Applied Radiation and Isotopes 104 (2015) 192–196
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Measurements of 2D distributions of absorbed dose in protontherapy with Gafchromic EBT3 films G. Gambarini a,n, V. Regazzoni a, E. Artuso a, D. Giove a, A. Mirandola b, M. Ciocca b a b
Department of Physics, Università degli Studi di Milano and INFN, Milano, Italy Medical Physics Unit, Centro Nazionale di Adroterapia Oncologica (CNAO), Pavia, Italy
H I G H L I G H T S
EBT3 films were calibrated with a proton pencil beam of 173.61 MeV. In-phantom depth-dose image in the SOBP region was measured with EBT3. A method to compensate for the EBT3 under-response, utilizing the file of irradiation planning data, was tested. The central depth-dose profile extracted from the image was compared with that calculated by the TPS. The inter-comparison of measured and calculated profiles has proven that satisfactory correction can be achieved with the proposed methods.
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2014 Received in revised form 30 June 2015 Accepted 30 June 2015 Available online 3 July 2015
A study of the response of EBT3 films to protons has been carried out with the aim of finding a simple modality to achieve dose images in which the effect of the film sensitivity dependence on radiation LET is amended. Light transmittance images (around 630 nm) were acquired by means of a CCD camera and the difference of optical density was assumed as dosimeter response. The calibration of EBT3 film was performed by means of protons of 173.61 MeV. Some EBT3 films were exposed, in a solid-water phantom, to proton beams of three different energies (89.17 MeV, 110.96 MeV and 130.57 MeV) and the obtained depth-dose profiles were compared with the calculated profiles. From the ratios of calculated and measured Bragg peaks, a trend of the decrease in EBT3 sensitivity with increasing peak depth has been deduced. A method for correcting the data measured with EBT3 films, utilizing the file of irradiation planning data, has been proposed and tested. The results confirm that the method can be advantageously applied for obtaining spatial distribution of the absorbed dose in proton therapy. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Gafchromic EBT3 film Dose imaging Proton therapy SOBP
1. Introduction Radiochromic film dosimeters are frequently used for measurements of planar dose distributions, because they have advantageous properties compared to other two-dimensional detectors, such as diodes, ionization chamber arrays, optical fibers and gel dosimeters (Palmer et al., 2013). Radiochromic films show great potentiality in particular in applications where there is a very high dose gradient and relatively high absorbed dose rates (Soares, 2007). Gafchromic EBT films have had a significant development in recent years and have proven to be advantageous for dosimetry of conventional radiotherapy. In radiotherapy with photons, after some preliminary studies concerning the analysis of the homogeneity of the response between the foils of the same batch and after accurate determination of the dose–response curve, precise n
Corresponding author.
http://dx.doi.org/10.1016/j.apradiso.2015.06.036 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
2D dose images can be achieved, with high spatial resolution. The utilization of EBT films in hadrontherapy is more complex and time consuming, because the response of such detectors to charged particle is energy dependent for low particle energy that is for high linear energy transfer (LET), so it is necessary to make suitable correction of the measured dose images. The problem lies in the fact that the correction procedure is not simple. Various methods have been suggested and investigated to make a proper correction of the dose profiles measured with EBT2 and EBT3 Gafchromic films (Park et al., 2011; Carnicer et al., 2013; Fiorini et al., 2014). The new Gafchromics EBT3 films were recently introduced to replace the previous EBT2 films, of which they are an upgrading. Many peculiarities of EBT2 are still existent in EBT3, as the yellow marker dye, incorporated in the active layer, which produces an absorption peak at 420 nm that might be adopted to amend possible differences in the sensitive layer thickness (Butson et al.,
G. Gambarini et al. / Applied Radiation and Isotopes 104 (2015) 192–196
2009). The support of the active layer is symmetric in EBT3, and this symmetry has brought great advantages. EBT3 films have shown negligible dependence of the response on the incidence direction of radiation and on photon energy, in the energy range of radiotherapies with external beams. Owing to the very advantageous characteristics of Gafchromics EBT3 films, which are near tissue-equivalent and allow achieving high spatial resolution, such dosimeters are of great interest in all conformal radiotherapies based on photons, as standard dosimeters for quality assurance or to perform depth-dose measurements. In the case of protons, the dependence of the dosimeter response on LET becomes critical. This is a typical characteristic of solid-state dosimeters. When protons penetrate in tissue, they lose energy mainly at the end of their path (Bragg peak) where the LET becomes higher and the dosimeter response becomes lower. EBT3 films will be a valid support also for proton therapy dosimetry, when effective methods of correction of the under-response will be implemented. The purpose of this work was to find a method as simple as possible to perform suitable correction of the effect of under-response of radiochromic film for high LET protons. The method has been tested on a depth-dose image obtained with EBT3 film placed in a water-equivalent phantom and irradiated with various energies and intensities designed to obtain a cubic region having homogeneous dose distribution. The correction was performed utilizing the file containing the irradiation data and inferring information about the film under-response from three Bragg peaks.
