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Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance
Physica Medica xxx (2015) 1e6
Contents lists available at ScienceDirect
Physica Medica journal homepage: http://www.physicamedica.com
Technical notes
Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance Mostafa Robatjazi a, Seied Rabi Mahdavi b, *, Abbas Takavr a, Hamid Reza Baghani c a
Department of Medical Physics, Tehran University of Medical Science, Poursina St, 1417614411 Tehran, Iran Department of Medical Physics, Iran University of Medical Science, Hemmat Exp. Way, 14496141525 Tehran, Iran c Department of Radiation Medicine, Shahid Beheshti University, Daneshjoo St, Velenjak, 1983963113 Tehran, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2014 Received in revised form 29 January 2015 Accepted 31 January 2015 Available online xxx
Purpose: Intraoperative radiation therapy (IORT) using electron beam is commonly done by mobile dedicated linacs that have a variable range of electron energies. This paper focuses on the evaluation of the EBT2 film response in the green and red colour channels for IORT quality assurance (QA). Methods: The calibration of the EBT2 films was done in two ranges; 0e8 Gy for machine QA by red channel and 8e24 Gy for patient-specific QA by green channel analysis. Irradiation of calibration films and relative dosimetries were performed in a water phantom. To evaluate the accuracy of the film response in relative dosimetry, gamma analysis was used to compare the results of the Monte Carlo simulation and ionometric dosimetry. Ten patients with early stage breast cancer were selected for in-vivo dosimetry using the green channel of the EBT2 film. Results: The calibration curves were obtained by linear fitting of the green channel and a third-order polynomial function in the red channel (R2 ¼ 0.99). The total dose uncertainty was up to 4.2% and 4.7% for the red and green channels, respectively. There was a good agreement between the relative dosimetries of films by the red channel, Monte Carlo simulations and ionometric values. The mean dose difference of the in-vivo dosimetry by green channel of this film and the expected values was about 1.98% ± 0.75. Conclusion: The results of this study showed that EBT2 film can be considered as an appropriate tool for machine and patient-specific QA in IORT. © 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Keywords: Gafchromic EBT2 film IORT Quality assurance
Introduction IORT is a treatment modality in cancer therapy where a singlefraction high radiation dose is delivered directly to the tumour bed during surgical intervention after the removal of a neoplastic mass. IORT using electron beams is commonly done using mobile dedicated linear accelerators that have a variable range of electron energies (3e12 MeV). It is very important to guarantee the delivery of an accurate radiation dose to the tumour area and maintain rigorous QA [1]. One suitable tool for dosimetry and QA in most radiotherapy techniques is Gafchromic EBT2 (ISP, Wayne, NJ), a secondgeneration of Gafchromic EBT film. The near-tissue equivalency of the composition, very weak energy dependency, insensitivity of
* Corresponding author. Tel./fax: þ98 21 88622647. E-mail address: [email protected] (S.R. Mahdavi).
dose response to changes in field size, depth, and dose rate, ability to be handled in room light, and the possibility of being immersed in water phantoms are all characteristics which make EBT2 film suitable for patient-specific and machine QA at a wide range of doses [2,3]. One of the important features of EBT2 film is the dose response curve in different dose ranges. Manufacturer recommendations state that any of the colour channels of the scanned films can be used for dose measurement; however, it is preferable to use the colour channel with the greatest response gradient. Using this criterion, for doses more than 10 Gy the response gradient is greatest in the green channel and at doses in the range of 5e10 Gy, the response gradients are almost similar in the red and green channels, and in doses range below 5 Gy, the red channel is the preferred channel for film dosimetry. This film is routinely used for patient and machine QA in different techniques of radiotherapy and dose response curves are often evaluated in a clinical dose range (up to 5 Gy) in the red
http://dx.doi.org/10.1016/j.ejmp.2015.01.020 1120-1797/© 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
2
M. Robatjazi et al. / Physica Medica xxx (2015) 1e6
channel. Since the higher dose range of the electron beams (up to 21 Gy) is delivered during IORT, the EBT2 film must be calibrated for this high dose range [4,5]. This paper evaluates EBT2 film response in the green and red channels for high dose ranges that normally used for IORT procedures and machine QA. Materials and methods Film The EBT2 films used in this study were 8 10 inch sheets from Lot No. A03091201. All films was handled according to manufacturer recommendations and those outlined in the American Association of Physicists in Medicine (AAPM) Task Group Report 55 [6,7]. Smaller pieces of films were required for calibration, so the film was divided into fragments 2 2.5 cm2. For machine QA and relative dosimetry, one sheet of film was cut into four sections 12.5 10 cm2 in size. For patient-specific QA, the film was cut using scissors to 2.5 2.5 cm2 in size. Film response in flatbed scannerbased dosimetry strongly depends on film orientation with respect to scanning direction; the film should be scanned in landscape orientation [8]. All film pieces were labelled using a fine marker pen to clarify the direction of the short axis of original sheet of film.
