- Email: [email protected]
Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners
Physica Medica xxx (2014) 1e5
Contents lists available at ScienceDirect
Physica Medica journal homepage: http://www.physicamedica.com
Technical notes
Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners Nicolas Farah a, b, Ziad Francis a, c, *, Marie Abboud a a
Physics Department, Faculty of Science, Saint Joseph University, B.P. 11-514 Riad El Solh, Beirut 1107 2050, Lebanon Department of Radiation-Oncology, Hôtel-Dieu de France Hospital, PO Box HDF 166830, Beirut, Lebanon c The Open University, Faculty of Science, Department of Physical Sciences, Walton Hall, MK7 6AA Milton Keynes, United Kingdom b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 4 April 2014 Accepted 25 April 2014 Available online xxx
We explore in our study the effects of electrons and X-rays irradiations on the newest version of the Gafchromic EBT3 film. Experiments are performed using the Varian “TrueBeam 1.6” medical accelerator delivering 6 MV X-ray photons and 6 MeV electron beams as desired. The main interest is to compare the responses of EBT3 films exposed to two separate beams of electrons and photons, for radiation doses ranging up to 500 cGy. The analysis is done on a flatbed EPSON 10000 XL scanner and cross checked on a HP Scanjet 4850 scanner. Both scanners are used in reflection mode taking into account landscape and portrait scanning positions. After thorough verifications, the reflective scanning method can be used on EBT3 as an economic alternative to the transmission method which was also one of the goals of this study. A comparison is also done between single scan configuration including all samples in a single A4 (HP) or A3 (EPSON) format area and multiple scan procedure where each sample is scanned separately on its own. The images analyses are done using the ImageJ software. Results show significant influence of the scanning configuration but no significant differences between electron and photon irradiations for both single and multiple scan configurations. In conclusion, the film provides a reliable relative dose measurement method for electrons and photons irradiations in the medical field applications. Ó 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Keywords: Gafchromic EBT3 Film dosimetry Radiotherapy
Introduction In the past years, using radiochromic films for dose assessment went through noticeable progress, and studies were emphasized on this type of measurements in the medical field for radiation dosimetry and radiotherapy control. Gafchromic film irradiation is seen today as a simple and quick method for dose profiling in different irradiation situations. However some properties should be considered for more reliable results taking into account the sensitivity of such type of films to several parameters e.g. radiation type and energy. Studies on the effect of read out light sources and ambient light on previous versions of Gafchromic films, e.g. MD-551 and MD-55-2 [1], showed an interesting color modification after few hours of exposure especially for fluorescent light sources, and
* Corresponding author. Physics Department, Faculty of Science, Saint Joseph University, B.P. 11-514 Riad El Solh, Beirut 1107 2050, Lebanon. Tel.: þ961 1 421 000ext3450; fax: þ961 4 532 657. E-mail addresses: [email protected] (Z. Francis), [email protected] (M. Abboud).
thus care should be taken during film handling and storage. Another study by Ref. [2] revealed a 9% post-irradiation coloration increase of MD-55-2 films up to 20 h after the irradiation, suggesting that films should be scanned for read out at least 6 h after exposure. The properties of EBT Gafchromic films exposed to X-rays radiation were also investigated by other authors e.g. Ref. [3] and the energy response of the EBT2 films was found to be independent of the incident photon energy over a wide range from 15 keV to w2.5 MeV for doses ranging between 25 cGy and 500 cGy. The absorption spectra variations during read out scanning of the EBT film showed a clear increase in net optical density for frequencies around 636 nm in comparison with older versions of Gafchromic films [4]. The improvements applied on the EBT made this type of films a powerful candidate for several applications like measuring solar UV radiation [5], and as a potential dosimeter for ion beams therapy [6], underlining the precision that can be achieved in practical cases like medical tomotherapy [7]. A detailed review by Ref. [8] summarizes the improvement of Gafchromic films through the recent years and underlines the importance of precision in dosimetry for medical applications. However, some performance issues were still reported for the EBT2 version, e.g., the
http://dx.doi.org/10.1016/j.ejmp.2014.04.010 1120-1797/Ó 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
2
N. Farah et al. / Physica Medica xxx (2014) 1e5
