OPTIMIZATION OF A RADIOPHOTOLUMINESCENT GLASS DOSEMETER FOR OCCUPATIONAL EYE LENS DOSIMETRY IN INTERVENTIONAL RADIOLOGY/CARDIOLOGY

OPTIMIZATION OF A RADIOPHOTOLUMINESCENT GLASS DOSEMETER FOR OCCUPATIONAL EYE LENS DOSIMETRY IN... Abstract Hospital based workers that perform interventional radiology are at risk of reaching the eye lens dose limit of 20 mSv/y. These workers are exposed to the radiation scattered by the patient, which creates a complex field, with low radiation energy reaching the eyes of the medical staff from wide angles. Therefore, the dosemeter used in the assessment of the eye lens dose of interventional radiologists needs to respond accurately in such conditions. In this study, the angular response of a commercially available radiophotoluminescent glass dosemeter, GD-352M, was optimized via Monte Carlo simulations, aiming at its use as eye lens dosemeter in interventional radiology. The improved dosemeter was manufactured and then characterized in terms of Hp(3), the quantity recommended for eye lens dosimetry. Its response was compared to the IEC 62387:2012 requirements for Hp(3) and to requirements proposed specifically for eye lens dosemeters used in interventional radiology. The improved dosemeter meets the IEC 62387:2012 requirements for energy and angular response for Hp(3) and also shows good agreement with the more strict requisites proposed for eye lens dosemeters to be used in interventional radiology. INTRODUCTION Since 2011, the International Commission on Radiological Protection has recommended an eye lens dose limit of 20 mSv per year, averaged over 5 years with no single year exceeding 50 mSv(1). This recommendation has been incorporated into the International Basic Safety Standards published in 2014(2) and is a response to studies indicating that lens opacifications and cataracts may occur following exposures to doses lower than the threshold previously established(3, 4). Literature has systematically shown that the dose for the eye lenses of interventional radiologists can be similar or even higher than the new annual limit(5, 6), especially in cardiac interventions(7). Consequently, this occupation imposes a challenge for dosimetry, because the radiation that reaches the eyes of the medical staff has low X-ray energies (20–150 keV) and comes from wide angles(8, 9). For this reason, it is crucial to use a dosemeter that can respond accurately in such exposure conditions. Moreover, because the dosemeter should be used on the head, close to the eyes or together with lead glasses, it should also be small in size, in order to not hinder the user(7). A silver activated phosphate glass detector, commercially available as GD-352M (Dose Ace, AGC Techno Glass, Japan), has shown good dosimetric properties in photons fields in terms of Hp(3), the quantity recommended for eye lens dosimetry(10). However, an undesirable angular dependence along its horizontal orientation for angles higher than 45° has been a limitation on its further application in this field. Therefore, the purpose of the present study is to redesign the filter in order to optimize the angular response of GD-352M, aiming at its use as an eye lens dosemeter in interventional radiology. MATERIALS AND METHODS This study was developed in three steps. First, GD-352M (AGC Techno Glass, Japan) was computationally modeled and its energy and angular response were calculated and validated against experimental data. Second, changes were made on the validated computational model of GD-352M, in order to improve its angular response. Finally, the improved dosemeter was manufactured and characterized experimentally. Details on all these steps are presented in the following sections. Numerical model of the rod glass detector The rod glass detector, GD-352M, of ϕ 1.5 mm × 12.0 mm, composed of 31.55% P, 51.16% O, 6.12% Al, 11.0% Na and 0.17% Ag and density 2.61 g/cm3 was modeled using the Monte Carlo code MCNPX(11). Surrounding the detector, a 0.75 mm thick filter composed of tin and lead (around 10%) was defined. As discussed by Silva et al.(10), this filter is necessary in applications where the dosemeter is exposed to radiation with energies lower than 100 keV, because of the effective atomic number of glass detector (Zeff = 12.04). Detector and filter were enclosed by a 0.5 mm thick ABS plastic holder (acrylonitrile butadiene styrene) of ϕ 4.3 mm × 14.5 mm and density of 1.05 g/cm3. The tally volume was defined within the glass detector, based on the actual readout volume of GD-352M, as shown in Figure 1a. This is determined by the diameter of the incident laser used in the readout process (ϕ 1.0 mm) and the length of the detector exposed to the photomultiplier tube (6.0 mm). In order to compare the simulations with experimental data, the ORAMED proposed cylindrical phantom(12) was also included in the simulations. It was modeled as a cylinder (ϕ 20 cm × 20 cm), with 0.5 cm thick walls made of polymethyl methacrylate (PMMA) and filled with water. For all the simulations, the dosemeters were placed at mid height of the surface of the phantom, which, in turn, remained at 1 m far from the radiation source. Figure 1. View largeDownload slide (a) Illustration of the readout volume of the glass dosemeter, with the reference point (black dot) and the localization of the identification (ID) present in the holder also indicated and (b) dosemeter placed in horizontal and vertical