scanned using Microtek scan wizard pro V 7.26 software at the maximum optical density (OD) range with all filters and image enhancement options turned off to cache the raw data without preprocessing. The films were scanned 24e48 h after irradiation in transmission and in 48 bit RGB mode. All images were scanned at 72 dots per in (dpi). The scanned films were saved as TIFF image files. Before irradiation, the unexposed films was scanned to measure the initial OD for netOD calculations. One source of uncertainty in film dosimetry is variation in scanner output. After each session of film scanning, an opaque and a blank scan were carried out to measure zero transmission and all the transmission light signals. Eq. (1) was used to eliminate this variation:
OD ¼ log10
PVfilm PVOpaque
PVBlank PVOpaque
Film scanning and analysis A Microtek 9800 XL CCD flatbed scanner equipped with a transparent media adaptor (TMA) was used to scan the film; this scanner can scan film in transmission mode. The films were
(1)
where the PVOpaque, PVBlank and PVfilm represent the pixel values measured in the zero-light transmitted images of the opaque sheet scan, blank screen and pixel value of the irradiated film, respectively. Since netOD was used for all film dosimetries and calibrations, the final netOD calculation was performed using Eq. (2):
netOD ¼ log10
Irradiation A mobile dedicated IORT accelerator, LIAC (Spa, Sordina, Italy, S/N: 0034), was used to irradiate the film using the reference applicator by 10 MeV electron beam. Irradiation of the calibration film was done in a MP3 water phantom (PTW, Freiburg) at the depth of the maximum dose (Dmax). The film was fixed to the hole of the device holder in the phantom for each dose level. The digital controller of the water phantom was used to precisely irradiate the film at Dmax. For machine QA, the film was fixed perpendicularly to the water surface and parallel to the electron beams that exit from the applicator. The film was fixed using paper tape to the end of the applicator and then immersed in the water phantom. It is critical in determination of the surface dose using this film that the edge of the film to be adjusted at the surface of the water. For this purpose, the water level was first raised until a small reflective gap was observed between the film edge, the distal end of the applicator and its reflection. Water was then slowly added with a syringe until the reflective-gap was reduced to zero. Each film was irradiated individually with 500 MUs of LIAC electron beams with different energies of 6, 8, 10 and 12 MeV. Between February and April 2014, 10 patients (5 full dose and 5 boost dose) with early stage breast cancer who underwent IORT after quadrantectomy were selected for in vivo dosimetry. IORT was carried out using electron beams with different applicator diameters (3e10 cm). To maintain sterile conditions during surgery, the film was wrapped in a thin sterile envelope by a nurse from the surgical staff. The wrapped film was then placed by the surgeon on top of the target to measure the surface dose. After the estimation of the target thickness and selection of the appropriate applicator by the surgeon, treatment planning was performed by a medical physicist according to the dosimetric data. Irradiation was carried out after approval by a radiotherapist.
PVexp PVOpaque PVBlank PVOpaque
log10
PVunexp PVUnopaque PVUnblank PVUnopaque
(2)
where PVunexp, PVunopaque, and PVunblnak represent the pixel values measured in zero-light transmitted images of the opaque sheet scan, blank screen and pixel value of the film, in the scanning of the unexposed film, respectively. Calibration curves Calibration of the EBT2 film was done at 0e8 Gy for machine QA by analysis of the red channel and 8e24 Gy for patient-specific QA by an analysis of the green channel. Calibration was carried out in increments of 0.5 Gy up to 8 Gy for red channel calibration and increments of 2 Gy up to 24 Gy for green channel calibration. To increase the accuracy of the calibration curve, each dose level was repeated three times using separate sections of film that placed at the centre of the field. The average netOD was considered as the film response to the corresponding dose level. The delivered dose to the calibration film was cross-calibrated using measurements from a calibrated Advanced Markus ion chamber (PTW, Freiburg) according to the IAEA TRS-398 protocol [9]. The dose uncertainties were calculated by error propagation as proposed by Devic [5]. Monte Carlo simulation To evaluate the performance of EBT2 film as a relative dosimeter in IORT and machine QA, the percentage of depth dose (PDD) and transverse dose profile (TDP) were compared with the experimental results using of ionometric dosimetry and Monte Carlo simulation. The machine head was simulated using the BEAMnrc code and the dose distribution in water was simulated using the DOSXYZnrc code [10]. For each simulation, the number of history was selected as 3 108. The cut-off energy of the electron and photon (ECUT and PCUT) were 0.521 and 0.01 MeV, respectively. The electron source was modelled as a Gaussian distributed intensity profile. The FWHM of distribution was 1 mm and the mean angular spread of the electron beam on exit window was selected
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
