Absolute dosimetric characterization of Gafchromic EBT3 and HDv2 films usingcommercial flat-bed scanners and evaluation of the scanner response functionvariabilityS. N. Chen, M. Gauthier, M. Bazalova-Carter, S. Bolanos, S. Glenzer, R. Riquier, G. Revet, P. Antici, A.Morabito, A. Propp, M. Starodubstev, and J. Fuchs Citation: Review of Scientific Instruments 87, 073301 (2016); doi: 10.1063/1.4954921 View online: http://dx.doi.org/10.1063/1.4954921 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/87/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characterization of GafChromic XR‐RV2 film and comparator strip using a flatbed scanner in reflection mode AIP Conf. Proc. 1310, 106 (2010); 10.1063/1.3531581 Computed tomography dose measurements with radiochromic films and a flatbed scanner Med. Phys. 37, 189 (2010); 10.1118/1.3271584 GafChromic EBT film dosimetry with flatbed CCD scanner: A novel background correction method and fulldose uncertainty analysis Med. Phys. 35, 3094 (2008); 10.1118/1.2938522 Precise radiochromic film dosimetry using a flat-bed document scanner Med. Phys. 32, 2245 (2005); 10.1118/1.1929253 The calibration of experimental self-developing Gafchromic® HXR film for the measurement of radiation dosein computed tomography Med. Phys. 32, 1010 (2005); 10.1118/1.1862802

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REVIEW OF SCIENTIFIC INSTRUMENTS 87, 073301 (2016)

Absolute dosimetric characterization of Gafchromic EBT3 and HDv2 filmsusing commercial flat-bed scanners and evaluation of the scannerresponse function variability

S. N. Chen,1,2 M. Gauthier,3 M. Bazalova-Carter,4 S. Bolanos,1 S. Glenzer,3 R. Riquier,1,5

G. Revet,1,2 P. Antici,6 A. Morabito,7 A. Propp,3 M. Starodubstev,2 and J. Fuchs1,21LULI-CNRS, Ecole Polytechnique, CEA: Universite Paris-Saclay, UPMC Univ Paris 06,Sorbonne Universities, F-91128 Palaiseau Cedex, France2Institute of Applied Physics, 46 Ulyanov Street, 603950 Nizhny Novgorod, Russia3SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA4Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8P 5C2, Canada5CEA, DAM, DIF, F-91297 Arpajon, France6INRS-EMT, Varennes, J3X1S2 Québec, Canada7ELI-ALPS, ELI-HU non profit kft, Dugonics ter 13, H-6720, Szeged, Hungary

(Received 17 March 2016; accepted 7 June 2016; published online 22 July 2016)

Radiochromic films (RCF) are commonly used in dosimetry for a wide range of radiation sources(electrons, protons, and photons) for medical, industrial, and scientific applications. They are multi-layered, which includes plastic substrate layers and sensitive layers that incorporate a radiation-sensitive dye. Quantitative dose can be retrieved by digitizing the film, provided that a prior cali-bration exists. Here, to calibrate the newly developed EBT3 and HDv2 RCFs from Gafchromic™,we used the Stanford Medical LINAC to deposit in the films various doses of 10 MeV photons, andby scanning the films using three independent EPSON Precision 2450 scanners, three independentEPSON V750 scanners, and two independent EPSON 11000XL scanners. The films were scannedin separate RGB channels, as well as in black and white, and film orientation was varied. We foundthat the green channel of the RGB scan and the grayscale channel are in fact quite consistent overthe different models of the scanner, although this comes at the cost of a reduction in sensitivity (bya factor ∼2.5 compared to the red channel). To allow any user to extend the absolute calibrationreported here to any other scanner, we furthermore provide a calibration curve of the EPSON 2450scanner based on absolutely calibrated, commercially available, optical density filters. Published byAIP Publishing. [http://dx.doi.org/10.1063/1.4954921]