inhomogeneity of the film response [9], showing the importance of the repetitive and complementary verification studies that are reported in the literature for the different types of Gafchromic films. In fact, comparisons between different versions of the EBT are of extreme importance, especially when it comes to assessing the improvements that are made for the medical applications dosimetry. Moreover, the analysis protocols that are reported in the literature can differ easily one from another leading to differences in the obtained results. Here comes the importance of reporting as much details as possible when it comes to analysis methods. In our study we use the most recent version of the EBT series namely the EBT3 film. The EBT3 is an improved version of the EBT2 film; it is made by laminating an active layer between two identical polyester layers [10]. Studies published in the literature showed some of the characteristics of the EBT3 e.g., energy dependence of the film reaction after X-rays irradiation [11], particle type dependence [12], specific IMRT applications [10] and comparisons with its predecessor the EBT2 in medical irradiation applications [13]. One of the advantages of the EBT3 is that the analysis is independent from the film scan orientation which was not the case for the EBT2. Here in this work, our interest lies mostly in comparing the response of the films to doses delivered by X-ray and electron beams, and using different analysis configurations with two flatbed reflective scanners. In fact, transmission scanners are more costly than reflective scanners and might be more difficult to acquire in some cases. One of the issues we also wanted to verify here is the reliability of the reflective analysis method comparing a relatively expensive scanner to the results obtained by a cheaper scanner model. Materials and methods Gafchromic film EBT3 The Gafchromic EBT3 (lot number A 01231202, Ashland Specialty Ingredients, NJ, USA) radiochromic film is available in sheets of 20.32 25.4 cm2. It consists of an active layer of w28 mm of thickness coated on both sides by a protective polyester layer of 100 mm. Samples are cut into pieces of 5 6 cm2 before the irradiation. This size order is acceptable and common to several studies in the literature [7,12]; it covers an area that is wide enough to limit statistical uncertainties during irradiation and analysis. Handling the samples from cutting phase to the scanning and folding is done using latex gloves to avoid stains and marks that can affect the scanning results. The samples are folded with care in a dark envelope whenever not in use in order to avoid ambient light and other sources effects during storage. Irradiation procedure The irradiations are accomplished using a Varian “TrueBeam 1.6” accelerator providing electron beams of 6 MeV average energy and photon beams of 6 MV acceleration potential. Relatively low energies are chosen and phantoms are used in order to achieve the best dose absorption rate and to minimize the irradiation time, without altering the quality of the results. The considered configuration is thus compatible with our study since the dose response of the film is independent of the depth in the phantom between 2 and 25 cm [14]. The irradiation field is considered uniform and limited within 10 10 cm2. The films are positioned between two water equivalent phantom layers of polystyrene; the build-up phantom (1.5 cm thickness as in Ref. [4]) is placed between the source field and the irradiated sample and the support phantom (10 cm thickness) laying behind the sample in the beam propagation direction. Additionally, no significant influence is observed
between low and high field sizes ranging from 3 3 cm2 up to 25 25 cm2 [14] on dose response so only the mentioned 10 10 cm2 configuration is adopted. The irradiation procedure is performed for doses ranging from 0 to 500 cGy with a step of 50 cGy for both electron and photon beams. The relative uncertainty of the delivered dose is around 2%; this is the standard conventional value that is clinically used when continuous quality control procedures are implemented using the ionization chamber methods. Scan procedure The irradiated films were folded in dark envelopes at least for 24 h to ensure the darkening effect stabilization. The scanning is performed in landscape and portrait positions using two different scanners: HP scanjet 4850 and EPSON 10000 XL with a resolution of 300 dots per inch (dpi) producing 48 bit RGB colored images in tiff format. When HP scanner is used, five blank scans are performed in order to warm up the lamp of the scanner and the sixth scan is saved for analysis. In the case of Epson 10000 XL scanner, an average of 2e3 preview scans was considered sufficient at the beginning of the scanning procedure to take into account the warm-up effect. According to Ref. [14] the impact of these effects on the results does not exceed 0.1%. We consider two approaches of scanning: (i) a single scan configuration where all the samples are placed according to increasing, decreasing or random dose order from left to right in order to ensure that the position on the scanner bed does not affect the results and then scanned at once fitting in an A4 or A3 format area for the HP and the EPSON scanners respectively and (ii) a ‘multiple scan’ configuration where every sample is placed in the center of the scanner bed and scanned on its own. The comparison between single and multiple scans approaches would show the effect of the scanning phase parameter on the results. As advised by the manufacturer, for the scanning, no filters or color correction functions are applied to raw pixel value results. Scan analysis Image measurement and analysis are performed on a region of interest (ROI) of 480 480 pixels2 around the film samples center. Three values are taken to obtain the mean pixel value with its standard deviation in the ROI. The scanned images are then analyzed using ImageJ [15] software. The RGB and the red component of the color are taken into account [5,16]. The net reflective optical density “net ROD” is calculated using the pixels values from the scanned images according to the following expression
