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Determination of the neutron spectra around an 18 MV medical LINAC with a passive Bonner sphere spectrometer based on gold foils and TLD pairs
Radiation Measurements 43 (2008) 1038 – 1043 www.elsevier.com/locate/radmeas
Determination of the neutron spectra around an 18 MV medical LINAC with a passive Bonner sphere spectrometer based on gold foils and TLD pairs A. Esposito a,∗ , R. Bedogni a , L. Lembo b , M. Morelli b a INFN–LNF Frascati National Laboratories, Via E. Fermi n. 40, 00044 Frascati, Italy b Hospital S. Maria della Scaletta, AUSL Ravenna, Italy
Abstract The increased use of LINACs with accelerating voltage higher than 10 MV in clinical radiotherapy is producing an increasing demand of accurate dosimetric measurements of the photon induced neutron contamination of the radiotherapy beams. The data available in literature have been obtained almost invariably with Bonner sphere spectrometers equipped with gold activation detectors. A collaboration between the Radiation Protection Group of the Frascati National Laboratories and the Ravenna Hospital allowed determining the neutron spectra in different significant points inside the treatment room of an 18 MV medical LINAC. Two independent sets of passive detectors have been employed inside a commercial set of Bonner spheres: gold activation foils (0.1 mm height × 10 mm diameter) and TLD pairs (TLD-600 and TLD-700 Harshaw type). Validated response matrices were used in both cases. The FRascati Unfolding Interactive Tool (FRUIT) code was used to unfold the experimental data. The spectra measured with the two systems are coherent, confirming the accuracy of both methods. The work also provides useful information for the radiation protection of the patient and the workers. © 2007 Elsevier Ltd. All rights reserved. Keywords: Bonner spheres; Neutron spectrometry; TLD pairs; Gold activation foils; Medical LINAC
1. Introduction Radiotherapy constitutes, in modern society, one of the major contributions to the collective dose to the population. It is in fact estimated (IAEA, 2006) that half of the cancer patients receive radiotherapy and the number of tele-therapy installations is increasing, especially in developing countries. A parasitic effect occurring in medical accelerators operating above 10 MV is the production of neutrons, mainly due to the giant dipole resonance reactions (, n) induced by high-energy photons in the materials constituting the accelerator head (collimators, filters and shields), the treatment room structures and the patient itself. The neutron term should be taken into account not only for the protection of the patient, but also for the optimization of the dose to workers. If a treatment room is designed to provide adequate photon shielding only, some undesired neutron doses could be delivered in the surrounding areas, especially if a suitable maze is not present. Very few and ∗ Corresponding author. Tel.: +39 0694032232; fax: +39 0694032364.
E-mail address: [email protected] (A. Esposito). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.035
insufficient information on the neutron term is provided by the accelerator manufacturers, and the measured data available in literature are usually valid for very specific machines and cannot be extended to other installations. It is therefore advisable, for any specific accelerator to be used in cancer therapy, to have an accurate evaluation of the neutron doses produced in the treatment room, not only on the patient plane (IEC, 1998) but also in the surrounding areas. Due to the dramatic energy dependence of the neutron fluence-to-dose equivalent conversion coefficients (ICRP, 1996), this determination is only possible using instruments able to derive sufficient information on the neutron energy distribution. Additional complications are the pulsed time structure of these neutron fields, the very intense photon component and the presence of radiofrequency fields, which could affect the operation of active instruments (Thomas et al., 2002). Previous measurements reported in literature were performed almost invariably with passive integrating detectors, especially passive Bonner sphere spectrometers (BSSs). BSS-based instruments have isotropic response, wide energy range and allow,
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
in principle, an accurate determination of the neutron spectrum. As central detectors, the most used are activation gold foils (Thomas et al., 2002; Fernández et al., 2007a) and TLD pairs (Barquero et al., 2005; Howell et al., 2005). TLD pairs and gold foils offer different performances, as briefly summarized below. TLDs are certainly more sensitive than gold foils, requiring less machine time. This is an appreciable advantage, especially taking into account the very intense time scheduling of radiotherapy machines. In addition, the read-out of TLDs is much more rapid than the gold foil counting procedure. On the other hand, TLDs are sensitive to photons, and a very accurate knowledge of their photon sensitivity is needed to correctly derive the “neutron signal”. Due to this reason, TLDs are probably only suited for measurements “off axis”. The gold foils technique relies on the post-irradiation counting of the 198 Au (411.8 keV photons with 95.5% yield) induced by thermal and epithermal neutrons in thin foils of pure 197 Au. An advantage of this system is the insensitivity to photons. However, the following effects have to be taken into account: • The photons of the (356 keV at 88%, 333 keV at 23% and 426 keV at 6.7%) generated by the 197 Au(, n)196 Au reactions (threshold photon energy 8.07 MeV) could alter the “genuine” thermal neutron signal, if the photon counting system has no or poor energy resolution, as in the case of a total counting. Even in this case, the two isotopes could be discriminated with a series of measurements at different times (198 Au and 196 Au have half lives 2.696 and 6.183 d, respectively), requiring a time consuming procedure. • The extra-neutrons coming from the (, n) reactions in the carbon of the spheres, when the photon energy exceeds 18 MeV. 196 Au
A relevant aspect to be considered is the experimental validation of the response matrix to be used for unfolding the raw data. Whilst the TLD-based system can be easily validated with common radionuclide sources, the gold-based BSS requires very intense neutron fields, only available in few calibration installations (Fernández et al., 2007b). It is worth mentioning that no TLD/gold comparative works have been presented, since all literature works used one technique only. In order to cover this lack, a double TLD/gold BSS system was developed and experimentally validated by the Radiation Protection Group of INFN–LNF (Frascati National Laboratories). Special aluminium holders were designed to expose four TLD pairs and a gold foil in the same sphere. Computational and experimental studies were performed in order to evaluate the perturbation due to different TLD pairs plus a gold foil in the same holder. In the framework of a collaboration with the Hospital S. Maria della Scaletta (AUSL Ravenna, Italy), this combined technique was used to determine the neutron fields in relevant points around an 18 MV Elekta Precise LINAC. The raw data were unfolded using the FRascati Unfolding Interactive Tool (FRUIT) (Bedogni et al., 2007a), a visual un-
1039
Fig. 1. The set-up of the treatment room of the Hospital S. Maria della Scaletta (AUSL Ravenna, Italy).
