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Feasibility study for the use of Ce3+-doped optical fibres in radiotherapy
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 562 (2006) 449–455 www.elsevier.com/locate/nima
Feasibility study for the use of Ce3+-doped optical fibres in radiotherapy E. Monesa,, I. Veroneseb, F. Morettic, M. Fasolic, G. Loia, E. Negria, M. Brambillaa, N. Chiodinic, G. Brambillad, A. Veddac a Medical Physics Department, Azienda Ospedaliera Maggiore della Carita`, Corso Mazzini 18, I-28100 Novara, Italy Dipartimento di Fisica, Universita` degli Studi di Milano, and INFN Sezione di Milano, Via Celoria 16, I-20133 Milano, Italy c Dipartimento di Scienza dei Materiali, Universita` degli Studi di Milano Bicocca, Via Cozzi 53, I-20125 Milano, Italy d Optoelectronics Research Centre, University of Southampton, Southampton S0171BJ, UK
b
Received 22 December 2005; received in revised form 1 February 2006; accepted 1 February 2006 Available online 28 February 2006
Abstract A feasibility study for the use of a Ce3+-doped optical fibre as a radioluminescent dosimeter in radiotherapy was carried out. The prototype showed a satisfactory reproducibility (1.7%) and a good linearity over a clinically relevant dose range (from few cGy to approximately 10 Gy). Moreover, the device enabled a reliable evaluation of the absorbed dose, independently of the dose rate and of the orientation of the incoming radiation. A slight energy dependence of the response was observed when the system was irradiated with electron beams of different energies. The results indicate that the employment of this new system might be very promising both for quality control measurements and ‘‘in vivo’’ dosimetry. Some improvements are nevertheless still required in order to allow a real-time evaluation of the Cherenkov radiation produced by the irradiated portion of the passive optical fibre, which influences some of the dosimetric properties of the system. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.n; 87.53.j; 85.60.jb Keywords: Radiotherapy dosimetry; Optical fibre; Radioluminescence
1. Introduction The recent development of new radiotherapy techniques, such as 3D Conformal Radiation Therapy (3DCRT) and Intensity Modulated Radiation Therapy (IMRT), calls for the improvement of instruments and methodologies employed for the maintenance of a satisfactory quality system. Similarly, a careful verification of the accuracy in the dose distributions, as planned by the Treatment Planning System (TPS) through mathematical models, is gaining importance due to the high dose gradients achievable with modern radiotherapy techniques. Corresponding author. Tel.: +39 0321 3733699; fax: +39 0321 3733327. E-mail address: [email protected] (E. Mones).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.003
A proper real-time dosimeter device, with high spatial resolution, can be an effective tool for both quality control measurements and ‘‘in vivo’’ dosimetry in all those situations in which there is a lack of charged particle equilibrium such as in the dosimetry of small-size radiation fields. Besides the punctual dimensions, the dosimeter should be characterised by satisfactory sensitivity, reproducibility and linearity over a broad dose interval. Moreover, the response of the device should be isotropic and independent of the type and energy of the radiation, as well as of the dose rate. The use of optical fibres for remote and real-time radiation-monitoring applications, including ‘‘in vivo’’ dosimetry, has been recently proposed by several research groups [1–7] with promising results. A prototype of a new dosimetric system based on a Ce3+-doped optical fibre was recently developed at the
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Department of Materials Science of the University of Milano-Bicocca (Italy), in cooperation with the Optoelectronics Research Centre of the University of Southampton (United Kingdom). Preliminary measurements of the radio luminescence (RL) and dosimetric properties of the device were determined by using photon beams of low energies (20 and 32 kV) demonstrating the potentialities of the system [8]. On the basis of these results, a feasibility test for the use of this dosimeter in radiotherapy applications is reported and discussed here. 2. Materials and methods 2.1. The dosimetry system The dosimetry system was based on a Ce3+-doped SiO2 optical fibre (1 cm length, 220 mm diameter), arc-fusion spliced with a 3 M commercial fibre with 0.48 numerical aperture. The extremity of the passive fibre was connected to a photomultiplier (PMT) tube (EMI 9125QB) operating in photon-counting mode by means of a suitable aluminium holder. The RL signal was recorded and processed using a dedicated code (LabView) developed by the authors. The procedure for the preparation of the doped optical fibre, using the sol–gel technique, described in Ref. [8], is briefly summarised here. The sol solution was prepared with the following components: TEOS 6 ml, 99.9% ethanol 18 ml, Ce(NO3)3 12H2O solution in ethanol (10 mg/ml) in the amount to obtain a doping of 600 ppm molar Ce in SiO2, water 3.6 ml. The sol–gel transition was reached after several days in thermostatic chamber at 40 1C. Xerogel powder was obtained by rapid drying in a rotating evaporator and grain size uniformity was achieved by grinding in an agate mortar. Slow sintering up to 1100 1C under an oxidising atmosphere in a quartz chamber gave the final glass powder. The powder was then subjected to a Rapid Thermal Treatment (RTT) at 1600–1800 1C for some minutes in order to improve Ce3+ scintillation efficiency and remove eventual OH content excess. After this treatment, the powder was introduced in a quartz tube at 104–105 mbar and voids were eliminated by using an ultrasonic bath. The tube was finally vacuum sealed and inserted in a furnace at 2100 1C. A fibre with 220 mm diameter (with a core of approximately 175 mm diameter) was released by using a proper pulling speed. 2.2. Characterization measurements 2.2.1. Reproducibility, dose response and dose rate dependence The reproducibility, dose response and dose rate dependence of the system response were studied using a 6 MV photon beam produced by a medical linear accelerator Clinac 2100 CD (Varian, USA). The beam field size was set to 10 10 cm2 and the dosimeter was placed on the
