Development of an EPR Dosimetry System Based on Hydroxyapatite in the Therapy Dose Level L. M. de Oliveira1, L. N. Rodrigues1, L. H. Bardella2 1
Instituto de Pesquisas Energéticas e Nucleares-IPEN/CNEN, Centro de Metrologia das Radiações Av. Lineu Prestes 2.242, Cidade Universitária, São Paulo, Brasil CEP 05508-900, FAX 55 11-3816 9209 E-mail:
[email protected] 2 Instituto Nacional do Câncer-INCA, Física Médica Praça Cruz Vermelha, 23 - Centro CEP 20230-130, Rio de Janeiro, Brasil.
Abstract Systematic investigation on A-type carbonated apatites allowed to establish the correlation between synthesis process, structural and dosimetric properties. The reproducibility of the EPR response for samples from eight batches was ± 0.5%. The overall uncertainty intrinsic to the A-type apatite powder/EPR dosimetry in the range of 2 Gy to 100 Gy was around 1% at 95% confidence level. However uncertainties can be still reduced applying numerical signal treatment mainly in the dose range less than 1Gy. Despite dosimeters based on hydroxyapatite should not be soft tissue-equivalent systems, results showed that the mass-energy absorption coefficient of this A-type apatite is equivalent to water at 6 MV. The EPR amplitude presented very low energy dependence between 6 MV and 1.25 MV. However the EPR response increased around 10% between 6 and 15 MV. Nevertheless, the values are lower than that could be expected. Results showed that the EPR response relative to the several photon energies available in medical accelerators must be determined. Otherwise, the low uncertainty and the long-term signal suggest the high potential of the ceramic apatite dosimeter for radiotherapy applications. Key words: EPR, dosimetry, A-type apatite, uncertainties, photon energy.
Introduction Hydroxyapatite of bones and teeth enamel has been applied successfully for dose reconstruction in accidents and archaeological dating using EPR spectroscopy. The method is based on the measurement of radiation-induced carbonate radicals in mineral phase of biological apatites. Carbonate ions are incorporated into hydroxyapatite (Ca10(PO4)6(OH)2) crystalline lattice of the calcified tissues substituting for both phosphate (B-type) and hydroxyl (A-type) ions. Synthetic carbonated apatites have been used as model systems for calcified tissues and the EPR spectrum of B-type carbonated apatites has been intensively investigated by some authors. However several reports (Callens et al., 1989, 1991, 1993; Moens et al., 1991) have shown that the EPR spectrum of B-type carbonated apatites depends severely on the samples preparation method. In anyway contributions arising from several radicals (CO2−, CO−, CO3−, CO33−, etc) overlap producing complex spectra. Systematic investigation on A-type carbonated apatites allowed to establish the correlation between synthesis process, structural and dosimetric properties, leading to a controlled material with interesting features for EPR dosimetry in the therapy dose level. This work presents the reproducibility of the dosimetric material obtained in this project and the uncertainty sources intrinsic to the A-type
apatite/EPR system in the therapy dose range. The response of A-type apatite powder dosimeter to 60Co gamma rays and high-energy photon beams produced by medical accelerators is also discussed. Method A-type carbonated apatites (Ca10(PO4)6(CO3)x(OH)2(1-x)) were prepared according to the method described by Bonel, 1972. Samples had been analyzed using infrared spectroscopy and X-ray diffraction. Infrared absorptions at 1534 cm−1 and 1465 cm−1 indicated that carbonate groups occupied OH− sites. Structural unit cell refinements revealed the expansion of the a parameter and the contraction of c parameter (a=b= 9.4854; c=6.8748) with respect to the hydroxyapatite structure, which confirmed the formation of A-type carbonated apatite. The dosimeters consist of cylindrical polyethylene capsules with approximately 4.9 mm external diameter and 25 mm height filled with polycrystalline material. Irradiations to gamma rays of 60Co were carried out in the National Laboratory for Metrology of Ionizing Radiation-LNMRI/IRD/CNEN in terms of air kerma in order to verify the uncertainties in the therapy dose range. Air kerma rate of 0.34 Gy/min was determined at the position of the dosimeters by secondary standards traceable to, respectively, BIPM, France, and NPL, UK. The relative EPR response to high energy photon beams was obtained by means of irradiations in a water phantom in accordance with TRS 398 code. A Theratron 780-C irradiator in National Cancer Institute-INCA was used and the dosimetry was performed with standards traceable to LNMRI at the position of the samples in water phantom. The absorbed dose to water rate was 2.12 Gy/min. High-energy photon irradiations were performed at 6 MV and 15 MV X-ray beam of a Varian Clinac 2300C/D linear accelerator available in the radiotherapy center of INCA. The absorbed dose at the dosimeters position was determined in a water phantom using the same standards traceable to LNMRI. For both energies, the dosimetry was carried out at the position of the samples in water phantom applying TRS 398. EPR measurements were carried out in a Bruker spectrometer model EMX operating in the X-band microwave range at the