2. Experimental setup Gafchromics EBT3 films (International Specialty Products, Wajne, NJ, USA) were utilized for the experiment. The EBT3 foils were carefully cut into square or rectangular pieces, depending on the experimental opportunity. All irradiations were performed at room temperature. A study of the response variation during the time after exposure has been carried out, in order to have a check and improvement of data found in literature. The active layer of EBT3 film is the same of the previous EBT2 film, and then the published data about the time evolution of the absorption spectra of EBT2 film after irradiation can be taken into consideration. Studies of postirradiation darkening time dependence of EBT2 films are found in literature (Andrès et al., 2010; Arjomandy et al., 2010). The EBT2 film response has shown to be unstable until 45 min after exposure; nevertheless variations (less than 3%) in optical densities between 2 h and 24 h after irradiation were observed. In this experiment, some EBT3 samples were exposed at different doses and the Δ(OD) of each sample was measured 20, 30 and 37 h after irradiation. No significant difference was recognized between the obtained values. However, in order to avoid any error due to the post-irradiation darkening of the films, all the optical analyses of films were performed 24 h after irradiation. Also the uniformity of response in the different regions of the films was studied, and the results were taken into account to choose properly the samples used in the experiment. 2.1. EBT3 film analysis Optical analysis of films was performed by means of light transmission detection. To this aim, samples were placed on a plane and uniform light source model LLUB (by PHLOXs, Aix en Provence, France). Transmittance images were acquired by means of a charge-coupled device (CCD, model uEye by IDSs, Obersulm, Germany) provided with optical lenses and filters. The EBT3 active component, after exposure to ionizing radiation, generates a blue polymer with two absorption peaks: a main peak at 636 nm and a
193
Fig. 1. Sketch of the instrumentation for EBT3 film optical analysis.
secondary one at 585 nm. The measurements reported in this paper were performed utilizing a band-pass filter peaked around 630 nm, attached to the optical system of the CCD camera. A sketch of the instrument is shown in Fig. 1. For the analysis, EBT samples were placed on the planar uniform white-light source, inside the proper space cut on a black screen settled on the light source, in which also a grey level (GL) reference strip is inserted. This strip gives way to check, and if necessary correct, any variations in the intensity of the light source. The absorbed dose is proportional to the optical absorbance that is obtained by the difference of optical density Δ(OD) between the two transmittance images acquired before and after irradiation. The GL transmittance images were processed by means of a dedicated software developed with the programming environment MATLAB (The MathWorks Inc, Natick MA, USA). The optical density difference is so evaluated:
Δ(OD) = ODaft − ODbef = Log (Ibef /Iaft ) where Ibef and Iaft are the intensities of transmitted light before and after irradiation. The GL of the acquired image is proportional to the transmitted light. Therefore:
Δ(OD) = Log (GL bef /GL aft ) and the obtained Δ(OD) values are converted into dose values by means of calibration data. 2.2. Film irradiation and calibration Exposures were carried out at the synchrotron of the Italian National Center for Oncological Hadron Therapy (CNAO) in Pavia (Italy), where proton beams with energies of 62–227 MeV are provided, whose corresponding Bragg peak depths in water are 3– 32 cm (Rossi, 2011). Films were carefully cut into square or rectangular pieces. For calibration, square pieces with 40 mm of side were prepared. Rectangular samples of 40 150 mm2 were set up for depth-dose measurements. All pieces were marked in order to maintain the correct orientation between irradiation and analysis. For calibration measurements, films were placed over a block of solid water (RW3, PTW, Freiburg, Germany) of 30 30 cm2, with a thickness of 20 cm and covered with a RW3 layer having the same side and a thickness of 19 mm, corresponding to 20 mm of water. The so obtained phantom was placed vertically and exposed to the horizontal beam line, in order to have the incident beam normal to the films. The irradiation setup for film calibration is shown in Fig. 2. Proton irradiation was attained with a proton pencil beam of
194
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Depth-dose profiles and Bragg peaks were reported as a function of depth in the RW3 phantom. Considering the phantom inclination at an angle α ¼3°, the depth is:
DepthRW 3 (d) = d/ cos α. Dose images have been obtained from the Δ(OD) images utilizing the calibration curve. From the so achieved dose images, dose profiles can be extracted. In order to compare the results with those obtained in a water phantom, the DepthRW3(d) values are converted into depth in water by means of the proper coefficient, equal to 1.048, corresponding to the RW3-to-water stopping power ratio.
