- Email: [email protected]
Calibration of portal imaging devices for radiotherapy in-vivo dosimetry
Nuclear Instruments and Methods in Physics Research A 623 (2010) 829–831
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Calibration of portal imaging devices for radiotherapy in-vivo dosimetry Angelo Piermattei a,b,, Savino Cilla b, Andrea Fidanzio a, Francesca Greco a, Domenico Sabatino b, Laura Gargiulo a, Luigi Azario a a b
Istituto di Fisica Universita Cattolica del Sacro Cuore (UCSC), Rome (RM), Italy U.O. di Fisica Sanitaria (UCSC), Campobasso (CB), Italy
a r t i c l e in fo
abstract
Available online 26 February 2010
The complexity of radiotherapy techniques requires an accurate verification of the dose delivered to the patient during treatment. Recently, the present authors have developed an in patient dose reconstruction method with X-ray beams for 3D conformal radiotherapy. The procedure is based on correlation functions defined by the ratios of the transit signal measured by an electronic portal imaging device (EPID) to the mid-plane dose measured by calibrated ion chambers in a solid water phantom. The dosimetric characterization of aSi EPIDs in terms of signal stability, linearity and dependence on field dimension pointed out that these detectors are useful for the transit dosimetry of photon beams. However, the aSi EPIDs manufactured by Varian, Elekta and Siemens for their linacs are at present used for the visual inspection of the patient’s set-up, and their use as transit dosimeters needs a special calibration that requires an effort for every beam. The aim of this paper has been the determination of a generalized EPID calibration that can be used by linacs of different manufacturers equipped with aSi EPIDs. The transit dosimetry method here proposed could supply for every linac the reconstruction in real time of the isocenter dose in patients with a tolerance level ranging between 7 4% and 7 6% depending on the treatment type and body district. & 2010 Elsevier B.V. All rights reserved.
Keywords: In-vivo dosimetry EPID Radiotherapy
1. Introduction The complexity of radiotherapy techniques requires an accurate verification of the dose delivered to the patient during treatment. The monitoring of discrepancies between the in-vivo reconstructed doses and the expected doses can track back possible errors due to (a) data transfer from the treatment plan system (TPS) to the radiotherapy unit, (b) incorrect beam delivery, (c) dose calculation algorithm accuracy, employed by the TPSs, (d) the patient’s set-up and (e) the patient’s morphological changes that may occur during the treatment. In the last years, some researches have been addressed to reconstruct the dose delivered to the patient during the treatment by means of electronic portal imaging devices (EPIDs). In particular, the new EPID generation, equipped with amorphous silicon (aSi) flat panels, supplies stable transit signals and it is suitable for transit dosimetry. Recently the present authors have proposed an in-vivo dosimetry method for 3D conformal radiotherapy (3D-CRT) based on correlation functions obtained by the ratios of the transit signals, st, measured by an aSi EPID, to mid-plane doses, D,
Corresponding author at: Istituto di Fisica Universita Cattolica del Sacro Cuore (UCSC), Rome (RM), Italy. E-mail address: [email protected] (A. Piermattei).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.131
measured in a solid water-phantom by a calibrated ion chamber [1]. The method, which allows a real-time in-vivo reconstruction of the isoceter dose, has been applied for 3D-CRTs of head, thorax, pelvic and breast pathologies, using dynamic stereobody irradiations [2], breath-hold irradiation [3] and adaptive radiotherapy strategies [4]. The tolerance/action levels of the method are estimated to be 4–6% depending on the body district. However, as in other transit in-vivo dosimetric methods proposed, this procedure requires implementation measurements that can discourage its use. The aim of this work was the determination of a generalized in-vivo dosimetry procedure for the X-ray beams supplied by linacs manufactured by the three international factories, Varian, Elekta and Siemens, which use aSi EPIDs. This way, the workload due to the implementation measurements of the method is avoided.
2. Materials and methods 2.1. Linac and EPID equipment Some characteristics of the linacs and the EPIDs used in this work are reported in Table 1. In particular, the Tissue Phantom Ratio (TPR) of the doses obtained at a depth of 20 cm to those at
830
A. Piermattei et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 829–831
Table 1 Principal characteristics of the linacs and EPIDs used in this work. Linacs aSi EPID # of pixels Pixel size (mm) Nominal Potential (MV) TPR beam quality
2100 Varian aS1000 512 384 784 6 10 15 0.666 0.736 0.764
Elekta Precise XRD 1640 1024 1024 400 6 10 15 0.687 0.727 0.760
Siemens Oncor OptiVue1000ST 512 512 800 6
ks ¼ 1=s t :
15 0.671 0.760
10 cm in water phantom was used as the X-ray beam quality index in this paper.