folding code developed at the INFN–LNF whose main characteristics are the reduced dependence of the results from the a priori information and the high level of flexibility and interactivity. TLD pairs or gold foils data were separately unfolded using validated response matrices, obtaining two “independent” spectra for each measurement point. The spectra measured with the two systems are coherent, in terms of spectrum shape and dosimetric quantities. This confirms the accuracy of both methods. In addition, the experimental data provide useful information for the protection of the patient and the workers as well. 2. The LINAC set-up The measurement points in the treatment room of the 18 MV Elekta Precise LINAC are shown in Fig. 1. All points are located in the isocentre plane. The isocentre point is P1. P2 and P3 (on the patient couch) are located at 1 and 1.5 m from P1, respectively. P4 is located at 5 m in the maze entrance. Whilst the combined technique was used in P2, P3 and P4, only gold foils were used at the isocentre (P1). Here the copious amount of photons should have probably masked the neutron signal on TLDs. 3. The INFN–LNF Bonner sphere system The BSS used in this work includes six polyethylene spheres (density 0.95 g cm−3 ) available from Ludlum Measurements, Inc. Their diameters are 2, 3, 5, 8, 10 and 12 in and are designed to hold a 4 × 4 6 LiI(Eu) active scintillator. 4. The passive BSS 4.1. Gold foils The activation detectors are circular gold foils with 99.9% chemical purity, diameter 10 mm and thickness 0.1 mm. Their
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A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
individual weight is taken into account in the calculations. The foils were counted using a high-purity coaxial germanium detector available at the Frascati Laboratories, with efficiency 0.069 ± 0.002 and FWHM 0.4% (“contact” geometry). Its very high energy resolution allows discriminating 196 Au from 198 Au. The response matrix for the gold foils into the Bonner spheres was calculated using MCNP-4C (Briesmeister, 2000) in the interval from 1.5E − 9 to 20 MeV and experimentally validated with an intense 252 Cf source available at the ENEA-Bologna Calibration Laboratory (Bedogni et al., this issue).
(Bedogni et al., 2007b), demonstrating that the expected perturbations are lower than the TLD pairs measurement uncertainties. Moreover, the perturbation in the gold activation due to the presence of the TLD pairs was experimentally evaluated. For the positions P2, P3 and P4, two exposures with the 5 in sphere were done. In the first the gold foil and the TLD pairs were irradiated together, whilst in the second the gold foil was exposed alone. The foils were counted and their saturation activities compared. Again, no appreciable differences were observed. 5. Unfolding
4.2. TLD pairs TLD pairs formed by TLD-600 and TLD-700 from Harshaw were used. The chips have dimensions 0.3175×0.3175× 0.0889 cm3 . The isotopic composition of lithium is 95.6% 6 Li for TLD-600 and 99.9% 7 Li for TLD-700. Whilst the photon sensitivity is of the same order of magnitude (TLD-600 was found to be 1.6 times more sensitive than TLD-700), the thermal neutron sensitivity of TLD-600 is about 103 times that of TLD-700 (Bedogni et al., 2006). This allows deriving the “thermal neutron signal” with the following simplified equation: Ln6 = L6 − S6 /S7 · L7 ,
(1)
where Ln6 , the neutron contribution to the TLD-600 reading, is the neutron signal to be introduced in the unfolding code; L6 and L7 are the readings of the TLD-600 and TLD-700; S7 and S6 are the photon sensitivity of the TLDs, measured in terms of TL units per unit air kerma. These data were obtained at the INFN–LNF Calibration Laboratory with a 137 Cs source. The read-out was performed with a Vinten Rialto reader. The response matrix of TLD pairs inside the Bonner spheres was considered to be proportional to the response matrix of the active 6 LiI(Eu) scintillator, for which several validated versions are available. In this work, the detailed Mares and Schraube matrix was chosen (Mares and Schraube, 1994). Furthermore, the appropriateness of this matrix for the TLD-based BSS was experimentally checked in well-characterized neutron fields, namely: • the INFN–LNF reference field of 241 Am–Be; • the 14.2 and 2.5 MeV quasimono-energetic sources available at the ENEA Frascati Neutron Generator (FNG) (Angelone et al., 1996). The results were satisfactory, allowing evaluating the overall uncertainty of this matrix for TLD pairs as ±6%. To achieve a higher accuracy, a re-calculation of the TLD-based BSS response matrix is planned. 4.3. TLDs/gold foils mutual perturbation Since four TLD pairs are simultaneously used in each exposure, the effect of the mutual shielding between TLDs needs to be evaluated. This was done by simulation in a previous work