Iu Net ROD ¼ log Ii
(1)
where Iu and Ii are the average pixel values of the reflected intensities through non irradiated and irradiated films respectively [17]. Results and discussion As already mentioned, we consider in our study several analysis configurations. The combination of different setups and approaches would lead to a high number of similar results that would be cumbersome to show at once, thus, for the sake of clarity only the interesting selection is displayed in this section. Furthermore, the error bars on experimental values represent less than 0.1% and they do not appear on the figures as they are smaller than the symbols.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
N. Farah et al. / Physica Medica xxx (2014) 1e5
3
Scan orientation comparison Figures 1 and 2 show the landscape and portrait scan orientation comparison for 6 MV photon and 6 MeV electron irradiations respectively, for doses between 0 and 500 cGy. The Epson 10000 XL scanner is used in reflective mode and the analysis is made in 2 different ways taking into account the complete RGB scanned image for one analysis and only the RED channel for the other. The landscape and portrait configurations lead to the same results for the RGB and the Red channel analyses; the differences of the ROD values are below 0.03% for the whole spectrum with an exception reaching 0.07% for doses above 400 cGy for electrons irradiations. Differences of around 0.45%e1.1% were reported in the literature for a dose range reaching up to 10 Gy [12]. In the previous EBT versions (EBT2) the scanning results showed noticeable differences between landscape and portrait scan positions, which is not the case for the EBT3 where the difference is negligible as mentioned by the manufacturer and verified by Ref. [13]. Furthermore, in previous studies on previous EBT films, the red channel showed a higher sensitivity than the RGB [18] and this tendency is still valid for the particular conditions applied in our study.
Particle type and energy dependence In order to verify the effect of the particle type on the EBT3 coloring, samples are irradiated using 2 different particles, photons and electrons. The scan of the films is done in a landscape configuration and a comparison between RGB and red channel analyses is conducted. Figure 3 shows a comparison of the net ROD for electron and photon irradiations. The obtained results show very close results for both radiation types and for the same doses. Discrepancies remain less than 0.07% for doses below 350 cGy and increase slightly up to 0.1% for doses around 500 cGy. These findings are in agreement with previous results from the literature where the EBT3 film was used for IMRT energy range e.g., 6 MVe15 MV and the energy dependence was proven to be negligible [10]. For high energy photon beams generally used in radiotherapy energy dependence of the EBT3 coloring is also weak [11] and the
Figure 1. Dose response of EBT3 Gafchromic film irradiated with 6 MV photon beam and scanned in landscape and portrait configurations using an Epson 10000 XL scanner. The analyses are shown using separately the RGB and the Red channel filtering.
Figure 2. Dose response of EBT3 Gafchromic film irradiated with 6 MeV electrons. The analysis details are the same as for Fig. 1.
dependence on the particle type was also reported to be nonsignificant by Refs. [12] and [19] for the EBT3 predecessors (The EBT2 and the EBT versions). Single versus multiple scan comparison As the results can vary according to the scanning details e.g. scan lamp temperature reference [20] and other parameters, we compare two scan configurations. In a multiple scan approach, each irradiated sample is scanned together with the non-irradiated one consecutively from the lower dose to the higher one, then in a decreasing order of dose and finally in a random order. In all three cases, the analyses of the scanned images yield to the same result. In a single scan approach, all the samples are placed in an A3 format area and scanned at once using the EPSON 10000 XL scanner.
Figure 3. Dose response curve for EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons, scanned in landscape orientation using an Epson 10000 XL scanner and analyzed using RGB and red color filtering.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
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N. Farah et al. / Physica Medica xxx (2014) 1e5
Figure 4. Superposition of red channel calibration curves for EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons, scanned in landscape orientation in single and multiple scans procedure using an Epson 10000 XL scanner.