orientation on the ORAMED cylindrical phantom. Figure 1. View largeDownload slide (a) Illustration of the readout volume of the glass dosemeter, with the reference point (black dot) and the localization of the identification (ID) present in the holder also indicated and (b) dosemeter placed in horizontal and vertical orientation on the ORAMED cylindrical phantom. Once GD-352M was modeled, its energy response was simulated for the ISO 4037 X-ray narrow spectrum series (mean energies from 12 up to 250 keV) and in addition for S-Cs (mononergetic, 661 keV)(13). The angular dependence was also simulated, both with the dosemeter placed in horizontal and vertical orientation on the surface of the phantom, as illustrated in Figure 1b. An N-60 beam was used and the response of the dosemeter was assessed at every 15°, from −90° up to 90°. Each angle was calculated separately, keeping the distance from the source to the phantom constant (1 m). The energy deposited in the readout volume was calculated with tally f6 (MeV/g), in kerma approximation for the N-series. This was done because the reading volume is 0.25 mm deep in the detector medium, hence, assuring charge-particle equilibrium. However, for S-Cs, secondary electrons were taken into account in the calculations. The number of particle histories for each calculation was in the order of 108, and statistical uncertainties were kept below 5%. Light efficiency of the detector was considered equal to unity for all energies(14). The response of the dosemeter, RE,α, was evaluated in terms of Hp(3) according to the following equation: RE,α[cts/mSv]=ME,αhp(3;E,α)∗Kα (1) where ME,α is the energy deposited in the dosemeter, hp(3; E, α) is the conversion coefficient from air kerma to Hp(3) for the energy and angles under evaluation(15, 16) and Ka refers to the air kerma. Ka was calculated using the same source parameters and distance as aforementioned, in a volume of air equal to the readout volume of the detector. Finally, RE,α was compared to experimental values obtained as described in a previous study(10). Optimization of the angular response by redesigning the filter GD-352M has good energy response in terms of Hp(3), good linearity and reproducibility(10, 17), besides being small in size. However, its angular response at angles higher than 45° along its horizontal orientation has been a drawback in its implementation as eye lens dosemeter. Therefore, using the validated computational model of GD-352M as a starting point, modifications were performed seeking improvement on its angular response. The material composition and dimensions of the glass detector remained unchanged and ABS plastic was also kept as the material of the holder. Conversely, the mixture of tin and lead of the filter was replaced by pure tin, whose K-edge (29.2 keV) coincides with the mean energy where the strongest absorption in the bare glass detector occurs (N-40, mean energy 33 keV)(10). The major modification occurred in the design of the filter. This was changed until no underestimation of Hp(3) would occur in any angle when irradiated with N-60, tied to the lowest variation in the response along the horizontal orientation, from −90° up to 90°, with respect to 0°. Once the final design was achieved, the energy response for the N-series and S-Cs and the angular response for its vertical and horizontal orientations were calculated. Characterization of the improved dosemeter The improved dosemeter developed with MCNPX was manufactured and then characterized experimentally at the Secondary Standard Dosimetry Laboratory at the Belgian Nuclear Research Center. All characterization was done using the ORAMED cylindrical phantom, at 1 m far from the X-ray source. The energy dependence of the improved dosemeter was assessed for the ISO N-series, S-Cs and IEC RQR series(13, 18). The angular response along the horizontal orientation of the improved dosemeter was evaluated with beams N-40, N-60 and N-80, from -90° up to 90°. These energies were selected based on the typical energies of scattered beams in interventional radiology(9). For the vertical orientation of the dosemeter, only N-60 was used, and the irradiations were done at 0°, 45° and 90°, because the changes in the filter do not affect the angular response of the dosemeter in this orientation. Experimental response of the improved dosemeter was evaluated in terms of Hp(3) using equation (1). Three dosemeters were used in each energy and angle of irradiation and the energy deposited, ME,α, was obtained as the light signal emitted during the readout process, averaged over the three dosemeters irradiated(19). RESULTS Validation of GD-352M modeled with MCNPX The energy response of GD-352M calculated with MCNPX and experimentally measured for the N-series and S-Cs is presented in Figure 2. Very good agreement between both sets of data was obtained from N-20 onwards (mean energy 16 keV). The simulated response at N-15 (mean energy 12 keV) was higher than the experimental data, because at this low energy there might be a lower efficiency in the detector in converting the deposited energy into light. The probability of creation of the luminescence centers depends on the ionization density of the charged particles depositing energy and decreases as the ionization density increases(20). This phenomenon is observed in thermoluminescent and optically stimulated dosemeters, being more important