beam axis at a depth of 5 cm in a water phantom, positioned at the standard Source–Surface Distance (SSD) of 100 cm. In order to assess the signal reproducibility, 10 consecutive measurements were collected delivering 100 Monitor Units (MU) for each irradiation, using a constant dose rate of 300 MU/min. In the reference conditions previously described, 1 MU corresponds to an absorbed dose of 1.15 cGy at the depth of maximum build-up. The dose response of the dosimetric system was investigated in a clinically relevant range extended from 4.9 cGy to 9.7 Gy. Three irradiations were repeated for each measurement point setting the dose rate to 300 MU/min. In order to investigate the dose rate dependence of the detector, a fixed dose of 2.26 Gy corresponding to 200 MU was delivered for each dose rate level available on the accelerator which spans from 100 to 600 MU/min, with 100 MU/min steps. At least three measurements were taken for each dose rate value. 2.2.2. Angular dependence As mentioned in Section 1, the investigated detector was a prototype, and in particular, the length of the active part of the fibre has not yet been optimised to reproduce the condition of ‘‘punctual detector’’. Consequently, the device was characterised by a cylindrical symmetry. The angular response of the detector along its symmetry axis was tested by positioning the fibre in air, along the axis of gantry rotation, without using any build-up material. The measurements were taken rotating the gantry from 01 to 3601, with 151 steps, delivering a constant dose of 100 MU for each irradiation, using electron beams of 6 MeV. 2.2.3. Stem effect The irradiation of optical fibres with high-energy beams, like those employed in radiotherapy, produces a luminescence signal due to the Cherenkov effect [9,10]. Cherenkov radiation, generated when a charged particle passes through a medium of refractive index n with a velocity greater than that of light in the medium, may become a significant source of stem effect, especially in the case of electron irradiation [11,12]. Since Cherenkov light is characterised by a broad spectral emission in the blueUV wavelength region, which overlaps the emission band at 450 nm of the RL produced by the Ce3+-dopant, its contribution cannot be simply subtracted by optical filtering [13]. A preliminary investigation of the Cherenkov effect and its influence in the field size dependence of the dosimeter response was carried out by irradiating the system with 6 MV photon beams of different field size. Field sizes ranging from 5 5 to 5 30 cm2 were used in order to directly expose increasing portions of the commercial optical fibre to the radiation beam. Each irradiation was performed with the active portion of the dosimeter positioned in the
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centre of the field, at 5 cm depth in the water phantom and with the passive part of the fibre stretched at the same depth in the direction of the increasing side of the photon field. In each irradiation, a constant dose of 1 Gy was delivered at the measurement point. 2.2.4. Energy dependence The intensity of Cherenkov light is dependent on the energy and the type of incident radiation [14]. Thus, this effect must be properly taken into account when energy dependence is characterised. Two approaches for eliminating the contribution of Cherenkov radiation to the RL signal, produced in an optical fibre, have been proposed in the literature. The first one was based on the simultaneous use of a blank fibre, originating a signal to subtract from that of the scintillator dosimeter fibre [1,11]. The second one consisted in the gated detection of the output of the optical fibre-coupled radiation dosimeter, exploiting the differences in the decay times of the Cherenkov emission and of the scintillator luminescence [7]. In this study, the first approach was considered and the Cherenkov radiation generated by an identical fibre, without the doped portion (i.e., reference fibre), irradiated in the same experimental conditions, was subtracted from the total luminescence signal emitted by the Ce3+-doped fibre. Photon and electron energies of 6 and 15 MV and 6, 9, 12, 15 and 18 MeV were considered, respectively. Photon beam measurements were performed with the same experimental set-up used for reproducibility measurements, while for the electron beams only the reference depth was changed by positioning the detector at the build-up depths of the different energies. For each energy level, the dosimeter and the commercial passive fibre were irradiated with a fixed dose of 1 Gy and a minimum set of three measurements were taken. 2.2.5. Depth–dose curve The depth–dose curve of a 6 MV photon beam with a size of 10 10 cm2 was measured by positioning the dosimeter in a water phantom at different depths, from 0.5 to 200 mm. Each irradiation was performed using a fixed dose rate of 300 MU/min. The measurement procedure was then repeated, under the same experimental conditions, using the reference fibre, in order to evaluate the Cherenkov light contribution for the various depths and to correct the dosimeter response. The results obtained were compared with those obtained using a diode (Scanditronix p-Si). 3. Results and discussion 3.1. Reproducibility Fig. 1 shows the RL signals versus time, measured during 10 consecutive irradiations. Time-scaling factors
Fig. 1. RL signals versus time, measured during 10 consecutive irradiations with a 6 MV photon beam.
were used to draw the curves in order to allow a better comparison among the various signals. The height of each curve (i.e., counts per seconds, cps) and the corresponding integral (i.e., total counts monitored during the irradiation time) are directly related to the dose rate and to the dose, respectively. The reproducibility of the dosimeter, calculated as the percentage error (1 standard deviation, 1SD) of the average of the 10 curve integrals was equal to 1.7%. Each integral was evaluated as the sum of the counts registered during the irradiation, after the subtraction of the background signal. The background, assessed by considering the cps monitored by the device in the absence of a direct exposition to the radiation beam, was equal to approximately 20 cps. The reproducibility of the system proved to be better than typical values achievable with other dosimeters commonly used for in vivo dosimetry such as MOSFETs (ffi 3%) [15] and TLDs (ffi5%) [16]. The value of 1.7% was nevertheless slightly higher than the one achievable by using diodes (ffi1%) [17], devices usually employed for the quality assurance measurements. 3.2. Dose response and dose rate dependence The dosimeter prototype exhibited a linear dose response over the whole dose range investigated, as shown in Fig. 2. The data points in Fig. 2 represent the integrals of the RL versus time curves, after background subtraction; the error bars correspond to 1SD. A straight line fitted to the experimental data (R2 ¼ 0.999) is also drawn in Fig. 2, together with the corresponding 95% confidence interval (CI). Fig. 3 shows RL versus time signals obtained by irradiating the fibre with the same dose and different dose rates. A systematic increase in the cps with increasing the irradiation time can be observed. The phenomenon, previously reported by other authors in different materials [5,6], might be due to the presence of defects in the
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Fig. 2. Dose response of the dosimeter measured using a 6 MV photon beam. The data points represent the integrals of the RL versus time curves, after the background subtraction. Error bars correspond to 1SD.