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to be 2 . To calculate PDD and TDP, the water phantom dimension was assumed as 30 30 15 cm3 and the voxel size as 2 2 2 mm3, which was the same as for the experimental conditions [11]. Gamma analysis was used as proposed by Low et al. [12] to evaluate agreement between the PDDs and TDPs derived from the film versus the Monte Carlo simulation and chamber. Gamma analysis is a comparison tool that simultaneously combines dose difference and distance to agreement (DTA) criteria. The dose difference was set as 3% and the DTA as 3 mm to calculate the gamma index. Results and discussion Calibration curves Figure 1 shows the calibration curves for the EBT2 film at two dose ranges for the electron beams of IORT machine. These curves were obtained by a linear fit for film response in green channel and a third order polynomial curve in red channel. The R-squared values for both green and red calibration curves were about 0.99. The total dose uncertainty was up to 4.2% for the red channel and 4.7% for the green channel calibration curves. Red channel for machine QA (relative dosimetry) AAPM TG Report 48 recommendations were used for machine QA, which prescribes that the MU be set as low as possible. Since low MUs were used to expose film for machine QA, red channel was used to convert the netOD of the film to the dose. The critical subject that the user of this film must consider in relative dosimetry is converting the netOD of the film to dose. At first glance, it seems that converting the netOD to dose is not necessary in such relative measurement; however, since there is no linear relationship between the netOD and dose values, it is necessary to convert the netOD to dose [4,13]. Measured PDDs by the red channel of EBT2 film, chamber, and Monte Carlo simulation for 6, 8, 10, and 12 MeV electron beams are shown in Fig. 2. Figure 3 shows the TDPs of four energies using the red channel dosimetry of film, chamber and Monte Carlo simulation at the depth of the maximum dose of each energy. There was a good agreement between the PDD and TDP curves of EBT2 film, the Monte Carlo simulations, and chamber at all beam energies. The corresponding gamma values for PDD and TDPs of the four beam energies passed in >95% of cases.
Figure 2. Measured (EBT2 and Advanced-Markus) and calculated (MC) PDDs in water along the central beam axis. The 6, 8, 10, 12 MeV PDDs are shown in Red, Blue, Pink, and Navy colours, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 shows the PDD parameters related to each energy of LIAC electron beam which measured by EBT2 film, Monte Carlo simulation, and Advanced Markus Chamber. Beam uniformity in TDPs varies with the electron energy, applicator size, and applicator wall material. High dose regions (horns) usually appear at the near of the filed edge. These horns are the result of electrons being scattered from the inner surface of the cone. Our results showed that the observed horns decrease with the increasing of the beam energy. This may be the result of the increase in electron scattering from the inner surface of the applicator as the energy decreased. Increasing the beam energy increased forward scattering which eliminated the horns in TDPs. These results were in good agreement with the results of Righi et al. [14]. Although they did not state the quantity of the beam TDPs in their study, it can be seen that the horns in the TDPs decreased as the beam energy increased. The scattering of the electrons in the medium widened the dose distribution in the target as the depth increased. One critical issue in treatment planning and evaluation of dose in peripheral zones and at various depths of the target in non-image based planning is the isodose curve. Isodose curve is very helpful for treatment planning of IORT. One recommended dosimeter for measuring the
Figure 1. Calibration curves for Gafchromic EBT2 film in two range and two colour channels. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
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M. Robatjazi et al. / Physica Medica xxx (2015) 1e6
Figure 3. Measured (EBT2 and Advanced-Markus) and calculated (MC) TDPs in water for all of the nominal beam energies of LIAC at the Dmax.
isodose curve is the film [15]. Figure 4 shows the isodose curves for EBT2 film at 6, 8, 10, and 12 MeV electron beam energies of the LIAC machine. It should be mentioned that the film response in red channel was employed to convert the netOD of these films to dose. It is important in IORT to assure an adequate surface dose, because the surface of the treatment volume has a high probability of containing tumour cells. The measurement of this dosimetric parameter should be done with great care [16]. It is recommended by the AAPM TG Report 106 that surface dose measurement should not be performed by scanning devices and application of an extrapolating chamber is time-consuming. Furthermore, the application of parallel plate chambers results in over-responses
because of the relatively large separation of the plate from the extrapolation chamber [15]. One of the most important characteristics of film dosimetry is the applicability of film for the measurement of surface doses. The surface dose measured by EBT2 film was 86.4% for 6 MeV, 88.1% for 8 MeV, 91.8% for 10 MeV and 93.5% for 12 MeV beams. Righi et al. [14] also reported the range of the surface doses for 4, 6, 8, and 10 MeV electron beams using film and Monte Carlo simulation for the LIAC (Model 10 MeV). This range was 88% for 4 and 91% for 10 MeV. It is likely that the difference in the values for low energies between studies resulted from the difference of the structure of the two models of LIAC (10 MeV Model versus 12 MeV Model).