INTRODUCTION

Radiochromic films (RCFs)1 are widely used for medical(see Ref. 2 and in particular Sec. V), industrial (food irradia-tion), as well as scientific applications, e.g., in laser-particleacceleration3 and nuclear physics.4 In both diagnostic andtherapeutic medical applications (e.g. in Intensity ModulatedRadiation Therapy or IMRT5), RCFs are used extensively forradiation dosimetry of various types of employed radiation,i.e., photon, electron, and proton. The particular interests ofRCF in this context are the following: (i) it is a passive detector,the color of which is immediately, permanently, and visiblychanged upon irradiation, without the need for any processingin a dedicated (dark) room;2 (ii) being a film, it has an intrinsicvery high spatial resolution; (iii) it is relatively inexpensive;(iv) it has a high sensitivity; (v) its response, which is neartissue-equivalent, is relatively energy-independent6 (except atlow energy7) and stable over time;8,9 (vi) it has a very highsaturation threshold (>kGy, as will be seen below) and thusa large dynamic range; and (vii) it is available in very largeformat, with a uniform response.1 On all these accounts, thenewest generation of Gafchromic™ RCF, namely, the EBT3(having a high sensitivity) and HDv2 (of low sensitivity) filmspresent improved performance compared to the earlier devel-oped RCF. Combined with the development of commercial

automatic analysis products,10 this has made RCF to become acommon tool in dosimetry. RCFs have also been instrumentalin spectral and spatial characterization of short pulse laserproduced ion beams that have an energy range of keV totens of MeV with high particle number.3 Many high energydensity (HED) plasma diagnostic techniques use RCF suchas in proton radiography of dense plasmas11 and magneticfield structures.12 Therefore, for these various communities,calibration of the films using different types of commerciallyavailable, and preferably inexpensive photoscanners is essen-tial. Among these new RCF films that have recently come tomarket, there has been a first calibration of the EBT3 by V.Borca et al. using a EPSON 10000XL scanner,13 but currentlythere is no published calibration of HDv2 that we know of.

As mentioned, several types of RCF by Gafchromic™have been produced over the years.1 Previous types (e.g. HD-810, HS, or MD) were characterized by a transparent (i.e.,spectrally neutral) substrate onto or into which a red-absorbingdye, the one sensitive to incoming radiation, was incorporated.Hence, they appeared blue, with the blue dye becoming darkerin proportion of the dose the films were exposed to. The mainmodification that has been brought to the newest types of RCFis that the substrate is no more neutral, but blue-absorbing,while the dose-sensitive dye remains red-absorbing.15 Hencethese newest films appear yellow when unexposed, and turning

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073301-2 Chen et al. Rev. Sci. Instrum. 87, 073301 (2016)

to dark green in the zones exposed to radiation. The modifiedsubstrate is polarization sensitive,1 where one needs to becautious with film orientation when scanning; for example,this is the case for older versions of the EBT films.1,14 TheEBT3 film11 is a variation on the EBT film family where theinternal layers are symmetric and has a polyester substrate withmicroscopic silica to help prevent Newton’s rings formation,e.g., when laid on the glass window of a flatbed scanner. Also,there is a cross-lamination of the substrate on the two sides ofthe films to eliminate scan orientation effects, i.e., it is globallypolarization insensitive.1,11 It has a sensitivity range of 1 cGyto greater than 40 Gy which makes it ideal for weak beams.The HDv2 film, which uses the same substrate and sensitivedye as the EBT3, is the next generation film that replaces theHD-810 film. It has a sensitivity range from 10 to 1000 Gywhich makes it ideal for high flux beams.

Due to the red-absorbing nature of the dose-sensitive dye,it was deemed15 that measuring the transmission (T) or theoptical density (OD, T = 10−OD) in the red channel of a color(RGB) scan of the films was the most sensitive way to retrieveinformation about the dose the film was exposed to. In thenewest films, since the substrate is blue-absorbing, it wasalso suggested1,21 that using the measurement of the OD inthe blue channel was a way to remove any non-uniformityfrom the substrate (since that measurement should be peculiarto the substrate and not affected by the change in the dyedue to exposition to radiation). Also, it was suggested1,21 thatmeasuring ODred/ODblue locally in each pixel was a way toremove any dependence on the substrate—of course in the limitof lowdose,otherwiseeven theODin thebluechannelbecomesmodified under irradiation, which would foil the technique.Alternatively, scanning in grayscale is possible and was in factalready found14 to lead to a smoother relationship of OD versusincident dose than for any of the color channels of an RGB scan.