The method to reconstruct the isoceter dose for a 3D-CRT is based on the measurements of the dose values on the beam central axis at mid-plane of a Solid Water phantom (SWP), RMI model 457 (Gammex, RMI Middelton, WI) and the transit signal by the EPID below SWP. The measurements were obtained using different SWPs of thicknesses w¼10, 22, 30 and 42 cm, and square field side, L, between 4 and 20 cm. The dose measurements were performed for the eight photon beams with the same calibrated ion chamber, Farmer type, model TM31010 (PTW-Freiburg, Germania), connected to an electrometer Tandem PTW, following the IAEA TRS 398 protocol [5]. To take into account the different MU calibrations carried out in the centers, a factor, k0, was defined as
ð4Þ
The ks factors, in terms of [CU/a.u.], were determined for each EPID and beam quality TPR. The transit signals st (TPR,w,L) in [a.u./MU] were obtained for every beam quality TPR using different SWP thicknesses, w, and square field L. The st(TPR,w,L) measured by aSi EPIDs of different manufacturers operating at the same SED were multiplied by ks, the sensitivity factors, to obtain the transit signals s0t : s0t ðTPR,w,LÞ ¼ st ðTPR,w,LÞks
2.2. Transit dosimetry method
k0 ¼ D0SAD =DSAD
An EPID calibration procedure has been implemented to obtain transit signals, st, at the SED independent of the MU calibration at the center and EPID sensitivity. This procedure uses a calibration factor, ks, defined as the ratio of 1 Calibration Unit (CU) per MU to the s t [a.u./MU], the EPID transit signal averaged over different measurement sessions using a 10 10 cm2 field size and a thickness w¼22 cm of the SWP centered at the SAD:
ð5Þ
in terms of CU/MU. These were independent of the EPID sensitivity as well as of the MU calibration of the center.
ð1Þ
D0SAD
where ¼ 1 cGy=MU is the reference dose per MU at the source axis distance (SAD), coincident with the depth dmax, in SWP, where the dose is maximum, for a 10 10 cm2 field and DSAD the dose in the same reference conditions determined at the center. This way the mid-plane dose D(TPR,w,L) values, obtained by different X-ray beam qualities (Table 1), were normalized by the factor k0: D0 ðTPR, w=2, LÞ ¼ DðTPR,w=2,LÞk0
ð2Þ
independent of the MU calibration of the radiotherapy center. All the EPID supplied DICOM images were obtained by integrating the EPID signals during all the beam irradiations. For the EPID manufactured by Elekta, each image pixel value s’ was subtracted from the number 65,535 (216 1) and then divided by the pixel scaling factor (specific of each image and used to optimize the image visualization) to obtain processed integrated pixel values s. For the EPID manufactured by Siemens, the image pixel values, s’, were multiplied by the number of subframes to obtain the signal values s. For the EPID manufactured by Varian, the image pixel values s’ were subtracted from the number 16,384 (214) and then multiplied by the number of subframes to obtain the s signal. All the EPIDs were positioned at a common source so that the EPID Distance SED ¼159 cm. The signal s, in terms of arbitrary unit (a.u.) on the beam central axis, was obtained as the average of the s values relative to the 6 6 central EPID pixels. The s linearity was evaluated in the MU range 20–400 MU and it was taken into account by the klin factor, defined as klin ¼ s=sMU
ð3Þ
where s and sMU are the signals per MU obtained for 100 MUs and for a different number of MUs, respectively.
Fig. 1. Surfaces used to fit the D0(TPR,w/2,L) and s0t(TPR,w,L) values relative to the eight photon beams of the Varian, Elekta and Siemens linacs. From the bottom to the top, the surfaces are relative to the square fields of side L¼4, 10 and 20 cm.
A. Piermattei et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 829–831
metric surface equations (Fig. 1). This way the Diso can be determined in a few seconds after the patient’s treatment.