Table 1 Comparable dosimetric parameters that extracted from PDD curves at all nominal energies of LIAC machine by Gafchromic EBT2 film, Advanced Markus chamber, and MC calculation. Energy (MeV)
6 8 10 12
EBT2 film
Adv-Markus chamber
MC calculations
R100 (mm)
R90 (mm)
RP (mm)
R50 (mm)
R100 (mm)
R90 (mm)
RP (mm)
R50 (mm)
R100 (mm)
R90 (mm)
RP (mm)
R50 (mm)
8.5 12.4 16.1 16.8
14.5 21.8 27.1 31.3
28.5 41.9 52.5 61.6
21.6 32.1 39.8 47.1
8.2 12.1 15.7 16.0
14.1 21.9 27.3 31.6
28.6 41.6 52.2 61.2
21.3 31.6 40.0 46.7
8.8 12.2 16.6 16.5
14.8 21.5 28.2 31.9
29.1 41.8 51.7 61.2
21.8 31.6 40.0 47.1
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
M. Robatjazi et al. / Physica Medica xxx (2015) 1e6
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Figure 4. Isodose curves of the four nominal electron beam energies of LIAC machine that measured by red channel dosimetry of Gafchromic EBT2 film. Normalization was done to the maximum value of central axis in each energy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Green channel for in-vivo dosimetry (absolute dosimetry) In-vivo dosimetry is a common way to perform overall check of the delivered dose in patient-specific QA for radiation treatment procedures. Since a high dose range of radiation was used in IORT, it was necessary to apply green channel calibration for the EBT2 film in patient-specific QA. Table 2 shows the results of film dosimetry for measuring the surface dose during breast IORT. The mean difference between measured and expected dose values was 1.98% ± 0.75. In 2003, Ciocca et al. [17] reported in-vivo dosimetry results for breast IORT using Gafchromic MD-55-2 film. The mean
Table 2 Results of in-vivo dosimetry by green channel of the Gafchromic EBT2 film in patient with breast cancer that underwent IORT. Beam Energy Applicator Predicted Measured Dose (MeV) diameter (cm) dose (Gy) dose (Gy) difference (%) Boost Dose
Full Dose
8 10 8 8 8 8 8 8 10 10
5 6 7 6 7 5 4 6 4 6
12.6 11.7 11.7 11.7 11.7 21.2 21 21 22.3 20.3
12.9 11.9 11.8 12 11.5 21 21.4 21.7 21.7 20
2.4 1.7 1 2.6 1.7 1 1.9 3.3 2.7 1.5
deviation between the measured and expected doses was 1.8% ± 4.7%. The results of the present in-vivo dosimetry using the EBT2 film after application of the green channel show good agreement with the results of Ciocca et al. Conclusion One of the primary concerns in the application of a dedicated IORT machine is radiation protection during treatment and machine QA. Dedicated IORT machines are designed for use in an unshielded operating room, which limits exposure for QA. On the other hand, the availability of complete dosimetric data is very helpful for treatment planning, but the measurement of these data in a scanning 3D water phantom requires high MUs. The present study applied EBT2 film and lower MUs than those MUs applied for scanning a 3D water phantom. This allows measurement of all dosimetric data for each nominal energy in the IORT machines. In line with the manufacturer's recommendations, we obtained the calibration curves that showed the application of red channel dosimetry has an acceptable response for doses up to 8 Gy while at the dose levels of greater than 10 Gy the green channel dosimetry would be preferable. A reliable response in the 0e8 Gy dose range was seen in the application of the EBT2 film for IORT machine QA that its data has a good agreement with the experimental and Monte Carlo calculated data. The results of green channel EBT2 film for in-vivo dosimetry showed that this system was reliable.
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
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References [1] Soriani A, Landoni V, Marzi S, Iaccarino G, Saracino B, Arcangeli G, et al. Setup verification and in vivo dosimetry during intraoperative radiation therapy (IORT) for prostate cancer. Med Phys 2007;34:3205. [2] Baghani HR, Aghamiri SMR, Mahdavi SR, Robatjazi M, Rahim Zadeh A, Akbari ME, et al. Dosimetric evaluation of Gafchromic EBT2 film for breast intraoperative electron radiotherapy verification. Phys Medica 2014;31(1): 37e42. [3] Carrasco M, Perucha M, Luis F, Baeza M, Herrador M. A comparison between radiochromic EBT2 film model and its predecessor EBT film model. Phys Medica 2013;29(4):412e22. [4] Arjomandy B, Tailor R, Anand A, Sahoo N, Gillin M, Prado K, et al. Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies. Med Phys 2010;37(5):1942e7. [5] Devic S, Seuntjens J, Sham E, Podgorsak EB, Schmidtlein CR, Kirov AS, et al. Precise radiochromic film dosimetry using a flat-bed document scanner. Med Phys 2005;32(7):2245e53. [6] ISP. Gafchromic EBT2 self-developing film for radiotherapy dosimetry. Accessible from: http://gafchromic.com; 2010. [7] Niroomand-Rad A, Blackwell CR, Coursey BM, Gall KP, Galvin JM, McLaughlin WL, et al. Radiochromic film dosimetry: recommendations of AAPM radiation therapy committee task group 55. Med Phys 1998;25(11):2093e115. [8] Butson M, Cheung T, Yu P. Scanning orientation effects on Gafchromic EBT film dosimetry. Australas Phys Eng Sci Med 2006;29(3):281e4. [9] TRS I. 398, Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water. Vienna: International Atomic Energy Agency; 2000.