Results of multiple scan and single scan approaches are reported in Fig. 4 for the red channel analysis. Differences between electron and photon irradiations are less than 0.01%. Although these discrepancies are insignificant, we recommend a single scan approach for the whole measurements set, which allows for faster analysis and avoids any potential doubts regarding the scan procedure as discussed in Ref. [21] e.g. scan to scan differences, temperature stability and other environmental issues. Scanner type effect In our study, two different scanners are used, the Epson 10000 XL which is commonly dedicated to this kind of analysis in the medical field and a desktop flatbed HP scanner (Scanjet 4850). We illustrate in Fig. 5 a comparison between both scanners. The displayed results are obtained for a landscape orientation and using a red channel filtered analysis. Notwithstanding the significant variations in sensitivity, both considered scanners exhibit good precision for Gafchromic film based dose measurements. Although results show a higher sensitivity of the Epson scanner reaching nearly 2 times the HP sensitivity for doses above 400 cGy, one can use relatively old and cost saving scanners as long as a suitable calibration is performed before use. Conclusion Based on our comparisons for the Gafchromic film dosimetry in the range 0e500 cGy, and supported by the results from Ref. [11] and those obtained by Ref. [12], this study suggests that the EBT3 film is suitable for reflective scanning analysis. Furthermore the results showed no difference between landscape and portrait scan orientation. In fact, the EBT3 properties should be investigated more thoroughly in future studies in order to confirm the reliability of the method described in this paper. Similarly to its predecessor the EBT2, the EBT3 film reaches larger values of optical density for the red channel, thus showing more sensitivity than with the RGB spectrum analysis as revealed previously by Fig. 3. The differences between photon and electron irradiation results can be neglected showing that the EBT3 response is independent of the particle type
Figure 5. Dose response curve for EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons, scanned in landscape orientation using 2 different scanners the EPSON 10000 XL and the HP Scanjet 4850. The analysis is done using a red channel filtering.
in this case as previously confirmed by Ref. [10]. Although the multiple and the single scan procedures did not show any significant difference in the results, we recommend to proceed in a single scan approach as long as the size and the number of the samples still fit within the scanning area. While comparing both scanners, the difference is relatively large between the 2 scanners analysis. This may depend on many parameters, mainly related to the scanner state and its design. Therefore it is impossible to compare results from different scanners. Despite the observed significant variation of sensitivity, the two scanners exhibit similar precision for Gafchromic film measurements. In conclusion, the EBT3 film is suitable for use as a dose control method in the clinical field for different radiation types and different doses. The analyses in reflective mode are reliable, and the use of the red channel filtering leads to more ROD sensitivity therefore to better dose measurement results. For a given system, pertinent methods should be developed as part of the dosimetry protocol during the calibration procedure. Acknowledgments Authors are grateful for the Research Council of the Saint Joseph University for funding this project (Funding FS45), and would like to thank the Department of Radiation-Oncology of the HDF for the technical support. References [1] Butson MJ, Yu PKN, Metcalfe PE. Effects of read-out light sources and ambient light on radiochromic film. Phys Med Biol 1998;43:2407e12. [2] Cheung T, Butson MJ, Yu PKN. Post-irradiation colouration of Gafchromic EBT radiochromic film. Phys Med Biol 2005;50:N281e5. [3] Butson MJ, Yu PKN, Cheung T, Alnawaf H. Energy response of the new EBT2 radiochromic film to X-ray radiation. Radiat Meas 2010;45:836e9. [4] Butson MJ, Cheung T, Yu PKN. Absorption spectra variations of EBT radiochromic film from radiation exposure. Phys Med Biol 2005;50:N135e40. [5] Butson ET, Cheung T, Yu PKN, Butson MJ. Measuring solar UV radiation with EBT radiochromic film. Phys Med Biol 2010;55:N487e93. [6] Martisíková M, Jakel O. Study of Gafchromic EBT film response over a large dose range. Phys Med Biol 2010;55:N281e90.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
N. Farah et al. / Physica Medica xxx (2014) 1e5 [7] Avanzo M, Drigo A, Ren Kaiser S, Roggio A, Sartor G, Chiovati P, et al. Dose to the skin in helical tomotherapy: results of in vivo measurements with radiochromic films. Phys Med 2013;29:304e11. [8] Devic S. Radiochromic film dosimetry: past, present, and future. Phys Med 2011;27:122e34. [9] Hartmann B, Martisíková M, Jäkel O. Technical note: homogeneity of gafchromic EBT2 film. Med Phys 2010;37:1753. [10] Casanova BV, Pasquino M, Russo G, Grosso P, Cante D, Sciacero P, et al. Dosimetric characterization and use of Gafchromic EBT3 film for IMRT dose verification. J Appl Clin Med Phys 2013;14:4111. [11] Massillon-JL G, Chiu-Tsao S-T, Domingo-Muñoz I, Chan MF. Energy dependence of the new Gafchromic EBT3 film: dose response curves for 50 kV, 6 and 15 MV X-ray beams. Int J Med Phys Clin Eng Radiat Oncol 2012;1:60e5. [12] Sorriaux J, Kacperek A, Rossomme S, Lee J, Bertrand D, Vynckier S, et al. Evaluation of Gafchromic EBT3 films characteristics in therapy photon, electron and proton beams. Phys Med 2013;29:599e606. [13] Reinhardt S, Hillbrand M, Wilkens JJ, Assmann W. Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams. Med Phys 2012;39:5257e62. [14] Fuss M, Sturtewagen E, De Wagter C, Georg D. Dosimetric characterization of GafChromic EBT film and its implication on film dosimetry quality assurance. Phys Med Biol 2007;52:4211e25.
