Radiochromic film calibration for low-energy seed brachytherapy dose measurement Hali Morrison, Geetha Menon, and Ron S. Sloboda Citation: Medical Physics 41, 072101 (2014); doi: 10.1118/1.4881146 View online: http://dx.doi.org/10.1118/1.4881146 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/41/7?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in On the feasibility of polyurethane based 3D dosimeters with optical CT for dosimetric verification of low energy photon brachytherapy seeds Med. Phys. 41, 071705 (2014); 10.1118/1.4883779 Comparison of methods for the measurement of radiation dose distributions in high dose rate (HDR) brachytherapy: Ge-doped optical fiber, EBT3 Gafchromic film, and PRESAGE® radiochromic plastic Med. Phys. 40, 061707 (2013); 10.1118/1.4805100 Response of lithium formate EPR dosimeters at photon energies relevant to the dosimetry of brachytherapy Med. Phys. 37, 4946 (2010); 10.1118/1.3475938 Film dosimetry calibration method for pulsed-dose-rate brachytherapy with an Ir 192 source Med. Phys. 34, 1678 (2007); 10.1118/1.2719366 A novel method of radiochromic film dosimetry using a color scanner Med. Phys. 33, 4085 (2006); 10.1118/1.2357019
Radiochromic film calibration for low-energy seed brachytherapy dose measurement Hali Morrison,a) Geetha Menon, and Ron S. Sloboda Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, University of Alberta, Edmonton, Alberta T6G 2R3, Canada
(Received 17 January 2014; revised 4 May 2014; accepted for publication 12 May 2014; published 9 June 2014) Purpose: Radiochromic film dosimetry is typically performed for high energy photons and moderate doses characterizing external beam radiotherapy (XRT). The purpose of this study was to investigate the accuracy of previously established film calibration procedures used in XRT when applied to low-energy, seed-based brachytherapy at higher doses, and to determine necessary modifications to achieve similar accuracy in absolute dose measurements. Methods: Gafchromic EBT3 film was used to measure radiation doses upwards of 35 Gy from 75 kVp, 200 kVp, 6 MV, and (∼28 keV) I-125 photon sources. For the latter irradiations a custom phantom was built to hold a single I-125 seed. Film pieces were scanned with an Epson 10000XL flatbed scanner and the resulting 48-bit RGB TIFF images were analyzed using both FilmQA Pro software and MATLAB. Calibration curves relating dose and optical density via a rational functional form for all three color channels at each irradiation energy were determined with and without the inclusion of uncertainties in the measured optical densities and dose values. The accuracy of calibration curve variations obtained using piecewise fitting, a reduced film measurement area for I-125 irradiation, and a reduced number of dose levels was also investigated. The energy dependence of the film lot used was also analyzed by calculating normalized optical density values. Results: Slight differences were found in the resulting calibration curves for the various fitting methods used. The accuracy of the calibration curves was found to improve at low doses and worsen at high doses when including uncertainties in optical densities and doses, which may better represent the variability that could be seen in film optical density measurements. When exposing the films to doses > 8 Gy, two-segment piecewise fitting was found to be necessary to achieve similar accuracies in absolute dose measurements as when using smaller dose ranges. When reducing the film measurement area for the I-125 irradiations, the accuracy of the calibration curve was degraded due to the presence of localized film heterogeneities. No degradation in the calibration curves was found when reducing the number of calibration points down to only 4, but with piecewise fitting, 6 calibration points as well as a blank film are required. Variations due to photon energy in film optical density of up to 3% were found above doses of 2 Gy. Conclusions: A modified procedure for performing EBT3 film calibration was established for use with low-energy brachytherapy seeds and high dose exposures. The energy dependence between 6 MV and I-125 photons is significant such that film calibrations should be done with an appropriately low-energy source when performing low-energy brachytherapy dose measurements. Twosegment piecewise fitting with the inclusion of errors in measured optical density as well as dose was found to result in the most accurate calibration curves. Above doses of 1 Gy, absolute dose measurements can be made with an accuracy of 1.6% for 6 MV beams and 5.7% for I-125 seed exposures if using the I-125 source for calibration, or 2.3% if using the 75 kVp photon beam for calibration. © 2014 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4881146] Key words: radiochromic film calibration, EBT3 film, brachytherapy, low-energy dosimetry
1. INTRODUCTION Previous work using Gafchromic film [EBT, EBT2, as well as EBT3 model films (Ashland Specialty Ingredients, Wayne, NJ)] has been primarily focused on its use for intensity modulated radiation therapy (IMRT) plan verification.1–5 Within this context, the film is typically exposed to MV energy photon beams and to moderate doses of 1–7 Gy, but in some cases has been exposed to high doses as well (up to 34 Gy) 072101-1
Med. Phys. 41 (7), July 2014
for stereotactic body radiotherapy.6, 7 The focus of the present work is to investigate the response of EBT3 film to exposures from a low-energy brachytherapy I-125 seeds and to higher doses of at least 35 Gy for dosimetry applications. Gafchromic film has been found to offer many advantages over other dosimetry systems employed for brachytherapy, such as TLD dosimetry which has been extensively described in TG-43 reports,8 mainly due to its availability for 2D dosimetry, and fine submillimetre resolution which is of
0094-2405/2014/41(7)/072101/11/$30.00
© 2014 Am. Assoc. Phys. Med.