3. Proposed method and results
Fig. 2. Irradiation setup for EBT3 film calibration with protons.
Fig. 3. Calibration of EBT3 films accomplished with 173.61 MeV protons.
173.61 MeV, properly scanned to obtain a homogeneous dose to a layer of 60 60 mm2 at the depth of EBT3 samples. Films were irradiated at seven different known doses, from 0.25 Gy to 6.87 Gy. For each dose, two films were exposed. The calibration curve was achieved by the averaged Δ(OD) in the central sample region of 10 10 mm2. In Fig. 3, the obtained calibration curve is shown. To carry out depth-dose measurements, the pieces of EBT3 (40 150 mm2) were placed in a RW3 phantom with dimension of 30 30 20 cm3. The phantom was placed on the treatment table and the horizontal proton beam line was used. Considering that the thin (0.28 mm) radiation sensitive component of EBT3 film does not have good tissue equivalence and also to avoid artifacts due to possible thin air gaps, the phantom was settled with a pitch of 3° with respect to the beam direction, obtained by tilting the treatment table itself. The choice of the pitch angle has been made according to the results of the experiment described by Zhao and Das (2010). A sketch of the irradiation configuration is shown in Fig. 4.
Fig. 4. Configuration of EBT3 films exposures for depth-dose measurements.
In order to investigate the Gafchromic EBT3 film response to protons of varying LET, a rectangular EBT3 film piece was placed in the RW3 phantom as shown in Fig. 4 and irradiated with a pencil beam scanning calculated by the treatment planning system (TPS) used at CNAO (Syngo RT Planning, Siemens Medical Solutions, Erlangen, Germany) to achieve a uniform dose in a region having the shape of a cube with 60 mm of side. This region was located between 60 mm and 121 mm in water phantom. Such a SpreadOut-Bragg-Peak (SOBP) was achieved using 31 different proton energies and 571 spot positions. The absorbed dose image has been achieved as described in Section 2 and the central depthdose profile was extracted from that image. As a reference, a dose profile was also extracted from the dose distribution calculated by the TPS. The experimental profile is reported in Fig. 5, together with that calculated by the TPS. As expected, the under-response of EBT3 film for protons with high LET is well evident. The irradiation has been repeated with another identical EBT3 film, and the same results have been obtained. A method has been studied to correct, in a conceptually simple but effective way, the acquired images, by compensating for the effect due to the dependence of the response of radiochromic film from the radiation LET. A starting point is the consideration that in each position of the profile, principally within the SOBP, the measured dose is due to the contributions of proton with different energies. The sensitivity of EBT3 film is lower for the component whose Bragg peak is in the considered position. Therefore, the searched method has been aimed to quantitatively assess the under-response of the film in each specific location, without the time consuming procedure of measuring Bragg peaks for every involved energy. The proposal was to use the data extracted from
Fig. 5. Central dose profile in water extracted from a dose image achieved with an EBT3 film (red) and profile calculated by the Syngo TPS (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
G. Gambarini et al. / Applied Radiation and Isotopes 104 (2015) 192–196
Fig. 6. Dept-dose image obtained with EBT3 film in a RW3 phantom irradiated with protons of 130.57 MeV.
the treatment plan combined with information drawn from a very limited number of Bragg peaks. The idea was to use three energies, corresponding to the minimum, average and maximum of the energies involved in the concerned irradiation. To this aim, EBT3 pieces with the same dimensions have been irradiated, in the same phantom configuration, with monoenergetic proton beams with energy 89.17 MeV, 110.96 MeV and 130.57 MeV. For each energy, the pencil beam was scanned to have a squared homogeneous dose region with 30 mm of side. The depth-dose image obtained with EBT3 film irradiated with 130.57 MeV protons is reported in Fig. 6, where the Bragg peak shape is clearly visible. For each energy, central depth-dose profiles were extracted from the dose images and compared with the profiles obtained, for the same energies, by Monte Carlo (MC) simulations. The FLUKA Monte Carlo code (Battistoni et al., 2007) was used, with the CNAO beam line accurately modeled (Tessonnier et al., 2014). The results are reported in Fig. 7. For each energy and depth, the ratio between the calculated profile and that measured with EBT3 film was evaluated. The obtained ratios, reported in Fig. 8, show a maximum at the Bragg peak depth. These maximum values of the ratio were then linearly correlated to the corresponding depth. In Fig. 9 the ratios versus depth are reported and the result of a linear fit is shown. In order to achieve approximate information of the contribution of high LET protons in every position of the profile, the file containing the irradiation data (energy for each slice and intensity of each spot) has been utilized. For each energy, except the last one, the depth of Bragg peak has been calculated and the averaged
195
Fig. 8. Ratio between the calculated profiles and the profiles obtained with EBT3 films.