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[15] ImageJ. http://imagej.nih.gov/ij/. [16] Richley L, John A, Coomber H, Fletcher S. Evaluation and optimization of the new EBT2 radiochromic film dosimetry system for patient dose verification in radiotherapy. Phys Med Biol 2010;55:2601e17. [17] Ohuchi H. High sensitivity radiochromic film dosimetry using an optical common-mode rejection and a reflective-mode flatbed color scanner. Med Phys 2007;34:4207e12. [18] Alva H, Mercado-Uribe H, Rodriguez-Villafuerte M, Brandan ME. The use of a reflective scanner to study radiochromic film response. Phys Med Biol 2002;47:2925e33. [19] 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: 1942e7. [20] Sorriaux J, Kacperek A, Rossomme S, Lee J, Bertrand D, Vynckier S, et al. Evaluation of Gafchromic EBT3 films characteristics in therapy photon, electron and proton beams. Phys Med 2012;12:S1120e797. [21] Lewis D, Micke A, Yu X, Chan MF. An efficient protocol for radiochromic film dosimetry combining calibration and measurement in a single scan. Med Phys 2012;39:6339e50.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
Contents lists available at ScienceDirect
Physica Medica journal homepage: http://www.physicamedica.com
Technical notes
Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners Nicolas Farah a, b, Ziad Francis a, c, *, Marie Abboud a a
Physics Department, Faculty of Science, Saint Joseph University, B.P. 11-514 Riad El Solh, Beirut 1107 2050, Lebanon Department of Radiation-Oncology, Hôtel-Dieu de France Hospital, PO Box HDF 166830, Beirut, Lebanon c The Open University, Faculty of Science, Department of Physical Sciences, Walton Hall, MK7 6AA Milton Keynes, United Kingdom b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 4 April 2014 Accepted 25 April 2014 Available online xxx
We explore in our study the effects of electrons and X-rays irradiations on the newest version of the Gafchromic EBT3 film. Experiments are performed using the Varian “TrueBeam 1.6” medical accelerator delivering 6 MV X-ray photons and 6 MeV electron beams as desired. The main interest is to compare the responses of EBT3 films exposed to two separate beams of electrons and photons, for radiation doses ranging up to 500 cGy. The analysis is done on a flatbed EPSON 10000 XL scanner and cross checked on a HP Scanjet 4850 scanner. Both scanners are used in reflection mode taking into account landscape and portrait scanning positions. After thorough verifications, the reflective scanning method can be used on EBT3 as an economic alternative to the transmission method which was also one of the goals of this study. A comparison is also done between single scan configuration including all samples in a single A4 (HP) or A3 (EPSON) format area and multiple scan procedure where each sample is scanned separately on its own. The images analyses are done using the ImageJ software. Results show significant influence of the scanning configuration but no significant differences between electron and photon irradiations for both single and multiple scan configurations. In conclusion, the film provides a reliable relative dose measurement method for electrons and photons irradiations in the medical field applications. Ó 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Keywords: Gafchromic EBT3 Film dosimetry Radiotherapy
Introduction In the past years, using radiochromic films for dose assessment went through noticeable progress, and studies were emphasized on this type of measurements in the medical field for radiation dosimetry and radiotherapy control. Gafchromic film irradiation is seen today as a simple and quick method for dose profiling in different irradiation situations. However some properties should be considered for more reliable results taking into account the sensitivity of such type of films to several parameters e.g. radiation type and energy. Studies on the effect of read out light sources and ambient light on previous versions of Gafchromic films, e.g. MD-551 and MD-55-2 [1], showed an interesting color modification after few hours of exposure especially for fluorescent light sources, and
* Corresponding author. Physics Department, Faculty of Science, Saint Joseph University, B.P. 11-514 Riad El Solh, Beirut 1107 2050, Lebanon. Tel.: þ961 1 421 000ext3450; fax: þ961 4 532 657. E-mail addresses: [email protected] (Z. Francis), [email protected] (M. Abboud).