072101-1
072101-2
Morrison, Menon, and Sloboda: Radiochromic film calibration for low-energy brachytherapy
particular benefit in small mm-sized regions of steep dose gradients which are observed in intravascular brachytherapy and eye plaque treatments.9 Many different methods have been developed for radiochromic film dosimetry, all of which have strengths and weaknesses. Early work with radiochromic film involved performing a double-exposure of the film, first to a known dose, then to the unknown dose to help correct for film heterogeneities,10 but these types of methods have been found to add noise and uncertainty to the film measurements.11 With subsequent improvements in film heterogeneity, a method now commonly used involves measuring the net optical density (netOD) of the film, and is extensively described by Devic et al.12 Another method which uses the measured scanner pixel values (PVs) directly and does not convert them to netOD has been extensively described by the film manufacturer.4 This method proposes the use of rational functions to relate dose to measured PV, as they were found to better represent the characteristics of the film over a wider dose range. Previous Gafchromic calibration methods using brachytherapy sources have been reported for Ir-192 using piecewise fitting with two different equations for lowand high-dose ranges,13 using the green channel only due to lower uncertainty up to 50 Gy,14 as well as using netODs for I-125 sources up to 35 Gy.9 Performing film calibrations with brachytherapy seeds poses difficult challenges including accurate seed-film placement, relatively low source strength resulting in long exposure times to obtain high doses from a single seed, and much higher uncertainties in radiation output than external beam units. The film manufacturer suggests using large irradiation areas for calibration to reduce the effect of film heterogeneities, which are also difficult to achieve with small brachytherapy sources, and often calibration is performed with a film region as small as 1 × 1 mm2 ,9 which may not accurately represent the average film response. In this work, Gafchromic EBT3 film was exposed to lowenergy photons from an I-125 brachytherapy seed and doses up to ∼35 Gy in order to assess various steps in the film calibration procedure to determine the modifications required to achieve high levels of accuracy when deviating from its typical use in radiotherapy – high energy exposure at moderate doses. The accuracy of the calibration curves over the broader dose range was assessed with and without the inclusion of uncertainty in both the measured optical density of the film as well as the reference dose. Accuracy of the calibration curve with a reduced number of dose levels over a larger dose range was also assessed. This work also establishes a novel film irradiation and measurement method when using brachytherapy seeds to obtain a larger region of constant dose to reduce the effects of film heterogeneities, and makes comparisons with previous methods. The extent of the energy dependence of the film batch used was also determined as it has been found to vary slightly between earlier film models (EBT and EBT2),15, 16 as well as from batch to batch.17–19 Though it is well accepted that the film is nearly energy independent above 100 keV,18 the variation in reported results warrants further investigation, and the energy dependence beMedical Physics, Vol. 41, No. 7, July 2014
072101-2
low an effective irradiation energy of 100 keV was of particular interest. Due to the aforementioned challenges when performing calibrations with brachytherapy seeds, the accuracy of using a low-energy orthovoltage photon beam of similar effective energy was investigated, and from the results of this work, recommendations are made for obtaining accurate film calibrations for low-energy brachytherapy dose measurement over 0–35 Gy range. 2. MATERIALS AND METHODS 2.A. Film R The film used in this study was Gafchromic EBT3 film (lot # A0318303), and is described by the vendor.4 When compared with previous Gafchromic film systems (for example MD-55 film), both contain monomer crystals; however, the organization of the crystals in EBT-type films is more rodor hair-like, which gives them an increased sensitivity (however, this is also what makes scanner responses dependent on film orientation). This difference is a result of the addition of a lithium salt to the pentacosa-10,12-dyinoic acid (PCDA) to form LiPCDA.20 This as well as additional medium atomic number additives make the film have nearly water-equivalent response over a large range of x-ray beam energies (100 keV– 18 MeV), but work by Sutherland et al. suggests EBT-type films have a nonunity intrinsic energy response at low energy (