Fig. 3. RL versus time signals obtained by irradiating the fibre with a 6 MV photon beam to the same dose, using different dose rates.
Ce-doped glass that, acting as carrier traps during irradiation, originated a slower phosphorescence component of the total luminescence signal. This effect is particularly evident at high dose rates, i.e., when the time intervals between the electron pulses generated by the accelerator are much shorter than the decay time of the phosphorescence. The effect of traps can be also observed at the end of the irradiation: when the beam is turned off, the signal does not drop immediately to the background value, but it shows slow phosphorescence decay. Studies are in progress to better explain the physical origin of the trapping defects and to eliminate, or at least minimize, their contribution. The consequence of this phenomenon on the practical use of the dosimeter is nevertheless of slight importance, since it implies only a
Fig. 4. RL output versus dose rate. The inset shows the dependence of the signal (expressed as total counts registered during the exposure to radiation, scaled with respect to the results obtained using the dose rate of 300 MU/min) on the dose rate.
slight increase in the background contribution that must be subtracted from the total counts, when repeated measurements are performed in a relatively short time. No significant consequences were on the contrary observed in the dose rate response, apart from the expected increase in the uncertainty of the RL estimates at a high dose rate, as shown in Fig. 4. Each data point in Fig. 4 represents the average of the cps registered during dosimeter irradiation at the same dose rate. The results of a linear fit to the experimental data, shown in the figure, clearly demonstrate the proportionality between the height of the signal and the dose rate. The dependence of the signal, expressed as total counts registered during the exposure to radiation, versus the dose rate is shown in the inset of Fig. 4. The various data were scaled with respect to the total counts registered using a dose rate of 300 MU/min. No significant differences can be observed among the various data, indicating that the dosimeter enables a reliable evaluation of the absorbed dose, independently of the dose rate used. 3.3. Angular dependence The results of the angular dependence test are shown in Fig. 5, where the total counts registered during the irradiation times are plotted versus the rotation angle of the gantry, in polar coordinates. Each data was scaled with respect to the signal measured in correspondence to the irradiations at 01. No significant angular dependence was observed, as could reasonably be expected by considering the cylindrical symmetry of the fibre and the experimental set-up. Although the Cherenkov emission strongly depends on the angle between the fibre and the incident electron direction [9], the measurements were performed by keeping the fibre orthogonal to the beam
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Fig. 6. Behaviour of the luminescence signal detected by the system with increasing portions of passive optical fibre directly exposed to irradiation. Each point was scaled with respect to the total counts registered with the smallest field size of 5 5 cm2. Fig. 5. Angular dependence of the dosimeter response measured using a 6 MeV electron beam. The distances between filled circles and the centre represent RL-integrated intensities (normalised to the value measured at 01) versus rotation angles, marked in the figure at 301 intervals.
direction. In these conditions, the amount of Cherenkov light was therefore constant, independently of the gantry position. By contrast, the angular response of the detector in any direction different from its axis of symmetry was expected to be anisotropic, as an effect of the non-negligible length of the portion of doped fibre. Moreover, if the fibre were not placed along the axis of the gantry rotation, different lengths of the fibre stem would be irradiated, with the consequent variation in the Cherenkov effect contribution (see Section 3.4). These features suggest the necessity of positioning the fibre aligned with the rotation axis of the gantry accelerator during the radiation therapy set-up, in order to enable a reliable dose (and dose rate) evaluation, independently of the orientation of the incoming beam. 3.4. Stem effect Fig. 6 shows the behaviour of the luminescence signal detected by the system with increasing the portion of passive optical fibre directly exposed to irradiation. Each data point was scaled with respect to the total counts registered with the smallest field size of 5 5 cm2. A constant growth of the dosimeter response can be observed, due to the increase in the number of high energy electrons crossing the fibre and producing Cherenkov radiation. An increase in the total detected signal in the order of 2.5% per cm of commercial fibre directly
irradiated was estimated by fitting a linear function (R2 ¼ 0.986) to the experimental data. The fitted line, together with the corresponding 95% CI, is shown in Fig. 6. This feature of the prototype could denote a limitation of the system, since its dosimetric response would depend on the position of the fibre in the radiation field. A subtraction of the Cherenkov radiation from the total luminescence signal detected by the photomultiplier tube is therefore required before using the dosimeter in clinical tests. As previously suggested, a simple solution of this problem could be based on the use of a second optical fibre, without the doped portion, able to generate an identical Cherenkov signal. This approach was employed in the measurement of the energy dependence of the device and of the depth–dose curve, as shown below. 3.5. Energy dependence Fig. 7 shows the percentage deviations from the average value of the RL counts (i.e., total counts minus the Cerenkov contribution) registered by the system during the irradiations with photon and electron beams of different energies (top and bottom x-axis, respectively). While the energy response is substantially independent of the energy of photon beams, a slight variation can be observed for electron beams: at high energies, the response of the dosimeter increases and a deviation from the average of approximately 5% is observed using 18 MeV electrons. This trend reflects the energy dependence of the mass collision stopping power of electrons in SiO2 (i.e., the main component of the fibre). The energy dependence of the signal can be easily corrected using proper calibration factors according to the energy employed for the irradiations, as commonly done for the diodes.
