples (about 60% over) have been 90
E.A. Ignatiev et al. / Applied Radiation and Isotopes 51 (1999) 151±159
obtained in comparison with other three symmetric teeth. Similar results took place for the PM2's and PM3's teeth (Table 2), i.e. for the tooth from the upper left position the considerably higher absorbed dose rate was found. Detailed visual investigation has shown the sites of serious trauma on the left side of the upper dog jaw. It is well known (see, for example, ICRP, 1973, Publication 20, or Starichenko et al., 1993) that the bone repair trauma is usually accompanied by prolonged remodeling with expanded blood supply, increased bone surface and other processes providing more intensive deposition of radionuclides. The dierence between the doses absorbed in root dentine of non-symmetric teeth (about 30%, compare data of Tables 1 and 2) is also essential. This fact may be explained by dierent blood supply of teeth in dierent positions and it needs to be ascertained in the course of future investigations. Comparison of the reconstructed doses for dierent tooth tissues showed a considerable dierence between the doses absorbed by tooth enamel, crown and root dentine. The mean ratio of these doses is 1:1.6:2.8 for enamel, crown and root dentine, respectively. These results generally agree with those obtained by Romanyukha et al. (1996a) for the teeth of Techa riverside residents who had consumed water polluted by 90 Sr (in the latter case the ratio of the absorbed doses in enamel and root dentine was 1:5). The observed dierence between the absorbed dose ratios for tooth enamel and root dentine of dog and humans can be explained by the dierence in the thickness of the tooth enamel layer. It was 0.3±0.5 mm for dog teeth and 1±2 mm for human teeth that is critical at limited depth penetration of b-particles emitting by 90 Sr and 90 Y. The evidence of the dose excess in dentine in comparison with tooth enamel can be used to distinguish the eect of internal exposure of 90 Sr from external exposure according to EPR dose reconstruction. Another important result is a signi®cant dierence between the doses absorbed by crown and root dentine. It can be explained by a better contact of root dentine with blood vessels, by root dentine next to the bone jaw, as well as by root partitions which have relatively high level of 90 Sr deposition. Similar results have been also obtained by chemical analysis of the teeth of Techa riverside residents for whom 90 Sr concentration in root dentine was higher in comparison with crown dentine as measured by Wieser et al. (1996a). Investigation of the contribution of self-irradiation in the absorbed doses carried out for two halves of the upper left PM4 has revealed the following ®ndings: During 86 days storage of its one half the dose absorbed in tooth enamel increased in a factor of (1.612 0.10); the dose absorbed in its crown dentine increased in a factor of (1.68 2 0.10), and the dose absorbed in root dentine remained essentially
157
unchanged (1.17 2 0.08). On the other hand, if this tooth had still remained in the jaw, the doses in all tooth tissues would have to increase by a factor of 141/86 = 1.64. Consequently, for tooth enamel and crown dentine the absorbed doses are determined mostly by self-irradiation from internal 90 Sr. The anomaly observed for root dentine may be due to contribution of the absorbed dose from the tooth alveolar bone walls, where a very high daily absorbed dose rate was reconstructed (Table 1). Another possible reason of small changing of the dose in root dentine is the in¯uence of the above-mentioned old bone trauma of the left upper jaw. Comparison of the absorbed dose rates in calci®ed tissues under investigation (Tables 1 and 3) allows the distinction of the following four groups of tissues: 1. 0.2±0.3 Gy per day: tooth enamel; 2. 0.4±0.5 Gy per day: crown dentine, diaphyseal cortical femur bone, femur yellow marrow, femur endostal bone surface; 3. 0.7±1.0 Gy per day: root dentine, cortical jaw bone, sinciput bones, sinciput bone adjacent to interparietale suture; 4. higher than 1.0 Gy per day: tooth alveolar bone walls, proximal and distal epiphysial femur bone, cortical and trabecular bone of vertebra. The proposed classi®cation underlines a special importance of dose reconstruction in the crown dentine for the case of 90 Sr internal exposure, since it is close to the absorbed doses in diaphyseal cortical femur bones, in cortical bones, as well as on the endostal bone surface and in femur yellow marrow measured by alanine dosimeters. Moreover, the lowest dose rate scattering (in the limits of 25%) among dierent teeth (see Table 1) has been found for the crown dentine. Thus, contrary to the dose absorbed in tooth enamel, the value of the dose absorbed in the crown dentine can easily be interpreted. The dose absorbed in root dentine is close to the dose absorbed by trabecular bone and cortical jaw bone. The observed dose saturation in vertebra at the dose higher than 350 Gy disagrees with the published results on the dose saturation at external g-exposure at 100 kGy (see, for example, Ikeya, 1993). Possible explanation of this discrepancy may be connected with a considerable dose gradient at internal b-exposure due to limited depth penetration (1±3 mm) and essential variation of 90 Sr deposition in the bone tissue. Direct comparison of the doses measured by alanine dosimeters and reconstructed doses in adjacent calci®ed tissues (Table 4) permits the selection of three dierent groups of the dosimetry results:
