Potential application of pure silica optical flat fibers for radiation therapy dosimetry

Radiation Physics and Chemistry 106 (2015) 73–76

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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem

Short Communication

Potential application of pure silica optical flat fibers for radiation therapy dosimetry A. Alawiah a,n, S. Bauk b, H.A. Abdul-Rashid c, W. Gieszczyk d, S. Hashim e, G.A. Mahdiraji f, N. Tamchek g, D.A. Bradley h,i a

Faculty of Engineering and Technology, Multimedia University, 75450 Malacca, Malaysia Physics Section, School of Distance Education, Universiti Sains Malaysia, 11800 Penang, Malaysia c Faculty of Engineering, Multimedia University, 20100 Cyberjaya, Selangor, Malaysia d Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego 152, 31-342 Krakow, Poland e Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia f Photonics Research Group, Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia g Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia h Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom i Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia b

H I G H L I G H T S







TL intensity (Im) is 1.5 times higher for 6 MeV electron than 6 MV photons beams. The Tm is not affected by the type of irradiation used at the constant dose. The FFs displayed a supralinear response from 2 Gy up to 10.0 Gy. The sensitivity of FFs to electrons is consistently higher than photons.

art ic l e i nf o

a b s t r a c t

Article history: Received 28 July 2013 Accepted 6 June 2014 Available online 16 June 2014

Pure silica optical flat fibers (FF) have been proposed as the basis for a novel radiation sensor by the measurement of the thermoluminescence (TL) produced. In this paper the TL performance of the FFs were studied. Using a linear accelerator (LINAC) delivering doses in the range 0.2–10.0 Gy, the TL dosimetric glow curves of the FFs were studied with respect to 6 MeV electron and 6 MV photon beams. When exposed to 6 MeV electron irradiation, the pure silica FFs displayed a supralinear response starting from 2 Gy up to 10.0 Gy. While for 6 MV photon irradiation, the FFs shows linear characteristic ðf ðDÞ ¼ 1Þ nearly up to 2 Gy. The TL intensity (Im) of the main peak of FFs is 1.5 times higher for 6 MeV electron beams than for 6 MV photon beams. The maximum peak temperature (Tm) it is not affected by the type of irradiation used at the same dose while the maximum TL intensity (Im) was found to be dependent on the type of radiation used. Overall results indicate that the pure silica FFs can be used as radiation sensors in the high-dose therapy dosimetry. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Thermoluminescence Flat fiber Pure silica Glow curve Kinetic parameters Dose response

1. Introduction The pure silica (SiO2) optical fiber has attracted much attention as a radiation sensor in dosimery. In a wider context, the potential of doped SiO2 optical fibers in measuring photon and charged particle beams can be found in a number of reports (Ong et al., 2009; Hashim et al., 2009). Ge-doped fibers have also been n

Corresponding author. E-mail address: [email protected] (A. Alawiah).

http://dx.doi.org/10.1016/j.radphyschem.2014.06.006 0969-806X/& 2014 Elsevier Ltd. All rights reserved.

demonstrated to offer excellent TL yield and spatial resolution, sufficient for use in high dose gradient situations as in Intensity Modulated Radiation Therapy (IMRT) (Noor et al., 2011). The same optical fibers also demonstrated reusability and low residual signal and fading (Abdul Rahman et al., 2010). Present investigation focuses on the TL response of pure silica FFs to determine the TL characteristics from the respective TL glow curves. The responses of these materials to photon and electron doses in the therapeutic energy and dose ranges were compared with that of TLD-100 (LiF:Mg,Ti).

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2. Materials and method 2.1. Samples preparation A typical pure silica FF used had an average mass of 19.33 70.01 mg, a thickness of 0.88 70.01 mm and a length of 0.5 70. 1 cm. The FF was fabricated using a conventional fiber drawing tower located at the Flat Fiber Laboratory, Department of Electrical Engineering, University of Malaya, Malaysia. The LiF based dosimeter (TLD-100) was used as a reference material in this study. The TLD-100 chips (3.0 mm  3.0 mm  0.89 mm) were obtained from the Harshaw-Bicron Company. In this study, the pure silica FFs were annealed using a Nabertherm Program Controlled S27 Furnace (Nabertherm, Germany). The fibers were wrapped in aluminum foil and annealed at 400 1C for 1 h. The pure silica FFs were then removed and allowed to cool to room temperature. The TLD-100 samples were annealed prior to irradiation at 400 1C for 1 h and 80 1C for 24 h without the post-irradiation anneal (International Atomic Energy Agency, 2005). The FFs were irradiated using a Siemens Mevatron MD2 (Siemens, Germany) linear accelerator (LINAC), which providing high-energy irradiation rays of 6 MeV electrons and 6 MV photons. For each dose a minimum of 10 fibers were irradiated, allowing assessment of statistical variation and reproducibility. The source to sample surface distance (SSD) was at 100 cm, with a field size of 15  15 cm2 selected for electron irradiations. In all cases, the fibers were placed at the center of the field. A Gammex RMI solid waterTM phantom (Gammex, U.S.A) (30 cm length  30 cm width) was used to ensure calibration conditions. The applicator size was 15  15 cm2. To provide for charged particle equilibrium at the sample position, the samples were located at the depth at which, in use of a single stationary beam, the maximum dose is deposited, Dmax; another 10 cm slab of the solid waterTM phantom was placed below the sample fibers to provide for full backscattering conditions (Fig. 1). The calibration of the clinical radiotherapy beams was carried out using a calibrated cavity ionization chamber model PTW Farmer 30004 (Aluminum electrode) with an internationally recognized calibration protocol of the International Atomic Energy Agency Technical Report Series 398 (IAEA TRS-398). The FFs were irradiated with doses ranging from 0.2 Gy to 10.0 Gy of 6 MeV electron and 6 MV photon, respectively. The TL yield as a function of temperature, referred to as the TL glow curve, were obtained herein, using a TLD reader, model Harshaw 3500 (Thermo Fisher Scientific Inc, U.S.A). During readout the following parameters were used: preheat temperature of 50 1C; acquisition temperature 400 1C for 14 s and a heating rate cycle of 25 1C/s for