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response of the dosimeter (grey triangles in Fig. 8) perfectly matches the results of the diode. 3.7. Conclusions
Fig. 7. Energy dependence of the dosimeter response obtained by irradiating the fibre with photons (top x-axis) and electrons (bottom xaxis) of different energies. Each data point represents the percent deviation from the average of the RL counts, after the subtraction of the Cherenkov light contribution.
Fig. 8. Relative depth–dose curves of a 10 10 cm2 6 MV photon beam measured with a diode and with the fibre. White circles, raw data measured by the fibre; grey triangles, corrected response of the fibre after the subtraction of the Cherenkov light contribution.
3.6. Depth–dose curve Fig. 8 shows the relative depth–dose curve, measured by the fibre (white circles) and compared with the one evaluated using a diode (dotted line). It can be observed that up to approximately 30 mm, the response of the fibre is very similar to that of the diode. In contrast, at higher depths, the stem effect becomes dominant, and influences the capability of the device to correctly assess the delivered dose. After the subtraction of the stem light contribution, using the reference (i.e., undoped) fibre, the corrected
A feasibility study was carried out for the use of a Ce3+doped optical fibre as a radioluminescent dosimeter in radiotherapy. The system allows a real-time monitoring of both dose and dose rate, and could be therefore suitable for a direct check of the absorbed dose during a radiotherapy treatment, providing a direct feedback to the medical physician. The prototype proved to have a satisfactory reproducibility (1.7%) and good linearity over the dose range of interest (from few cGy to approximately 10 Gy). Moreover, the device enabled a reliable evaluation of the absorbed dose, independently of the dose rate and of the orientation of the incoming radiation. A slight energy dependence of the response was observed when the system was irradiated with electron beams of different energies. Some improvements are still required before using the dosimeter in clinical applications. The origin of a slow phosphorescence component of the signal, which adds to the prompt RL, deserves further investigation. The reduction of this contribution could improve the capability of the system in assessing high dose rate values with satisfactory precision, and will enable to perform repeated acquisitions in short times, without a consequent increase in the background signal. At present, the main limitation of the system is represented by the Cherenkov radiation generated in the passive optical fibre, which influences some of the dosimetric properties of the system. This disadvantage might be overcome through a proper improvement in the design of the system. The addition of a second optical fibre, parallel to the first one and lacking the doped portion, could be used for a real-time evaluation of the Cherenkov radiation that, in turn, may be subtracted from the total luminescence signal acquired by the doped fibre, without compromising the punctual dimensions of the dosimeter. This approach, tested during the study of the energy response of the dosimeter and of its capability to correctly measure the depth–dose profile, proved to give satisfactory results. References [1] D. Le´tourneau, J. Pouliot, R. Roy, Med. Phys. 26 (1999) 2555. [2] G. Erfut, M.R. Krbetschek., T. Trautmann, W. Stolz, Radiat. Meas. 32 (2000) 735. [3] A.L. Huston, B.L. Justus, P.L. Falkenstein, R.W. Miller, H. Ning, R. Altemus, Nucl. Instr. and Meth. B. 184 (2001) 55. [4] M.C. Aznar, C.E. Andersen, L. Bøtter-Jensen, S.A˚.J. Ba¨ck, S. Mattsson, F. Kjær-Kristoffersen, J. Medin, Phys. Med. Biol. 49 (2004) 1655. [5] R. Gaza, S.W.S. McKeever, M.S. Akselrod, A. Akselrod, T. Underwood, C. Yoder, C.E. Andersen, M.C. Aznar, C.J. Marckmann, L. Bøtter-Jensen, Radiat. Meas. 38 (2004) 809.
ARTICLE IN PRESS E. Mones et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 449–455 [6] J.C. Polf, E.G. Yukihara, M.S. Akselrod, S.W.S. McKeever, Radiat. Meas. 38 (2004) 227. [7] B.L. Justin, P. Falkenstein, A.L. Huston, M.C. Plazas, H. Ning, R.W. Miller, Appl. Opt. 43 (2004) 1663. [8] A. Vedda, N. Chiodini, D. Di Martino, M. Fasoli, S. Keffer, A. Lauria, M. Martini, F. Moretti G. Spinolo, M. Nikl, N. Solovieva, G. Brambilla, Appl. Phys. Lett. 85 (2004) 6356. [9] A.S. Beddar, T.R. Mackie, F.H. Attix, Phys. Med. Biol. 37 (1992) 925. [10] M.A. Clift, R.A. Sutton, D.V. Webb, Phys. Med. Biol. 45 (2000) 1165. [11] A.S. Beddar, T.R. Mackie, F.H. Attix, Phys. Med. Biol. 37 (1992) 1901.
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[12] L. Archambault, J. Arsenault, L. Gingras, Med. Phys. 32 (2005) 2271. [13] S.F. de Boer, A.S. Beddar, J.A. Rawlinson, Phys. Med. Biol. 38 (1993) 945. [14] J.V. Jelley, Cerenkov Radiation and its Applications, Pergamon press, NewYork, 1958. [15] P. Scalchi, P. Francescon, Int. J. Radiat. Oncol. Biol. Phys. 40 (1998) 987. [16] P. McGhee, S. Humphreys, P. Dunscombe, Med. Dosim. 18 (1993) 187. [17] J. Van Dam, G. Marinello, in: G. Garant (Ed.), Methods for In Vivo Dosimetry in External Radiotherapy, ESTRO Booklet no. I, Apeldoorn, Leuven, 1994.