[10] Rogers D, Ma C, Walters B, Ding G, Sheikh-Bagheri D, Zhang G. BEAMnrc users Manual National Research Council of Canada. 2002. NRCC Rep PIRS-0509 (A) revK. [11] Iaccarino G, Strigari L, D'Andrea M, Bellesi L, Felici G, Ciccotelli A, et al. Monte Carlo simulation of electron beams generated by a 12 MeV dedicated mobile IORT accelerator. Phys Med Biol 2011;56(14):4579. [12] Low DA, Dempsey JF. Evaluation of the gamma dose distribution comparison method. Med Phys 2003;30(9):2455e64. [13] Devic S, Tomic N, Aldelaijan S, DeBlois F, Seuntjens J, Chan MF, et al. Linearization of doseeresponse curve of the radiochromic film dosimetry system. Med Phys 2012;39(8):4850e7. [14] Righi S, Karaj E, Felici G, Di Martino F. Dosimetric characteristics of electron beams produced by two mobile accelerators, Novac7 and Liac, for intraoperative radiation therapy through Monte Carlo simulation. J Appl Clin Med Phys 2013;(1):14. € A, Gibbons J, Li XA, et al. Accelerator [15] Das IJ, Cheng C-W, Watts RJ, Ahnesjo beam data commissioning equipment and procedures: report of the TG-106 of the Therapy Physics Committee of the AAPM. Med Phys 2008;35(9): 4186e215. [16] Palta JR, Biggs PJ, Hazle JD, Huq MS, Dahl RA, Ochran TG, et al. Intraoperative electron beam radiation therapy: technique, dosimetry, and dose specification: report of Task Force 48 of the Radiation Therapy Committee, American Association of Physicists in Medicine. Int J Radiat Oncol Biol Phys 1995;33(3): 725e46. [17] Ciocca M, Orecchia R, Garibaldi C, Rondi E, Luini A, Gatti G, et al. In vivo dosimetry using radiochromic films during intraoperative electron beam radiation therapy in early-stage breast cancer. Radiother Oncol 2003;69(3): 285e9.
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
Contents lists available at ScienceDirect
Physica Medica journal homepage: http://www.physicamedica.com
Technical notes
Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance Mostafa Robatjazi a, Seied Rabi Mahdavi b, *, Abbas Takavr a, Hamid Reza Baghani c a
Department of Medical Physics, Tehran University of Medical Science, Poursina St, 1417614411 Tehran, Iran Department of Medical Physics, Iran University of Medical Science, Hemmat Exp. Way, 14496141525 Tehran, Iran c Department of Radiation Medicine, Shahid Beheshti University, Daneshjoo St, Velenjak, 1983963113 Tehran, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2014 Received in revised form 29 January 2015 Accepted 31 January 2015 Available online xxx
Purpose: Intraoperative radiation therapy (IORT) using electron beam is commonly done by mobile dedicated linacs that have a variable range of electron energies. This paper focuses on the evaluation of the EBT2 film response in the green and red colour channels for IORT quality assurance (QA). Methods: The calibration of the EBT2 films was done in two ranges; 0e8 Gy for machine QA by red channel and 8e24 Gy for patient-specific QA by green channel analysis. Irradiation of calibration films and relative dosimetries were performed in a water phantom. To evaluate the accuracy of the film response in relative dosimetry, gamma analysis was used to compare the results of the Monte Carlo simulation and ionometric dosimetry. Ten patients with early stage breast cancer were selected for in-vivo dosimetry using the green channel of the EBT2 film. Results: The calibration curves were obtained by linear fitting of the green channel and a third-order polynomial function in the red channel (R2 ¼ 0.99). The total dose uncertainty was up to 4.2% and 4.7% for the red and green channels, respectively. There was a good agreement between the relative dosimetries of films by the red channel, Monte Carlo simulations and ionometric values. The mean dose difference of the in-vivo dosimetry by green channel of this film and the expected values was about 1.98% ± 0.75. Conclusion: The results of this study showed that EBT2 film can be considered as an appropriate tool for machine and patient-specific QA in IORT. © 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Keywords: Gafchromic EBT2 film IORT Quality assurance
Introduction IORT is a treatment modality in cancer therapy where a singlefraction high radiation dose is delivered directly to the tumour bed during surgical intervention after the removal of a neoplastic mass. IORT using electron beams is commonly done using mobile dedicated linear accelerators that have a variable range of electron energies (3e12 MeV). It is very important to guarantee the delivery of an accurate radiation dose to the tumour area and maintain rigorous QA [1]. One suitable tool for dosimetry and QA in most radiotherapy techniques is Gafchromic EBT2 (ISP, Wayne, NJ), a secondgeneration of Gafchromic EBT film. The near-tissue equivalency of the composition, very weak energy dependency, insensitivity of
* Corresponding author. Tel./fax: þ98 21 88622647. E-mail address: [email protected] (S.R. Mahdavi).