The correlation ratios are defined in terms of [CU/Gy] by s0 ðTPR,w,LÞ FðTPR,w,LÞ ¼ 0t D ðTPR,w=2,LÞ
ð6Þ
The F(TPR,w, L) ratios are dependent only on the TPR, the SWP thickness, w, and the equivalent square field side L. The s0t(TPR,w,L) and D0(TPR,w/2,L) values were fitted with parametric surface equations. Following the formalism adopted in a previous work [1] the invivo isocenter dose can be obtained for every beam by ks klin f ðMV,d,LÞ iso Diso ¼ St TMRw ð7Þ w=2 k0 FðTPR,w,LÞ where St (a.u.) is the EPID integral signal (obtained by a generic MU’s number) that is initially converted by the sensitivity factor ks in CUs, to take into account the EPID sensitivity and divided by k0 to take into account the MU calibration of the center; f(MV,d,L) is a factor that depends on the Mega Voltage (MV), which takes into account the beam scattered on the EPID as a function of the distance, d, between the SWP mid-plane and the isocenter [1]; iso TMRw w=2 is the ratio of the tissue maximum ratio [1] determined at the isocenter depth, wiso, to the SWP half thickness w/2, and F(TPR,w,L) is the correlation ratio defined by Eq. (6).
3. Results Fig. 1 shows the surfaces used to fit the D0(TPR,w/2,L) and s t(TPR,w,L) values relative to the eight photon beams. The fit standard errors were within 2% (2SD). The in-vivo isoceter dose Diso, was determined by the D0(TPR,w/2,L) and the s0t(TPR,w,L) values selected by the para0
831
4. Conclusions In some countries, the in-vivo dosimetry is required for all patients treated with external beams and many researchers today are studying new methods based on the use of EPIDs that are easy to implement, simple, efficient in their daily use and sufficiently accurate for the purpose they are serving. The aim of this work was the determination of a generalized procedure for the in-vivo dosimetry by aSi EPIDs by Varian, Elekta and Siemens, which are the only manufacturers of clinical linacs. In particular, the D0(TPR,w/2,L) and s0t(TPR,w,L) were fitted with surface equations depending on the TPR, w and L. The preliminary results confirm that the tolerance levels of the reconstructed isocenter dose are within 4% for head treatments, 5% for pelvic treatments and 6% for lung treatments. The generalized procedure here reported satisfies the requirements of efficiency and accuracy of a real time method that can be easily included in the quality assurance program of the center. References [1] [2] [3] [4] [5]
A. Piermattei, et al., Physics in Medicine and Biology 52 (2007) 5101. A. Piermattei, et al., Medical Physics 35 (2008) 1830. A. Piermattei, et al., Acta Oncologica 47 (2008) 1414. A. Piermattei, et al., Medical Physics 36 (2009) 2206. IAEA, Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water, IAEA TRS 398, 2004.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Calibration of portal imaging devices for radiotherapy in-vivo dosimetry Angelo Piermattei a,b,, Savino Cilla b, Andrea Fidanzio a, Francesca Greco a, Domenico Sabatino b, Laura Gargiulo a, Luigi Azario a a b
Istituto di Fisica Universita Cattolica del Sacro Cuore (UCSC), Rome (RM), Italy U.O. di Fisica Sanitaria (UCSC), Campobasso (CB), Italy
a r t i c l e in fo
abstract
Available online 26 February 2010
The complexity of radiotherapy techniques requires an accurate verification of the dose delivered to the patient during treatment. Recently, the present authors have developed an in patient dose reconstruction method with X-ray beams for 3D conformal radiotherapy. The procedure is based on correlation functions defined by the ratios of the transit signal measured by an electronic portal imaging device (EPID) to the mid-plane dose measured by calibrated ion chambers in a solid water phantom. The dosimetric characterization of aSi EPIDs in terms of signal stability, linearity and dependence on field dimension pointed out that these detectors are useful for the transit dosimetry of photon beams. However, the aSi EPIDs manufactured by Varian, Elekta and Siemens for their linacs are at present used for the visual inspection of the patient’s set-up, and their use as transit dosimeters needs a special calibration that requires an effort for every beam. The aim of this paper has been the determination of a generalized EPID calibration that can be used by linacs of different manufacturers equipped with aSi EPIDs. The transit dosimetry method here proposed could supply for every linac the reconstruction in real time of the isocenter dose in patients with a tolerance level ranging between 7 4% and 7 6% depending on the treatment type and body district. & 2010 Elsevier B.V. All rights reserved.