in low X-ray energies, due to the increase of the linear energy transfer (LET)(21, 22) and it has also been reported in glass dosemeters irradiated with charged particles(20). Therefore, while the simulation accounts for the energy deposited in the readout area, it did not consider the variation in the probability of creation of luminescence centers according to the ionization density, which may be the reason for the difference observed in Figure 2. Besides, the thickness of the detector may also play a role at this low energy(23). Figure 2. View largeDownload slide Energy dependence of GD-352M obtained experimentally and via Monte Carlo calculations for the N-series and S-Cs. Figure 2. View largeDownload slide Energy dependence of GD-352M obtained experimentally and via Monte Carlo calculations for the N-series and S-Cs. Figure 3 presents the angular response along the vertical and horizontal orientations of GD-352M for N-60, from both experimental and calculated data. Once again, the two data sets show very good agreement. The difference observed at ±75° along its horizontal orientation might have been either due to the unavoidable uncertainties in the dimensions of the filters present in GD-352M used in the experimental study, or due to the uncertainty of 5° in the determination of the angle during the experimental irradiation. Figure 3. View largeDownload slide Angular dependence of GD-352M irradiated with N-60, obtained experimentally and via Monte Carlo calculations. Figure 3. View largeDownload slide Angular dependence of GD-352M irradiated with N-60, obtained experimentally and via Monte Carlo calculations. The major drawbacks in implementing GD-352M as eye lens dosemeter can be identified in Figure 3 as well: the very high response at ± 90° and the underestimation at ±60° and ±75°, when irradiated along its horizontal orientation. These are caused by the shape of the filters surrounding the detector and have been discussed in a previous study(10). Optimization of the angular response An illustration of the final design of the improved dosemeter is shown in Figure 4, together with the current design of GD-352M. Extra filters were added to the extremities of the dosemeter and the filters along the detector were shortened. Although the improved dosemeter has extra filters at the top and bottom, its length is only 0.5 mm longer than GD-352M, whilst the diameter remained the same. The small dimensions of GD-352M, which were kept in the improved dosemeter, are important specially regarding its applications in eye lens dosimetry, where large dosemeters might disturb the user, since they should be worn close to the eye(7). A detailed drawing with the dimensions of both dosemeters is included in the Appendix. Figure 4. View largeDownload slide (a) Illustration, not to scale, of GD-352M and improved dosemeter and (b) comparison of the size of both dosemeters. Detailed dimensions are presented in the Appendix. Figure 4. View largeDownload slide (a) Illustration, not to scale, of GD-352M and improved dosemeter and (b) comparison of the size of both dosemeters. Detailed dimensions are presented in the Appendix. Validation of the improved dosemeter Energy response The energy response of the improved dosemeter and current GD-352M are presented in Figure 5 for the ISO narrow series and the IEC RQR series. The lack of lead in the filter of the improved dosemeter accounts for the higher response observed around 90 keV. This occurs because the K-edge of lead at 88 keV increases the absorption in the filter of GD-352M around this energy, in comparison to the filter made of pure tin in the improved dosemeter. Furthermore, the shorter length of filters in the improved dosemeter exposes a larger area of the detector to radiation, which also contributes to the higher response, again in comparison to GD-352M. Anyhow, the response of the two dosemeters lies within the IEC 62 387:2012 limits for Hp(3)(24). In the energy range of interest in interventional radiology (20 up to 150 keV), both dosemeters also perform within the limits proposed by Bordy et al.(25). Compared to LiF based dosemeters, which are typically used for eye lens dosimetry, such as the EyeD™, the improved glass dosemeter performs slightly better. Its response varies within ±15% from the unity between 24 and 150 keV, with respect to Cs, whereas the LiF dosemeters have a ±20% variation reported for the same energy range(26, 27). Figure 5. View largeDownload slide Energy response of GD-352M and of the improved dosemeter to the ISO N-series and IEC RQR series. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. Figure 5. View largeDownload slide Energy response of GD-352M and of the improved dosemeter to the ISO N-series and IEC RQR series. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. Due to the fact that producing the filter using pure tin is a cumbersome task, a final dosemeter with the filter composed by a mixture of tin and lead similar to the one in GD-352M and, therefore, with similar energy response, is foreseen. In the energy range of the IEC RQR series, which have very similar physical characteristics to the radiation beam used in interventional procedures(18), the improved dosemeter responds mostly independent of the energy of the incident beam, varying <10% from unity. Angular response The angular response of the improved dosemeter irradiated with N-60, along its horizontal orientation is presented in Figure 6. For comparison, the response of current GD-352M is also shown. Two main improvements were observed: (1) the underestimation between ±60° and ±75° is no longer present, because of the shorter length of the filters surrounding the detector and (2) the high absorption of the bare detector at ±90°, observed in GD-352M, is offset with the filters included at the top and bottom of the dosemeter. Figure 6. View largeDownload slide Angular response of GD-352M and of the improved dosemeter along its horizontal orientation, when irradiated with N-60 (mean energy: 48 keV). Figure 6. View largeDownload slide Angular response of GD-352M and of the improved dosemeter along its horizontal orientation, when irradiated with N-60 (mean energy: 48 keV). Figure 7 presents the angular response of the improved dosemeter when irradiated from −90° up to 90° along its horizontal orientation with N-40, N-60 and N-80 beams. Data is also shown for the response in its vertical orientation, when irradiated at 0°, 45° and 90°, with N-60. Furthermore, the limits established for passive dosemeters by IEC 62 387:2012(24) and those proposed by Bordy et al.(25) for Hp(3) are also included. Figure 7. View largeDownload slide Angular response of the improved dosemeter in its horizontal orientation, when irradiated with N-40, N-60 and N-80. Its response in the vertical orientation is shown for N-60. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. Figure 7. View largeDownload slide Angular response of the improved dosemeter in its horizontal orientation, when irradiated with N-40, N-60 and N-80. Its response in the vertical orientation is shown for N-60. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. In the energy range studied, the improved dosemeter responds within the IEC 62 387:2012 limits. However, at certain angles (90°) and energies (N-60, −45°) it exceeds the more strict limits proposed by Bordy et al.(25). Nevertheless, it is interesting to notice that the improved dosemeter shows similar response both in its horizontal and vertical orientations, with a coefficient of variation lower than 15% for all energies and angles considered in Figure 7. This could be achieved, despite the rod shape of the dosemeter, mostly by redesigning the filters that were already present in GD-352M. Compared to the EyeD™, which has around −15% variation between 0° and 75° for a mean energy of 65 keV(28), with respect to 0°, the improved glass dosemeter has the advantage of not underestimating Hp(3) with a maximum +15% variation for the same parameters. In interventional radiology, the major contribution to the radiation dose received by the medical staff comes from the radiation scattered in the patient(29). Moreover, the projection of the primary beam may change along a single procedure(30). For these reasons, the radiation that reaches the eyes of the medical staff has low energies and comes from a wide range of angles(8, 9). The improved dosemeter proposed in this study is better suited to be used in this demanding context and has shown similar characteristics to other eye lens dosemeters(27, 31). Moreover, because of its small size, it is also suitable to be used in conjunction with lead glasses(32). CONCLUSION In this study the angular response of a commercially available glass dosemeter, GD-352M, was improved aiming at its use as eye lens dosemeter in interventional radiology. The improved dosemeter meets the energy and angular responses required by IEC 62 387:2012 for Hp(3) measurements in photons fields regardless its orientation, without underestimating the dose for energies higher than N-60, and with a maximum 1.5 overestimation. In addition, it also shows good agreement with the more strict requirements proposed by Bordy et al.(25) for eye lens dosimetry. FUNDING This study was partially supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES—Brasília, Brazil (process number BEX 0961/13-2 to Silva E.H.). Ueno S. and Koguchi Y. are employees at Chiyoda Technol Corporation. All the other authors have nothing to disclosure. REFERENCES 1 International Commission on Radiological Protection . 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X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—Part 1: Radiation characteristics and production methods. ISO4037-1, 58 ( 1996 ). 14 Kadoya , N. et al. . Dosimetric properties of radiophotoluminescent glass detector in low-energy photon beams . Med. Phys. 39 ( 10 ), 5910 – 5916 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 15 Behrens , R. Air kerma to Hp(3) conversion coefficients for a new cylinder phantom for photon reference radiation qualities . Radiat. Prot. Dosim. 151 ( 3 ), 450 – 455 ( 2012 ). Google Scholar CrossRef Search ADS 16 Principi , S. , Guardiola , C. , Duch , M. A. and Ginjaume , M. Air kerma to Hp(3) conversion coefficients for IEC 61267 RQR X-RAY radiation qualitites: application to dose monitoring of the lens of the eye in medical diagnostics . Radiat. Prot. Dosim. 170 ( 1–4 ), 45 – 48 ( 2016 ). Google Scholar CrossRef Search ADS 17 Wesolowska , P. 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Estimating head and neck tissue dose from x-ray scatter to physicians performing x-ray guided cardiovascular procedures: a phantom study . J. Radiol. Prot. 37 , 43 – 58 ( 2017 ). Google Scholar CrossRef Search ADS PubMed 30 Cornetto , A. P. , Aimonetto , S. , Pisano , F. , Giudice , M. , Sicuro , M. , Meloni , T. and Tofani , S. The contribution of interventional cardiology procedures to the population radiation dose in a ‘health-care level i’ representative region . Radiat. Prot. Dosim. 