room temperature. The spectrometer setting was: 328 ms time constant, 84 s sweep time, 50 mW microwave power, 0.25 mT modulation amplitude and 5 mT sweep width. The measurements were performed not before 1 hour that the spectrometer had been switched on. Results and Discussion Sources of uncertainties intrinsic to the apatite powder dosimeter and the EPR technique were evaluated experimentally. Apatite samples were irradiated at several absorbed dose levels from 100 to 2 Gy with 60Co source at LNMRI/IRD. In this first evaluation, numerical signal treatment was not applied on the spectra. Long-term sensitivity variations of the spectrometer were taken into account by regular checks using an apatite sample irradiated to 100 Gy absorbed dose. In addition an 11 hours stability test was performed during a measurement session without considering a warm-up time for the equipment. The spectrometer was stable within ± 0.12%. Similar result was found for another Bruker spectrometer using an alanine sample irradiated to 100 kGy (Regulla et al., 1993). In order to investigate the reproducibility of the apatite powder dosimeter in the therapy range, five dose values were selected (2, 6, 50, 100 Gy). One sample was irradiated at each dose level. The EPR spectrum of each dosimeter was recorded
ten times with the sample being removed from the cavity, randomly replaced on the same position inside it and all the spectrometer parameters reset for each time. Results are presented in Table 1. The contribution coming from the spectrometer was also evaluated without moving the sample but re-tuning the cavity and the parameters being reset for each time. In this case amplitudes could be reproduced within ± 0.08 % to samples irradiated from 50 to 100 Gy and within ± 0.42 % to 2 Gy. In this project, the dependence on slight position variations of the sample in the cavity was also minimized in function of the apatite mass which fills almost the entire effective length of the microwave cavity, 18 mm long (Regulla et al., 1993). Fluctuations in the EPR response intrinsic to each apatite powder dosimeter in the same batch was evaluated with ten samples irradiated to 10 Gy absorbed dose. The interspecimen scattering was ± 0.48 % (Table 1). This result is slightly better than that reported by Regulla et al., 1993 for alanine pellets from GSF and ISS irradiated at higher doses. Naturally there are other factors which contribute for uncertainties in the EPR response of pellets. However this evaluation was performed to lower doses and using only one batch. In addition the production process of these samples can be still improved. In a first evaluation, the overall uncertainty intrinsic to the apatite powder/EPR dosimetry irradiated between 100 and 2 Gy is similar to the same sources uncertainties obtained for alanine/EPR dosimetry in the high-dose range and much lower than uncertainties reported for alanine in the therapy dose range (Bartolotta et al., 1993; Wieser et al., 1993; Ruckerbauer et al., 1996). However, it should be noted that the alanine has been tested in the therapy dose level applying mathematical signal treatment and other efforts to reduce uncertainties. Table 1 - Uncertainty sources for the Apatite powder/EPR dosimetry system evaluated using EMX spectrometer. Absorbed dose (Gy)
100
50
6
2
Reproducibility/repeatability (%)
0.14
0.14
0.35
0.44
Interspecimen scattering (%)
0.48
0.48
0.48
0.48
Mass determination (%)
0.20
0.20
0.20
0.20
Spectrometer stability (%)
0.12
0.12
0.12
0.12
1.10
1.10
1.28
1.38
Overall uncertainty for EPR response (σ=2) (%)
* Uncertainty related to the calibration curve was not taken into account in this table. Uncertainty from the air kerma rate in LNMRI (secondary standard ionization chamber): ± 0.6% at 95% confidence level.
Several reports (Callens et al., 1989, 1991, 1993; Moens et al., 1991) have shown that the EPR spectrum of B-type carbonated apatites depends severely on the samples preparation method. Thus, the relationship between dosimetric properties and synthesis process of the A-type carbonated apatite was systematically investigated. After obtaining a controlled material with suitable properties for EPR dosimetry, the reproducibility of the synthesis process was evaluated with eight batches of sample produced under the same conditions and irradiated to 2 Gy (Fig. 1). The EPR signal amplitude was reproduced within ± 0.51 %. Characteristics intrinsic to EPR dosimetry are especially advantageous for
radiotherapy applications, as non-destructive readout technique, which allows the integration of the fractionated dose with the same dosimeter and repeated evaluations of the total dose delivered. Since the dosimeter present long-term radiation-induced radicals, the system could provide an archive report of the total dose delivered to patients (Oliveira et al., 2002). However the EPR response to high energy photon beams is an important aspect of this dosimeter. Despite the similarity of the A-type apatite to a biological tissue in terms of the radiation energy absorption, dosimeters based on hydroxyapatite should not be soft tissueequivalent systems. Nevertheless, results showed that the mass-energy absorption coefficient of this A-type apatite is equivalent to water at 6 MV (fig. 2). 10 9
EPR amplitude (a.u.)