Many techniques and procedures,15 either working inreflection or transmission, have been tested and used forreading the RCF: LED-illuminated scanners, laser-illuminatedscanners, or photo-type flatbed scanners. The later type ofscanners utilize an incoherent fluorescent light that can illu-minate the film to be scanned in transmission mode, a linearCCD array measuring the light transmitted through the film.Such photoscanners present the advantage of being the mosteconomical solution for reading the films, while being fast andstill offering high spatial resolution (1600 dpi, or 63 dots/mm,which results in a resolution of 16 µm, in most cases) and16-bits per channel. They are also less noisy than laser-baseddensitometers.16 Calibrations have been published on manytypes of RCF (see Refs. 13 and 17 and references therein),which are of course only valid for a certain model of scannerdue to the variation in the lamps they are equipped with.14

However, calibration curves of some newer models of RCF,like the HDv2, have not been reported yet. Furthermore, thequestion remains of the variation of such a response functiondepending on the peculiar scanner one can use and how it canbe transferred to another scanner model. It has been previouslyshown18 that sophisticated scanners, which are expensivebecause they can scan very large (A3) format, offer only aslight improvement over low-cost scanners (able to scan upto the A4 format) in terms of uniformity and reproducibility;

however, no test across of sensitivity and sensitivity reproduc-ibility was reported.

Since these flatbed scanners are mainstream commercialproducts, produced in various locations and likely equippedwith lamps and photodetectors that are outsourced fromvarious manufacturers at different times, the consistency inthe scanner response function can be compromised. On thecontrary, flatbed scanners are less noisy than densitometersand readily available. We have measured not only the responsefunction of HDv2 and EBT3 models of RCF for various typesof scanners (of the Epson-brand as they are very commonand broadly used in photographic, scientific, and medicalapplications), but also we have done so using several scannersof the same nominal type in order to see the variability of thescanner response function depending on the particular scannerone has. Furthermore, we have also absolutely calibrated theEPSON 2450 scanner using commercially available absorptiveneutral density filters. This allows any user to transpose, usingthe same set of filters, the film calibration curves given in thisarticle to any other scanner.

MATERIALS AND METHODS

The calibration of the EBT3 and the HDv2 films wasperformed at the Stanford Medical LINAC. The films werecut to 2 × 2 cm2 and irradiated with 10 MeV photons with thefollowing doses:

EBT32.2, 5.0, 10.2, 20.2, 50.1, 100.1, 250.2, 500.0, 999.9,

1499.9, 2000.1, 3000.2, 4000.1 cGy,

HDV21, 2, 5, 10, 20, 50, 110, 250, 550 Gy.

The dose was calculated assuming the deposited dose onthe RCF to be similar to human tissue (Mylar) and was cali-brated with an ionization chamber according to the AmericanAssociation of Physicists in Medicine (AAPM) Task Group51 protocol for clinical reference dosimetry of high-energyphoton and electron beams.19 We also used a phantom to makeuniform the effective dose on the RCFs and flatten the filmduring irradiation. The uncertainty in the dose deposited in thefilms was estimated to be 0.2 cGy according to the ionizationchamber. The region of interest (ROI) was a 1.5 × 1.5 cm2

patch in the middle of each film to avoid the edges for the cali-bration points presented below and at least five non-irradiatedfilms were scanned at the same time to give the reference level.The energy deposition is assumed to be uniform over the entireRCF area as the variation of the irradiation was 0.3%.