168 ( 2 ), 261 – 270 ( 2016 ). 31 Gilvin , P. J. , Baker , S. T. , Gibbens , N. J. , Roberts , G. H. , Tanner , R. J. , Eakins , J. S. , Hager , L. G. and Daniels , T. J. Type testing of a head band dosemeter for measuring eye lens dose in terms of Hp(3) . Radiat. Prot. Dosim. 157 ( 3 ), 430 – 436 ( 2013 ). Google Scholar CrossRef Search ADS 32 Silva , E. H. , da, Struelens , L. , Covens , P. , Ueno , S. , Ube , M. , Vanhavere , F. and Buls , N. Where is the best position to place a dosemeter in order to assess the eye lens dose when lead glasses are used? Radiat. Meas. 106 , 257 – 261 ( 2017 ). Google Scholar CrossRef Search ADS 33 Mori , H. 2015 . AGC Techno Glass, private communication. APPENDIX DIMENSIONS OF GD-352M AND OF THE IMPROVED DOSEMETER Figure A.1. View largeDownload slide Dimensions of GD-352M, in mm(33). Figure A.1. View largeDownload slide Dimensions of GD-352M, in mm(33). Figure A.2. View largeDownload slide MCNPX model of GD-352M. Figure A.2. View largeDownload slide MCNPX model of GD-352M. Figure A.3. View largeDownload slide Dimensions of the improved dosemeter, in mm. Figure A.3. View largeDownload slide Dimensions of the improved dosemeter, in mm. Figure A.4. View largeDownload slide MCNPX model of the improved dosemeter. Figure A.4. View largeDownload slide MCNPX model of the improved dosemeter. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

OPTIMIZATION OF A RADIOPHOTOLUMINESCENT GLASS DOSEMETER FOR OCCUPATIONAL EYE LENS DOSIMETRY IN INTERVENTIONAL RADIOLOGY/CARDIOLOGY

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Abstract

Abstract Hospital based workers that perform interventional radiology are at risk of reaching the eye lens dose limit of 20 mSv/y. These workers are exposed to the radiation scattered by the patient, which creates a complex field, with low radiation energy reaching the eyes of the medical staff from wide angles. Therefore, the dosemeter used in the assessment of the eye lens dose of interventional radiologists needs to respond accurately in such conditions. In this study, the angular response of a commercially available radiophotoluminescent glass dosemeter, GD-352M, was optimized via Monte Carlo simulations, aiming at its use as eye lens dosemeter in interventional radiology. The improved dosemeter was manufactured and then characterized in terms of Hp(3), the quantity recommended for eye lens dosimetry. Its response was compared to the IEC 62387:2012 requirements for Hp(3) and to requirements proposed specifically for eye lens dosemeters used in interventional radiology. The improved dosemeter meets the IEC 62387:2012 requirements for energy and angular response for Hp(3) and also shows good agreement with the more strict requisites proposed for eye lens dosemeters to be used in interventional radiology. INTRODUCTION Since 2011, the International Commission on Radiological Protection has recommended an eye lens dose limit of 20 mSv per year, averaged over 5 years with no single year exceeding 50 mSv(1). This recommendation has been incorporated into the International Basic Safety Standards published in 2014(2) and is a response to studies indicating that lens opacifications and cataracts may occur following exposures to doses lower than the threshold previously established(3, 4). Literature has systematically shown that the dose for the eye lenses of interventional radiologists can be similar or even higher than the new annual limit(5, 6), especially in cardiac interventions(7). Consequently, this occupation imposes a challenge for dosimetry, because the radiation that reaches the eyes of the medical staff has low X-ray energies (20–150 keV) and comes from wide angles(8, 9). For this reason, it is crucial to use a dosemeter that can respond accurately in such exposure conditions. Moreover, because the dosemeter should be used on the head, close to the eyes or together with lead glasses, it should also be small in size, in order to not hinder the user(7). A silver activated phosphate glass detector, commercially available as GD-352M (Dose Ace, AGC Techno Glass, Japan), has shown good dosimetric properties in photons fields in terms of Hp(3), the quantity recommended for eye lens dosimetry(10). However, an undesirable angular dependence along its horizontal orientation for angles higher than 45° has been a limitation on its further application in this field. Therefore, the purpose of the present study is to redesign the filter in order to optimize the angular response of GD-352M, aiming at its use as an eye lens dosemeter in interventional radiology. MATERIALS AND METHODS This study was developed in three steps. First, GD-352M (AGC Techno Glass, Japan) was computationally modeled and its energy and angular response were calculated and validated against experimental data. Second, changes were made on the validated computational model of GD-352M, in order to improve its angular response. Finally, the improved dosemeter was manufactured and characterized experimentally. Details on all these steps are presented in the following sections. Numerical model of the rod glass detector The rod glass detector, GD-352M, of ϕ 1.5 mm × 12.0 mm, composed of 31.55% P, 51.16% O, 6.12% Al, 11.0% Na and 0.17% Ag and density 2.61 g/cm3 was modeled using the Monte Carlo code MCNPX(11). Surrounding the detector, a 0.75 mm thick filter composed of tin and lead (around 10%) was defined. As discussed by Silva et al.