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Dosimetric control of 90Sr–90Y source trains for endovascular brachytherapy by radiochromic film
Nuclear Instruments and Methods in Physics Research B 213 (2004) 658–661 www.elsevier.com/locate/nimb
Dosimetric control of 90Sr–90Y source trains for endovascular brachytherapy by radiochromic film q Francßois Tondeur a
a,*
, Isabelle Gerardy a, Joseph Guillaume a, Michel Van Dycke b
Institut Sup erieur Industriel de Bruxelles, 150 Rue Royale, B1000 Brussels, Belgium b Clinique g en erale St. Jean, Brussels, Belgium
Abstract We examine the dosimetric control of 90 Sr–90 Y source trains used for the prevention of restenosis by endovascular irradiation. Beta sources have many advantages in this respect, but their dosimetric control is not easy, because of the very steep dose gradients. Radiochromic films are exposed to the b radiation in a phantom, at the reference distance of 2 mm. Exposure to 60 Co gamma-rays and 4.5 MeV electron linac beam are used for dose calibration. No significant difference is found between 60 Co and electron calibrations. A bad reproducibility (up to 8%) is observed in dose measurements with the source train, attributed to fluctuations of the position of the individual sources in the catheter through which the sources are conveyed. This problem is solved by simultaneously exposing two films, on the two sides of a 4-mm thick phantom, with the catheter at the centre. After film digitisation and conversion to dose, the geometrical mean of the corresponding doses at 2 mm in the two images is calculated. A much better reproducibility is obtained (2%). A software has been written for the analysis and averaging of the images. The results are consistent with Monte-Carlo calculations for a source of the same activity. They disagree with the initial dose calibration of the source train, although traceable to NIST. Ó 2003 Elsevier B.V. All rights reserved. PACS: 87.53.H; 87.53.J; 87.56
1. Introduction Prevention of restenosis by endovascular irradiation has become very popular in the last years
q
This work was supported by Novoste Europe. Corresponding author. Tel.: +32-2-217-4540; fax: +32-2217-4609. E-mail address: [email protected] (F. Tondeur). *
[1]. Beta sources have many advantages in this respect, due to their short range, but their dosimetric control is not easy because of the very steep dose gradients. According to the Belgian rules, the hospital physicist is responsible for the proper use of the device and should thus be able to make an independent control of the conformity of the sources. In this work, we explore the possibility of dosimetric control of Sr–Y source trains by radiochromic films.
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01681-1
F. Tondeur et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 658–661
659
2. Material and methods 2.1. Sources We examine a Novosteâ 90 Sr–90 Y source train that contains 12 cylindrical ceramic sources with a total length of 30 mm, plus two end gold seed of different length easily identified in X-ray images. The source train is provided with a calibration certificate traceable to NIST, indicating an activity of 1.55 GBq 90 Sr (4 November 1997) and a doserate of 81 mGy/s at 2 mm (27 April 1997). 2.2. Film exposition For irradiation, the sources are conveyed in a catheter by water pressure. Gafchromicâ MD-55 films are exposed to the beta rays of the sources at the reference distance of 2 mm (measured from the train axis) in a solid water phantom. Five films are exposed at each dose. For reasons that will appear hereafter, a second phantom is also used, in which two films may be simultaneously exposed at 2 mm in opposite directions, the catheter being sealed within a 4 mm slab. 2.3. Film calibration For calibration, the films are exposed to 60 Co gamma-rays and 4.5 MeV electron beam from a Saturne 43 linac, both controlled with ion chambers calibrated at the Belgian standard dosimetry laboratory at the university of Gent. Five films have been exposed at each dose.
Fig. 1. Typical color-inverted grey image.
As for the scanners, we choose two kinds of devices that should easily be found in all hospitals: one Vidar VXR12+ radiographic film scanner, and one simple PC scanner Canoscan D660U. After digitisation and colour inversion, the images are converted into a table of grey levels by software. A typical image is shown in Fig. 1. The calibration then allows one to convert the data into absorbed dose. The data are further analysed as follows: for each row orthogonal to the train axis, the maximum is calculated by making a Gaussian fit to the dose data. The set of maximums is the dose profile along the axis at the reference distance of 2 mm.
3. Results
2.4. Measurement devices
3.1. Comparison between scanners
Manual densitometers as well as scanning devices are used. The densitometers are a Macbeth TD501 and a Victoreen 34-443. Their use is found unpractical, because of an insufficient spatial resolution, of the difficulty to accurately position the films, and to the need of many repeated point measurements. The data obtained in this way will not be presented here; they are fully consistent with those obtained with scanning devices.