thus care should be taken during film handling and storage. Another study by Ref. [2] revealed a 9% post-irradiation coloration increase of MD-55-2 films up to 20 h after the irradiation, suggesting that films should be scanned for read out at least 6 h after exposure. The properties of EBT Gafchromic films exposed to X-rays radiation were also investigated by other authors e.g. Ref. [3] and the energy response of the EBT2 films was found to be independent of the incident photon energy over a wide range from 15 keV to w2.5 MeV for doses ranging between 25 cGy and 500 cGy. The absorption spectra variations during read out scanning of the EBT film showed a clear increase in net optical density for frequencies around 636 nm in comparison with older versions of Gafchromic films [4]. The improvements applied on the EBT made this type of films a powerful candidate for several applications like measuring solar UV radiation [5], and as a potential dosimeter for ion beams therapy [6], underlining the precision that can be achieved in practical cases like medical tomotherapy [7]. A detailed review by Ref. [8] summarizes the improvement of Gafchromic films through the recent years and underlines the importance of precision in dosimetry for medical applications. However, some performance issues were still reported for the EBT2 version, e.g., the
http://dx.doi.org/10.1016/j.ejmp.2014.04.010 1120-1797/Ó 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
2
N. Farah et al. / Physica Medica xxx (2014) 1e5
inhomogeneity of the film response [9], showing the importance of the repetitive and complementary verification studies that are reported in the literature for the different types of Gafchromic films. In fact, comparisons between different versions of the EBT are of extreme importance, especially when it comes to assessing the improvements that are made for the medical applications dosimetry. Moreover, the analysis protocols that are reported in the literature can differ easily one from another leading to differences in the obtained results. Here comes the importance of reporting as much details as possible when it comes to analysis methods. In our study we use the most recent version of the EBT series namely the EBT3 film. The EBT3 is an improved version of the EBT2 film; it is made by laminating an active layer between two identical polyester layers [10]. Studies published in the literature showed some of the characteristics of the EBT3 e.g., energy dependence of the film reaction after X-rays irradiation [11], particle type dependence [12], specific IMRT applications [10] and comparisons with its predecessor the EBT2 in medical irradiation applications [13]. One of the advantages of the EBT3 is that the analysis is independent from the film scan orientation which was not the case for the EBT2. Here in this work, our interest lies mostly in comparing the response of the films to doses delivered by X-ray and electron beams, and using different analysis configurations with two flatbed reflective scanners. In fact, transmission scanners are more costly than reflective scanners and might be more difficult to acquire in some cases. One of the issues we also wanted to verify here is the reliability of the reflective analysis method comparing a relatively expensive scanner to the results obtained by a cheaper scanner model. Materials and methods Gafchromic film EBT3 The Gafchromic EBT3 (lot number A 01231202, Ashland Specialty Ingredients, NJ, USA) radiochromic film is available in sheets of 20.32 25.4 cm2. It consists of an active layer of w28 mm of thickness coated on both sides by a protective polyester layer of 100 mm. Samples are cut into pieces of 5 6 cm2 before the irradiation. This size order is acceptable and common to several studies in the literature [7,12]; it covers an area that is wide enough to limit statistical uncertainties during irradiation and analysis. Handling the samples from cutting phase to the scanning and folding is done using latex gloves to avoid stains and marks that can affect the scanning results. The samples are folded with care in a dark envelope whenever not in use in order to avoid ambient light and other sources effects during storage. Irradiation procedure The irradiations are accomplished using a Varian “TrueBeam 1.6” accelerator providing electron beams of 6 MeV average energy and photon beams of 6 MV acceleration potential. Relatively low energies are chosen and phantoms are used in order to achieve the best dose absorption rate and to minimize the irradiation time, without altering the quality of the results. The considered configuration is thus compatible with our study since the dose response of the film is independent of the depth in the phantom between 2 and 25 cm [14]. The irradiation field is considered uniform and limited within 10 10 cm2. The films are positioned between two water equivalent phantom layers of polystyrene; the build-up phantom (1.5 cm thickness as in Ref. [4]) is placed between the source field and the irradiated sample and the support phantom (10 cm thickness) laying behind the sample in the beam propagation direction. Additionally, no significant influence is observed