dose response to changes in field size, depth, and dose rate, ability to be handled in room light, and the possibility of being immersed in water phantoms are all characteristics which make EBT2 film suitable for patient-specific and machine QA at a wide range of doses [2,3]. One of the important features of EBT2 film is the dose response curve in different dose ranges. Manufacturer recommendations state that any of the colour channels of the scanned films can be used for dose measurement; however, it is preferable to use the colour channel with the greatest response gradient. Using this criterion, for doses more than 10 Gy the response gradient is greatest in the green channel and at doses in the range of 5e10 Gy, the response gradients are almost similar in the red and green channels, and in doses range below 5 Gy, the red channel is the preferred channel for film dosimetry. This film is routinely used for patient and machine QA in different techniques of radiotherapy and dose response curves are often evaluated in a clinical dose range (up to 5 Gy) in the red
http://dx.doi.org/10.1016/j.ejmp.2015.01.020 1120-1797/© 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020
2
M. Robatjazi et al. / Physica Medica xxx (2015) 1e6
channel. Since the higher dose range of the electron beams (up to 21 Gy) is delivered during IORT, the EBT2 film must be calibrated for this high dose range [4,5]. This paper evaluates EBT2 film response in the green and red channels for high dose ranges that normally used for IORT procedures and machine QA. Materials and methods Film The EBT2 films used in this study were 8 10 inch sheets from Lot No. A03091201. All films was handled according to manufacturer recommendations and those outlined in the American Association of Physicists in Medicine (AAPM) Task Group Report 55 [6,7]. Smaller pieces of films were required for calibration, so the film was divided into fragments 2 2.5 cm2. For machine QA and relative dosimetry, one sheet of film was cut into four sections 12.5 10 cm2 in size. For patient-specific QA, the film was cut using scissors to 2.5 2.5 cm2 in size. Film response in flatbed scannerbased dosimetry strongly depends on film orientation with respect to scanning direction; the film should be scanned in landscape orientation [8]. All film pieces were labelled using a fine marker pen to clarify the direction of the short axis of original sheet of film.
scanned using Microtek scan wizard pro V 7.26 software at the maximum optical density (OD) range with all filters and image enhancement options turned off to cache the raw data without preprocessing. The films were scanned 24e48 h after irradiation in transmission and in 48 bit RGB mode. All images were scanned at 72 dots per in (dpi). The scanned films were saved as TIFF image files. Before irradiation, the unexposed films was scanned to measure the initial OD for netOD calculations. One source of uncertainty in film dosimetry is variation in scanner output. After each session of film scanning, an opaque and a blank scan were carried out to measure zero transmission and all the transmission light signals. Eq. (1) was used to eliminate this variation:
OD ¼ log10
PVfilm PVOpaque
PVBlank PVOpaque
Film scanning and analysis A Microtek 9800 XL CCD flatbed scanner equipped with a transparent media adaptor (TMA) was used to scan the film; this scanner can scan film in transmission mode. The films were
(1)
where the PVOpaque, PVBlank and PVfilm represent the pixel values measured in the zero-light transmitted images of the opaque sheet scan, blank screen and pixel value of the irradiated film, respectively. Since netOD was used for all film dosimetries and calibrations, the final netOD calculation was performed using Eq. (2):
netOD ¼ log10
Irradiation A mobile dedicated IORT accelerator, LIAC (Spa, Sordina, Italy, S/N: 0034), was used to irradiate the film using the reference applicator by 10 MeV electron beam. Irradiation of the calibration film was done in a MP3 water phantom (PTW, Freiburg) at the depth of the maximum dose (Dmax). The film was fixed to the hole of the device holder in the phantom for each dose level. The digital controller of the water phantom was used to precisely irradiate the film at Dmax. For machine QA, the film was fixed perpendicularly to the water surface and parallel to the electron beams that exit from the applicator. The film was fixed using paper tape to the end of the applicator and then immersed in the water phantom. It is critical in determination of the surface dose using this film that the edge of the film to be adjusted at the surface of the water. For this purpose, the water level was first raised until a small reflective gap was observed between the film edge, the distal end of the applicator and its reflection. Water was then slowly added with a syringe until the reflective-gap was reduced to zero. Each film was irradiated individually with 500 MUs of LIAC electron beams with different energies of 6, 8, 10 and 12 MeV. Between February and April 2014, 10 patients (5 full dose and 5 boost dose) with early stage breast cancer who underwent IORT after quadrantectomy were selected for in vivo dosimetry. IORT was carried out using electron beams with different applicator diameters (3e10 cm). To maintain sterile conditions during surgery, the film was wrapped in a thin sterile envelope by a nurse from the surgical staff. The wrapped film was then placed by the surgeon on top of the target to measure the surface dose. After the estimation of the target thickness and selection of the appropriate applicator by the surgeon, treatment planning was performed by a medical physicist according to the dosimetric data. Irradiation was carried out after approval by a radiotherapist.
