- Email: [email protected]
In-phantom dose imaging with polymer gel layer dosimeters
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) S195–S198
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
In-phantom dose imaging with polymer gel layer dosimeters E. Vanossi a,b, M. Carrara c, G. Gambarini d,b,, A. Negri d, M. Mariani a a
Department of Energy, Nuclear Engineering Division, Polytechnic of Milan, Via Ponzio 34/3, 20133 Milano, Italy INFN Milan, Via Celoria 16, 20133 Milano, Italy Fondazione IRCCS ‘‘Istituto Nazionale Tumori’’, Via Venezian 1, 20133 Milano, Italy d Department of Physics, University of Milan, Via Celoria 16, 20133 Milano, Italy b c
a r t i c l e in fo
abstract
Keywords: BNCT dosimetry Gel dosimetry Polymer gel Dose imaging
Gel dosimeters in form of layers have shown noticeable potentiality for in-phantom dose profiling and imaging in BNCT neutron fields. Such dosimeters give the possibility of achieving spatial dose distributions of each dose contribution in neutron fields. The various dose components are separated by means of pixel-to-pixel manipulations of pairs of images acquired with gel dosimeters having different isotopic compositions. The reliability of polymer-gel-layer dosimeters (PGLD) for BNCT has been studied and their utilisation limits have been inspected. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction The method for spatial determination of absorbed doses in thermal or epithermal neutron fields, based on gel dosimeters in form of layers, has revealed to be very convenient. In fact, by using gel layer dosimeters it is possible, by means of a properly studied procedure, to obtain the spatial dose distribution of each dose contribution in thermal or epithermal neutron fields (photon and charged particle doses). BNCT dosimetry using Fricke-xylenol-orange-infused gel dosimeters has been widely studied and experimented, giving reliable results. Polymer-gel-layer dosimeters (PGLD), in which a polymerisation process appears as a consequence of absorbed dose, have been recently tested, because of their characteristic absence of diffusion. In fact, due to the diffusion of ferric ions, Fricke-gel dosimeters require prompt analysis after exposure to avoid loss of spatial information. In this work the recent results of a study about the reliability of polymer gel in BNCT dosimetry are discussed. In tissue exposed in the thermal column of a nuclear reactor, after injection of the boron carrier, the absorbed dose results from three main contributions: the therapeutic dose due to alpha and lithium particles released in the reaction of thermal neutrons with 10 B (10B(n,a)7Li), the dose from protons due to the reaction of thermal neutrons with nitrogen (14N(n,p)14C) and the gamma dose from the reaction of thermal neutrons with hydrogen (1H(n,g)2H and background, if not negligible). The fast neutron
Corresponding author. Dipartimento di Fisica dell’Universita` e INFN – Sezione di Milano, Via Celoria 16, 20133 Milano, Italy. Tel: +39 02 50317243; fax: +39 02 50317618. E-mail address: [email protected] (G. Gambarini).
0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.03.048
dose mainly due to recoil protons from elastic scattering of fast neutrons with hydrogen nuclei is negligible in reactor thermal columns. 2. Materials and methods The experiments were carried out by exposing a phantom containing the dosimeters in the thermal column of the TRIGA MARK II reactor of the University of Pavia. This is the reactor in which BNCT had been successfully applied to treat multiple and diffused liver metastases by means of extracorporeal exposure (Pinelli et al., 2002). The aim of this work is the study of the feasibility of utilising gel dosimeters. To this purpose, a simple shape (cylindrical) was chosen for the phantom, consisting of a tissue equivalent (TE) material with a total mass similar to that of a liver. Polymer gel dosimeters in form of layers were used to measure the spatial distribution of the absorbed dose in a phantom irradiated in the thermal column. The dosimeters were rectangular (12 6 cm2) and had a thickness of 3 mm. The layer holders were composed of a rectangular frame between two transparent sheets in order to allow the optical analysis of light transmittance. In Fig. 1 some polymer gel layers after irradiation are shown. The polymer gel dosimeter PAG (Polymer Acrylamide Gelatine), suitably synthesised in the laboratory, was used for the measurements shown here. The samples were always irradiated one day after the preparation. The gel composition is the same utilised by others (Baldock et al., 1998) with a variation in the amount of the gelling agent that was lowered to make the compound more fluid to facilitate the gel introduction in the holder by means of a siring, through apposite holes in the frame.
ARTICLE IN PRESS S196
E. Vanossi et al. / Applied Radiation and Isotopes 67 (2009) S195–S198
Fig. 1. Layers of polymer gel after irradiation.