between low and high field sizes ranging from 3 3 cm2 up to 25 25 cm2 [14] on dose response so only the mentioned 10 10 cm2 configuration is adopted. The irradiation procedure is performed for doses ranging from 0 to 500 cGy with a step of 50 cGy for both electron and photon beams. The relative uncertainty of the delivered dose is around 2%; this is the standard conventional value that is clinically used when continuous quality control procedures are implemented using the ionization chamber methods. Scan procedure The irradiated films were folded in dark envelopes at least for 24 h to ensure the darkening effect stabilization. The scanning is performed in landscape and portrait positions using two different scanners: HP scanjet 4850 and EPSON 10000 XL with a resolution of 300 dots per inch (dpi) producing 48 bit RGB colored images in tiff format. When HP scanner is used, five blank scans are performed in order to warm up the lamp of the scanner and the sixth scan is saved for analysis. In the case of Epson 10000 XL scanner, an average of 2e3 preview scans was considered sufficient at the beginning of the scanning procedure to take into account the warm-up effect. According to Ref. [14] the impact of these effects on the results does not exceed 0.1%. We consider two approaches of scanning: (i) a single scan configuration where all the samples are placed according to increasing, decreasing or random dose order from left to right in order to ensure that the position on the scanner bed does not affect the results and then scanned at once fitting in an A4 or A3 format area for the HP and the EPSON scanners respectively and (ii) a ‘multiple scan’ configuration where every sample is placed in the center of the scanner bed and scanned on its own. The comparison between single and multiple scans approaches would show the effect of the scanning phase parameter on the results. As advised by the manufacturer, for the scanning, no filters or color correction functions are applied to raw pixel value results. Scan analysis Image measurement and analysis are performed on a region of interest (ROI) of 480 480 pixels2 around the film samples center. Three values are taken to obtain the mean pixel value with its standard deviation in the ROI. The scanned images are then analyzed using ImageJ [15] software. The RGB and the red component of the color are taken into account [5,16]. The net reflective optical density “net ROD” is calculated using the pixels values from the scanned images according to the following expression
Iu Net ROD ¼ log Ii
(1)
where Iu and Ii are the average pixel values of the reflected intensities through non irradiated and irradiated films respectively [17]. Results and discussion As already mentioned, we consider in our study several analysis configurations. The combination of different setups and approaches would lead to a high number of similar results that would be cumbersome to show at once, thus, for the sake of clarity only the interesting selection is displayed in this section. Furthermore, the error bars on experimental values represent less than 0.1% and they do not appear on the figures as they are smaller than the symbols.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
N. Farah et al. / Physica Medica xxx (2014) 1e5
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Scan orientation comparison Figures 1 and 2 show the landscape and portrait scan orientation comparison for 6 MV photon and 6 MeV electron irradiations respectively, for doses between 0 and 500 cGy. The Epson 10000 XL scanner is used in reflective mode and the analysis is made in 2 different ways taking into account the complete RGB scanned image for one analysis and only the RED channel for the other. The landscape and portrait configurations lead to the same results for the RGB and the Red channel analyses; the differences of the ROD values are below 0.03% for the whole spectrum with an exception reaching 0.07% for doses above 400 cGy for electrons irradiations. Differences of around 0.45%e1.1% were reported in the literature for a dose range reaching up to 10 Gy [12]. In the previous EBT versions (EBT2) the scanning results showed noticeable differences between landscape and portrait scan positions, which is not the case for the EBT3 where the difference is negligible as mentioned by the manufacturer and verified by Ref. [13]. Furthermore, in previous studies on previous EBT films, the red channel showed a higher sensitivity than the RGB [18] and this tendency is still valid for the particular conditions applied in our study.
Particle type and energy dependence In order to verify the effect of the particle type on the EBT3 coloring, samples are irradiated using 2 different particles, photons and electrons. The scan of the films is done in a landscape configuration and a comparison between RGB and red channel analyses is conducted. Figure 3 shows a comparison of the net ROD for electron and photon irradiations. The obtained results show very close results for both radiation types and for the same doses. Discrepancies remain less than 0.07% for doses below 350 cGy and increase slightly up to 0.1% for doses around 500 cGy. These findings are in agreement with previous results from the literature where the EBT3 film was used for IMRT energy range e.g., 6 MVe15 MV and the energy dependence was proven to be negligible [10]. For high energy photon beams generally used in radiotherapy energy dependence of the EBT3 coloring is also weak [11] and the
Figure 1. Dose response of EBT3 Gafchromic film irradiated with 6 MV photon beam and scanned in landscape and portrait configurations using an Epson 10000 XL scanner. The analyses are shown using separately the RGB and the Red channel filtering.
Figure 2. Dose response of EBT3 Gafchromic film irradiated with 6 MeV electrons. The analysis details are the same as for Fig. 1.
dependence on the particle type was also reported to be nonsignificant by Refs. [12] and [19] for the EBT3 predecessors (The EBT2 and the EBT versions). Single versus multiple scan comparison As the results can vary according to the scanning details e.g. scan lamp temperature reference [20] and other parameters, we compare two scan configurations. In a multiple scan approach, each irradiated sample is scanned together with the non-irradiated one consecutively from the lower dose to the higher one, then in a decreasing order of dose and finally in a random order. In all three cases, the analyses of the scanned images yield to the same result. In a single scan approach, all the samples are placed in an A3 format area and scanned at once using the EPSON 10000 XL scanner.