8 7 6 5 4 3 2 1 0 B1
B2
B3
B4
B5
B6
B7
B8
Batches
Figure 1. EPR signal amplitude of samples from eight batches irradiated to absorbed dose of 2 Gy in the same conditions.
Ratio Apatite dose/Tissue dose
1,14 1,12
15 MV
1,10 1,08 1,06 1,04 1,02
60
Co
6 MV
1,00 0,98 0,96 0,55
0,60
0,65
0,70
0,75
0,80
TPR20/10
Figure 2. Photon energy dependence for EPR response in terms of ratio of dose in dosimeter to water, normalized to 6 MV. Conclusions The dosimetric properties of the A-type carbonated apatite depend severely on the synthesis process, however preliminary results indicated that a well-controlled
synthesis process had already been reached in this project. Improvements can be performed in terms of the mathematical signal treatment mainly in the dose range less than 1Gy. Nevertheless, the apatite/EPR dosimetry system can already offer high precision for applications in the therapy dose range. Dosimeters based on hydroxyapatite should not be soft tissue-equivalent systems. However the evaluation of the mass-energy absorption coefficients showed that the A-type apatite is equivalent to water at 6 MV. In addition, the EPR amplitude presented very low energy dependence between 1.25 and 6 MV, which increases between 6 and 15 MV being nevertheless, lower than that could be expected. This result indicates that the EPR response relative to the several photon energies available in medical accelerators needs to be determined. Otherwise, the low uncertainty and the long-term signal suggest the high potential for application of the ceramic apatite dosimeter in radiotherapy. Acknowledgements Authors thank Dr. Maria Tereza Lammy of USP/IF for using of EPR spectrometer. References Callens, F. J., Verbeeck, R. M. H., Naessens, D. E., et al. (1989) Effect of Carbonate Content on the ESR Spectrum Near g = 2 of Carbonated Calciumapatites Synthesized from Aqueous Media. Calcif. Tissue Int., 44, pp. 114-124. Callens, F. J., Verbeeck, R. M. H., Naessens, D. E., et al. (1991) Effect of Carbonate Content and Drying Temperature on the ESR Spectrum Near g = 2 of Carbonated Calciumapatites Synthesized from Aqueous Media. Calcif. Tissue Int., 48, pp. 249-259. Callens, F. J., Verbeeck, R. M. H., Naessens, D. E., et al. (1993) ESR Study of 13 C-Enriched Carbonated Calciumapatites Precipitated from Aqueous Solutions. Calcif. Tissue Int., 52, pp. 386-391. Moens, P., Callens, F., Matthys, P., et al. (1991) Adsorption of Carbonate-Derived Molecules on the Surface of Carbonate-Containing Apatites. J. Chem. Soc. Faraday Trans., 87, pp. 3137-3141. Bonel, G. (1972) Contribution a l’étude de la Carbonation des Apatites. Part I and II Ann. Chim., 7, pp. 65-88 and pp. 127-144. Regulla, D., Bartolotta, A., Deffner, U., et al. (1993) Calibration Network Based on Alanine/ESR Dosimetry. Appl. Radiat. Isot., 44 (1/2), pp.23-31. Bartolotta, A., Fattibene, P., Onori, S. et al. (1993) Sources of Uncertainties in Therapy Level Alanine Dosimetry. Appl. Radiat. Isot., 44 (1/2), pp. 13-17. Wieser, A., Lettau, C., Fill, U. et al. (1993) The Influence of Non-radiation Induced ESR Background Signal from Paraffin-Alanine Probes for Dosimetry in the Radiotherapy Dose Range. Appl. Radiat. Isot., 44 (1/2), pp.59-65. Ruckerbauer, F., Sprunck, M., Regulla, D. (1996) Numerical Signal Treatment for Optimized Alanine/ESR Dosimetry in the Therapy-level Dose Range. Appl. Radiat. Isot., 47 (11/12), pp. 1263-1268. de Oliveira, L. , Rossi, A., Lopes, R., Rodrigues, L. (2002) The influence of unstable signals for Electron Spin Resonance dosimetry with A-type carbonated apatite. Radiat. Prot. Dosim., 101(1-4), pp. 539-544. IAEA (2000) Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetrybased on Standards of Absorbed Dose to Water. IAEA Tecnical Reports Series No. 398.