0.2
Δ (OD)
0.16
0.12
0.08
0.04
Standard gel Borated gel
0 0
10
20
30
40
Absorbed Dose [Gy] Fig. 3. Calibration curves of standard (~) and dosimeters.
Fig. 2. Phantom utilised for the irradiation of gel dosimeter layers.
The gel dosimeters are prepared with the following chemical reagents: gelatine powder as gelling agent, acrylamide and N,N’ methylene-bisacrylamide as monomers, tetrakis (hydroxymethyl) phosphonium chloride (THP) as antioxidant and highly purified and deionised water. The amount of each compound was the same reported in previous papers (Mariani et al., 2007; Vanossi et al., 2008), reporting the results of the first studies about the feasibility of performing BNCT dosimetry with PGLD. The method had been improved for both the dosimeter preparation protocols and the image elaboration software. Couples of gel dosimeters were put in a cylindrical phantom and exposed in the thermal column of the nuclear reactor TRIGA MARK II of Pavia. The phantom, shown in Fig. 2, was composed of two polyethylene shells (1 mm thick) having the shape of about half-cylinder, filled with a tissue-equivalent gel obtained by dissolving in purified and deionised water the gelling agent agar in the amount of 4% of the final weight. The two parts were assembled around the dosimeters in order to finally obtain a TE cylindrical phantom with the dosimeters in the central plane. The so obtained phantom had a height of 11 cm and a diameter of 12 cm. In order to determine the gamma dose and the dose due to the charged particles emitted during the reactions of neutrons with 10 B, couples of gel dosimeters were prepared, one having the standard chemical composition and the other containing also a
10
B-added (’) polymer gel
40 ppm of 10B. The neutron transport is determined by the whole phantom and a change in the isotopic content of the dosimeters that are layers having a thickness of only 3 mm, do not give measurable differences. The method for separating the various dose contributions developed using Fricke-gel dosimeters (Gambarini et al., 2004) was applied to determine the different contributions of the absorbed dose. The polymer gel dosimeters were optically analysed by imaging the samples placed on a plane light source by means of a CCD camera. The difference D(OD) of optical density detected before and after irradiation is proportional to the absorbed dose that was therefore evaluated by means of dosimeter calibrations. The algorithms for the separation of the dose contributions take into account that the sensitivity of the dosimeters changes with LET of the radiation. For the sensitivity to the products of the reactions with 10B the value 0.41 was assumed (Gambarini et al., 2002). Gel dosimeter calibration was performed for each gel preparation. To this aim some dosimeters of each group were irradiated at different doses with a calibrated 137Cs source.
3. Results An example of the gel calibration is shown in Fig. 3 where the calibration curves for a standard polymer gel and a gel added with 10 B are reported. Some exposures of the cylindrical phantom containing couples of polymer gel dosimeters (standard and 10B-added) in the thermal column of TRIGA MARK II reactor were carried out. From the light-transmittance images of the two dosimeters by means of pixel-to-pixel elaborations using the proper algorithm, both boron
ARTICLE IN PRESS E. Vanossi et al. / Applied Radiation and Isotopes 67 (2009) S195–S198
S197
and gamma dose images were obtained. An example of such results is shown in Figs. 4 and 5. In one case, the phantom was rotated 1801 for similarity with the modality of the liver treatment. The obtained dose images are reported in Figs. 6 and 7.
Fig. 7. Boron dose image measured by means of a PGLD placed in the phantom that was 1801 rotated during the irradiation.
60 Boron Dose Fig. 4. Image of the gamma dose measured by means of a PGLD.
Absorbed Dose [Gy]
50
Polymer Fricke
40 30 20
Fricke
10 Polymer Gamma Dose 0 0
2
4 6 8 Depth in phantom [cm]
10
12
Fig. 8. Boron dose and gamma dose profiles in the central axis extracted from a polymer gel and a Fricke-gel.
Fig. 5. Boron dose image measured by means of a PGLD.
From these last dose images the profiles along the beam axis were extracted. In order to check the reliability of the here studied polymer dosimeter the profiles were compared with the corresponding profiles extracted from the images obtained by means of Fricke-gel dosimeters exposed in the same phantom and in the same reactor configuration. In fact, Fricke-gel dosimeters had shown to give good results both for the shapes of spatial dose distributions and their quantitative values. All profiles are reported in Fig. 8. The agreement obtained for the boron dose is noticeable. Concerning the gamma dose, the good agreement is visible for the shape of the profiles and some discordance in the absolute values. More precise results could be obtained performing more than one measurement and working with averaged images.
4. Conclusions
Fig. 6. Image of the gamma dose measured by means of a PGLD placed in the phantom that was 1801 rotated during the irradiation.