Fig. 9. Values of the maximum of each ratio between calculated and measured dose profiles versus the corresponding depth in water phantom and linear fit of such values.
Fig. 10. Function of the averaged intensity of protons giving Bragg peak at a given depth.
Fig. 7. Central depth-dose profiles extracted from the dose images achieved with EBT3 films (red) and profiles obtained, for the same energies, by Monte Carlo simulations (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
intensity (i.e. number of particles) of the 571 spots has been evaluated. The intensity of the higher energy (31st slice) was much higher than the others and then its value has been attributed also to the three channels before the position of the peak. The intensity values have been plotted versus the Bragg peak coordinate and a function has been achieved with a fitting procedure (see Fig. 10). The so found function has been multiplied, pixel to pixel, by the linear function reported in Fig. 9 in order to obtain the averaged under-sensitivity that has to be adjusted. The dose deficit in the measured profile has been deduced multiplying this last function by the measured dose profile achieved with the EBT3 film. The results are reported in Fig. 11. Finally, the obtained deficit of dose has been added to the
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position (Bragg peak). However, the results appear to prove that the approximations adopted are acceptable. The described method can be applied to the whole measured dose image. The reported results relate to a central image region. A check of the reliability of results obtainable in peripheral regions, as well as for more complex and realistic treatment plans, has been planned.
Acknowledgments
Fig. 11. Averaged under-sensitivity (solid blue line) and deficit in the dose profile (dashed red line) measured with EBT3 film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
This work was partially supported by the National Institute of Nuclear Physics (INFN), Italy. The authors would like to thank A. Mairani (CNAO Foundation, Pavia) and G. Magro (University and INFN of Pavia) for providing Monte Carlo simulation data.
References
Fig. 12. Measured dose profiles (red), corrected profile (green) and profile calculated by the TPS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
measured dose profile and the resulting profile has been compared with that calculated by the TPS. In Fig. 12, the three profiles (calculated, measured with EBT3 film and corrected) are shown.
4. Conclusions The results of this study show that satisfactory correction of EBT3 images and profiles can be achieved using the data extracted from the treatment plan combined with information drawn from a very limited number of measured Bragg peaks. The method is based on various approximations, because it uses mediated beam intensities and considers, in each position, only the contribution of the protons that have the maximum release of energy in that
Andrès, C., del Castillo, A., Tortosa, R., Alonso, D., Barquero, R., 2010. A comprehensive study of the Gafchromic EBT2 radiochromic film. A comparison with EBT. Med. Phys. 37, 6271–6278. Arjomandy, B., Tailor, R., Anand, A., Sahoo, N., Gillin, M., Prado, K., Vicic, M., 2010. Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron and proton beam energies. Med. Phys. 37, 1942–1947. Battistoni, G., Muraro, S., Sala, P.R., Cerutti, F., Ferrari, A., Roesler, S., Fassò, A., Ranft, J., 2007. The FLUKA code: description and benchmarking. AIP Conf. Proc. 896, 31–49. Butson, M.J., Cheung, T., Yu, P.K.N., Alnawaf, H., 2009. Dose and absorption spectra response of EBT2 Gafchromic film to high energy x-rays. Australas. Phys. Eng. Sci. Med. 32, 196–202. Carnicer, A., Angellier, G., Gérard, A., Garnier, N., Dubois, C., Amblard, R., Hérault, J., 2013. Development and validation of radiochromic film dosimetry and Monte Carlo simulation tools for acquisition of absolute, high-spatial resolution longitudinal dose distributions in ocular proton therapy. Radiat. Meas. 59, 225–232. Fiorini, F., Kirby, D., Thompson, J., Green, S., Parker, D.J., Jones, B., Hill, M.A., 2014. Under-response correction for EBT3 films in the presence of