The “neutron signal” to be used in the unfolding procedures (also termed as “raw data”) is Ln6 for the TLD pairs and the saturation-specific activity Asat for the gold foils. Whilst Ln6 is proportional to the time integrated neutron fluence delivered during the exposure, Asat is proportional to the neutron fluence rate. As already mentioned, the gold or TLD pair-based BSSs are independently validated and calibrated. This implies that each technique, alone, should be able to provide a complete characterization of the neutron spectrum in a given point. The neutron spectra were derived from the raw data using the FRUIT unfolding code, developed for the needs of the workplace neutron monitoring. The main characteristics of this code are briefly listed: • FRUIT is a parametric code that models a generic neutron spectrum in terms of few physical parameters. For the spectra analysed in this work, an evaporative model was selected; • the only “pre-information” needed is a qualitative indication of the “radiation environment”; • the user can easily control the state of the convergence procedure; • the uncertainties on the neutron fluence and ambient dose equivalent are derived from the uncertainties on the sphere counts and on the response matrix. 6. Results and discussion To determine the neutron spectrum in a given point of the treatment room, all spheres were subsequently irradiated to a corresponding isocentre photon dose of 1000 monitor units (10 Gy) with a square 15 cm×15 cm field at the isocentre plane. The yield of the accelerator was (161 ± 3) MU min−1 . The spectra determined from the gold foils and the TLD pairs are reported in Figs. 2 and 3, respectively. Fig. 4 reports the TLDs and gold foils spectra in the same point, P2. All fluence spectra are normalized to an isocentre in water of 1 Gy. The following quantities, considered important for the radiation protection of either patient or workers, were derived for all studied points and reported in Table 1: • the total neutron fluence per unit photon absorbed dose at the isocentre, , measured in cm−2 Gy−1 ;
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
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Fig. 2. Neutron spectra derived with the gold foils-based BSS in measurement points at different distances from the isocentre. All spectra are normalized to 1 Gy photon dose at the isocentre.
Fig. 3. Neutron spectra derived with the TLD pairs-based BSS in measurement points at different distances from the isocentre. All spectra are normalized to 1 Gy photon dose at the isocentre.
• the evaporation, epithermal and thermal components of the neutron fluence (expressed as a fraction of the total fluence) Pev , Pepi and Pth ; • the ambient dose equivalent per unit photon absorbed dose at the isocentre, H ∗ (10), measured in mSv Gy−1 ; • the fluence to ambient dose equivalent average conversion coefficient, h∗ (10) = H ∗ (10)/, measured in pSv cm2 . As shown in Figs. 2 and 3 and confirmed by the numerical values of Pev , Pepi and Pth (Table 1), the spectra in the treatment
room become softer as the distance from the isocentre increases. This is also confirmed by the change in h∗ (10), that decreases from about 200 at the isocentre down to 100 at 5 m distance. It is worth noting that, whilst the fluence due to the direct “evaporation” component roughly decreases with the inverse square distance from the isocentre, the thermal fluence is roughly constant. This agrees with the formulation from McGinley (1998). All spectra have an evaporation peak at 0.3–0.4 MeV, in agreement with most of the literature works (Thomas et al.,
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A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
Fig. 4. Neutron spectra derived with the TLD pairs or gold foils-based BSSs at point P2.
Table 1 Dosimetric and field quantities derived in the measurement points with the gold foil or TLD pairs-based BSSs BSS type
Point
(cm−2 Gy−1 )
Pev (%)
Pepi (%)
Pth (%)
h∗ (10) (pSv cm2 )
H ∗ (10) (mSv Gy−1 )
Gold foils
1 2 3 4
9.11 × 106 ± 2.1% 4.36 × 106 ± 2.1% 3.98 × 106 ± 2.4% 1.35 × 106 ± 4%
64 48 36 33
27 38 47 33
9 14 17 34
214 ± 3% 141 ± 7% 125 ± 7% 98 ± 11%
1.95 ± 0.07 0.61 ± 0.05 0.50 ± 0.04 0.130 ± 0.015
TLDs
2 3 4
4.15 × 106 ± 4% 3.89 × 106 ± 4% 1.33 × 106 ± 4%
45 37 32
41 47 35
14 16 33
144 ± 14% 136 ± 10% 90 ± 11%
0.60 ± 0.09 0.53 ± 0.05 0.120 ± 0.014
The fluence and the ambient dose equivalent are normalized to 1 Gy photon dose at the isocentre. The uncertainties on and h ∗ (10) are provided by the FRUIT unfolding code on the basis of the uncertainties on the response matrix and sphere counts.