The above reported results show that the studied PGLD can be satisfactorily utilised for imaging both gamma and boron doses in TE phantoms exposed in a reactor thermal column designed for BNCT treatments. In particular such dosimeters can be fruitfully used instead of Fricke-gel dosimeters when it is not practical to install the gel imaging instrumentation near to the irradiation facility as it is mandatory for Fricke-gel.
ARTICLE IN PRESS S198
E. Vanossi et al. / Applied Radiation and Isotopes 67 (2009) S195–S198
Acknowledgement The work was partially supported by the National Institute of Nuclear Physics (INFN), Italy. References Baldock, C., Burford, R.P., Billingham, N., Wagner, G.S., Patval, S., Badawi, R.D., Keevil, S.F., 1998. Experimental procedure for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry. Phys. Med. Biol. 43, 695–702. Gambarini, G., Birattari, C., Colombi, C., Pirola, L., Rosi, G., 2002. Fricke-gel dosimetry in boron neutron capture therapy. Radiat. Prot. Dosim. 101, 419–422.
Gambarini, G., Colli, V., Gay, S., Petrovich, C., Pirola, L., Rosi, G., 2004. In-phantom imaging of all dose components in boron neutron capture therapy by means of gel dosimeters. Appl. Radiat. Isot. 61, 759–763. Mariani, M., Vanossi, E., Gambarini, G., Carrara, M., Valente, M., 2007. Preliminary results from a polymer gel dosimeter for absorbed dose imaging in radiotherapy. Radiat. Phys. Chem. 76, 1507–1510. Pinelli, T., Zonta, A., Altieri, S., Barni, S., Brughieri, A., Pedroni, P., Bruschi, P., Chiari, P., Ferrari, C., Fossati, F., Nano, R., Ngnitejeu Tata, S., Prati, U., Ricevuti, G., Riveda, L., Zonta, C., 2002. TAOrMINA: from the first idea to the application to the human liver. In: Sauerwein, W., Moss, R., Wittig, A. (Eds.), Research and Development in Neutron Capture Therapy. Monduzzi, Bologna Italy, pp. 1065–1072. Vanossi, E., Carrara, M., Gambarini, G., Mariani, M., Valente, M., 2008. Study of polymer gel for dose imaging in radiotherapy. Radiat. Meas. 43, 442–445.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
In-phantom dose imaging with polymer gel layer dosimeters E. Vanossi a,b, M. Carrara c, G. Gambarini d,b,, A. Negri d, M. Mariani a a
Department of Energy, Nuclear Engineering Division, Polytechnic of Milan, Via Ponzio 34/3, 20133 Milano, Italy INFN Milan, Via Celoria 16, 20133 Milano, Italy Fondazione IRCCS ‘‘Istituto Nazionale Tumori’’, Via Venezian 1, 20133 Milano, Italy d Department of Physics, University of Milan, Via Celoria 16, 20133 Milano, Italy b c
a r t i c l e in fo
abstract
Keywords: BNCT dosimetry Gel dosimetry Polymer gel Dose imaging
Gel dosimeters in form of layers have shown noticeable potentiality for in-phantom dose profiling and imaging in BNCT neutron fields. Such dosimeters give the possibility of achieving spatial dose distributions of each dose contribution in neutron fields. The various dose components are separated by means of pixel-to-pixel manipulations of pairs of images acquired with gel dosimeters having different isotopic compositions. The reliability of polymer-gel-layer dosimeters (PGLD) for BNCT has been studied and their utilisation limits have been inspected. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction The method for spatial determination of absorbed doses in thermal or epithermal neutron fields, based on gel dosimeters in form of layers, has revealed to be very convenient. In fact, by using gel layer dosimeters it is possible, by means of a properly studied procedure, to obtain the spatial dose distribution of each dose contribution in thermal or epithermal neutron fields (photon and charged particle doses). BNCT dosimetry using Fricke-xylenol-orange-infused gel dosimeters has been widely studied and experimented, giving reliable results. Polymer-gel-layer dosimeters (PGLD), in which a polymerisation process appears as a consequence of absorbed dose, have been recently tested, because of their characteristic absence of diffusion. In fact, due to the diffusion of ferric ions, Fricke-gel dosimeters require prompt analysis after exposure to avoid loss of spatial information. In this work the recent results of a study about the reliability of polymer gel in BNCT dosimetry are discussed. In tissue exposed in the thermal column of a nuclear reactor, after injection of the boron carrier, the absorbed dose results from three main contributions: the therapeutic dose due to alpha and lithium particles released in the reaction of thermal neutrons with 10 B (10B(n,a)7Li), the dose from protons due to the reaction of thermal neutrons with nitrogen (14N(n,p)14C) and the gamma dose from the reaction of thermal neutrons with hydrogen (1H(n,g)2H and background, if not negligible). The fast neutron
Corresponding author. Dipartimento di Fisica dell’Universita` e INFN – Sezione di Milano, Via Celoria 16, 20133 Milano, Italy. Tel: +39 02 50317243; fax: +39 02 50317618. E-mail address: [email protected] (G. Gambarini).