Nuclear Instruments and Methods in Physics Research A 562 (2006) 449–455 www.elsevier.com/locate/nima
Feasibility study for the use of Ce3+-doped optical fibres in radiotherapy E. Monesa,, I. Veroneseb, F. Morettic, M. Fasolic, G. Loia, E. Negria, M. Brambillaa, N. Chiodinic, G. Brambillad, A. Veddac a Medical Physics Department, Azienda Ospedaliera Maggiore della Carita`, Corso Mazzini 18, I-28100 Novara, Italy Dipartimento di Fisica, Universita` degli Studi di Milano, and INFN Sezione di Milano, Via Celoria 16, I-20133 Milano, Italy c Dipartimento di Scienza dei Materiali, Universita` degli Studi di Milano Bicocca, Via Cozzi 53, I-20125 Milano, Italy d Optoelectronics Research Centre, University of Southampton, Southampton S0171BJ, UK
b
Received 22 December 2005; received in revised form 1 February 2006; accepted 1 February 2006 Available online 28 February 2006
Abstract A feasibility study for the use of a Ce3+-doped optical fibre as a radioluminescent dosimeter in radiotherapy was carried out. The prototype showed a satisfactory reproducibility (1.7%) and a good linearity over a clinically relevant dose range (from few cGy to approximately 10 Gy). Moreover, the device enabled a reliable evaluation of the absorbed dose, independently of the dose rate and of the orientation of the incoming radiation. A slight energy dependence of the response was observed when the system was irradiated with electron beams of different energies. The results indicate that the employment of this new system might be very promising both for quality control measurements and ‘‘in vivo’’ dosimetry. Some improvements are nevertheless still required in order to allow a real-time evaluation of the Cherenkov radiation produced by the irradiated portion of the passive optical fibre, which influences some of the dosimetric properties of the system. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.n; 87.53.j; 85.60.jb Keywords: Radiotherapy dosimetry; Optical fibre; Radioluminescence
1. Introduction The recent development of new radiotherapy techniques, such as 3D Conformal Radiation Therapy (3DCRT) and Intensity Modulated Radiation Therapy (IMRT), calls for the improvement of instruments and methodologies employed for the maintenance of a satisfactory quality system. Similarly, a careful verification of the accuracy in the dose distributions, as planned by the Treatment Planning System (TPS) through mathematical models, is gaining importance due to the high dose gradients achievable with modern radiotherapy techniques. Corresponding author. Tel.: +39 0321 3733699; fax: +39 0321 3733327. E-mail address: [email protected] (E. Mones).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.003
A proper real-time dosimeter device, with high spatial resolution, can be an effective tool for both quality control measurements and ‘‘in vivo’’ dosimetry in all those situations in which there is a lack of charged particle equilibrium such as in the dosimetry of small-size radiation fields. Besides the punctual dimensions, the dosimeter should be characterised by satisfactory sensitivity, reproducibility and linearity over a broad dose interval. Moreover, the response of the device should be isotropic and independent of the type and energy of the radiation, as well as of the dose rate. The use of optical fibres for remote and real-time radiation-monitoring applications, including ‘‘in vivo’’ dosimetry, has been recently proposed by several research groups [1–7] with promising results. A prototype of a new dosimetric system based on a Ce3+-doped optical fibre was recently developed at the
ARTICLE IN PRESS 450
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Department of Materials Science of the University of Milano-Bicocca (Italy), in cooperation with the Optoelectronics Research Centre of the University of Southampton (United Kingdom). Preliminary measurements of the radio luminescence (RL) and dosimetric properties of the device were determined by using photon beams of low energies (20 and 32 kV) demonstrating the potentialities of the system [8]. On the basis of these results, a feasibility test for the use of this dosimeter in radiotherapy applications is reported and discussed here. 2. Materials and methods 2.1. The dosimetry system The dosimetry system was based on a Ce3+-doped SiO2 optical fibre (1 cm length, 220 mm diameter), arc-fusion spliced with a 3 M commercial fibre with 0.48 numerical aperture. The extremity of the passive fibre was connected to a photomultiplier (PMT) tube (EMI 9125QB) operating in photon-counting mode by means of a suitable aluminium holder. The RL signal was recorded and processed using a dedicated code (LabView) developed by the authors. The procedure for the preparation of the doped optical fibre, using the sol–gel technique, described in Ref. [8], is briefly summarised here. The sol solution was prepared with the following components: TEOS 6 ml, 99.9% ethanol 18 ml, Ce(NO3)3 12H2O solution in ethanol (10 mg/ml) in the amount to obtain a doping of 600 ppm molar Ce in SiO2, water 3.6 ml. The sol–gel transition was reached after several days in thermostatic chamber at 40 1C. Xerogel powder was obtained by rapid drying in a rotating evaporator and grain size uniformity was achieved by grinding in an agate mortar. Slow sintering up to 1100 1C under an oxidising atmosphere in a quartz chamber gave the final glass powder. The powder was then subjected to a Rapid Thermal Treatment (RTT) at 1600–1800 1C for some minutes in order to improve Ce3+ scintillation efficiency and remove eventual OH content excess. After this treatment, the powder was introduced in a quartz tube at 104–105 mbar and voids were eliminated by using an ultrasonic bath. The tube was finally vacuum sealed and inserted in a furnace at 2100 1C. A fibre with 220 mm diameter (with a core of approximately 175 mm diameter) was released by using a proper pulling speed. 2.2. Characterization measurements 2.2.1. Reproducibility, dose response and dose rate dependence The reproducibility, dose response and dose rate dependence of the system response were studied using a 6 MV photon beam produced by a medical linear accelerator Clinac 2100 CD (Varian, USA). The beam field size was set to 10 10 cm2 and the dosimeter was placed on the