proton spread out Bragg peaks. Phys. Med. 30, 454–461. Palmer, A.L., Di Pietro, P., Alobaidli, S., Issa, F., Doran, S., Bradley, D., Nisbet, A., 2013. Comparison of methods for the measurement of radiation dose distributions in high dose rate (HDR) brachytherapy: Ge-doped optical fiber, EBT3 Gafchromic film, and PRESAGEs radiochromic plastic. Med. Phys. 40, 061707. Park, S.A., Kwak, J.W., Yoon, M.G., Shin, D.H., Lee, S.B., Cho, K.H., Kang, S.K., Kim, K.J., Bae, H.S., Park, S.Y., 2011. Dose verification of proton beam therapy using the Gafchromic EBT film. Radiat. Meas. 46, 717–721. Rossi, S., 2011. The status of CNAO. Eur. Phys. J. Plus 126, 78. Soares, C.G., 2007. Radiochromic film dosimetry. Radiat. Meas. 41, S100–S116. Tessonnier, T., Mairani, A., Cappucci, F., Mirandola, A., Vilches Freixas, G., Molinelli, S., Ciocca, M., 2014. Development and application of tools for Monte Carlo based simulations in a particle beam radiotherapy facility. Appl. Radiat. Isot. 83, 155–158. Zhao, L., Das, I.J., 2010. Gafchromic EBT film dosimetry in proton beams. Phys. Med. Biol. 55, N291–N301.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Measurements of 2D distributions of absorbed dose in protontherapy with Gafchromic EBT3 films G. Gambarini a,n, V. Regazzoni a, E. Artuso a, D. Giove a, A. Mirandola b, M. Ciocca b a b
Department of Physics, Università degli Studi di Milano and INFN, Milano, Italy Medical Physics Unit, Centro Nazionale di Adroterapia Oncologica (CNAO), Pavia, Italy
H I G H L I G H T S
EBT3 films were calibrated with a proton pencil beam of 173.61 MeV. In-phantom depth-dose image in the SOBP region was measured with EBT3. A method to compensate for the EBT3 under-response, utilizing the file of irradiation planning data, was tested. The central depth-dose profile extracted from the image was compared with that calculated by the TPS. The inter-comparison of measured and calculated profiles has proven that satisfactory correction can be achieved with the proposed methods.
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2014 Received in revised form 30 June 2015 Accepted 30 June 2015 Available online 3 July 2015
A study of the response of EBT3 films to protons has been carried out with the aim of finding a simple modality to achieve dose images in which the effect of the film sensitivity dependence on radiation LET is amended. Light transmittance images (around 630 nm) were acquired by means of a CCD camera and the difference of optical density was assumed as dosimeter response. The calibration of EBT3 film was performed by means of protons of 173.61 MeV. Some EBT3 films were exposed, in a solid-water phantom, to proton beams of three different energies (89.17 MeV, 110.96 MeV and 130.57 MeV) and the obtained depth-dose profiles were compared with the calculated profiles. From the ratios of calculated and measured Bragg peaks, a trend of the decrease in EBT3 sensitivity with increasing peak depth has been deduced. A method for correcting the data measured with EBT3 films, utilizing the file of irradiation planning data, has been proposed and tested. The results confirm that the method can be advantageously applied for obtaining spatial distribution of the absorbed dose in proton therapy. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Gafchromic EBT3 film Dose imaging Proton therapy SOBP
1. Introduction Radiochromic film dosimeters are frequently used for measurements of planar dose distributions, because they have advantageous properties compared to other two-dimensional detectors, such as diodes, ionization chamber arrays, optical fibers and gel dosimeters (Palmer et al., 2013). Radiochromic films show great potentiality in particular in applications where there is a very high dose gradient and relatively high absorbed dose rates (Soares, 2007). Gafchromic EBT films have had a significant development in recent years and have proven to be advantageous for dosimetry of conventional radiotherapy. In radiotherapy with photons, after some preliminary studies concerning the analysis of the homogeneity of the response between the foils of the same batch and after accurate determination of the dose–response curve, precise n
Corresponding author.