0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.03.048
dose mainly due to recoil protons from elastic scattering of fast neutrons with hydrogen nuclei is negligible in reactor thermal columns. 2. Materials and methods The experiments were carried out by exposing a phantom containing the dosimeters in the thermal column of the TRIGA MARK II reactor of the University of Pavia. This is the reactor in which BNCT had been successfully applied to treat multiple and diffused liver metastases by means of extracorporeal exposure (Pinelli et al., 2002). The aim of this work is the study of the feasibility of utilising gel dosimeters. To this purpose, a simple shape (cylindrical) was chosen for the phantom, consisting of a tissue equivalent (TE) material with a total mass similar to that of a liver. Polymer gel dosimeters in form of layers were used to measure the spatial distribution of the absorbed dose in a phantom irradiated in the thermal column. The dosimeters were rectangular (12 6 cm2) and had a thickness of 3 mm. The layer holders were composed of a rectangular frame between two transparent sheets in order to allow the optical analysis of light transmittance. In Fig. 1 some polymer gel layers after irradiation are shown. The polymer gel dosimeter PAG (Polymer Acrylamide Gelatine), suitably synthesised in the laboratory, was used for the measurements shown here. The samples were always irradiated one day after the preparation. The gel composition is the same utilised by others (Baldock et al., 1998) with a variation in the amount of the gelling agent that was lowered to make the compound more fluid to facilitate the gel introduction in the holder by means of a siring, through apposite holes in the frame.
ARTICLE IN PRESS S196
E. Vanossi et al. / Applied Radiation and Isotopes 67 (2009) S195–S198
Fig. 1. Layers of polymer gel after irradiation.
0.2
Δ (OD)
0.16
0.12
0.08
0.04
Standard gel Borated gel
0 0
10
20
30
40
Absorbed Dose [Gy] Fig. 3. Calibration curves of standard (~) and dosimeters.
Fig. 2. Phantom utilised for the irradiation of gel dosimeter layers.
The gel dosimeters are prepared with the following chemical reagents: gelatine powder as gelling agent, acrylamide and N,N’ methylene-bisacrylamide as monomers, tetrakis (hydroxymethyl) phosphonium chloride (THP) as antioxidant and highly purified and deionised water. The amount of each compound was the same reported in previous papers (Mariani et al., 2007; Vanossi et al., 2008), reporting the results of the first studies about the feasibility of performing BNCT dosimetry with PGLD. The method had been improved for both the dosimeter preparation protocols and the image elaboration software. Couples of gel dosimeters were put in a cylindrical phantom and exposed in the thermal column of the nuclear reactor TRIGA MARK II of Pavia. The phantom, shown in Fig. 2, was composed of two polyethylene shells (1 mm thick) having the shape of about half-cylinder, filled with a tissue-equivalent gel obtained by dissolving in purified and deionised water the gelling agent agar in the amount of 4% of the final weight. The two parts were assembled around the dosimeters in order to finally obtain a TE cylindrical phantom with the dosimeters in the central plane. The so obtained phantom had a height of 11 cm and a diameter of 12 cm. In order to determine the gamma dose and the dose due to the charged particles emitted during the reactions of neutrons with 10 B, couples of gel dosimeters were prepared, one having the standard chemical composition and the other containing also a
10
B-added (’) polymer gel
40 ppm of 10B. The neutron transport is determined by the whole phantom and a change in the isotopic content of the dosimeters that are layers having a thickness of only 3 mm, do not give measurable differences. The method for separating the various dose contributions developed using Fricke-gel dosimeters (Gambarini et al., 2004) was applied to determine the different contributions of the absorbed dose. The polymer gel dosimeters were optically analysed by imaging the samples placed on a plane light source by means of a CCD camera. The difference D(OD) of optical density detected before and after irradiation is proportional to the absorbed dose that was therefore evaluated by means of dosimeter calibrations. The algorithms for the separation of the dose contributions take into account that the sensitivity of the dosimeters changes with LET of the radiation. For the sensitivity to the products of the reactions with 10B the value 0.41 was assumed (Gambarini et al., 2002). Gel dosimeter calibration was performed for each gel preparation. To this aim some dosimeters of each group were irradiated at different doses with a calibrated 137Cs source.