Presented below are the measured transmission (in termsof OD) of the films from the three different models ofscanners: we used three independent EPSON Precision 2450scanners, three independent EPSON V750 scanners, andtwo independent EPSON 11000XL scanners. The films werescanned in transmission mode, in 48-bits color as well as in16-bits grayscale, in different orientations (as detailed below).Thesettingsof thescannersoftware (providedbyEPSON)wereExposure = −1, Gamma = 1, Highlight = 255, Shadow = 0and the files were saved as TIFF. Note that it is important

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073301-3 Chen et al. Rev. Sci. Instrum. 87, 073301 (2016)

to impose these parameters manually, otherwise the softwarewill optimize the rendering of the scans depending on the filmexposure. We would like to note that all the scanners are locatedin North America and Europe, with the three different types onboth continents. Note also that at least one of each scanner typeis on a different continent compared to the other scanners ofthe same type. Intentionally, we also used scanners that werebought far apart (up to ten years) from each other to test theimpact of the age of the scanner on the results. For each scan,the lamp used for the transmission measurement was allowedto warm up (for half-hour) so that a stable illumination wasreached. The first scans were made more than 48 h after the filmirradiation, afterwhichnegligiblevariationover timeof thefilmresponse, when stored in a light-tight envelope, is observed.11,14

Note that the effect of multiple scans of a given film was alreadyinvestigated,12,14 tofindthat thishadaminimal impact (less than1%) on the measured OD. The small size of the films, whichwere centered on the scanner windows, allows to neglect thewell-known “lateral artifact” that causes the apparent film ODto increase with the distance from the scan axis.20,21

Since the scans have been made at different periods afterexposure, for each scan, the OD of the control (non-irradiated)films, which was scanned as well each time, was removedfrom the OD of the exposed film in order to remove back-ground accumulating over time. Hence, what is presented inhere is ODnet = −log(signalfilm/signalcontrol_film). We did notapply here the robust procedure detailed in Ref. 15 that allowsone to reduce to minimum errors in the determination of thecalibration since our aim is mostly to evaluate, and show, thedifferences that arise in such calibration from using variousscanning methods, as well as various scanners of the samemodels. This procedure basically uses multiple scanning ofthe same films as well as the detection of dead pixels inthe scanner. It should obviously be applied for a careful andprecise calibration at one’s workplace.

Furthermore, the RCFs have been scanned for calibrationin the USA as well as in Europe, and we checked that air travelwas not significantly influencing the films themselves. Thiswas done by sending by airmail (as the calibrated RCF later on)a batch of unexposed films from the USA to Europe, and back totheUSA.Thesewerescannedthree times:before leaving, inEu-rope, and when returning. No significant change, compared tothe noise level, and compared to films of the same batch and thatdid not travel, was found in the films having travelled this way.

RESULTS

First, in Figure 1(a), we present the OD vs. Dose of theGREEN channel of EBT3 scanned in RGB color and in Fig-ure 1(b), we present the OD vs. Dose of the GREEN channelof the HDv2 scanned in RGB. The OD values shown in thefigures are measured by averaging the OD across the entirefilm. A profile across one scan reveals that the variation ofthe OD is 0.3% (similar for all the scanners employed inthis investigation, consistently with Ref. 12), of which can beattributed to the dust on the flatbed scanner, the photon beamnon-uniformities, and digital noise inherent to any scan.

A fitting function of the following can be used to representthe data for the range of dose values that were used in thecalibration only.

EBT3 green channel

OD (Dose) = 0.012 324 + (0.000 529 08 x)+ (−1.1 × 10−7 x2) + (8.5924 × 10−12 x3). (1)

HDv2 green channel

OD (Dose) = 0.008 32 + (0.000 831 x) + (−4.4 × 10−7 x2).(2)

FIG. 1. Optical density versus deposited dose of the GREEN channel of the irradiated (a) EBT3 films with the fitting function of Equation (1) and (b) HDv2films with fitting the function of Equation (2), both scanned in RGB. Note that the width of the fitting functions is only for illustration purposes and does notreflect any quantitative values.

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073301-4 Chen et al. Rev. Sci. Instrum. 87, 073301 (2016)

FIG. 2. Optical density versus deposited dose of the irradiated (a) EBT3 films with the fitting function of Equation (3) and (b) HDv2 films with the fittingfunction of Equation (3), both scanned in the grayscale. Note that the width of the fitting functions is only for illustration purposes and does not reflect anyquantitative values.