PVexp PVOpaque PVBlank PVOpaque
log10
PVunexp PVUnopaque PVUnblank PVUnopaque
(2)
where PVunexp, PVunopaque, and PVunblnak represent the pixel values measured in zero-light transmitted images of the opaque sheet scan, blank screen and pixel value of the film, in the scanning of the unexposed film, respectively. Calibration curves Calibration of the EBT2 film was done at 0e8 Gy for machine QA by analysis of the red channel and 8e24 Gy for patient-specific QA by an analysis of the green channel. Calibration was carried out in increments of 0.5 Gy up to 8 Gy for red channel calibration and increments of 2 Gy up to 24 Gy for green channel calibration. To increase the accuracy of the calibration curve, each dose level was repeated three times using separate sections of film that placed at the centre of the field. The average netOD was considered as the film response to the corresponding dose level. The delivered dose to the calibration film was cross-calibrated using measurements from a calibrated Advanced Markus ion chamber (PTW, Freiburg) according to the IAEA TRS-398 protocol [9]. The dose uncertainties were calculated by error propagation as proposed by Devic [5]. Monte Carlo simulation To evaluate the performance of EBT2 film as a relative dosimeter in IORT and machine QA, the percentage of depth dose (PDD) and transverse dose profile (TDP) were compared with the experimental results using of ionometric dosimetry and Monte Carlo simulation. The machine head was simulated using the BEAMnrc code and the dose distribution in water was simulated using the DOSXYZnrc code [10]. For each simulation, the number of history was selected as 3 108. The cut-off energy of the electron and photon (ECUT and PCUT) were 0.521 and 0.01 MeV, respectively. The electron source was modelled as a Gaussian distributed intensity profile. The FWHM of distribution was 1 mm and the mean angular spread of the electron beam on exit window was selected
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to be 2 . To calculate PDD and TDP, the water phantom dimension was assumed as 30 30 15 cm3 and the voxel size as 2 2 2 mm3, which was the same as for the experimental conditions [11]. Gamma analysis was used as proposed by Low et al. [12] to evaluate agreement between the PDDs and TDPs derived from the film versus the Monte Carlo simulation and chamber. Gamma analysis is a comparison tool that simultaneously combines dose difference and distance to agreement (DTA) criteria. The dose difference was set as 3% and the DTA as 3 mm to calculate the gamma index. Results and discussion Calibration curves Figure 1 shows the calibration curves for the EBT2 film at two dose ranges for the electron beams of IORT machine. These curves were obtained by a linear fit for film response in green channel and a third order polynomial curve in red channel. The R-squared values for both green and red calibration curves were about 0.99. The total dose uncertainty was up to 4.2% for the red channel and 4.7% for the green channel calibration curves. Red channel for machine QA (relative dosimetry) AAPM TG Report 48 recommendations were used for machine QA, which prescribes that the MU be set as low as possible. Since low MUs were used to expose film for machine QA, red channel was used to convert the netOD of the film to the dose. The critical subject that the user of this film must consider in relative dosimetry is converting the netOD of the film to dose. At first glance, it seems that converting the netOD to dose is not necessary in such relative measurement; however, since there is no linear relationship between the netOD and dose values, it is necessary to convert the netOD to dose [4,13]. Measured PDDs by the red channel of EBT2 film, chamber, and Monte Carlo simulation for 6, 8, 10, and 12 MeV electron beams are shown in Fig. 2. Figure 3 shows the TDPs of four energies using the red channel dosimetry of film, chamber and Monte Carlo simulation at the depth of the maximum dose of each energy. There was a good agreement between the PDD and TDP curves of EBT2 film, the Monte Carlo simulations, and chamber at all beam energies. The corresponding gamma values for PDD and TDPs of the four beam energies passed in >95% of cases.
Figure 2. Measured (EBT2 and Advanced-Markus) and calculated (MC) PDDs in water along the central beam axis. The 6, 8, 10, 12 MeV PDDs are shown in Red, Blue, Pink, and Navy colours, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 shows the PDD parameters related to each energy of LIAC electron beam which measured by EBT2 film, Monte Carlo simulation, and Advanced Markus Chamber. Beam uniformity in TDPs varies with the electron energy, applicator size, and applicator wall material. High dose regions (horns) usually appear at the near of the filed edge. These horns are the result of electrons being scattered from the inner surface of the cone. Our results showed that the observed horns decrease with the increasing of the beam energy. This may be the result of the increase in electron scattering from the inner surface of the applicator as the energy decreased. Increasing the beam energy increased forward scattering which eliminated the horns in TDPs. These results were in good agreement with the results of Righi et al. [14]. Although they did not state the quantity of the beam TDPs in their study, it can be seen that the horns in the TDPs decreased as the beam energy increased. The scattering of the electrons in the medium widened the dose distribution in the target as the depth increased. One critical issue in treatment planning and evaluation of dose in peripheral zones and at various depths of the target in non-image based planning is the isodose curve. Isodose curve is very helpful for treatment planning of IORT. One recommended dosimeter for measuring the
Figure 1. Calibration curves for Gafchromic EBT2 film in two range and two colour channels. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Figure 3. Measured (EBT2 and Advanced-Markus) and calculated (MC) TDPs in water for all of the nominal beam energies of LIAC at the Dmax.