2002; Zanini et al., 2004; Kralik and Turek, 2004; Howell et al., 2005). The gold or TLD-based BSSs show very similar results, in terms of either the spectra shape (see Fig. 4) or the value of the integral quantities (see Table 1). These results are highly satisfactory because the two systems were independently tested and calibrated: whilst the TLD-based BSS was calibrated at the INFN Am–Be source, the gold-based BSS was calibrated at the Cf source of the ENEA-Bologna Calibration Laboratory. This coherence confirms the accuracy and the suitability of both methods. To complete the comparison between the two techniques, some comments on the uncertainties are needed. The gold foils were counted from 3 to 20 h in a germanium spectrometer, providing typical counting uncertainties of 4% (isocentre), 8% (P2 and P3) or 12% (P4). This depends on the gold foil activity and the amount of available “counting time”. The gold-based BSS response matrix has an evaluated overall uncertainty of ±2%.
The TLD pairs were read in a commercial reader with uncertainty 5– 8%, nearly independent of the measurement position. This figure could be certainly reduced by taking into account the individual thermal neutron sensitivity of each TLD-600. The TLD pairs response matrix is affected by a ±6% overall uncertainty. The read-out procedure takes about 1 min per TLD. Compared with the TLD-based system, the gold provides results (particularly or h∗ (10)) with lower uncertainties, mainly due to the lower uncertainty on the response matrix. When the fluence rate is small (point P4), the two systems provide the same uncertainties. It is likely that the use of individual neutron sensitivity factors for the TLD-600 and the improvement of the TLD response matrix would result in uncertainties of the same order, or lower, than the gold-based system. Moreover, compared with the gold foils, the TLD system is very time effective. These arguments qualify the TLD pair-based BSS as a rapid and accurate method for characterizing the neutron fields around the medical LINACs. This kind of measurements could
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
be also recommended as an acceptance test for radiotherapy machines. 7. Conclusions A passive BSS based on TLD pairs or gold foils was developed to characterize the neutron spectra around high-energy electron medical accelerators. Both kinds of passive detectors can be exposed in the same Bonner sphere, allowing determining the neutron spectra using a limited amount of accelerator time. Moreover, the spectrum in a given position can be derived with two methods independently validated. This combined technique was used at an 18 MV medical LINAC, obtaining coherent results in terms of both spectra shapes and integral quantities (fluence and dose equivalent in all studied points). The results confirm the suitability of both TLD and gold foil-based BSSs. Attractive characteristics of the TLD system are the high sensitivity and the rapid read-out. With some improvements, as the individual neutron characterization of the TLD-600 and the reduction of the uncertainty on the response matrix, the TLD-based technique could be highly suited for the needs of operational measurements. References Angelone, M., Pillon, M., Battistoni, P., Martini, M., Martone, M., Rado, V., 1996. Experimental and numerical calibration of the 14 MeV Frascati neutron source. Rev. Sci. Instrum. 67 (6), 2189. Barquero, R., Mendez, R., Vega-Carrillo, H.R., Iñiguez, M.P., Edwards, T.M., 2005. Neutron spectra and dosimetric features around an 18 MV LINAC accelerator. Health Phys. 88 (1), 48–58. Bedogni, R., Esposito, A., Angelone, M., Chiti, M., 2006. Determination of the response to photons and thermal neutrons of new LiF based TL materials for radiation protection purposes. IEEE Trans. Nucl. Sci. 53 (3), 1367–1370. Bedogni, R., Domingo, C., Esposito, A., Fernández, F., 2007a. FRUIT: an operational tool for multisphere neutron spectrometry in workplaces. Nucl. Instrum. Methods A 580, 1301–1309. Bedogni, R., Esposito, A., Chiti, M., 2007b. Neutron spectrometry around a high energy electron–positron collider using a multi-sphere system with passive detectors. Radiat. Prot. Dosim., in press, doi:10.1093/rpd/ncm109.