Here, we only present only the GREEN channel of thescans in RGB of both types of film. We had found that withthe different software and scan settings that are possible (e.g.Gamma correction, exposure, etc.), and even with no colorcorrection settings mentioned above, the resulting OD of theRED and BLUE channels can vary up to 60%. However,the measurements in the GREEN channel remains consistent(varying by 16% in the various models of scanners we tested)even when scanning, for example, using automatic settingsused by the software of the scanner. Also, we found in the REDchannel an unusual slope change on some scanners (like the

EPSON 11000XL #2), which makes it difficult to scale the datafrom one scanner to another. We also observe that the oldestand most inexpensive (EPSON 2450) scanner model is the oneyielding the least dispersion, in particular for the red channelof the EBT3 film.

Since we observed these variations of response functionfor the RGB channel, we decided to test the scanners’ responsefunctions in grayscale. This is shown in Figure 2 which presentsthe scans of the EBT3 and the HDv2 in grayscale. Although thevariations of the response curve between the various scannersare higher than when using the GREEN channel, the differences

FIG. 3. Optical density versus deposited dose of the irradiated EBT3 and HDv2 films scanned in the grayscale comparing the orientation of the film in thescanner.

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073301-5 Chen et al. Rev. Sci. Instrum. 87, 073301 (2016)

FIG. 4. The optical density of scanning in Black and White (BW) and in color RGB of known absorptive neutral density glass filters by Newport Corporationand Thorlabs as recorded by the scanner EPSON 2450.

within a given model family are only of several percentagepoints. Here, we could test only one scanner of the 11000XLmodel since the RGB and grayscale scans were not performedat the same period, and since the other 11000XL that we used totest its response function in the RGB channel had unfortunatelya mechanical malfunction when testing the grayscale responseof the films. We moreover observe that for the EBT3, there isin fact a very little change in the sensitivity when using thegrayscale compared to using the most sensitive (red or green)channels of the RGB map. For the HDv2, the sensitivity of thegrayscale is comparable to the one of the green channels ofthe RGB, but with a superiority in the reproducibility amongscanners.

A fitting function of the following can be used to representthe data for the range of dose values that were used in thecalibration only.

EBT3 grayscale

OD (Dose) = 0.012 324 + (0.000 55362 x)+ (−1.59 × 10−7 x2) + (1.75 × 10−11 x3). (3)

HDv2 grayscale

OD (Dose) = 0.010 98 + (0.000 9597 x)+ (−6.0413 × 10−7 x2). (4)

Finally, since the EBT3 has been purposely designed tohave a lesser sensitivity to the scanning orientation comparedto the EBT2,11 we have tested the influence of the orientationof the film on the reading of the dose on the film. We per-formed a scan of the films in black and white, and the resultsare presented in Figure 3. We did not observe any differencebetween the two orientations with the EPSON 2540 scanner,but a slight difference with the EPSON v750 scanner, as wasobserved when using the 10000XL scanner.11

In order to allow anyone to transpose the calibrationcurves shown in Figure 3 to any other scanner model, wehave absolutely calibrated the EPSON 2450 scanner. This wasdone by measuring the detected optical density of calibratedabsorptive neutral density filters made by Newport Corpora-

tion and Thorlabs. This measurement is shown in Figure 4.We note here that the slope is not unity, but has a factor of twofor ODNewport/Thorlabs < 3. For ODNewport/Thorlabs > 3, the curvechanges of slope. As mentioned above, for the doses that weare interested in, those used in the calibration, the OD of theRCF films is less than one, hence rests in the linear regime ofthe scanners.