Keywords: In-vivo dosimetry EPID Radiotherapy
1. Introduction The complexity of radiotherapy techniques requires an accurate verification of the dose delivered to the patient during treatment. The monitoring of discrepancies between the in-vivo reconstructed doses and the expected doses can track back possible errors due to (a) data transfer from the treatment plan system (TPS) to the radiotherapy unit, (b) incorrect beam delivery, (c) dose calculation algorithm accuracy, employed by the TPSs, (d) the patient’s set-up and (e) the patient’s morphological changes that may occur during the treatment. In the last years, some researches have been addressed to reconstruct the dose delivered to the patient during the treatment by means of electronic portal imaging devices (EPIDs). In particular, the new EPID generation, equipped with amorphous silicon (aSi) flat panels, supplies stable transit signals and it is suitable for transit dosimetry. Recently the present authors have proposed an in-vivo dosimetry method for 3D conformal radiotherapy (3D-CRT) based on correlation functions obtained by the ratios of the transit signals, st, measured by an aSi EPID, to mid-plane doses, D,
Corresponding author at: Istituto di Fisica Universita Cattolica del Sacro Cuore (UCSC), Rome (RM), Italy. E-mail address: [email protected] (A. Piermattei).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.131
measured in a solid water-phantom by a calibrated ion chamber [1]. The method, which allows a real-time in-vivo reconstruction of the isoceter dose, has been applied for 3D-CRTs of head, thorax, pelvic and breast pathologies, using dynamic stereobody irradiations [2], breath-hold irradiation [3] and adaptive radiotherapy strategies [4]. The tolerance/action levels of the method are estimated to be 4–6% depending on the body district. However, as in other transit in-vivo dosimetric methods proposed, this procedure requires implementation measurements that can discourage its use. The aim of this work was the determination of a generalized in-vivo dosimetry procedure for the X-ray beams supplied by linacs manufactured by the three international factories, Varian, Elekta and Siemens, which use aSi EPIDs. This way, the workload due to the implementation measurements of the method is avoided.
2. Materials and methods 2.1. Linac and EPID equipment Some characteristics of the linacs and the EPIDs used in this work are reported in Table 1. In particular, the Tissue Phantom Ratio (TPR) of the doses obtained at a depth of 20 cm to those at
830
A. Piermattei et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 829–831
Table 1 Principal characteristics of the linacs and EPIDs used in this work. Linacs aSi EPID # of pixels Pixel size (mm) Nominal Potential (MV) TPR beam quality
2100 Varian aS1000 512 384 784 6 10 15 0.666 0.736 0.764
Elekta Precise XRD 1640 1024 1024 400 6 10 15 0.687 0.727 0.760
Siemens Oncor OptiVue1000ST 512 512 800 6
ks ¼ 1=s t :
15 0.671 0.760
10 cm in water phantom was used as the X-ray beam quality index in this paper.
The method to reconstruct the isoceter dose for a 3D-CRT is based on the measurements of the dose values on the beam central axis at mid-plane of a Solid Water phantom (SWP), RMI model 457 (Gammex, RMI Middelton, WI) and the transit signal by the EPID below SWP. The measurements were obtained using different SWPs of thicknesses w¼10, 22, 30 and 42 cm, and square field side, L, between 4 and 20 cm. The dose measurements were performed for the eight photon beams with the same calibrated ion chamber, Farmer type, model TM31010 (PTW-Freiburg, Germania), connected to an electrometer Tandem PTW, following the IAEA TRS 398 protocol [5]. To take into account the different MU calibrations carried out in the centers, a factor, k0, was defined as
ð4Þ
The ks factors, in terms of [CU/a.u.], were determined for each EPID and beam quality TPR. The transit signals st (TPR,w,L) in [a.u./MU] were obtained for every beam quality TPR using different SWP thicknesses, w, and square field L. The st(TPR,w,L) measured by aSi EPIDs of different manufacturers operating at the same SED were multiplied by ks, the sensitivity factors, to obtain the transit signals s0t : s0t ðTPR,w,LÞ ¼ st ðTPR,w,LÞks
2.2. Transit dosimetry method
k0 ¼ D0SAD =DSAD
An EPID calibration procedure has been implemented to obtain transit signals, st, at the SED independent of the MU calibration at the center and EPID sensitivity. This procedure uses a calibration factor, ks, defined as the ratio of 1 Calibration Unit (CU) per MU to the s t [a.u./MU], the EPID transit signal averaged over different measurement sessions using a 10 10 cm2 field size and a thickness w¼22 cm of the SWP centered at the SAD:
ð5Þ
in terms of CU/MU. These were independent of the EPID sensitivity as well as of the MU calibration of the center.