beam axis at a depth of 5 cm in a water phantom, positioned at the standard Source–Surface Distance (SSD) of 100 cm. In order to assess the signal reproducibility, 10 consecutive measurements were collected delivering 100 Monitor Units (MU) for each irradiation, using a constant dose rate of 300 MU/min. In the reference conditions previously described, 1 MU corresponds to an absorbed dose of 1.15 cGy at the depth of maximum build-up. The dose response of the dosimetric system was investigated in a clinically relevant range extended from 4.9 cGy to 9.7 Gy. Three irradiations were repeated for each measurement point setting the dose rate to 300 MU/min. In order to investigate the dose rate dependence of the detector, a fixed dose of 2.26 Gy corresponding to 200 MU was delivered for each dose rate level available on the accelerator which spans from 100 to 600 MU/min, with 100 MU/min steps. At least three measurements were taken for each dose rate value. 2.2.2. Angular dependence As mentioned in Section 1, the investigated detector was a prototype, and in particular, the length of the active part of the fibre has not yet been optimised to reproduce the condition of ‘‘punctual detector’’. Consequently, the device was characterised by a cylindrical symmetry. The angular response of the detector along its symmetry axis was tested by positioning the fibre in air, along the axis of gantry rotation, without using any build-up material. The measurements were taken rotating the gantry from 01 to 3601, with 151 steps, delivering a constant dose of 100 MU for each irradiation, using electron beams of 6 MeV. 2.2.3. Stem effect The irradiation of optical fibres with high-energy beams, like those employed in radiotherapy, produces a luminescence signal due to the Cherenkov effect [9,10]. Cherenkov radiation, generated when a charged particle passes through a medium of refractive index n with a velocity greater than that of light in the medium, may become a significant source of stem effect, especially in the case of electron irradiation [11,12]. Since Cherenkov light is characterised by a broad spectral emission in the blueUV wavelength region, which overlaps the emission band at 450 nm of the RL produced by the Ce3+-dopant, its contribution cannot be simply subtracted by optical filtering [13]. A preliminary investigation of the Cherenkov effect and its influence in the field size dependence of the dosimeter response was carried out by irradiating the system with 6 MV photon beams of different field size. Field sizes ranging from 5 5 to 5 30 cm2 were used in order to directly expose increasing portions of the commercial optical fibre to the radiation beam. Each irradiation was performed with the active portion of the dosimeter positioned in the
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centre of the field, at 5 cm depth in the water phantom and with the passive part of the fibre stretched at the same depth in the direction of the increasing side of the photon field. In each irradiation, a constant dose of 1 Gy was delivered at the measurement point. 2.2.4. Energy dependence The intensity of Cherenkov light is dependent on the energy and the type of incident radiation [14]. Thus, this effect must be properly taken into account when energy dependence is characterised. Two approaches for eliminating the contribution of Cherenkov radiation to the RL signal, produced in an optical fibre, have been proposed in the literature. The first one was based on the simultaneous use of a blank fibre, originating a signal to subtract from that of the scintillator dosimeter fibre [1,11]. The second one consisted in the gated detection of the output of the optical fibre-coupled radiation dosimeter, exploiting the differences in the decay times of the Cherenkov emission and of the scintillator luminescence [7]. In this study, the first approach was considered and the Cherenkov radiation generated by an identical fibre, without the doped portion (i.e., reference fibre), irradiated in the same experimental conditions, was subtracted from the total luminescence signal emitted by the Ce3+-doped fibre. Photon and electron energies of 6 and 15 MV and 6, 9, 12, 15 and 18 MeV were considered, respectively. Photon beam measurements were performed with the same experimental set-up used for reproducibility measurements, while for the electron beams only the reference depth was changed by positioning the detector at the build-up depths of the different energies. For each energy level, the dosimeter and the commercial passive fibre were irradiated with a fixed dose of 1 Gy and a minimum set of three measurements were taken. 2.2.5. Depth–dose curve The depth–dose curve of a 6 MV photon beam with a size of 10 10 cm2 was measured by positioning the dosimeter in a water phantom at different depths, from 0.5 to 200 mm. Each irradiation was performed using a fixed dose rate of 300 MU/min. The measurement procedure was then repeated, under the same experimental conditions, using the reference fibre, in order to evaluate the Cherenkov light contribution for the various depths and to correct the dosimeter response. The results obtained were compared with those obtained using a diode (Scanditronix p-Si). 3. Results and discussion 3.1. Reproducibility Fig. 1 shows the RL signals versus time, measured during 10 consecutive irradiations. Time-scaling factors
Fig. 1. RL signals versus time, measured during 10 consecutive irradiations with a 6 MV photon beam.
were used to draw the curves in order to allow a better comparison among the various signals. The height of each curve (i.e., counts per seconds, cps) and the corresponding integral (i.e., total counts monitored during the irradiation time) are directly related to the dose rate and to the dose, respectively. The reproducibility of the dosimeter, calculated as the percentage error (1 standard deviation, 1SD) of the average of the 10 curve integrals was equal to 1.7%. Each integral was evaluated as the sum of the counts registered during the irradiation, after the subtraction of the background signal. The background, assessed by considering the cps monitored by the device in the absence of a direct exposition to the radiation beam, was equal to approximately 20 cps. The reproducibility of the system proved to be better than typical values achievable with other dosimeters commonly used for in vivo dosimetry such as MOSFETs (ffi 3%) [15] and TLDs (ffi5%) [16]. The value of 1.7% was nevertheless slightly higher than the one achievable by using diodes (ffi1%) [17], devices usually employed for the quality assurance measurements. 3.2. Dose response and dose rate dependence The dosimeter prototype exhibited a linear dose response over the whole dose range investigated, as shown in Fig. 2. The data points in Fig. 2 represent the integrals of the RL versus time curves, after background subtraction; the error bars correspond to 1SD. A straight line fitted to the experimental data (R2 ¼ 0.999) is also drawn in Fig. 2, together with the corresponding 95% confidence interval (CI). Fig. 3 shows RL versus time signals obtained by irradiating the fibre with the same dose and different dose rates. A systematic increase in the cps with increasing the irradiation time can be observed. The phenomenon, previously reported by other authors in different materials [5,6], might be due to the presence of defects in the
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Fig. 2. Dose response of the dosimeter measured using a 6 MV photon beam. The data points represent the integrals of the RL versus time curves, after the background subtraction. Error bars correspond to 1SD.