(10), this filter is necessary in applications where the dosemeter is exposed to radiation with energies lower than 100 keV, because of the effective atomic number of glass detector (Zeff = 12.04). Detector and filter were enclosed by a 0.5 mm thick ABS plastic holder (acrylonitrile butadiene styrene) of ϕ 4.3 mm × 14.5 mm and density of 1.05 g/cm3. The tally volume was defined within the glass detector, based on the actual readout volume of GD-352M, as shown in Figure 1a. This is determined by the diameter of the incident laser used in the readout process (ϕ 1.0 mm) and the length of the detector exposed to the photomultiplier tube (6.0 mm). In order to compare the simulations with experimental data, the ORAMED proposed cylindrical phantom(12) was also included in the simulations. It was modeled as a cylinder (ϕ 20 cm × 20 cm), with 0.5 cm thick walls made of polymethyl methacrylate (PMMA) and filled with water. For all the simulations, the dosemeters were placed at mid height of the surface of the phantom, which, in turn, remained at 1 m far from the radiation source. Figure 1. View largeDownload slide (a) Illustration of the readout volume of the glass dosemeter, with the reference point (black dot) and the localization of the identification (ID) present in the holder also indicated and (b) dosemeter placed in horizontal and vertical orientation on the ORAMED cylindrical phantom. Figure 1. View largeDownload slide (a) Illustration of the readout volume of the glass dosemeter, with the reference point (black dot) and the localization of the identification (ID) present in the holder also indicated and (b) dosemeter placed in horizontal and vertical orientation on the ORAMED cylindrical phantom. Once GD-352M was modeled, its energy response was simulated for the ISO 4037 X-ray narrow spectrum series (mean energies from 12 up to 250 keV) and in addition for S-Cs (mononergetic, 661 keV)(13). The angular dependence was also simulated, both with the dosemeter placed in horizontal and vertical orientation on the surface of the phantom, as illustrated in Figure 1b. An N-60 beam was used and the response of the dosemeter was assessed at every 15°, from −90° up to 90°. Each angle was calculated separately, keeping the distance from the source to the phantom constant (1 m). The energy deposited in the readout volume was calculated with tally f6 (MeV/g), in kerma approximation for the N-series. This was done because the reading volume is 0.25 mm deep in the detector medium, hence, assuring charge-particle equilibrium. However, for S-Cs, secondary electrons were taken into account in the calculations. The number of particle histories for each calculation was in the order of 108, and statistical uncertainties were kept below 5%. Light efficiency of the detector was considered equal to unity for all energies(14). The response of the dosemeter, RE,α, was evaluated in terms of Hp(3) according to the following equation: RE,α[cts/mSv]=ME,αhp(3;E,α)∗Kα (1) where ME,α is the energy deposited in the dosemeter, hp(3; E, α) is the conversion coefficient from air kerma to Hp(3) for the energy and angles under evaluation(15, 16) and Ka refers to the air kerma. Ka was calculated using the same source parameters and distance as aforementioned, in a volume of air equal to the readout volume of the detector. Finally, RE,α was compared to experimental values obtained as described in a previous study(10). Optimization of the angular response by redesigning the filter GD-352M has good energy response in terms of Hp(3), good linearity and reproducibility(10, 17), besides being small in size. However, its angular response at angles higher than 45° along its horizontal orientation has been a drawback in its implementation as eye lens dosemeter. Therefore, using the validated computational model of GD-352M as a starting point, modifications were performed seeking improvement on its angular response. The material composition and dimensions of the glass detector remained unchanged and ABS plastic was also kept as the material of the holder. Conversely, the mixture of tin and lead of the filter was replaced by pure tin, whose K-edge (29.2 keV) coincides with the mean energy where the strongest absorption in the bare glass detector occurs (N-40, mean energy 33 keV)(10). The major modification occurred in the design of the filter. This was changed until no underestimation of Hp(3) would occur in any angle when irradiated with N-60, tied to the lowest variation in the response along the horizontal orientation, from −90° up to 90°, with respect to 0°. Once the final design was achieved, the energy response for the N-series and S-Cs and the angular response for its vertical and horizontal orientations were calculated. Characterization of the improved dosemeter The improved dosemeter developed with MCNPX was manufactured and then characterized experimentally at the Secondary Standard Dosimetry Laboratory at the Belgian Nuclear Research Center. All characterization was done using the ORAMED cylindrical phantom, at 1 m far from the X-ray source. The energy dependence of the improved dosemeter was assessed for the ISO N-series, S-Cs and IEC RQR series(13, 18). The angular