Fig. 2(a) and (b) show a comparison between the dose profiles across the image axis obtained with the Vidar hospital scanner and Canon PC scanner respectively. It is shown that the PC scanner is a bit less accurate. However, the Gaussian fit made on the upper part of the curve is an efficient way to get rid of the fluctuations, and working with the simple, inexpensive PC scanner is found to be sufficient, although the radiographic
660
F. Tondeur et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 658–661 250
OD (arbitrary unit)
250
intensity
200 150 100
200 150 100 50
50
0 0
20
40
60
80
0 0
50
100
pixels
(a)
Dose (Gy)
150
Fig. 3. Calibration curve with Vidar VXR12 scanner.
700 600 40
400
dose (Gy)
intensity
500
300 200 100 0 0
(b)
50
100
150
200
pixels
Fig. 2. (a) Grey-level profile across the image axis: Vidar VXR12 scanner and (b) same as 2(a), but with Canoscan D660U.
image scanner may be recommended when it is available. 3.2. Calibration For each calibration dose, the average dose in the 25% central part of the film is calculated, and averaged over five films. Fig. 3 simultaneously shows calibration points for gamma rays and for electrons, obtained with the Vidar scanner. No significant difference is found between the two calibrations. The results are well reproduced by a simple quadratic fit. 3.3. Dose along axis profiles When exposing the films at 2 mm, a significant lack of reproducibility (up to 8%) is noted, with
30
20 -200
-100
0
100
200
pixels
Fig. 4. Dose profile along axis for 5 min expositions.
fluctuations at a small scale shown in Fig. 4. These fluctuations are much larger than in the films exposed to uniform calibration dose, because of the variations of the film sensitivity. They have been rather attributed to variations in the position of the sources in the catheter. To solve this problem, two films are simultaneously exposed at 2 mm in opposite directions. The geometrical mean of the opposite dose profiles along the axis is calculated, a procedure that is expected to largely compensate the fluctuations in the distance between the sources and the films. Indeed, the resulting mean profiles show a much better reproducibility (2%), that is sufficient to detect for example the inhomogeneity of the dose profile at 2 mm (4% in the train we examine) due to the differences in activity between the 12 sources of the train.
F. Tondeur et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 658–661
3.4. Average dose at 2 mm The dose at 2 mm along the axis is averaged on the central 20 mm (i.e. 2/3 of the train length) for comparison with the doserate calibration provided with the sources. All our measurements fall into a 92 3 mGy/s range, in contradiction to 74 mGy/s from the certified value (traceable to NIST), corrected for the decay.
4. Monte-Carlo calculation In order to control the consistency of our doserate result with the certified activity of the source train, we perform a Monte-Carlo simulation with the EGS4 code [2]. The geometrical model includes the lateral inox cover of the SrTiO3 sources, but neglects their end-face inox cover, as well as the differences between the gold end seeds The train is supposed to be surrounded by pure water with a density of 1, i.e. the catheter is supposed to behave like water. The results given here are obtained by runs of 10 million histories. The statistical accuracy of the dose at the reference distance is about 0.2%. The spectra of 90 Sr and 90 Y are taken from the JEF-PC library [3]. The calculation gives an evaluation of 33.7 pGy/s/Bq, leading to 97.7 mGy/s for the certified activity (traceable to NIST) with decay correction. Taking into account the uncertainties in the model as well as in the certified activity, this value is compatible with the mean doserate at 2 mm ob-
661
tained from our film measurements, but contradictory with the certified doserate. The problem has been submitted to NIST by the manufacturer. NIST admits an improper use of the extrapolation ion chamber method in the initial doserate calibration.
5. Conclusion Dosimetric control of beta sources by radiochromic films is an easy and cheap method that may be used by hospital physicists to perform an independent control. The only material needed is a simple phantom, pieces of radiochromic film, and a PC-scanner (or a radiographic film scanner if available). A software has been written that performs all image analysis and dose averaging.
Acknowledgements We thank Novoste Europe for a financial support and for providing the source train used in the present study.
References [1] R. Waksman, S.B. King, I.R. Crocker, R.F. Mould (Eds.), Vascular Brachytherapy, Nucletron B.V., Netherlands, 1996. [2] http://ehssun.lbl.gov/egs/egs.html. [3] JEF-PC, OECD Nuclear Energy Agency Data Bank, 92130 Issy les Moulineaux, France.