Radiochromic film calibration for low-energy seed brachytherapy dose measurement Hali Morrison,a) Geetha Menon, and Ron S. Sloboda Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, University of Alberta, Edmonton, Alberta T6G 2R3, Canada
(Received 17 January 2014; revised 4 May 2014; accepted for publication 12 May 2014; published 9 June 2014) Purpose: Radiochromic film dosimetry is typically performed for high energy photons and moderate doses characterizing external beam radiotherapy (XRT). The purpose of this study was to investigate the accuracy of previously established film calibration procedures used in XRT when applied to low-energy, seed-based brachytherapy at higher doses, and to determine necessary modifications to achieve similar accuracy in absolute dose measurements. Methods: Gafchromic EBT3 film was used to measure radiation doses upwards of 35 Gy from 75 kVp, 200 kVp, 6 MV, and (∼28 keV) I-125 photon sources. For the latter irradiations a custom phantom was built to hold a single I-125 seed. Film pieces were scanned with an Epson 10000XL flatbed scanner and the resulting 48-bit RGB TIFF images were analyzed using both FilmQA Pro software and MATLAB. Calibration curves relating dose and optical density via a rational functional form for all three color channels at each irradiation energy were determined with and without the inclusion of uncertainties in the measured optical densities and dose values. The accuracy of calibration curve variations obtained using piecewise fitting, a reduced film measurement area for I-125 irradiation, and a reduced number of dose levels was also investigated. The energy dependence of the film lot used was also analyzed by calculating normalized optical density values. Results: Slight differences were found in the resulting calibration curves for the various fitting methods used. The accuracy of the calibration curves was found to improve at low doses and worsen at high doses when including uncertainties in optical densities and doses, which may better represent the variability that could be seen in film optical density measurements. When exposing the films to doses > 8 Gy, two-segment piecewise fitting was found to be necessary to achieve similar accuracies in absolute dose measurements as when using smaller dose ranges. When reducing the film measurement area for the I-125 irradiations, the accuracy of the calibration curve was degraded due to the presence of localized film heterogeneities. No degradation in the calibration curves was found when reducing the number of calibration points down to only 4, but with piecewise fitting, 6 calibration points as well as a blank film are required. Variations due to photon energy in film optical density of up to 3% were found above doses of 2 Gy. Conclusions: A modified procedure for performing EBT3 film calibration was established for use with low-energy brachytherapy seeds and high dose exposures. The energy dependence between 6 MV and I-125 photons is significant such that film calibrations should be done with an appropriately low-energy source when performing low-energy brachytherapy dose measurements. Twosegment piecewise fitting with the inclusion of errors in measured optical density as well as dose was found to result in the most accurate calibration curves. Above doses of 1 Gy, absolute dose measurements can be made with an accuracy of 1.6% for 6 MV beams and 5.7% for I-125 seed exposures if using the I-125 source for calibration, or 2.3% if using the 75 kVp photon beam for calibration. © 2014 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4881146] Key words: radiochromic film calibration, EBT3 film, brachytherapy, low-energy dosimetry
1. INTRODUCTION Previous work using Gafchromic film [EBT, EBT2, as well as EBT3 model films (Ashland Specialty Ingredients, Wayne, NJ)] has been primarily focused on its use for intensity modulated radiation therapy (IMRT) plan verification.1–5 Within this context, the film is typically exposed to MV energy photon beams and to moderate doses of 1–7 Gy, but in some cases has been exposed to high doses as well (up to 34 Gy) 072101-1
Med. Phys. 41 (7), July 2014
for stereotactic body radiotherapy.6, 7 The focus of the present work is to investigate the response of EBT3 film to exposures from a low-energy brachytherapy I-125 seeds and to higher doses of at least 35 Gy for dosimetry applications. Gafchromic film has been found to offer many advantages over other dosimetry systems employed for brachytherapy, such as TLD dosimetry which has been extensively described in TG-43 reports,8 mainly due to its availability for 2D dosimetry, and fine submillimetre resolution which is of
0094-2405/2014/41(7)/072101/11/$30.00
© 2014 Am. Assoc. Phys. Med.