3. Results An example of the gel calibration is shown in Fig. 3 where the calibration curves for a standard polymer gel and a gel added with 10 B are reported. Some exposures of the cylindrical phantom containing couples of polymer gel dosimeters (standard and 10B-added) in the thermal column of TRIGA MARK II reactor were carried out. From the light-transmittance images of the two dosimeters by means of pixel-to-pixel elaborations using the proper algorithm, both boron
ARTICLE IN PRESS E. Vanossi et al. / Applied Radiation and Isotopes 67 (2009) S195–S198
S197
and gamma dose images were obtained. An example of such results is shown in Figs. 4 and 5. In one case, the phantom was rotated 1801 for similarity with the modality of the liver treatment. The obtained dose images are reported in Figs. 6 and 7.
Fig. 7. Boron dose image measured by means of a PGLD placed in the phantom that was 1801 rotated during the irradiation.
60 Boron Dose Fig. 4. Image of the gamma dose measured by means of a PGLD.
Absorbed Dose [Gy]
50
Polymer Fricke
40 30 20
Fricke
10 Polymer Gamma Dose 0 0
2
4 6 8 Depth in phantom [cm]
10
12
Fig. 8. Boron dose and gamma dose profiles in the central axis extracted from a polymer gel and a Fricke-gel.
Fig. 5. Boron dose image measured by means of a PGLD.
From these last dose images the profiles along the beam axis were extracted. In order to check the reliability of the here studied polymer dosimeter the profiles were compared with the corresponding profiles extracted from the images obtained by means of Fricke-gel dosimeters exposed in the same phantom and in the same reactor configuration. In fact, Fricke-gel dosimeters had shown to give good results both for the shapes of spatial dose distributions and their quantitative values. All profiles are reported in Fig. 8. The agreement obtained for the boron dose is noticeable. Concerning the gamma dose, the good agreement is visible for the shape of the profiles and some discordance in the absolute values. More precise results could be obtained performing more than one measurement and working with averaged images.
4. Conclusions
Fig. 6. Image of the gamma dose measured by means of a PGLD placed in the phantom that was 1801 rotated during the irradiation.
The above reported results show that the studied PGLD can be satisfactorily utilised for imaging both gamma and boron doses in TE phantoms exposed in a reactor thermal column designed for BNCT treatments. In particular such dosimeters can be fruitfully used instead of Fricke-gel dosimeters when it is not practical to install the gel imaging instrumentation near to the irradiation facility as it is mandatory for Fricke-gel.
ARTICLE IN PRESS S198
E. Vanossi et al. / Applied Radiation and Isotopes 67 (2009) S195–S198
Acknowledgement The work was partially supported by the National Institute of Nuclear Physics (INFN), Italy. References Baldock, C., Burford, R.P., Billingham, N., Wagner, G.S., Patval, S., Badawi, R.D., Keevil, S.F., 1998. Experimental procedure for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry. Phys. Med. Biol. 43, 695–702. Gambarini, G., Birattari, C., Colombi, C., Pirola, L., Rosi, G., 2002. Fricke-gel dosimetry in boron neutron capture therapy. Radiat. Prot. Dosim. 101, 419–422.
Gambarini, G., Colli, V., Gay, S., Petrovich, C., Pirola, L., Rosi, G., 2004. In-phantom imaging of all dose components in boron neutron capture therapy by means of gel dosimeters. Appl. Radiat. Isot. 61, 759–763. Mariani, M., Vanossi, E., Gambarini, G., Carrara, M., Valente, M., 2007. Preliminary results from a polymer gel dosimeter for absorbed dose imaging in radiotherapy. Radiat. Phys. Chem. 76, 1507–1510. Pinelli, T., Zonta, A., Altieri, S., Barni, S., Brughieri, A., Pedroni, P., Bruschi, P., Chiari, P., Ferrari, C., Fossati, F., Nano, R., Ngnitejeu Tata, S., Prati, U., Ricevuti, G., Riveda, L., Zonta, C., 2002. TAOrMINA: from the first idea to the application to the human liver. In: Sauerwein, W., Moss, R., Wittig, A. (Eds.), Research and Development in Neutron Capture Therapy. Monduzzi, Bologna Italy, pp. 1065–1072. Vanossi, E., Carrara, M., Gambarini, G., Mariani, M., Valente, M., 2008. Study of polymer gel for dose imaging in radiotherapy. Radiat. Meas. 43, 442–445.