Fig. 3. RL versus time signals obtained by irradiating the fibre with a 6 MV photon beam to the same dose, using different dose rates.
Ce-doped glass that, acting as carrier traps during irradiation, originated a slower phosphorescence component of the total luminescence signal. This effect is particularly evident at high dose rates, i.e., when the time intervals between the electron pulses generated by the accelerator are much shorter than the decay time of the phosphorescence. The effect of traps can be also observed at the end of the irradiation: when the beam is turned off, the signal does not drop immediately to the background value, but it shows slow phosphorescence decay. Studies are in progress to better explain the physical origin of the trapping defects and to eliminate, or at least minimize, their contribution. The consequence of this phenomenon on the practical use of the dosimeter is nevertheless of slight importance, since it implies only a
Fig. 4. RL output versus dose rate. The inset shows the dependence of the signal (expressed as total counts registered during the exposure to radiation, scaled with respect to the results obtained using the dose rate of 300 MU/min) on the dose rate.
slight increase in the background contribution that must be subtracted from the total counts, when repeated measurements are performed in a relatively short time. No significant consequences were on the contrary observed in the dose rate response, apart from the expected increase in the uncertainty of the RL estimates at a high dose rate, as shown in Fig. 4. Each data point in Fig. 4 represents the average of the cps registered during dosimeter irradiation at the same dose rate. The results of a linear fit to the experimental data, shown in the figure, clearly demonstrate the proportionality between the height of the signal and the dose rate. The dependence of the signal, expressed as total counts registered during the exposure to radiation, versus the dose rate is shown in the inset of Fig. 4. The various data were scaled with respect to the total counts registered using a dose rate of 300 MU/min. No significant differences can be observed among the various data, indicating that the dosimeter enables a reliable evaluation of the absorbed dose, independently of the dose rate used. 3.3. Angular dependence The results of the angular dependence test are shown in Fig. 5, where the total counts registered during the irradiation times are plotted versus the rotation angle of the gantry, in polar coordinates. Each data was scaled with respect to the signal measured in correspondence to the irradiations at 01. No significant angular dependence was observed, as could reasonably be expected by considering the cylindrical symmetry of the fibre and the experimental set-up. Although the Cherenkov emission strongly depends on the angle between the fibre and the incident electron direction [9], the measurements were performed by keeping the fibre orthogonal to the beam
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Fig. 6. Behaviour of the luminescence signal detected by the system with increasing portions of passive optical fibre directly exposed to irradiation. Each point was scaled with respect to the total counts registered with the smallest field size of 5 5 cm2. Fig. 5. Angular dependence of the dosimeter response measured using a 6 MeV electron beam. The distances between filled circles and the centre represent RL-integrated intensities (normalised to the value measured at 01) versus rotation angles, marked in the figure at 301 intervals.
direction. In these conditions, the amount of Cherenkov light was therefore constant, independently of the gantry position. By contrast, the angular response of the detector in any direction different from its axis of symmetry was expected to be anisotropic, as an effect of the non-negligible length of the portion of doped fibre. Moreover, if the fibre were not placed along the axis of the gantry rotation, different lengths of the fibre stem would be irradiated, with the consequent variation in the Cherenkov effect contribution (see Section 3.4). These features suggest the necessity of positioning the fibre aligned with the rotation axis of the gantry accelerator during the radiation therapy set-up, in order to enable a reliable dose (and dose rate) evaluation, independently of the orientation of the incoming beam. 3.4. Stem effect Fig. 6 shows the behaviour of the luminescence signal detected by the system with increasing the portion of passive optical fibre directly exposed to irradiation. Each data point was scaled with respect to the total counts registered with the smallest field size of 5 5 cm2. A constant growth of the dosimeter response can be observed, due to the increase in the number of high energy electrons crossing the fibre and producing Cherenkov radiation. An increase in the total detected signal in the order of 2.5% per cm of commercial fibre directly
irradiated was estimated by fitting a linear function (R2 ¼ 0.986) to the experimental data. The fitted line, together with the corresponding 95% CI, is shown in Fig. 6. This feature of the prototype could denote a limitation of the system, since its dosimetric response would depend on the position of the fibre in the radiation field. A subtraction of the Cherenkov radiation from the total luminescence signal detected by the photomultiplier tube is therefore required before using the dosimeter in clinical tests. As previously suggested, a simple solution of this problem could be based on the use of a second optical fibre, without the doped portion, able to generate an identical Cherenkov signal. This approach was employed in the measurement of the energy dependence of the device and of the depth–dose curve, as shown below. 3.5. Energy dependence Fig. 7 shows the percentage deviations from the average value of the RL counts (i.e., total counts minus the Cerenkov contribution) registered by the system during the irradiations with photon and electron beams of different energies (top and bottom x-axis, respectively). While the energy response is substantially independent of the energy of photon beams, a slight variation can be observed for electron beams: at high energies, the response of the dosimeter increases and a deviation from the average of approximately 5% is observed using 18 MeV electrons. This trend reflects the energy dependence of the mass collision stopping power of electrons in SiO2 (i.e., the main component of the fibre). The energy dependence of the signal can be easily corrected using proper calibration factors according to the energy employed for the irradiations, as commonly done for the diodes.