response along the horizontal orientation of the improved dosemeter was evaluated with beams N-40, N-60 and N-80, from -90° up to 90°. These energies were selected based on the typical energies of scattered beams in interventional radiology(9). For the vertical orientation of the dosemeter, only N-60 was used, and the irradiations were done at 0°, 45° and 90°, because the changes in the filter do not affect the angular response of the dosemeter in this orientation. Experimental response of the improved dosemeter was evaluated in terms of Hp(3) using equation (1). Three dosemeters were used in each energy and angle of irradiation and the energy deposited, ME,α, was obtained as the light signal emitted during the readout process, averaged over the three dosemeters irradiated(19). RESULTS Validation of GD-352M modeled with MCNPX The energy response of GD-352M calculated with MCNPX and experimentally measured for the N-series and S-Cs is presented in Figure 2. Very good agreement between both sets of data was obtained from N-20 onwards (mean energy 16 keV). The simulated response at N-15 (mean energy 12 keV) was higher than the experimental data, because at this low energy there might be a lower efficiency in the detector in converting the deposited energy into light. The probability of creation of the luminescence centers depends on the ionization density of the charged particles depositing energy and decreases as the ionization density increases(20). This phenomenon is observed in thermoluminescent and optically stimulated dosemeters, being more important in low X-ray energies, due to the increase of the linear energy transfer (LET)(21, 22) and it has also been reported in glass dosemeters irradiated with charged particles(20). Therefore, while the simulation accounts for the energy deposited in the readout area, it did not consider the variation in the probability of creation of luminescence centers according to the ionization density, which may be the reason for the difference observed in Figure 2. Besides, the thickness of the detector may also play a role at this low energy(23). Figure 2. View largeDownload slide Energy dependence of GD-352M obtained experimentally and via Monte Carlo calculations for the N-series and S-Cs. Figure 2. View largeDownload slide Energy dependence of GD-352M obtained experimentally and via Monte Carlo calculations for the N-series and S-Cs. Figure 3 presents the angular response along the vertical and horizontal orientations of GD-352M for N-60, from both experimental and calculated data. Once again, the two data sets show very good agreement. The difference observed at ±75° along its horizontal orientation might have been either due to the unavoidable uncertainties in the dimensions of the filters present in GD-352M used in the experimental study, or due to the uncertainty of 5° in the determination of the angle during the experimental irradiation. Figure 3. View largeDownload slide Angular dependence of GD-352M irradiated with N-60, obtained experimentally and via Monte Carlo calculations. Figure 3. View largeDownload slide Angular dependence of GD-352M irradiated with N-60, obtained experimentally and via Monte Carlo calculations. The major drawbacks in implementing GD-352M as eye lens dosemeter can be identified in Figure 3 as well: the very high response at ± 90° and the underestimation at ±60° and ±75°, when irradiated along its horizontal orientation. These are caused by the shape of the filters surrounding the detector and have been discussed in a previous study(10). Optimization of the angular response An illustration of the final design of the improved dosemeter is shown in Figure 4, together with the current design of GD-352M. Extra filters were added to the extremities of the dosemeter and the filters along the detector were shortened. Although the improved dosemeter has extra filters at the top and bottom, its length is only 0.5 mm longer than GD-352M, whilst the diameter remained the same. The small dimensions of GD-352M, which were kept in the improved dosemeter, are important specially regarding its applications in eye lens dosimetry, where large dosemeters might disturb the user, since they should be worn close to the eye(7). A detailed drawing with the dimensions of both dosemeters is included in the Appendix. Figure 4. View largeDownload slide (a) Illustration, not to scale, of GD-352M and improved dosemeter and (b) comparison of the size of both dosemeters. Detailed dimensions are presented in the Appendix. Figure 4. View largeDownload slide (a) Illustration, not to scale, of GD-352M and improved dosemeter and (b) comparison of the size of both dosemeters. Detailed dimensions are presented in the Appendix. Validation of the improved dosemeter Energy response The energy response of the improved dosemeter and current GD-352M are presented in Figure 5 for the ISO narrow series and the IEC RQR series. The lack of lead in the filter of the improved dosemeter accounts for the higher response observed around 90 keV. This occurs because the K-edge of lead at 88 keV increases the absorption in the filter of GD-352M around this energy, in comparison to the filter made of pure tin in the improved dosemeter. Furthermore, the shorter length of filters in the improved dosemeter exposes a larger area of the detector to radiation, which also contributes to the higher response, again in comparison to GD-352M. Anyhow, the response of the two dosemeters lies within the IEC 62 387:2012 limits for Hp(3)(24). In the energy range of interest in interventional radiology (20 up to 150 keV), both dosemeters also perform within the limits proposed by Bordy et al.