FF and 10 1C/s for TLD-100. All readings were taken under N2 gas flow, suppressing oxidation and potential triboluminescence. The TL readings were normalized to the mass of fiber, to obtain results in C/mg. For the homogeneity test, a total of 200 pieces FFs were irradiated at 6 MeV electrons giving the dose of 0.8 Gy by LINAC at the dose rate of 400 cGy/min.

3. Results and discussion 3.1. Experimental TL glow curve The mean of peak integral of FFs was 10.6  106 (a.u) with the standard deviation of 1.5  105. The uncertainty has been determined as 71 standard error of the mean and the coefficient variation was not exceeded 75%, as required for radiotherapy clinical applications (ICRU Report No. 24, 1976) and the homogeneity of FFs are maintained. The pure silica FFs also show good reproducibility with a standard deviation less than 3.0%. Fig. 2 shows the maximum TL intensity (Im) of the main TL glow peak of pure silica FF as a function of the source of irradiation with the constant dose of 0.8 Gy. There was no significant difference in the value of maximum peak temperature (Tm), since each irradiation quality gives the same location of Tm for the constant dose of 0.8 Gy. The Im shows a significant difference when the type of irradiation changes from photon of 6 MV to electron of 6 MeV. Overall, the observed result confirmed that the Tm was not affected by the type of irradiation. The 6 MeV electron irradiation shows the TL intensity 1.5 times higher compared to that of the 6 MV photons, which can be attributed to the greater linear energy transfer (LET) for electrons. This result shows that the electrons have imparted more energy to the fiber material in the Coulomb-force interactions along their tracks. In contrast, a 6 MV photon may penetrate the fiber material with less interaction and less energy being transferred to the materials. 3.2. Deconvolved TL glow peaks The experimental glow curves of FF can be deconvoluted using a curve fitting computer program of WinGCF as shown in Fig. 3. A total of 300 glow curves were analysed for the determination of the kinetic parameters which include the activation energy, Ea and Tm of pure silica FFs and TLD-100. The FOM values obtained in this work are between 0.5% and 2.5% and are exhibited a good fit with the experimental result (Balian and Eddy, 1977). Considering the main dosimetric peak of FFs is Peak 2, while for TLD-100 is Peak 5, the kinetic parameters

3.0E+05

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Fig. 1. The dosimeter and sample experimental set-up on a solid water phantom. Another solid water layer was placed on top of the set-up to provide for chargedparticle equilibrium.

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Temperature ( o C) Fig. 2. TL glow curve of pure silica FFs subjected to 6 MeV electron and 6 MV photon beams with the the maximum TL intensity (Im), at 210 1C.

A. Alawiah et al. / Radiation Physics and Chemistry 106 (2015) 73–76

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Peak 3 Peak 2 Peak 4

Peak 1

Peak 5

Fig. 3. The output of WinGCF analysis for pure silica FF following 0.8 Gy electron irradiation of 6 MeV. The deconvoluted TL glow curve consists of five glow peaks with the FOM of 1.997%.

5000

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Dose (Gy) Fig. 4. Dose response of pure silica FFs after doses of 6 MeV electrons and 6 MV photons.

are as following: FFs (Ea; (0.92 70.07) eV, Tm (210.2 70.5) 1C; TLD100 (Ea; (1.74 70.09) eV, Tm (209.4 70.3) 1C. These findings of TLD-100 are consistent with those of Liang et al. who found that the Tm of peak 5 was 210 1C and Ea was 2.15 eV (Liang et al., 2011), while Horowitz et al. (2002) found that Tm was 209 1C and Ea was 2.24 eV of peak 5.

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Fig. 5. TL sensitivity of pure silica FFs after doses of 6 MeV electrons and 6 MV photons.

6 MV photons as shown in Fig. 5. The sensitivity of the pure silica FFs to electrons is consistently higher as compared to photons, especially in the higher dose region where their sensitivities are not significantly affected by the increase in dose. At the dose of 6 Gy, the sensitivity of FFs to 6 MeV electrons is 1.4 times greater as compared to 6 MV photons.

