- Email: [email protected]
Verification of high dose rate 192Ir source position during brachytherapy treatment
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 617 (2010) 206–208
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Verification of high dose rate brachytherapy treatment
192
Ir source position during
M. Baticˇ a,, J. Burger b, V. Cindro a, G. Kramberger a, I. Mandicˇ a, M. Mikuzˇ a,c, A. Studen a, M. Zavrtanik a a
Experimental Particle Physics Department, Institute Jozˇef Stefan, Ljubljana, Slovenia Department of Radiophysics, Institute of Oncology, Ljubljana, Slovenia c Department of Physics, University of Ljubljana, Ljubljana, Slovenia b
a r t i c l e in f o
a b s t r a c t
Available online 9 October 2009
A system for in vivo tracking of 1 Ci 192Ir source during brachytherapy treatment has been built using high resistivity silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. The sensors consist of 256 pads arranged in 32 8 grid with pad size 1:4 1:4 mm2 and 1 mm thickness. The sensors have two metal layers, enabling connection of readout electronics (VATAGP3_1 chips) at the edge of the detector. With source self-images obtained from a dual-pinhole system, location of the source can be reconstructed in three dimensions in real time, allowing on-line detection of deviations from planned treatment. The system was tested with 1 Ci 192Ir clinical source in air and plexi-glass phantom. The movements of the source could be tracked in a field of view of approximately 20 20 20 cm3 with absolute precision of about 5 mm. Positions of the source, relative to the first measured source position, could be mapped with precision of around 3 mm. & 2009 Elsevier B.V. All rights reserved.
Keywords: Source localization Brachytherapy Silicon pixel detectors Verification
1. Introduction Basic gamma camera employs source self-imaging through the pinhole onto the image detector. Detection system with a single pinhole provides only two-dimensional projection of the threedimensional dwell position of the source, since only a line connecting the pinhole and the detected image of the source can be reconstructed. In order to provide information on the third dimension, at least two such systems need to be employed. Due to measurement error in the positions of pinholes and detectors, the source position may be represented by the point closest to two lines, connecting both pinholes with corresponding detected images. Fig. 1 illustrates the method of reconstruction. Several authors have proposed different methods for such source location monitoring [1–5]. Since 192Ir spectrum has many lines with majority in the region above 350 keV, large pinhole aperture was constructed in order to compensate for low detector efficiency for high energy photons. It was determined with simulation that lead block of 2 cm thickness has sufficient stopping power for all gamma rays from 192 Ir spectrum. Knife-edge angle was calculated to provide good source image within chosen field of view. The shield with pinhole schematics is shown in Fig. 2.
Corresponding author.
E-mail address: [email protected] (M. Baticˇ). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.09.122
There are four silicon pad detectors in the setup, two for each pinhole system, resulting in field of view of about 20 20 20 cm3 approximately 40 cm away from the pinhole. The sensor, processed by SINTEF, consists of 256 pads arranged in 32 8 grid. The pad size is 1:4 1:4 mm2 and detector thickness is 1 mm with p þ n n þ doping profile. The VATAGP3_1 chip, a 128 channel self triggering ASIC produced by IDEAS, is used to read out the detector signals [6]. Fig. 3 shows a hybrid with the silicon sensor attached to two read-out chips.
2. Measurements with
192
Ir source
Measurements were taken at the Institute of Oncology in Ljubljana. Source was a standard 192Ir brachytherapy seed with nominal activity 1 Ci. Setup of the measurements can be seen in Fig. 4. The source was moved by afterloading machine through the catheters placed in different locations inside the field of view, with distances between dwell positions varying from few millimeters to few centimeters. Data of some positions are given in Table 1. Calculation of residuals for absolute reconstruction for all measurements of source positions r¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ðx xrec Þ þ ðy yrec Þ þ ðz zrec Þ , where x, y and z are true source positions and the subscript ‘‘rec’’ stands for corresponding reconstructed source positions, yields average 4.6 mm and rms value 2.3 mm. However, the main goal of this system is not to
ARTICLE IN PRESS M. Baticˇ et al. / Nuclear Instruments and Methods in Physics Research A 617 (2010) 206–208
207
Fig. 1. Diagram illustrating reconstruction of source position S from two lines, connecting images on the detectors and corresponding pinholes.
2mm Fig. 4. Afterloading machine with two channels connected to two needles. Two one-pinhole setups can be seen mounted on the support structure.