072101-1
072101-2
Morrison, Menon, and Sloboda: Radiochromic film calibration for low-energy brachytherapy
particular benefit in small mm-sized regions of steep dose gradients which are observed in intravascular brachytherapy and eye plaque treatments.9 Many different methods have been developed for radiochromic film dosimetry, all of which have strengths and weaknesses. Early work with radiochromic film involved performing a double-exposure of the film, first to a known dose, then to the unknown dose to help correct for film heterogeneities,10 but these types of methods have been found to add noise and uncertainty to the film measurements.11 With subsequent improvements in film heterogeneity, a method now commonly used involves measuring the net optical density (netOD) of the film, and is extensively described by Devic et al.12 Another method which uses the measured scanner pixel values (PVs) directly and does not convert them to netOD has been extensively described by the film manufacturer.4 This method proposes the use of rational functions to relate dose to measured PV, as they were found to better represent the characteristics of the film over a wider dose range. Previous Gafchromic calibration methods using brachytherapy sources have been reported for Ir-192 using piecewise fitting with two different equations for lowand high-dose ranges,13 using the green channel only due to lower uncertainty up to 50 Gy,14 as well as using netODs for I-125 sources up to 35 Gy.9 Performing film calibrations with brachytherapy seeds poses difficult challenges including accurate seed-film placement, relatively low source strength resulting in long exposure times to obtain high doses from a single seed, and much higher uncertainties in radiation output than external beam units. The film manufacturer suggests using large irradiation areas for calibration to reduce the effect of film heterogeneities, which are also difficult to achieve with small brachytherapy sources, and often calibration is performed with a film region as small as 1 × 1 mm2 ,9 which may not accurately represent the average film response. In this work, Gafchromic EBT3 film was exposed to lowenergy photons from an I-125 brachytherapy seed and doses up to ∼35 Gy in order to assess various steps in the film calibration procedure to determine the modifications required to achieve high levels of accuracy when deviating from its typical use in radiotherapy – high energy exposure at moderate doses. The accuracy of the calibration curves over the broader dose range was assessed with and without the inclusion of uncertainty in both the measured optical density of the film as well as the reference dose. Accuracy of the calibration curve with a reduced number of dose levels over a larger dose range was also assessed. This work also establishes a novel film irradiation and measurement method when using brachytherapy seeds to obtain a larger region of constant dose to reduce the effects of film heterogeneities, and makes comparisons with previous methods. The extent of the energy dependence of the film batch used was also determined as it has been found to vary slightly between earlier film models (EBT and EBT2),15, 16 as well as from batch to batch.17–19 Though it is well accepted that the film is nearly energy independent above 100 keV,18 the variation in reported results warrants further investigation, and the energy dependence beMedical Physics, Vol. 41, No. 7, July 2014
072101-2
low an effective irradiation energy of 100 keV was of particular interest. Due to the aforementioned challenges when performing calibrations with brachytherapy seeds, the accuracy of using a low-energy orthovoltage photon beam of similar effective energy was investigated, and from the results of this work, recommendations are made for obtaining accurate film calibrations for low-energy brachytherapy dose measurement over 0–35 Gy range. 2. MATERIALS AND METHODS 2.A. Film R The film used in this study was Gafchromic EBT3 film (lot # A0318303), and is described by the vendor.4 When compared with previous Gafchromic film systems (for example MD-55 film), both contain monomer crystals; however, the organization of the crystals in EBT-type films is more rodor hair-like, which gives them an increased sensitivity (however, this is also what makes scanner responses dependent on film orientation). This difference is a result of the addition of a lithium salt to the pentacosa-10,12-dyinoic acid (PCDA) to form LiPCDA.20 This as well as additional medium atomic number additives make the film have nearly water-equivalent response over a large range of x-ray beam energies (100 keV– 18 MeV), but work by Sutherland et al. suggests EBT-type films have a nonunity intrinsic energy response at low energy (