4. Conclusion 3.3. Dose response The dose dependence effect of FFs was evaluated using the modified supralinearity index,f ðDÞ, where, D1 is a normalization dose in the initial linear range and So is the intercept on the TL integral axis of the extrapolation of the linear region (Chen and McKeever, 1994).     SðDÞ  So SðD1 Þ  So = f ðDÞ ¼ ð1Þ D D1 The dose response function of f ðDÞ versus the dose, as displayed in terms of the integral TL signal in Fig. 4. It can be seen from this figure that the FFs shows linear characteristic ðf ðDÞ ¼ 1Þ nearly up to 1 Gy followed by a region of supralinear growth ðf ðDÞ 4 1Þ for the doses up to 10 Gy for 6 MeV electrons. While for 6 MV photon irradiation, the FFs shows linear characteristic ðf ðDÞ ¼ 1Þ nearly up to 2 Gy. The FFs under 6 MeV electron irradiation shows the more supralinear response than 6 MV photon irradiation. The sensitivity of the pure silica FFs was calculated from the dose response of the integral glow curve to 6 MeV electrons and

The pure silica FFs show good reproducibility with a standard deviation less than 3.0%. The TL response of the pure silica FFs to electrons is consistently higher as compared to photons. The 6 MeV electron irradiation shows the TL intensity 1.5 times higher as compared to that of 6 MV photons. The Tm was not affected by the type of irradiation used at the constant dose. The Im was found to be dependent on the type of radiation. When exposed to 6 MeV electron irradiation, the pure silica FFs displayed a supralinear response from 2 Gy up to 10.0 Gy. While for 6 MV photon irradiation, the FFs shows linear characteristic ðf ðDÞ ¼ 1Þ nearly up to 2 Gy. Kinetic parameters evaluation carried out using WinGCF of glow curve fitting analysis with an acceptable FOM range between 0.5% and 2.5% and exhibit a good fit with the experimental results. The sensitivity of the pure silica FFs to electrons is consistently higher than photons, especially in the higher dose region where their sensitivities are not significantly affected by the increase in dose. In conclusion, the preliminary results indicate that the FFs show promising results to be used as a radiation sensor in the high-dose therapy dosimetry.

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Acknowledgements We are indebted to the Medical Physics Group of the Radiotherapy Unit of Mount Miriam Cancer Hospital in Penang, Malaysia for the use of a LINAC. This work was supported by FRGS Research Grant EP20120521004 under the Ministry of Higher Education (MOHE), Malaysia Q3. The authors would like to acknowledge receipt of a University of Malaya-Ministry of Higher Education UM-MOHE High Impact Research Grant H-21001-00-F000033. In addition, the authors would like to acknowledge the University of Malaya fiber pulling group and the MOHE HIR Grant A00000750001, which supported the use of the fiber pulling tower system. References Abdul Rahman, A.T., Nisbet, A., Bradley, D.A., 2010. Dose-rate and the reciprocity law: TL response of Ge-doped SiO2 optical fibers at therapeutic radiation doses. Nucl. Instr. Meth. A 619, 157–162. Balian, H.G., Eddy, N.W., 1977. Figure-of-merit (FOM). An improved criterion over the normalized Chi-squared test for assessing goodness-of-fit-of gamma-ray spectral peaks. Nucl. Instrum. Methods 145, 389–395.

Chen, R., McKeever, S.W.S., 1994. Characterization of nonlinearities in the dose dependence of thermoluminescence. Rad. Meas 23, 667–673. Hashim, S., Al-Ahbabi, S., Bradley, D.A., Webb, M., Jeynes, C., Ramli, A.T., Wagiran, H., 2009. The thermoluminescence response of doped SiO2 optical fibers subjected to photon and electron irradiation. Appl. Radiat. Isot 67, 423–427. Horowitz, Y.S., Oster, L., Satinger, D., Biderman, S., Einav, Y., 2002. The composite structure of peak 5 in the glow curve of LiF:Mg,Ti (TLD-100): confirmation of peak 5a arising from a locally trapped electron–hole configuration. Radiat. Prot. Dosim. 100 (1-4), 123–126. ICRU Report No. 24., 1976. Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures. International Commission on Radiation Units and Measurements (ICRU), Washington, DC. International Atomic Energy Agency, 2005. Implementation of the International Code of Practice on Dosimetry in Radiotherapy (TRS 398): Review of Testing Results, IAEA-TECDOC. IAEA, Vienna p. 1455. Liang, Q., Davis, S.D., Horowitz, Y.S., DeWerd, L.A., 2011. Investigation of the relative TL response for low-energy X-rays relative to 60Co for TLD-100. Radiat. Meas. 46 (12), 1453–1456. Noor, N.M., Hussein, M., Bradley, D.A., Nisbet, A., 2011. The potential of Ge-doped optical fiber TL dosimetry for 3D verification of high energy IMRT photon beams. Nucl. Instrum. Methods Phys. Res., Sect. A 652, 891–895. Ong, C.L., Kandaiya, S., Kho, H.T., Chong, M.T., 2009. Segments of a commercial Gedoped optical fiber as a thermoluminescence dosimeter in radiotherapy. Radiat. Meas. 44, 158–162.