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Bedogni, R., Esposito, A., Gentile, A., Angelone, M., Gualdrini, G., this issue. Determination and validation of a response matrix for a passive Bonner spere spectrometer based on gold foils., in press, doi:10.1016/j.radmeas. 2007.11.016. Briesmeister, J.F. (Ed.), 2000. MCNP4—A General Monte Carlo N-Particle Transport Code. Version 4C. LA-13709-M. Los Alamos National Laboratory, Los Alamos, USA. Fernández, F., Bouassoule, T., Amgarou, K., Domingo, C., Garcia, M.J., Lacoste, V., Gressier, V, Muller, H., 2007a. Monte Carlo calculation and validation of a gold foil-based Bonner sphere system. Radiat. Prot. Dosim., in press, doi:10.1093/rpd/ncm075. Fernández, F., Domingo, C., Amgarou, K., Castelo, J., Bouassoule, T., Garcia, M.J., Luguera, E., 2007b. Neutron measurements in a Varian 2100 LINAC facility using a Bonner sphere system based on passive gold activation detectors. Radiat. Prot. Dosim., in press, doi:10.1093/rpd/ncm076. Howell, R.M., Ferenci, M.S., Hertel, N., Fullerton, G.D., Fox, T., Davis, L.W., 2005. Measurements of secondary neutron dose from 15 MV and 18 MV IMRT. Radiat. Prot. Dosim. 115 (1–4), 508–512. IAEA International Atomic Energy Agency, 2006. Radiation Protection in the Design of Radiotherapy Facilities. IAEA Safety Report Series No. 47. ICRP International Commission on Radiological Protection, 1996. Conversion Coefficients for Use in Radiological Protection Against External Radiation. Publication, vol. 74. Pergamon Press, Oxford. IEC International Electrotechnical Commission, 1998. Medical electrical equipment—part 2–1: particular requirements for the safety of electron accelerators in the range 1 MeV to 50 MeV. IEC Standard Report 60601-2-1. Kralik, M., Turek, K., 2004. Characterization of neutron fields around highenergy X-ray radiotherapy machines. Radiat. Prot. Dosim. 110 (1–4), 503–507. Mares, V., Schraube, H., 1994. Evaluation of the response matrix of a Bonner sphere spectrometer with LiI detector from thermal energy to 100 MeV. Nucl. Instrum. Methods A 337, 461–473. McGinley, P.H., 1998. Shielding Techniques. Medical Physics Publishing, Madison, WI. Thomas, D.J., Bardell, A.G., Macaulay, E.M., 2002. Characterisation of a gold foil-based Bonner sphere set and measurements of neutron spectra at a medical accelerator. Nucl. Instrum. Methods A 476, 31–35. Zanini, A., Durisi, E., Fasolo, F., Visca, L., Ongaro, C., Nastasi, U., Burn, K.W., Annand, J.R.M., et al., 2004. Neutron spectra in a tissue equivalent phantom during photon radiotherapy treatment by LINACs. Radiat. Prot. Dosim. 110 (1–4), 157–160.
Determination of the neutron spectra around an 18 MV medical LINAC with a passive Bonner sphere spectrometer based on gold foils and TLD pairs A. Esposito a,∗ , R. Bedogni a , L. Lembo b , M. Morelli b a INFN–LNF Frascati National Laboratories, Via E. Fermi n. 40, 00044 Frascati, Italy b Hospital S. Maria della Scaletta, AUSL Ravenna, Italy
Abstract The increased use of LINACs with accelerating voltage higher than 10 MV in clinical radiotherapy is producing an increasing demand of accurate dosimetric measurements of the photon induced neutron contamination of the radiotherapy beams. The data available in literature have been obtained almost invariably with Bonner sphere spectrometers equipped with gold activation detectors. A collaboration between the Radiation Protection Group of the Frascati National Laboratories and the Ravenna Hospital allowed determining the neutron spectra in different significant points inside the treatment room of an 18 MV medical LINAC. Two independent sets of passive detectors have been employed inside a commercial set of Bonner spheres: gold activation foils (0.1 mm height × 10 mm diameter) and TLD pairs (TLD-600 and TLD-700 Harshaw type). Validated response matrices were used in both cases. The FRascati Unfolding Interactive Tool (FRUIT) code was used to unfold the experimental data. The spectra measured with the two systems are coherent, confirming the accuracy of both methods. The work also provides useful information for the radiation protection of the patient and the workers. © 2007 Elsevier Ltd. All rights reserved. Keywords: Bonner spheres; Neutron spectrometry; TLD pairs; Gold activation foils; Medical LINAC
1. Introduction Radiotherapy constitutes, in modern society, one of the major contributions to the collective dose to the population. It is in fact estimated (IAEA, 2006) that half of the cancer patients receive radiotherapy and the number of tele-therapy installations is increasing, especially in developing countries. A parasitic effect occurring in medical accelerators operating above 10 MV is the production of neutrons, mainly due to the giant dipole resonance reactions (, n) induced by high-energy photons in the materials constituting the accelerator head (collimators, filters and shields), the treatment room structures and the patient itself. The neutron term should be taken into account not only for the protection of the patient, but also for the optimization of the dose to workers. If a treatment room is designed to provide adequate photon shielding only, some undesired neutron doses could be delivered in the surrounding areas, especially if a suitable maze is not present. Very few and ∗ Corresponding author. Tel.: +39 0694032232; fax: +39 0694032364.