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response of the dosimeter (grey triangles in Fig. 8) perfectly matches the results of the diode. 3.7. Conclusions
Fig. 7. Energy dependence of the dosimeter response obtained by irradiating the fibre with photons (top x-axis) and electrons (bottom xaxis) of different energies. Each data point represents the percent deviation from the average of the RL counts, after the subtraction of the Cherenkov light contribution.
Fig. 8. Relative depth–dose curves of a 10 10 cm2 6 MV photon beam measured with a diode and with the fibre. White circles, raw data measured by the fibre; grey triangles, corrected response of the fibre after the subtraction of the Cherenkov light contribution.
3.6. Depth–dose curve Fig. 8 shows the relative depth–dose curve, measured by the fibre (white circles) and compared with the one evaluated using a diode (dotted line). It can be observed that up to approximately 30 mm, the response of the fibre is very similar to that of the diode. In contrast, at higher depths, the stem effect becomes dominant, and influences the capability of the device to correctly assess the delivered dose. After the subtraction of the stem light contribution, using the reference (i.e., undoped) fibre, the corrected
A feasibility study was carried out for the use of a Ce3+doped optical fibre as a radioluminescent dosimeter in radiotherapy. The system allows a real-time monitoring of both dose and dose rate, and could be therefore suitable for a direct check of the absorbed dose during a radiotherapy treatment, providing a direct feedback to the medical physician. The prototype proved to have a satisfactory reproducibility (1.7%) and good linearity over the dose range of interest (from few cGy to approximately 10 Gy). Moreover, the device enabled a reliable evaluation of the absorbed dose, independently of the dose rate and of the orientation of the incoming radiation. A slight energy dependence of the response was observed when the system was irradiated with electron beams of different energies. Some improvements are still required before using the dosimeter in clinical applications. The origin of a slow phosphorescence component of the signal, which adds to the prompt RL, deserves further investigation. The reduction of this contribution could improve the capability of the system in assessing high dose rate values with satisfactory precision, and will enable to perform repeated acquisitions in short times, without a consequent increase in the background signal. At present, the main limitation of the system is represented by the Cherenkov radiation generated in the passive optical fibre, which influences some of the dosimetric properties of the system. This disadvantage might be overcome through a proper improvement in the design of the system. The addition of a second optical fibre, parallel to the first one and lacking the doped portion, could be used for a real-time evaluation of the Cherenkov radiation that, in turn, may be subtracted from the total luminescence signal acquired by the doped fibre, without compromising the punctual dimensions of the dosimeter. This approach, tested during the study of the energy response of the dosimeter and of its capability to correctly measure the depth–dose profile, proved to give satisfactory results. References [1] D. Le´tourneau, J. Pouliot, R. Roy, Med. Phys. 26 (1999) 2555. [2] G. Erfut, M.R. Krbetschek., T. Trautmann, W. Stolz, Radiat. Meas. 32 (2000) 735. [3] A.L. Huston, B.L. Justus, P.L. Falkenstein, R.W. Miller, H. Ning, R. Altemus, Nucl. Instr. and Meth. B. 184 (2001) 55. [4] M.C. Aznar, C.E. Andersen, L. Bøtter-Jensen, S.A˚.J. Ba¨ck, S. Mattsson, F. Kjær-Kristoffersen, J. Medin, Phys. Med. Biol. 49 (2004) 1655. [5] R. Gaza, S.W.S. McKeever, M.S. Akselrod, A. Akselrod, T. Underwood, C. Yoder, C.E. Andersen, M.C. Aznar, C.J. Marckmann, L. Bøtter-Jensen, Radiat. Meas. 38 (2004) 809.
ARTICLE IN PRESS E. Mones et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 449–455 [6] J.C. Polf, E.G. Yukihara, M.S. Akselrod, S.W.S. McKeever, Radiat. Meas. 38 (2004) 227. [7] B.L. Justin, P. Falkenstein, A.L. Huston, M.C. Plazas, H. Ning, R.W. Miller, Appl. Opt. 43 (2004) 1663. [8] A. Vedda, N. Chiodini, D. Di Martino, M. Fasoli, S. Keffer, A. Lauria, M. Martini, F. Moretti G. Spinolo, M. Nikl, N. Solovieva, G. Brambilla, Appl. Phys. Lett. 85 (2004) 6356. [9] A.S. Beddar, T.R. Mackie, F.H. Attix, Phys. Med. Biol. 37 (1992) 925. [10] M.A. Clift, R.A. Sutton, D.V. Webb, Phys. Med. Biol. 45 (2000) 1165. [11] A.S. Beddar, T.R. Mackie, F.H. Attix, Phys. Med. Biol. 37 (1992) 1901.
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[12] L. Archambault, J. Arsenault, L. Gingras, Med. Phys. 32 (2005) 2271. [13] S.F. de Boer, A.S. Beddar, J.A. Rawlinson, Phys. Med. Biol. 38 (1993) 945. [14] J.V. Jelley, Cerenkov Radiation and its Applications, Pergamon press, NewYork, 1958. [15] P. Scalchi, P. Francescon, Int. J. Radiat. Oncol. Biol. Phys. 40 (1998) 987. [16] P. McGhee, S. Humphreys, P. Dunscombe, Med. Dosim. 18 (1993) 187. [17] J. Van Dam, G. Marinello, in: G. Garant (Ed.), Methods for In Vivo Dosimetry in External Radiotherapy, ESTRO Booklet no. I, Apeldoorn, Leuven, 1994.