http://dx.doi.org/10.1016/j.apradiso.2015.06.036 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
2D dose images can be achieved, with high spatial resolution. The utilization of EBT films in hadrontherapy is more complex and time consuming, because the response of such detectors to charged particle is energy dependent for low particle energy that is for high linear energy transfer (LET), so it is necessary to make suitable correction of the measured dose images. The problem lies in the fact that the correction procedure is not simple. Various methods have been suggested and investigated to make a proper correction of the dose profiles measured with EBT2 and EBT3 Gafchromic films (Park et al., 2011; Carnicer et al., 2013; Fiorini et al., 2014). The new Gafchromics EBT3 films were recently introduced to replace the previous EBT2 films, of which they are an upgrading. Many peculiarities of EBT2 are still existent in EBT3, as the yellow marker dye, incorporated in the active layer, which produces an absorption peak at 420 nm that might be adopted to amend possible differences in the sensitive layer thickness (Butson et al.,
G. Gambarini et al. / Applied Radiation and Isotopes 104 (2015) 192–196
2009). The support of the active layer is symmetric in EBT3, and this symmetry has brought great advantages. EBT3 films have shown negligible dependence of the response on the incidence direction of radiation and on photon energy, in the energy range of radiotherapies with external beams. Owing to the very advantageous characteristics of Gafchromics EBT3 films, which are near tissue-equivalent and allow achieving high spatial resolution, such dosimeters are of great interest in all conformal radiotherapies based on photons, as standard dosimeters for quality assurance or to perform depth-dose measurements. In the case of protons, the dependence of the dosimeter response on LET becomes critical. This is a typical characteristic of solid-state dosimeters. When protons penetrate in tissue, they lose energy mainly at the end of their path (Bragg peak) where the LET becomes higher and the dosimeter response becomes lower. EBT3 films will be a valid support also for proton therapy dosimetry, when effective methods of correction of the under-response will be implemented. The purpose of this work was to find a method as simple as possible to perform suitable correction of the effect of under-response of radiochromic film for high LET protons. The method has been tested on a depth-dose image obtained with EBT3 film placed in a water-equivalent phantom and irradiated with various energies and intensities designed to obtain a cubic region having homogeneous dose distribution. The correction was performed utilizing the file containing the irradiation data and inferring information about the film under-response from three Bragg peaks.
2. Experimental setup Gafchromics EBT3 films (International Specialty Products, Wajne, NJ, USA) were utilized for the experiment. The EBT3 foils were carefully cut into square or rectangular pieces, depending on the experimental opportunity. All irradiations were performed at room temperature. A study of the response variation during the time after exposure has been carried out, in order to have a check and improvement of data found in literature. The active layer of EBT3 film is the same of the previous EBT2 film, and then the published data about the time evolution of the absorption spectra of EBT2 film after irradiation can be taken into consideration. Studies of postirradiation darkening time dependence of EBT2 films are found in literature (Andrès et al., 2010; Arjomandy et al., 2010). The EBT2 film response has shown to be unstable until 45 min after exposure; nevertheless variations (less than 3%) in optical densities between 2 h and 24 h after irradiation were observed. In this experiment, some EBT3 samples were exposed at different doses and the Δ(OD) of each sample was measured 20, 30 and 37 h after irradiation. No significant difference was recognized between the obtained values. However, in order to avoid any error due to the post-irradiation darkening of the films, all the optical analyses of films were performed 24 h after irradiation. Also the uniformity of response in the different regions of the films was studied, and the results were taken into account to choose properly the samples used in the experiment. 2.1. EBT3 film analysis Optical analysis of films was performed by means of light transmission detection. To this aim, samples were placed on a plane and uniform light source model LLUB (by PHLOXs, Aix en Provence, France). Transmittance images were acquired by means of a charge-coupled device (CCD, model uEye by IDSs, Obersulm, Germany) provided with optical lenses and filters. The EBT3 active component, after exposure to ionizing radiation, generates a blue polymer with two absorption peaks: a main peak at 636 nm and a
193
Fig. 1. Sketch of the instrumentation for EBT3 film optical analysis.