20mm 40mm 12mm 30mm
50mm
Fig. 2. Cross-section of the lead shield with pinhole insert (marked green). (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Dwell positions of four measurements (in one needle) with reconstructed positions and residuals (rx ¼ x xrec , similar for ry and rz ). needle 1
1 2 3 4
Source positions (mm)
Reconstructed (mm)
Residuals (mm)
x
y
z
xrec
yrec
zrec
rx
ry
rz
70.0 70.0 70.0 70.0
65.5 23.5 18.5 60.5
0.0 0.0 0.0 0.0
73.8 74.5 73.7 72.2
64.3 23.2 19.8 60.6
1.6 3.1 5.2 4.4
3.8 4.5 3.7 2.2
1.3 0.3 1.3 0.1
1.6 3.1 5.2 4.4
Table 2 Average residuals for all measurements, calculated for absolute and relative type of reconstruction. Residuals (mm) rx Absolute reconstruction Relative reconstruction
1.47 0.25
ry 0.22 0.23
Total (mm) rz 0.99 0.95
4:57 7 2:34 2:81 7 2:74
The uncertainty of total values is the rms value of the distribution.
Fig. 3. Photograph of a silicon module. A 1 mm thick, 256-pad silicon sensor (top) is bonded to a set of two VATAGP3_1 ASICs.
reconstruct absolute coordinates of source, but to follow the movements of source during therapy. So it is much more important for the system to be able to correctly reconstruct coordinates of source locations relative to nearby dwell position. Therefore origin of the coordinate system can be set to the first point of treatment and then the following source positions determined relative to that origin. For relative reconstruction of the source positions average distance (from real to reconstructed point) is 2.8 mm with rms value of 2.7 mm. Measurements were also taken with 192Ir source located inside 20 20 20 cm3 plexiglass phantom and it was found that the phantom does not influence the precision of the reconstructed source positions. Final results are displayed in Table 2.
Fig. 5. Simulation of the 192Ir source in air. Blue lines are tracks of photons emitted by source. Only events that result in energy deposit in the Si detectors are shown. The simulation and measurements are in agreement. (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)
ARTICLE IN PRESS 208
M. Baticˇ et al. / Nuclear Instruments and Methods in Physics Research A 617 (2010) 206–208
Geant4 simulation of the system was done using nordugrid.org computing power. Simulation was found to be in good agreement with the measurements. Fig. 5 shows the results of the simulation, illustrating the imaging method. References [1] T. Nakano, Phys. Med. Biol. 48 (2003) 2133.
[2] [3] [4] [5] [6]
D. Sheikh-Bagheri, P. Munro, Med. Phys. 25 (1998) 404. J. Duan, Med. Phys. 28 (2001) 167. S. Majewski, Nucl. Sci. Symp. 3 (1999) 1613. M.F. Smith, IEEE Trans. Nucl. Sci. NS-44 (1997) 1154. A. Studen, Nucl. Instr. and Meth. A 501 (2003) 273.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Verification of high dose rate brachytherapy treatment
192
Ir source position during
M. Baticˇ a,, J. Burger b, V. Cindro a, G. Kramberger a, I. Mandicˇ a, M. Mikuzˇ a,c, A. Studen a, M. Zavrtanik a a
Experimental Particle Physics Department, Institute Jozˇef Stefan, Ljubljana, Slovenia Department of Radiophysics, Institute of Oncology, Ljubljana, Slovenia c Department of Physics, University of Ljubljana, Ljubljana, Slovenia b
a r t i c l e in f o
a b s t r a c t
Available online 9 October 2009
A system for in vivo tracking of 1 Ci 192Ir source during brachytherapy treatment has been built using high resistivity silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. The sensors consist of 256 pads arranged in 32 8 grid with pad size 1:4 1:4 mm2 and 1 mm thickness. The sensors have two metal layers, enabling connection of readout electronics (VATAGP3_1 chips) at the edge of the detector. With source self-images obtained from a dual-pinhole system, location of the source can be reconstructed in three dimensions in real time, allowing on-line detection of deviations from planned treatment. The system was tested with 1 Ci 192Ir clinical source in air and plexi-glass phantom. The movements of the source could be tracked in a field of view of approximately 20 20 20 cm3 with absolute precision of about 5 mm. Positions of the source, relative to the first measured source position, could be mapped with precision of around 3 mm. & 2009 Elsevier B.V. All rights reserved.
Keywords: Source localization Brachytherapy Silicon pixel detectors Verification
1. Introduction Basic gamma camera employs source self-imaging through the pinhole onto the image detector. Detection system with a single pinhole provides only two-dimensional projection of the threedimensional dwell position of the source, since only a line connecting the pinhole and the detected image of the source can be reconstructed. In order to provide information on the third dimension, at least two such systems need to be employed. Due to measurement error in the positions of pinholes and detectors, the source position may be represented by the point closest to two lines, connecting both pinholes with corresponding detected images. Fig. 1 illustrates the method of reconstruction. Several authors have proposed different methods for such source location monitoring [1–5]. Since 192Ir spectrum has many lines with majority in the region above 350 keV, large pinhole aperture was constructed in order to compensate for low detector efficiency for high energy photons. It was determined with simulation that lead block of 2 cm thickness has sufficient stopping power for all gamma rays from 192 Ir spectrum. Knife-edge angle was calculated to provide good source image within chosen field of view. The shield with pinhole schematics is shown in Fig. 2.