ð1Þ
D0SAD
where ¼ 1 cGy=MU is the reference dose per MU at the source axis distance (SAD), coincident with the depth dmax, in SWP, where the dose is maximum, for a 10 10 cm2 field and DSAD the dose in the same reference conditions determined at the center. This way the mid-plane dose D(TPR,w,L) values, obtained by different X-ray beam qualities (Table 1), were normalized by the factor k0: D0 ðTPR, w=2, LÞ ¼ DðTPR,w=2,LÞk0
ð2Þ
independent of the MU calibration of the radiotherapy center. All the EPID supplied DICOM images were obtained by integrating the EPID signals during all the beam irradiations. For the EPID manufactured by Elekta, each image pixel value s’ was subtracted from the number 65,535 (216 1) and then divided by the pixel scaling factor (specific of each image and used to optimize the image visualization) to obtain processed integrated pixel values s. For the EPID manufactured by Siemens, the image pixel values, s’, were multiplied by the number of subframes to obtain the signal values s. For the EPID manufactured by Varian, the image pixel values s’ were subtracted from the number 16,384 (214) and then multiplied by the number of subframes to obtain the s signal. All the EPIDs were positioned at a common source so that the EPID Distance SED ¼159 cm. The signal s, in terms of arbitrary unit (a.u.) on the beam central axis, was obtained as the average of the s values relative to the 6 6 central EPID pixels. The s linearity was evaluated in the MU range 20–400 MU and it was taken into account by the klin factor, defined as klin ¼ s=sMU
ð3Þ
where s and sMU are the signals per MU obtained for 100 MUs and for a different number of MUs, respectively.
Fig. 1. Surfaces used to fit the D0(TPR,w/2,L) and s0t(TPR,w,L) values relative to the eight photon beams of the Varian, Elekta and Siemens linacs. From the bottom to the top, the surfaces are relative to the square fields of side L¼4, 10 and 20 cm.
A. Piermattei et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 829–831
metric surface equations (Fig. 1). This way the Diso can be determined in a few seconds after the patient’s treatment.
The correlation ratios are defined in terms of [CU/Gy] by s0 ðTPR,w,LÞ FðTPR,w,LÞ ¼ 0t D ðTPR,w=2,LÞ
ð6Þ
The F(TPR,w, L) ratios are dependent only on the TPR, the SWP thickness, w, and the equivalent square field side L. The s0t(TPR,w,L) and D0(TPR,w/2,L) values were fitted with parametric surface equations. Following the formalism adopted in a previous work [1] the invivo isocenter dose can be obtained for every beam by ks klin f ðMV,d,LÞ iso Diso ¼ St TMRw ð7Þ w=2 k0 FðTPR,w,LÞ where St (a.u.) is the EPID integral signal (obtained by a generic MU’s number) that is initially converted by the sensitivity factor ks in CUs, to take into account the EPID sensitivity and divided by k0 to take into account the MU calibration of the center; f(MV,d,L) is a factor that depends on the Mega Voltage (MV), which takes into account the beam scattered on the EPID as a function of the distance, d, between the SWP mid-plane and the isocenter [1]; iso TMRw w=2 is the ratio of the tissue maximum ratio [1] determined at the isocenter depth, wiso, to the SWP half thickness w/2, and F(TPR,w,L) is the correlation ratio defined by Eq. (6).
3. Results Fig. 1 shows the surfaces used to fit the D0(TPR,w/2,L) and s t(TPR,w,L) values relative to the eight photon beams. The fit standard errors were within 2% (2SD). The in-vivo isoceter dose Diso, was determined by the D0(TPR,w/2,L) and the s0t(TPR,w,L) values selected by the para0
831
4. Conclusions In some countries, the in-vivo dosimetry is required for all patients treated with external beams and many researchers today are studying new methods based on the use of EPIDs that are easy to implement, simple, efficient in their daily use and sufficiently accurate for the purpose they are serving. The aim of this work was the determination of a generalized procedure for the in-vivo dosimetry by aSi EPIDs by Varian, Elekta and Siemens, which are the only manufacturers of clinical linacs. In particular, the D0(TPR,w/2,L) and s0t(TPR,w,L) were fitted with surface equations depending on the TPR, w and L. The preliminary results confirm that the tolerance levels of the reconstructed isocenter dose are within 4% for head treatments, 5% for pelvic treatments and 6% for lung treatments. The generalized procedure here reported satisfies the requirements of efficiency and accuracy of a real time method that can be easily included in the quality assurance program of the center. References [1] [2] [3] [4] [5]
A. Piermattei, et al., Physics in Medicine and Biology 52 (2007) 5101. A. Piermattei, et al., Medical Physics 35 (2008) 1830. A. Piermattei, et al., Acta Oncologica 47 (2008) 1414. A. Piermattei, et al., Medical Physics 36 (2009) 2206. IAEA, Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water, IAEA TRS 398, 2004.