Dosimetric control of 90Sr–90Y source trains for endovascular brachytherapy by radiochromic film q Francßois Tondeur a
a,*
, Isabelle Gerardy a, Joseph Guillaume a, Michel Van Dycke b
Institut Sup erieur Industriel de Bruxelles, 150 Rue Royale, B1000 Brussels, Belgium b Clinique g en erale St. Jean, Brussels, Belgium
Abstract We examine the dosimetric control of 90 Sr–90 Y source trains used for the prevention of restenosis by endovascular irradiation. Beta sources have many advantages in this respect, but their dosimetric control is not easy, because of the very steep dose gradients. Radiochromic films are exposed to the b radiation in a phantom, at the reference distance of 2 mm. Exposure to 60 Co gamma-rays and 4.5 MeV electron linac beam are used for dose calibration. No significant difference is found between 60 Co and electron calibrations. A bad reproducibility (up to 8%) is observed in dose measurements with the source train, attributed to fluctuations of the position of the individual sources in the catheter through which the sources are conveyed. This problem is solved by simultaneously exposing two films, on the two sides of a 4-mm thick phantom, with the catheter at the centre. After film digitisation and conversion to dose, the geometrical mean of the corresponding doses at 2 mm in the two images is calculated. A much better reproducibility is obtained (2%). A software has been written for the analysis and averaging of the images. The results are consistent with Monte-Carlo calculations for a source of the same activity. They disagree with the initial dose calibration of the source train, although traceable to NIST. Ó 2003 Elsevier B.V. All rights reserved. PACS: 87.53.H; 87.53.J; 87.56
1. Introduction Prevention of restenosis by endovascular irradiation has become very popular in the last years
q
This work was supported by Novoste Europe. Corresponding author. Tel.: +32-2-217-4540; fax: +32-2217-4609. E-mail address: [email protected] (F. Tondeur). *
[1]. Beta sources have many advantages in this respect, due to their short range, but their dosimetric control is not easy because of the very steep dose gradients. According to the Belgian rules, the hospital physicist is responsible for the proper use of the device and should thus be able to make an independent control of the conformity of the sources. In this work, we explore the possibility of dosimetric control of Sr–Y source trains by radiochromic films.
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01681-1
F. Tondeur et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 658–661
659
2. Material and methods 2.1. Sources We examine a Novosteâ 90 Sr–90 Y source train that contains 12 cylindrical ceramic sources with a total length of 30 mm, plus two end gold seed of different length easily identified in X-ray images. The source train is provided with a calibration certificate traceable to NIST, indicating an activity of 1.55 GBq 90 Sr (4 November 1997) and a doserate of 81 mGy/s at 2 mm (27 April 1997). 2.2. Film exposition For irradiation, the sources are conveyed in a catheter by water pressure. Gafchromicâ MD-55 films are exposed to the beta rays of the sources at the reference distance of 2 mm (measured from the train axis) in a solid water phantom. Five films are exposed at each dose. For reasons that will appear hereafter, a second phantom is also used, in which two films may be simultaneously exposed at 2 mm in opposite directions, the catheter being sealed within a 4 mm slab. 2.3. Film calibration For calibration, the films are exposed to 60 Co gamma-rays and 4.5 MeV electron beam from a Saturne 43 linac, both controlled with ion chambers calibrated at the Belgian standard dosimetry laboratory at the university of Gent. Five films have been exposed at each dose.
Fig. 1. Typical color-inverted grey image.
As for the scanners, we choose two kinds of devices that should easily be found in all hospitals: one Vidar VXR12+ radiographic film scanner, and one simple PC scanner Canoscan D660U. After digitisation and colour inversion, the images are converted into a table of grey levels by software. A typical image is shown in Fig. 1. The calibration then allows one to convert the data into absorbed dose. The data are further analysed as follows: for each row orthogonal to the train axis, the maximum is calculated by making a Gaussian fit to the dose data. The set of maximums is the dose profile along the axis at the reference distance of 2 mm.
3. Results
2.4. Measurement devices
3.1. Comparison between scanners
Manual densitometers as well as scanning devices are used. The densitometers are a Macbeth TD501 and a Victoreen 34-443. Their use is found unpractical, because of an insufficient spatial resolution, of the difficulty to accurately position the films, and to the need of many repeated point measurements. The data obtained in this way will not be presented here; they are fully consistent with those obtained with scanning devices.