CONCLUSIONS

We have calibrated the EBT3 and HDv2 radiochromic(from Gafchromic) film. This was done for three differentflat-bed scanner models, as well as various scanners of thesame model to test scanner response variability. Absolutedose measurement using commercial flat-bed scanner hasthe advantage of being easy, fast, and low-cost, while alsoallowing to scan large-size films (typically up to A4 format)and exploiting the intrinsic high spatial resolution of the RCF,hence allowing for the determination of steep gradients inradiation deposition. We found that depending on the needs ofindividual medical physicists or researchers in terms of accept-able error bars, etc., calibration of the EBT3 and the HDv2may need to be done for the type of scanner used. With thatsaid, the green channel and grayscale can give results that aresimilar for all of the EPSON scanners surveyed here. The redchannel, which has been recommended to be used for readingthe RCF owing to its optimum sensitivity (the absorbance ofthe radiation-sensitive dye being peaked in the red), is foundto be the less reliable, displaying an extremely large variabilityof the response function for a given scanner model. Of course,using the green and grayscale as the preferred channel comesat the cost of reduced sensitivity. This effect is minimal forthe EBT3, but of a factor ∼2.5 for the HDv2. However, usingthe green or grayscale channels, this comes alongside gainingunmatched reproducibility across various scanners. We alsofound that orientation of the film during scanning gives slightlydifferent results, but all one needs to do is to scan twice inthe two orientations for a more accurate result. In summary,we find that flatbed scanners can be indeed used as a reliable

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073301-6 Chen et al. Rev. Sci. Instrum. 87, 073301 (2016)

absolute dosimetric measurement system, the most reliable,quite a priori unexpectedly, being the most inexpensive one.Finally, it seems that the only advantage of the most expensivescanner model we tested (Epson 11000XL) is its ability toscan film sizes up to A3 format when the other scanners (thesimplest one being cheaper by a factor of several tens) arelimited to scanning up to A4 format.

ACKNOWLEDGMENTS

This project has received funding from the EuropeanUnion’s Horizon 2020 research and innovation programmeunder grant agreement no 654148 Laserlab-Europe. This workwas supported in part by Grant Agreement No. 633053 fromthe EURO fusion consortium and by the Ministry of Educationand Science of the Russian Federation under Contract No.14.Z50.31.0007. This work was supported by the DOE Officeof Science, Fusion Energy Science under FWP No. 100182.And finally, this work was also supported by FRQNT Grant2016-PR-189974 and NSERC Discovery Grant 435416.

1See http://www.aapm.org/meetings/09SS/documents/23Soares-RadiochromicFilm.pdf for an overview of the use of radiochromic films; See http://www.filmqapro.com/Documents/Lewis_Radiochromic_Film_20101020.pdf for general characteristics of radiochromic films and of their use.

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6G. Massillon-JL, S.-T. Chiu-Tsao, I. Domingo-Munoz, and M. F. Chan,“Energy dependence of the new GafChromic EBT3 film: Dose responsecurves for 50 KV, 6 and 15 MV x-ray beams,” Int. J. Med. Phys., Clin. Eng.Radiat. Oncol. 1, 60–65 (2012).

7H. Bekerat et al., “Improving the energy response of external beam therapy(EBT) GafChromicTM dosimetry films at low energies (≤100 keV),” Med.Phys. 41, 022101 (2014).

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10See http://www.filmqapro.com for an analysis software commercially avail-able specifically to scan and analyze exposed RCF film made by Gafchromicusing several types of EPSON made photo scanners.

11A. J. Mackinnon, P. K. Patel, R. P. Town, M. J. Edwards, T. Phillips, S.C. Lerner, D. W. Price, D. Hicks, M. H. Key, S. Hatchett, S. C. Wilks,M. Borghesi, L. Romagnani, S. Kar, T. Toncian, G. Pretzler, O. Willi, M.Koenig, E. Martinolli, S. Lepape, A. Benuzzi-Mounaix, P. Audebert, J. C.Gauthier, J. King, R. Snavely, R. R. Freeman, and T. Boehlly, Rev. Sci.Instrum. 75, 3531 (2004).

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14J. E. Matney, B. C. Parker, D. W. Neck, G. Henkelmann, and I. I. Rosen,“Evaluation of a commercial flatbed document scanner and radiographicfilm scanner for radiochromic EBT film dosimetry,” J. Appl. Clin. Med.Phys. 11(2), 198 (2010).

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