isodose curve is the film [15]. Figure 4 shows the isodose curves for EBT2 film at 6, 8, 10, and 12 MeV electron beam energies of the LIAC machine. It should be mentioned that the film response in red channel was employed to convert the netOD of these films to dose. It is important in IORT to assure an adequate surface dose, because the surface of the treatment volume has a high probability of containing tumour cells. The measurement of this dosimetric parameter should be done with great care [16]. It is recommended by the AAPM TG Report 106 that surface dose measurement should not be performed by scanning devices and application of an extrapolating chamber is time-consuming. Furthermore, the application of parallel plate chambers results in over-responses
because of the relatively large separation of the plate from the extrapolation chamber [15]. One of the most important characteristics of film dosimetry is the applicability of film for the measurement of surface doses. The surface dose measured by EBT2 film was 86.4% for 6 MeV, 88.1% for 8 MeV, 91.8% for 10 MeV and 93.5% for 12 MeV beams. Righi et al. [14] also reported the range of the surface doses for 4, 6, 8, and 10 MeV electron beams using film and Monte Carlo simulation for the LIAC (Model 10 MeV). This range was 88% for 4 and 91% for 10 MeV. It is likely that the difference in the values for low energies between studies resulted from the difference of the structure of the two models of LIAC (10 MeV Model versus 12 MeV Model).
Table 1 Comparable dosimetric parameters that extracted from PDD curves at all nominal energies of LIAC machine by Gafchromic EBT2 film, Advanced Markus chamber, and MC calculation. Energy (MeV)
6 8 10 12
EBT2 film
Adv-Markus chamber
MC calculations
R100 (mm)
R90 (mm)
RP (mm)
R50 (mm)
R100 (mm)
R90 (mm)
RP (mm)
R50 (mm)
R100 (mm)
R90 (mm)
RP (mm)
R50 (mm)
8.5 12.4 16.1 16.8
14.5 21.8 27.1 31.3
28.5 41.9 52.5 61.6
21.6 32.1 39.8 47.1
8.2 12.1 15.7 16.0
14.1 21.9 27.3 31.6
28.6 41.6 52.2 61.2
21.3 31.6 40.0 46.7
8.8 12.2 16.6 16.5
14.8 21.5 28.2 31.9
29.1 41.8 51.7 61.2
21.8 31.6 40.0 47.1
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Figure 4. Isodose curves of the four nominal electron beam energies of LIAC machine that measured by red channel dosimetry of Gafchromic EBT2 film. Normalization was done to the maximum value of central axis in each energy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Green channel for in-vivo dosimetry (absolute dosimetry) In-vivo dosimetry is a common way to perform overall check of the delivered dose in patient-specific QA for radiation treatment procedures. Since a high dose range of radiation was used in IORT, it was necessary to apply green channel calibration for the EBT2 film in patient-specific QA. Table 2 shows the results of film dosimetry for measuring the surface dose during breast IORT. The mean difference between measured and expected dose values was 1.98% ± 0.75. In 2003, Ciocca et al. [17] reported in-vivo dosimetry results for breast IORT using Gafchromic MD-55-2 film. The mean
Table 2 Results of in-vivo dosimetry by green channel of the Gafchromic EBT2 film in patient with breast cancer that underwent IORT. Beam Energy Applicator Predicted Measured Dose (MeV) diameter (cm) dose (Gy) dose (Gy) difference (%) Boost Dose
Full Dose
8 10 8 8 8 8 8 8 10 10
5 6 7 6 7 5 4 6 4 6
12.6 11.7 11.7 11.7 11.7 21.2 21 21 22.3 20.3
12.9 11.9 11.8 12 11.5 21 21.4 21.7 21.7 20
2.4 1.7 1 2.6 1.7 1 1.9 3.3 2.7 1.5
deviation between the measured and expected doses was 1.8% ± 4.7%. The results of the present in-vivo dosimetry using the EBT2 film after application of the green channel show good agreement with the results of Ciocca et al. Conclusion One of the primary concerns in the application of a dedicated IORT machine is radiation protection during treatment and machine QA. Dedicated IORT machines are designed for use in an unshielded operating room, which limits exposure for QA. On the other hand, the availability of complete dosimetric data is very helpful for treatment planning, but the measurement of these data in a scanning 3D water phantom requires high MUs. The present study applied EBT2 film and lower MUs than those MUs applied for scanning a 3D water phantom. This allows measurement of all dosimetric data for each nominal energy in the IORT machines. In line with the manufacturer's recommendations, we obtained the calibration curves that showed the application of red channel dosimetry has an acceptable response for doses up to 8 Gy while at the dose levels of greater than 10 Gy the green channel dosimetry would be preferable. A reliable response in the 0e8 Gy dose range was seen in the application of the EBT2 film for IORT machine QA that its data has a good agreement with the experimental and Monte Carlo calculated data. The results of green channel EBT2 film for in-vivo dosimetry showed that this system was reliable.
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Please cite this article in press as: Robatjazi M, et al., Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance, Physica Medica (2015), http://dx.doi.org/10.1016/j.ejmp.2015.01.020