E-mail address: [email protected] (A. Esposito). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.035
insufficient information on the neutron term is provided by the accelerator manufacturers, and the measured data available in literature are usually valid for very specific machines and cannot be extended to other installations. It is therefore advisable, for any specific accelerator to be used in cancer therapy, to have an accurate evaluation of the neutron doses produced in the treatment room, not only on the patient plane (IEC, 1998) but also in the surrounding areas. Due to the dramatic energy dependence of the neutron fluence-to-dose equivalent conversion coefficients (ICRP, 1996), this determination is only possible using instruments able to derive sufficient information on the neutron energy distribution. Additional complications are the pulsed time structure of these neutron fields, the very intense photon component and the presence of radiofrequency fields, which could affect the operation of active instruments (Thomas et al., 2002). Previous measurements reported in literature were performed almost invariably with passive integrating detectors, especially passive Bonner sphere spectrometers (BSSs). BSS-based instruments have isotropic response, wide energy range and allow,
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
in principle, an accurate determination of the neutron spectrum. As central detectors, the most used are activation gold foils (Thomas et al., 2002; Fernández et al., 2007a) and TLD pairs (Barquero et al., 2005; Howell et al., 2005). TLD pairs and gold foils offer different performances, as briefly summarized below. TLDs are certainly more sensitive than gold foils, requiring less machine time. This is an appreciable advantage, especially taking into account the very intense time scheduling of radiotherapy machines. In addition, the read-out of TLDs is much more rapid than the gold foil counting procedure. On the other hand, TLDs are sensitive to photons, and a very accurate knowledge of their photon sensitivity is needed to correctly derive the “neutron signal”. Due to this reason, TLDs are probably only suited for measurements “off axis”. The gold foils technique relies on the post-irradiation counting of the 198 Au (411.8 keV photons with 95.5% yield) induced by thermal and epithermal neutrons in thin foils of pure 197 Au. An advantage of this system is the insensitivity to photons. However, the following effects have to be taken into account: • The photons of the (356 keV at 88%, 333 keV at 23% and 426 keV at 6.7%) generated by the 197 Au(, n)196 Au reactions (threshold photon energy 8.07 MeV) could alter the “genuine” thermal neutron signal, if the photon counting system has no or poor energy resolution, as in the case of a total counting. Even in this case, the two isotopes could be discriminated with a series of measurements at different times (198 Au and 196 Au have half lives 2.696 and 6.183 d, respectively), requiring a time consuming procedure. • The extra-neutrons coming from the (, n) reactions in the carbon of the spheres, when the photon energy exceeds 18 MeV. 196 Au
A relevant aspect to be considered is the experimental validation of the response matrix to be used for unfolding the raw data. Whilst the TLD-based system can be easily validated with common radionuclide sources, the gold-based BSS requires very intense neutron fields, only available in few calibration installations (Fernández et al., 2007b). It is worth mentioning that no TLD/gold comparative works have been presented, since all literature works used one technique only. In order to cover this lack, a double TLD/gold BSS system was developed and experimentally validated by the Radiation Protection Group of INFN–LNF (Frascati National Laboratories). Special aluminium holders were designed to expose four TLD pairs and a gold foil in the same sphere. Computational and experimental studies were performed in order to evaluate the perturbation due to different TLD pairs plus a gold foil in the same holder. In the framework of a collaboration with the Hospital S. Maria della Scaletta (AUSL Ravenna, Italy), this combined technique was used to determine the neutron fields in relevant points around an 18 MV Elekta Precise LINAC. The raw data were unfolded using the FRascati Unfolding Interactive Tool (FRUIT) (Bedogni et al., 2007a), a visual un-
1039
Fig. 1. The set-up of the treatment room of the Hospital S. Maria della Scaletta (AUSL Ravenna, Italy).
folding code developed at the INFN–LNF whose main characteristics are the reduced dependence of the results from the a priori information and the high level of flexibility and interactivity. TLD pairs or gold foils data were separately unfolded using validated response matrices, obtaining two “independent” spectra for each measurement point. The spectra measured with the two systems are coherent, in terms of spectrum shape and dosimetric quantities. This confirms the accuracy of both methods. In addition, the experimental data provide useful information for the protection of the patient and the workers as well. 2. The LINAC set-up The measurement points in the treatment room of the 18 MV Elekta Precise LINAC are shown in Fig. 1. All points are located in the isocentre plane. The isocentre point is P1. P2 and P3 (on the patient couch) are located at 1 and 1.5 m from P1, respectively. P4 is located at 5 m in the maze entrance. Whilst the combined technique was used in P2, P3 and P4, only gold foils were used at the isocentre (P1). Here the copious amount of photons should have probably masked the neutron signal on TLDs. 3. The INFN–LNF Bonner sphere system The BSS used in this work includes six polyethylene spheres (density 0.95 g cm−3 ) available from Ludlum Measurements, Inc. Their diameters are 2, 3, 5, 8, 10 and 12 in and are designed to hold a 4 × 4 6 LiI(Eu) active scintillator. 4. The passive BSS 4.1. Gold foils The activation detectors are circular gold foils with 99.9% chemical purity, diameter 10 mm and thickness 0.1 mm. Their
1040
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
individual weight is taken into account in the calculations. The foils were counted using a high-purity coaxial germanium detector available at the Frascati Laboratories, with efficiency 0.069 ± 0.002 and FWHM 0.4% (“contact” geometry). Its very high energy resolution allows discriminating 196 Au from 198 Au. The response matrix for the gold foils into the Bonner spheres was calculated using MCNP-4C (Briesmeister, 2000) in the interval from 1.5E − 9 to 20 MeV and experimentally validated with an intense 252 Cf source available at the ENEA-Bologna Calibration Laboratory (Bedogni et al., this issue).