secondary one at 585 nm. The measurements reported in this paper were performed utilizing a band-pass filter peaked around 630 nm, attached to the optical system of the CCD camera. A sketch of the instrument is shown in Fig. 1. For the analysis, EBT samples were placed on the planar uniform white-light source, inside the proper space cut on a black screen settled on the light source, in which also a grey level (GL) reference strip is inserted. This strip gives way to check, and if necessary correct, any variations in the intensity of the light source. The absorbed dose is proportional to the optical absorbance that is obtained by the difference of optical density Δ(OD) between the two transmittance images acquired before and after irradiation. The GL transmittance images were processed by means of a dedicated software developed with the programming environment MATLAB (The MathWorks Inc, Natick MA, USA). The optical density difference is so evaluated:
Δ(OD) = ODaft − ODbef = Log (Ibef /Iaft ) where Ibef and Iaft are the intensities of transmitted light before and after irradiation. The GL of the acquired image is proportional to the transmitted light. Therefore:
Δ(OD) = Log (GL bef /GL aft ) and the obtained Δ(OD) values are converted into dose values by means of calibration data. 2.2. Film irradiation and calibration Exposures were carried out at the synchrotron of the Italian National Center for Oncological Hadron Therapy (CNAO) in Pavia (Italy), where proton beams with energies of 62–227 MeV are provided, whose corresponding Bragg peak depths in water are 3– 32 cm (Rossi, 2011). Films were carefully cut into square or rectangular pieces. For calibration, square pieces with 40 mm of side were prepared. Rectangular samples of 40 150 mm2 were set up for depth-dose measurements. All pieces were marked in order to maintain the correct orientation between irradiation and analysis. For calibration measurements, films were placed over a block of solid water (RW3, PTW, Freiburg, Germany) of 30 30 cm2, with a thickness of 20 cm and covered with a RW3 layer having the same side and a thickness of 19 mm, corresponding to 20 mm of water. The so obtained phantom was placed vertically and exposed to the horizontal beam line, in order to have the incident beam normal to the films. The irradiation setup for film calibration is shown in Fig. 2. Proton irradiation was attained with a proton pencil beam of
194
G. Gambarini et al. / Applied Radiation and Isotopes 104 (2015) 192–196
Depth-dose profiles and Bragg peaks were reported as a function of depth in the RW3 phantom. Considering the phantom inclination at an angle α ¼3°, the depth is:
DepthRW 3 (d) = d/ cos α. Dose images have been obtained from the Δ(OD) images utilizing the calibration curve. From the so achieved dose images, dose profiles can be extracted. In order to compare the results with those obtained in a water phantom, the DepthRW3(d) values are converted into depth in water by means of the proper coefficient, equal to 1.048, corresponding to the RW3-to-water stopping power ratio.
3. Proposed method and results
Fig. 2. Irradiation setup for EBT3 film calibration with protons.
Fig. 3. Calibration of EBT3 films accomplished with 173.61 MeV protons.
173.61 MeV, properly scanned to obtain a homogeneous dose to a layer of 60 60 mm2 at the depth of EBT3 samples. Films were irradiated at seven different known doses, from 0.25 Gy to 6.87 Gy. For each dose, two films were exposed. The calibration curve was achieved by the averaged Δ(OD) in the central sample region of 10 10 mm2. In Fig. 3, the obtained calibration curve is shown. To carry out depth-dose measurements, the pieces of EBT3 (40 150 mm2) were placed in a RW3 phantom with dimension of 30 30 20 cm3. The phantom was placed on the treatment table and the horizontal proton beam line was used. Considering that the thin (0.28 mm) radiation sensitive component of EBT3 film does not have good tissue equivalence and also to avoid artifacts due to possible thin air gaps, the phantom was settled with a pitch of 3° with respect to the beam direction, obtained by tilting the treatment table itself. The choice of the pitch angle has been made according to the results of the experiment described by Zhao and Das (2010). A sketch of the irradiation configuration is shown in Fig. 4.
Fig. 4. Configuration of EBT3 films exposures for depth-dose measurements.
In order to investigate the Gafchromic EBT3 film response to protons of varying LET, a rectangular EBT3 film piece was placed in the RW3 phantom as shown in Fig. 4 and irradiated with a pencil beam scanning calculated by the treatment planning system (TPS) used at CNAO (Syngo RT Planning, Siemens Medical Solutions, Erlangen, Germany) to achieve a uniform dose in a region having the shape of a cube with 60 mm of side. This region was located between 60 mm and 121 mm in water phantom. Such a SpreadOut-Bragg-Peak (SOBP) was achieved using 31 different proton energies and 571 spot positions. The absorbed dose image has been achieved as described in Section 2 and the central depthdose profile was extracted from that image. As a reference, a dose profile was also extracted from the dose distribution calculated by the TPS. The experimental profile is reported in Fig. 5, together with that calculated by the TPS. As expected, the under-response of EBT3 film for protons with high LET is well evident. The irradiation has been repeated with another identical EBT3 film, and the same results have been obtained. A method has been studied to correct, in a conceptually simple but effective way, the acquired images, by compensating for the effect due to the dependence of the response of radiochromic film from the radiation LET. A starting point is the consideration that in each position of the profile, principally within the SOBP, the measured dose is due to the contributions of proton with different energies. The sensitivity of EBT3 film is lower for the component whose Bragg peak is in the considered position. Therefore, the searched method has been aimed to quantitatively assess the under-response of the film in each specific location, without the time consuming procedure of measuring Bragg peaks for every involved energy. The proposal was to use the data extracted from
Fig. 5. Central dose profile in water extracted from a dose image achieved with an EBT3 film (red) and profile calculated by the Syngo TPS (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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Fig. 6. Dept-dose image obtained with EBT3 film in a RW3 phantom irradiated with protons of 130.57 MeV.