Figure 3. Dose response curve for EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons, scanned in landscape orientation using an Epson 10000 XL scanner and analyzed using RGB and red color filtering.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
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N. Farah et al. / Physica Medica xxx (2014) 1e5
Figure 4. Superposition of red channel calibration curves for EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons, scanned in landscape orientation in single and multiple scans procedure using an Epson 10000 XL scanner.
Results of multiple scan and single scan approaches are reported in Fig. 4 for the red channel analysis. Differences between electron and photon irradiations are less than 0.01%. Although these discrepancies are insignificant, we recommend a single scan approach for the whole measurements set, which allows for faster analysis and avoids any potential doubts regarding the scan procedure as discussed in Ref. [21] e.g. scan to scan differences, temperature stability and other environmental issues. Scanner type effect In our study, two different scanners are used, the Epson 10000 XL which is commonly dedicated to this kind of analysis in the medical field and a desktop flatbed HP scanner (Scanjet 4850). We illustrate in Fig. 5 a comparison between both scanners. The displayed results are obtained for a landscape orientation and using a red channel filtered analysis. Notwithstanding the significant variations in sensitivity, both considered scanners exhibit good precision for Gafchromic film based dose measurements. Although results show a higher sensitivity of the Epson scanner reaching nearly 2 times the HP sensitivity for doses above 400 cGy, one can use relatively old and cost saving scanners as long as a suitable calibration is performed before use. Conclusion Based on our comparisons for the Gafchromic film dosimetry in the range 0e500 cGy, and supported by the results from Ref. [11] and those obtained by Ref. [12], this study suggests that the EBT3 film is suitable for reflective scanning analysis. Furthermore the results showed no difference between landscape and portrait scan orientation. In fact, the EBT3 properties should be investigated more thoroughly in future studies in order to confirm the reliability of the method described in this paper. Similarly to its predecessor the EBT2, the EBT3 film reaches larger values of optical density for the red channel, thus showing more sensitivity than with the RGB spectrum analysis as revealed previously by Fig. 3. The differences between photon and electron irradiation results can be neglected showing that the EBT3 response is independent of the particle type
Figure 5. Dose response curve for EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons, scanned in landscape orientation using 2 different scanners the EPSON 10000 XL and the HP Scanjet 4850. The analysis is done using a red channel filtering.
in this case as previously confirmed by Ref. [10]. Although the multiple and the single scan procedures did not show any significant difference in the results, we recommend to proceed in a single scan approach as long as the size and the number of the samples still fit within the scanning area. While comparing both scanners, the difference is relatively large between the 2 scanners analysis. This may depend on many parameters, mainly related to the scanner state and its design. Therefore it is impossible to compare results from different scanners. Despite the observed significant variation of sensitivity, the two scanners exhibit similar precision for Gafchromic film measurements. In conclusion, the EBT3 film is suitable for use as a dose control method in the clinical field for different radiation types and different doses. The analyses in reflective mode are reliable, and the use of the red channel filtering leads to more ROD sensitivity therefore to better dose measurement results. For a given system, pertinent methods should be developed as part of the dosimetry protocol during the calibration procedure. Acknowledgments Authors are grateful for the Research Council of the Saint Joseph University for funding this project (Funding FS45), and would like to thank the Department of Radiation-Oncology of the HDF for the technical support. References [1] Butson MJ, Yu PKN, Metcalfe PE. Effects of read-out light sources and ambient light on radiochromic film. Phys Med Biol 1998;43:2407e12. [2] Cheung T, Butson MJ, Yu PKN. Post-irradiation colouration of Gafchromic EBT radiochromic film. Phys Med Biol 2005;50:N281e5. [3] Butson MJ, Yu PKN, Cheung T, Alnawaf H. Energy response of the new EBT2 radiochromic film to X-ray radiation. Radiat Meas 2010;45:836e9. [4] Butson MJ, Cheung T, Yu PKN. Absorption spectra variations of EBT radiochromic film from radiation exposure. Phys Med Biol 2005;50:N135e40. [5] Butson ET, Cheung T, Yu PKN, Butson MJ. Measuring solar UV radiation with EBT radiochromic film. Phys Med Biol 2010;55:N487e93. [6] Martisíková M, Jakel O. Study of Gafchromic EBT film response over a large dose range. Phys Med Biol 2010;55:N281e90.
Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010
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Please cite this article in press as: Farah N, et al., Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners, Physica Medica (2014), http://dx.doi.org/10.1016/j.ejmp.2014.04.010