(Bedogni et al., 2007b), demonstrating that the expected perturbations are lower than the TLD pairs measurement uncertainties. Moreover, the perturbation in the gold activation due to the presence of the TLD pairs was experimentally evaluated. For the positions P2, P3 and P4, two exposures with the 5 in sphere were done. In the first the gold foil and the TLD pairs were irradiated together, whilst in the second the gold foil was exposed alone. The foils were counted and their saturation activities compared. Again, no appreciable differences were observed. 5. Unfolding
4.2. TLD pairs TLD pairs formed by TLD-600 and TLD-700 from Harshaw were used. The chips have dimensions 0.3175×0.3175× 0.0889 cm3 . The isotopic composition of lithium is 95.6% 6 Li for TLD-600 and 99.9% 7 Li for TLD-700. Whilst the photon sensitivity is of the same order of magnitude (TLD-600 was found to be 1.6 times more sensitive than TLD-700), the thermal neutron sensitivity of TLD-600 is about 103 times that of TLD-700 (Bedogni et al., 2006). This allows deriving the “thermal neutron signal” with the following simplified equation: Ln6 = L6 − S6 /S7 · L7 ,
(1)
where Ln6 , the neutron contribution to the TLD-600 reading, is the neutron signal to be introduced in the unfolding code; L6 and L7 are the readings of the TLD-600 and TLD-700; S7 and S6 are the photon sensitivity of the TLDs, measured in terms of TL units per unit air kerma. These data were obtained at the INFN–LNF Calibration Laboratory with a 137 Cs source. The read-out was performed with a Vinten Rialto reader. The response matrix of TLD pairs inside the Bonner spheres was considered to be proportional to the response matrix of the active 6 LiI(Eu) scintillator, for which several validated versions are available. In this work, the detailed Mares and Schraube matrix was chosen (Mares and Schraube, 1994). Furthermore, the appropriateness of this matrix for the TLD-based BSS was experimentally checked in well-characterized neutron fields, namely: • the INFN–LNF reference field of 241 Am–Be; • the 14.2 and 2.5 MeV quasimono-energetic sources available at the ENEA Frascati Neutron Generator (FNG) (Angelone et al., 1996). The results were satisfactory, allowing evaluating the overall uncertainty of this matrix for TLD pairs as ±6%. To achieve a higher accuracy, a re-calculation of the TLD-based BSS response matrix is planned. 4.3. TLDs/gold foils mutual perturbation Since four TLD pairs are simultaneously used in each exposure, the effect of the mutual shielding between TLDs needs to be evaluated. This was done by simulation in a previous work
The “neutron signal” to be used in the unfolding procedures (also termed as “raw data”) is Ln6 for the TLD pairs and the saturation-specific activity Asat for the gold foils. Whilst Ln6 is proportional to the time integrated neutron fluence delivered during the exposure, Asat is proportional to the neutron fluence rate. As already mentioned, the gold or TLD pair-based BSSs are independently validated and calibrated. This implies that each technique, alone, should be able to provide a complete characterization of the neutron spectrum in a given point. The neutron spectra were derived from the raw data using the FRUIT unfolding code, developed for the needs of the workplace neutron monitoring. The main characteristics of this code are briefly listed: • FRUIT is a parametric code that models a generic neutron spectrum in terms of few physical parameters. For the spectra analysed in this work, an evaporative model was selected; • the only “pre-information” needed is a qualitative indication of the “radiation environment”; • the user can easily control the state of the convergence procedure; • the uncertainties on the neutron fluence and ambient dose equivalent are derived from the uncertainties on the sphere counts and on the response matrix. 6. Results and discussion To determine the neutron spectrum in a given point of the treatment room, all spheres were subsequently irradiated to a corresponding isocentre photon dose of 1000 monitor units (10 Gy) with a square 15 cm×15 cm field at the isocentre plane. The yield of the accelerator was (161 ± 3) MU min−1 . The spectra determined from the gold foils and the TLD pairs are reported in Figs. 2 and 3, respectively. Fig. 4 reports the TLDs and gold foils spectra in the same point, P2. All fluence spectra are normalized to an isocentre in water of 1 Gy. The following quantities, considered important for the radiation protection of either patient or workers, were derived for all studied points and reported in Table 1: • the total neutron fluence per unit photon absorbed dose at the isocentre, , measured in cm−2 Gy−1 ;
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
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Fig. 2. Neutron spectra derived with the gold foils-based BSS in measurement points at different distances from the isocentre. All spectra are normalized to 1 Gy photon dose at the isocentre.
Fig. 3. Neutron spectra derived with the TLD pairs-based BSS in measurement points at different distances from the isocentre. All spectra are normalized to 1 Gy photon dose at the isocentre.
• the evaporation, epithermal and thermal components of the neutron fluence (expressed as a fraction of the total fluence) Pev , Pepi and Pth ; • the ambient dose equivalent per unit photon absorbed dose at the isocentre, H ∗ (10), measured in mSv Gy−1 ; • the fluence to ambient dose equivalent average conversion coefficient, h∗ (10) = H ∗ (10)/, measured in pSv cm2 . As shown in Figs. 2 and 3 and confirmed by the numerical values of Pev , Pepi and Pth (Table 1), the spectra in the treatment
room become softer as the distance from the isocentre increases. This is also confirmed by the change in h∗ (10), that decreases from about 200 at the isocentre down to 100 at 5 m distance. It is worth noting that, whilst the fluence due to the direct “evaporation” component roughly decreases with the inverse square distance from the isocentre, the thermal fluence is roughly constant. This agrees with the formulation from McGinley (1998). All spectra have an evaporation peak at 0.3–0.4 MeV, in agreement with most of the literature works (Thomas et al.,
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A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
Fig. 4. Neutron spectra derived with the TLD pairs or gold foils-based BSSs at point P2.