Corresponding author.
E-mail address: [email protected] (M. Baticˇ). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.09.122
There are four silicon pad detectors in the setup, two for each pinhole system, resulting in field of view of about 20 20 20 cm3 approximately 40 cm away from the pinhole. The sensor, processed by SINTEF, consists of 256 pads arranged in 32 8 grid. The pad size is 1:4 1:4 mm2 and detector thickness is 1 mm with p þ n n þ doping profile. The VATAGP3_1 chip, a 128 channel self triggering ASIC produced by IDEAS, is used to read out the detector signals [6]. Fig. 3 shows a hybrid with the silicon sensor attached to two read-out chips.
2. Measurements with
192
Ir source
Measurements were taken at the Institute of Oncology in Ljubljana. Source was a standard 192Ir brachytherapy seed with nominal activity 1 Ci. Setup of the measurements can be seen in Fig. 4. The source was moved by afterloading machine through the catheters placed in different locations inside the field of view, with distances between dwell positions varying from few millimeters to few centimeters. Data of some positions are given in Table 1. Calculation of residuals for absolute reconstruction for all measurements of source positions r¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ðx xrec Þ þ ðy yrec Þ þ ðz zrec Þ , where x, y and z are true source positions and the subscript ‘‘rec’’ stands for corresponding reconstructed source positions, yields average 4.6 mm and rms value 2.3 mm. However, the main goal of this system is not to
ARTICLE IN PRESS M. Baticˇ et al. / Nuclear Instruments and Methods in Physics Research A 617 (2010) 206–208
207
Fig. 1. Diagram illustrating reconstruction of source position S from two lines, connecting images on the detectors and corresponding pinholes.
2mm Fig. 4. Afterloading machine with two channels connected to two needles. Two one-pinhole setups can be seen mounted on the support structure.
20mm 40mm 12mm 30mm
50mm
Fig. 2. Cross-section of the lead shield with pinhole insert (marked green). (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Dwell positions of four measurements (in one needle) with reconstructed positions and residuals (rx ¼ x xrec , similar for ry and rz ). needle 1
1 2 3 4
Source positions (mm)
Reconstructed (mm)
Residuals (mm)
x
y
z
xrec
yrec
zrec
rx
ry
rz
70.0 70.0 70.0 70.0
65.5 23.5 18.5 60.5
0.0 0.0 0.0 0.0
73.8 74.5 73.7 72.2
64.3 23.2 19.8 60.6
1.6 3.1 5.2 4.4
3.8 4.5 3.7 2.2
1.3 0.3 1.3 0.1
1.6 3.1 5.2 4.4
Table 2 Average residuals for all measurements, calculated for absolute and relative type of reconstruction. Residuals (mm) rx Absolute reconstruction Relative reconstruction
1.47 0.25
ry 0.22 0.23
Total (mm) rz 0.99 0.95
4:57 7 2:34 2:81 7 2:74
The uncertainty of total values is the rms value of the distribution.
Fig. 3. Photograph of a silicon module. A 1 mm thick, 256-pad silicon sensor (top) is bonded to a set of two VATAGP3_1 ASICs.
reconstruct absolute coordinates of source, but to follow the movements of source during therapy. So it is much more important for the system to be able to correctly reconstruct coordinates of source locations relative to nearby dwell position. Therefore origin of the coordinate system can be set to the first point of treatment and then the following source positions determined relative to that origin. For relative reconstruction of the source positions average distance (from real to reconstructed point) is 2.8 mm with rms value of 2.7 mm. Measurements were also taken with 192Ir source located inside 20 20 20 cm3 plexiglass phantom and it was found that the phantom does not influence the precision of the reconstructed source positions. Final results are displayed in Table 2.
Fig. 5. Simulation of the 192Ir source in air. Blue lines are tracks of photons emitted by source. Only events that result in energy deposit in the Si detectors are shown. The simulation and measurements are in agreement. (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)
ARTICLE IN PRESS 208
M. Baticˇ et al. / Nuclear Instruments and Methods in Physics Research A 617 (2010) 206–208
Geant4 simulation of the system was done using nordugrid.org computing power. Simulation was found to be in good agreement with the measurements. Fig. 5 shows the results of the simulation, illustrating the imaging method. References [1] T. Nakano, Phys. Med. Biol. 48 (2003) 2133.
[2] [3] [4] [5] [6]
D. Sheikh-Bagheri, P. Munro, Med. Phys. 25 (1998) 404. J. Duan, Med. Phys. 28 (2001) 167. S. Majewski, Nucl. Sci. Symp. 3 (1999) 1613. M.F. Smith, IEEE Trans. Nucl. Sci. NS-44 (1997) 1154. A. Studen, Nucl. Instr. and Meth. A 501 (2003) 273.