the treatment plan combined with information drawn from a very limited number of Bragg peaks. The idea was to use three energies, corresponding to the minimum, average and maximum of the energies involved in the concerned irradiation. To this aim, EBT3 pieces with the same dimensions have been irradiated, in the same phantom configuration, with monoenergetic proton beams with energy 89.17 MeV, 110.96 MeV and 130.57 MeV. For each energy, the pencil beam was scanned to have a squared homogeneous dose region with 30 mm of side. The depth-dose image obtained with EBT3 film irradiated with 130.57 MeV protons is reported in Fig. 6, where the Bragg peak shape is clearly visible. For each energy, central depth-dose profiles were extracted from the dose images and compared with the profiles obtained, for the same energies, by Monte Carlo (MC) simulations. The FLUKA Monte Carlo code (Battistoni et al., 2007) was used, with the CNAO beam line accurately modeled (Tessonnier et al., 2014). The results are reported in Fig. 7. For each energy and depth, the ratio between the calculated profile and that measured with EBT3 film was evaluated. The obtained ratios, reported in Fig. 8, show a maximum at the Bragg peak depth. These maximum values of the ratio were then linearly correlated to the corresponding depth. In Fig. 9 the ratios versus depth are reported and the result of a linear fit is shown. In order to achieve approximate information of the contribution of high LET protons in every position of the profile, the file containing the irradiation data (energy for each slice and intensity of each spot) has been utilized. For each energy, except the last one, the depth of Bragg peak has been calculated and the averaged
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Fig. 8. Ratio between the calculated profiles and the profiles obtained with EBT3 films.
Fig. 9. Values of the maximum of each ratio between calculated and measured dose profiles versus the corresponding depth in water phantom and linear fit of such values.
Fig. 10. Function of the averaged intensity of protons giving Bragg peak at a given depth.
Fig. 7. Central depth-dose profiles extracted from the dose images achieved with EBT3 films (red) and profiles obtained, for the same energies, by Monte Carlo simulations (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
intensity (i.e. number of particles) of the 571 spots has been evaluated. The intensity of the higher energy (31st slice) was much higher than the others and then its value has been attributed also to the three channels before the position of the peak. The intensity values have been plotted versus the Bragg peak coordinate and a function has been achieved with a fitting procedure (see Fig. 10). The so found function has been multiplied, pixel to pixel, by the linear function reported in Fig. 9 in order to obtain the averaged under-sensitivity that has to be adjusted. The dose deficit in the measured profile has been deduced multiplying this last function by the measured dose profile achieved with the EBT3 film. The results are reported in Fig. 11. Finally, the obtained deficit of dose has been added to the
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position (Bragg peak). However, the results appear to prove that the approximations adopted are acceptable. The described method can be applied to the whole measured dose image. The reported results relate to a central image region. A check of the reliability of results obtainable in peripheral regions, as well as for more complex and realistic treatment plans, has been planned.
Acknowledgments
Fig. 11. Averaged under-sensitivity (solid blue line) and deficit in the dose profile (dashed red line) measured with EBT3 film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
This work was partially supported by the National Institute of Nuclear Physics (INFN), Italy. The authors would like to thank A. Mairani (CNAO Foundation, Pavia) and G. Magro (University and INFN of Pavia) for providing Monte Carlo simulation data.
References
Fig. 12. Measured dose profiles (red), corrected profile (green) and profile calculated by the TPS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
measured dose profile and the resulting profile has been compared with that calculated by the TPS. In Fig. 12, the three profiles (calculated, measured with EBT3 film and corrected) are shown.
4. Conclusions The results of this study show that satisfactory correction of EBT3 images and profiles can be achieved using the data extracted from the treatment plan combined with information drawn from a very limited number of measured Bragg peaks. The method is based on various approximations, because it uses mediated beam intensities and considers, in each position, only the contribution of the protons that have the maximum release of energy in that
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