Table 1 Dosimetric and field quantities derived in the measurement points with the gold foil or TLD pairs-based BSSs BSS type
Point
(cm−2 Gy−1 )
Pev (%)
Pepi (%)
Pth (%)
h∗ (10) (pSv cm2 )
H ∗ (10) (mSv Gy−1 )
Gold foils
1 2 3 4
9.11 × 106 ± 2.1% 4.36 × 106 ± 2.1% 3.98 × 106 ± 2.4% 1.35 × 106 ± 4%
64 48 36 33
27 38 47 33
9 14 17 34
214 ± 3% 141 ± 7% 125 ± 7% 98 ± 11%
1.95 ± 0.07 0.61 ± 0.05 0.50 ± 0.04 0.130 ± 0.015
TLDs
2 3 4
4.15 × 106 ± 4% 3.89 × 106 ± 4% 1.33 × 106 ± 4%
45 37 32
41 47 35
14 16 33
144 ± 14% 136 ± 10% 90 ± 11%
0.60 ± 0.09 0.53 ± 0.05 0.120 ± 0.014
The fluence and the ambient dose equivalent are normalized to 1 Gy photon dose at the isocentre. The uncertainties on and h ∗ (10) are provided by the FRUIT unfolding code on the basis of the uncertainties on the response matrix and sphere counts.
2002; Zanini et al., 2004; Kralik and Turek, 2004; Howell et al., 2005). The gold or TLD-based BSSs show very similar results, in terms of either the spectra shape (see Fig. 4) or the value of the integral quantities (see Table 1). These results are highly satisfactory because the two systems were independently tested and calibrated: whilst the TLD-based BSS was calibrated at the INFN Am–Be source, the gold-based BSS was calibrated at the Cf source of the ENEA-Bologna Calibration Laboratory. This coherence confirms the accuracy and the suitability of both methods. To complete the comparison between the two techniques, some comments on the uncertainties are needed. The gold foils were counted from 3 to 20 h in a germanium spectrometer, providing typical counting uncertainties of 4% (isocentre), 8% (P2 and P3) or 12% (P4). This depends on the gold foil activity and the amount of available “counting time”. The gold-based BSS response matrix has an evaluated overall uncertainty of ±2%.
The TLD pairs were read in a commercial reader with uncertainty 5– 8%, nearly independent of the measurement position. This figure could be certainly reduced by taking into account the individual thermal neutron sensitivity of each TLD-600. The TLD pairs response matrix is affected by a ±6% overall uncertainty. The read-out procedure takes about 1 min per TLD. Compared with the TLD-based system, the gold provides results (particularly or h∗ (10)) with lower uncertainties, mainly due to the lower uncertainty on the response matrix. When the fluence rate is small (point P4), the two systems provide the same uncertainties. It is likely that the use of individual neutron sensitivity factors for the TLD-600 and the improvement of the TLD response matrix would result in uncertainties of the same order, or lower, than the gold-based system. Moreover, compared with the gold foils, the TLD system is very time effective. These arguments qualify the TLD pair-based BSS as a rapid and accurate method for characterizing the neutron fields around the medical LINACs. This kind of measurements could
A. Esposito et al. / Radiation Measurements 43 (2008) 1038 – 1043
be also recommended as an acceptance test for radiotherapy machines. 7. Conclusions A passive BSS based on TLD pairs or gold foils was developed to characterize the neutron spectra around high-energy electron medical accelerators. Both kinds of passive detectors can be exposed in the same Bonner sphere, allowing determining the neutron spectra using a limited amount of accelerator time. Moreover, the spectrum in a given position can be derived with two methods independently validated. This combined technique was used at an 18 MV medical LINAC, obtaining coherent results in terms of both spectra shapes and integral quantities (fluence and dose equivalent in all studied points). The results confirm the suitability of both TLD and gold foil-based BSSs. Attractive characteristics of the TLD system are the high sensitivity and the rapid read-out. With some improvements, as the individual neutron characterization of the TLD-600 and the reduction of the uncertainty on the response matrix, the TLD-based technique could be highly suited for the needs of operational measurements. References Angelone, M., Pillon, M., Battistoni, P., Martini, M., Martone, M., Rado, V., 1996. Experimental and numerical calibration of the 14 MeV Frascati neutron source. Rev. Sci. Instrum. 67 (6), 2189. Barquero, R., Mendez, R., Vega-Carrillo, H.R., Iñiguez, M.P., Edwards, T.M., 2005. Neutron spectra and dosimetric features around an 18 MV LINAC accelerator. Health Phys. 88 (1), 48–58. Bedogni, R., Esposito, A., Angelone, M., Chiti, M., 2006. Determination of the response to photons and thermal neutrons of new LiF based TL materials for radiation protection purposes. IEEE Trans. Nucl. Sci. 53 (3), 1367–1370. Bedogni, R., Domingo, C., Esposito, A., Fernández, F., 2007a. FRUIT: an operational tool for multisphere neutron spectrometry in workplaces. Nucl. Instrum. Methods A 580, 1301–1309. Bedogni, R., Esposito, A., Chiti, M., 2007b. Neutron spectrometry around a high energy electron–positron collider using a multi-sphere system with passive detectors. Radiat. Prot. Dosim., in press, doi:10.1093/rpd/ncm109.
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