Fig. 2(a) and (b) show a comparison between the dose profiles across the image axis obtained with the Vidar hospital scanner and Canon PC scanner respectively. It is shown that the PC scanner is a bit less accurate. However, the Gaussian fit made on the upper part of the curve is an efficient way to get rid of the fluctuations, and working with the simple, inexpensive PC scanner is found to be sufficient, although the radiographic
660
F. Tondeur et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 658–661 250
OD (arbitrary unit)
250
intensity
200 150 100
200 150 100 50
50
0 0
20
40
60
80
0 0
50
100
pixels
(a)
Dose (Gy)
150
Fig. 3. Calibration curve with Vidar VXR12 scanner.
700 600 40
400
dose (Gy)
intensity
500
300 200 100 0 0
(b)
50
100
150
200
pixels
Fig. 2. (a) Grey-level profile across the image axis: Vidar VXR12 scanner and (b) same as 2(a), but with Canoscan D660U.
image scanner may be recommended when it is available. 3.2. Calibration For each calibration dose, the average dose in the 25% central part of the film is calculated, and averaged over five films. Fig. 3 simultaneously shows calibration points for gamma rays and for electrons, obtained with the Vidar scanner. No significant difference is found between the two calibrations. The results are well reproduced by a simple quadratic fit. 3.3. Dose along axis profiles When exposing the films at 2 mm, a significant lack of reproducibility (up to 8%) is noted, with
30
20 -200
-100
0
100
200
pixels
Fig. 4. Dose profile along axis for 5 min expositions.
fluctuations at a small scale shown in Fig. 4. These fluctuations are much larger than in the films exposed to uniform calibration dose, because of the variations of the film sensitivity. They have been rather attributed to variations in the position of the sources in the catheter. To solve this problem, two films are simultaneously exposed at 2 mm in opposite directions. The geometrical mean of the opposite dose profiles along the axis is calculated, a procedure that is expected to largely compensate the fluctuations in the distance between the sources and the films. Indeed, the resulting mean profiles show a much better reproducibility (2%), that is sufficient to detect for example the inhomogeneity of the dose profile at 2 mm (4% in the train we examine) due to the differences in activity between the 12 sources of the train.
F. Tondeur et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 658–661
3.4. Average dose at 2 mm The dose at 2 mm along the axis is averaged on the central 20 mm (i.e. 2/3 of the train length) for comparison with the doserate calibration provided with the sources. All our measurements fall into a 92 3 mGy/s range, in contradiction to 74 mGy/s from the certified value (traceable to NIST), corrected for the decay.
4. Monte-Carlo calculation In order to control the consistency of our doserate result with the certified activity of the source train, we perform a Monte-Carlo simulation with the EGS4 code [2]. The geometrical model includes the lateral inox cover of the SrTiO3 sources, but neglects their end-face inox cover, as well as the differences between the gold end seeds The train is supposed to be surrounded by pure water with a density of 1, i.e. the catheter is supposed to behave like water. The results given here are obtained by runs of 10 million histories. The statistical accuracy of the dose at the reference distance is about 0.2%. The spectra of 90 Sr and 90 Y are taken from the JEF-PC library [3]. The calculation gives an evaluation of 33.7 pGy/s/Bq, leading to 97.7 mGy/s for the certified activity (traceable to NIST) with decay correction. Taking into account the uncertainties in the model as well as in the certified activity, this value is compatible with the mean doserate at 2 mm ob-
661
tained from our film measurements, but contradictory with the certified doserate. The problem has been submitted to NIST by the manufacturer. NIST admits an improper use of the extrapolation ion chamber method in the initial doserate calibration.
5. Conclusion Dosimetric control of beta sources by radiochromic films is an easy and cheap method that may be used by hospital physicists to perform an independent control. The only material needed is a simple phantom, pieces of radiochromic film, and a PC-scanner (or a radiographic film scanner if available). A software has been written that performs all image analysis and dose averaging.
Acknowledgements We thank Novoste Europe for a financial support and for providing the source train used in the present study.
References [1] R. Waksman, S.B. King, I.R. Crocker, R.F. Mould (Eds.), Vascular Brachytherapy, Nucletron B.V., Netherlands, 1996. [2] http://ehssun.lbl.gov/egs/egs.html. [3] JEF-PC, OECD Nuclear Energy Agency Data Bank, 92130 Issy les Moulineaux, France.