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1. Dose measured by alanine dosimeter is approximately equal to that reconstructed in the adjacent calci®ed tissue (dosimeter No. 10); 2. Dose measured by alanine dosimeter is about half of the reconstructed one (dosimeters No. 1, 3, 6); 3. Dose measured by alanine dosimeter is more than factor of 2 less than the reconstructed one (dosimeters No. 2 and 5). It is reasonable to assume that in the ®rst selected group of dosimeters there are exactly the same geometries of the exposure of dosimeter and bone sample, that in the second group there are hemispherical geometries of the dosimeter's exposure because it was ®xed on the bone surface, and for the third group of two dosimeters the considerable dierence in doses can probably be explained by the bad contact between dosimeter and bone, which is very important for b-exposure. The latter assumption allows to us to estimate the mean dose deviation as measured by alanine dosimeter and reconstructed in calci®ed tissues on the base comparison of the resulting doses measured by dosimeters of ®rst two selected groups (1, 3, 6, 10) and the respective reconstructed doses. For the second group of dosimeter the factor 2 was used for the comparison with results of EPR reconstruction. The simple calculation shows that dose measured by alanine is factor of 1.01 2 0.15 less than reconstructed one. Thus, we can conclude that EPR dose reconstruction in calci®ed tissues gives a dose which is very close to the tissue equivalent value. 5. Conclusions The reconstructed doses at real accidents (e.g. Techa riverside) have been de®ned with the help of tooth reconstruction dosimetry. The results allow us to estimate the important role of the EPR method in the dose reconstruction of calci®ed tissues for internal exposure. One of the most interesting and unexpected results is the lack of correlation between the doses reconstructed in tooth enamel with EPR and 90 Sr chemical content in the samples and in root dentine for 14 Techa riverside residents (Wieser et al., 1996b). On the other hand, a close correlation between the doses measured by alanine dosimeters characterizing 90 Sr content and the doses absorbed in the same bone tissue has been found for the dog (Table 4). There are two essential distinguishing features concerning conditions of exposure and the dose reconstruction for dog and Techa riverside residences, namely the regime of 90 Sr ingestion and the period of time between the dose reconstruction performance and 90 Sr ingestion. As dierentiated from dog, the population of Techa river-
side had consumed 90 Sr over a period of years, and EPR measurements were performed after the initial ingestion 40 years ago, when the main consumption of 90 Sr had taken place. Consequently, the 90 Sr content and the dose reconstructed by EPR in the corresponding calci®ed tissue dier from each other and can be considered as complementary parameters. The former parameter overlooks the simply accumulated dose and the latter parameter neglects both anatomical regime of injection and metabolism of 90 Sr. In other words, the EPR measurements of the samples of calci®ed tissues oer a means of reconstruction the doses obtained individually taking into account the real regime of injection and peculiarities of metabolism. Therefore, a knowledge of both parameters (the reconstructed dose and 90 Sr content) as well as the model of metabolism gives a principal possibility to reconstruct not only the accumulated dose but also the regime of its injection. There are also some other important technical conclusions concerning the method of EPR retrospective dosimetry with calci®ed tissues: 1. There is essentially no scattering of the doses values absorbed in tissues of teeth in symmetric positions in the mouth. However, the bone trauma can in¯uence the dose absorbed due to internal exposure; 2. There is an appreciable dierence between the doses values absorbed in tissues of teeth in non-symmetric positions in the dog mouth. Maximal dierence (up to 33%) was observed for root dentine; 3. In the case of 90 Sr internal exposure the dose reconstruction in crown dentine plays an important role because its quantity is close to the doses absorbed in diaphyseal cortical femur bone, dose on the endostal bone surface and femur yellow marrow. The greater dose absorbed in crown dentine when compared with the dose absorbed in tooth enamel points out the predominant dose contribution of internal exposure. Moreover, the dose absorbed in crown dentine has the greatest reproducibility for dierent teeth in comparison with the doses absorbed in other tooth tissues. The dose absorbed in root dentine is close to the dose absorbed in trabecular bones and cortical jaw bones; 4. The highest absorbed doses have been found in metaphysis trabecular femur bones, tooth alveolar bone walls, cortical and trabecular vertebra. Therefore, these tissues are promising candidates as biomarkers of internal 90 Sr exposure for post-mortal autopsy; 5. Direct comparison of the doses measured by alanine dosimeters and reconstructed doses in adjacent calci®ed tissues have shown that reconstructed doses are close to tissue equivalence; 6. The conclusions have been formulated based on the experiments with dog sacri®ced 52 h after injection
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by 90 Sr. Taking into account the in¯uence of 90 Sr metabolism on the dose distribution in the dog skeleton, it is advantageous to extend the time before sacri®ce. Finally, the experiments give us unique information by means of EPR retrospective dosimetry with calci®ed tissues for internal exposure, since such information can not be derived directly from whole-body counter or chemical analysis. Acknowledgements The authors thank Dr. D. Regulla and Dr. A. Wieser for providing alanine dosimeters and associated dose measurements. We also thank Dr. G. Kenner for editing the manuscript and helpful discussions. AAR would like also to express his gratitude to Dr. M.F. Desrosiers for fruitful discussion of these results. References Degteva, M.O., Kozheurov, V.P., Vorobiova, M.I., 1994. General approach to dose reconstruction in the population exposed as a result of the release of radioactive wastes into Techa river. Sci. Total Environ. 142, 49±61. Desrosiers, N., Avila, M.J., Schauer, D.A., Coursey, B.M., Parks, N.J., 1993. Experimental validation of radiopharmaceutical absorbed dose to mineralized bone tissue. Appl. Radiat. lsot. 44, 459±463. ICRP, 1973. Publication 20, Alkaline Earth Metabolism in Adult Man. Pergamon Press, Oxford. Ignatiev, E.A., Romanyukha, A.A., Koshta, A.A., Wieser, A., 1996. Selective saturation method for EPR dosimetry with tooth enamel. Appl. Radiat. Isot. 47, 333±337. Ikeya, M., 1993. In: New Application of Electron Spin Resonance. Dating, Dosimetry and Microscopy. World Scienti®c, Singapore. Kossenko, M.M., Degteva, M.O., Petrushova, N.A., 1992. Leukemia risk estimate to those exposed as a result of nuclear incidents in the Southern Urals. PSR Q. 2, 187± 197.
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