(25). Compared to LiF based dosemeters, which are typically used for eye lens dosimetry, such as the EyeD™, the improved glass dosemeter performs slightly better. Its response varies within ±15% from the unity between 24 and 150 keV, with respect to Cs, whereas the LiF dosemeters have a ±20% variation reported for the same energy range(26, 27). Figure 5. View largeDownload slide Energy response of GD-352M and of the improved dosemeter to the ISO N-series and IEC RQR series. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. Figure 5. View largeDownload slide Energy response of GD-352M and of the improved dosemeter to the ISO N-series and IEC RQR series. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. Due to the fact that producing the filter using pure tin is a cumbersome task, a final dosemeter with the filter composed by a mixture of tin and lead similar to the one in GD-352M and, therefore, with similar energy response, is foreseen. In the energy range of the IEC RQR series, which have very similar physical characteristics to the radiation beam used in interventional procedures(18), the improved dosemeter responds mostly independent of the energy of the incident beam, varying <10% from unity. Angular response The angular response of the improved dosemeter irradiated with N-60, along its horizontal orientation is presented in Figure 6. For comparison, the response of current GD-352M is also shown. Two main improvements were observed: (1) the underestimation between ±60° and ±75° is no longer present, because of the shorter length of the filters surrounding the detector and (2) the high absorption of the bare detector at ±90°, observed in GD-352M, is offset with the filters included at the top and bottom of the dosemeter. Figure 6. View largeDownload slide Angular response of GD-352M and of the improved dosemeter along its horizontal orientation, when irradiated with N-60 (mean energy: 48 keV). Figure 6. View largeDownload slide Angular response of GD-352M and of the improved dosemeter along its horizontal orientation, when irradiated with N-60 (mean energy: 48 keV). Figure 7 presents the angular response of the improved dosemeter when irradiated from −90° up to 90° along its horizontal orientation with N-40, N-60 and N-80 beams. Data is also shown for the response in its vertical orientation, when irradiated at 0°, 45° and 90°, with N-60. Furthermore, the limits established for passive dosemeters by IEC 62 387:2012(24) and those proposed by Bordy et al.(25) for Hp(3) are also included. Figure 7. View largeDownload slide Angular response of the improved dosemeter in its horizontal orientation, when irradiated with N-40, N-60 and N-80. Its response in the vertical orientation is shown for N-60. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. Figure 7. View largeDownload slide Angular response of the improved dosemeter in its horizontal orientation, when irradiated with N-40, N-60 and N-80. Its response in the vertical orientation is shown for N-60. Limits established by IEC 62 387:2012 (dashed line) and proposed by Bordy et al.(25) (solid line) are also included. In the energy range studied, the improved dosemeter responds within the IEC 62 387:2012 limits. However, at certain angles (90°) and energies (N-60, −45°) it exceeds the more strict limits proposed by Bordy et al.(25). Nevertheless, it is interesting to notice that the improved dosemeter shows similar response both in its horizontal and vertical orientations, with a coefficient of variation lower than 15% for all energies and angles considered in Figure 7. This could be achieved, despite the rod shape of the dosemeter, mostly by redesigning the filters that were already present in GD-352M. Compared to the EyeD™, which has around −15% variation between 0° and 75° for a mean energy of 65 keV(28), with respect to 0°, the improved glass dosemeter has the advantage of not underestimating Hp(3) with a maximum +15% variation for the same parameters. In interventional radiology, the major contribution to the radiation dose received by the medical staff comes from the radiation scattered in the patient(29). Moreover, the projection of the primary beam may change along a single procedure(30). For these reasons, the radiation that reaches the eyes of the medical staff has low energies and comes from a wide range of angles(8, 9). The improved dosemeter proposed in this study is better suited to be used in this demanding context and has shown similar characteristics to other eye lens dosemeters(27, 31). Moreover, because of its small size, it is also suitable to be used in conjunction with lead glasses(32). CONCLUSION In this study the angular response of a commercially available glass dosemeter, GD-352M, was improved aiming at its use as eye lens dosemeter in interventional radiology. The improved dosemeter meets the energy and angular responses required by IEC 62 387:2012 for Hp(3) measurements in photons fields regardless its orientation, without underestimating the dose for energies higher than N-60, and with a maximum 1.5 overestimation. In addition, it also shows good agreement with the more strict requirements proposed by Bordy et al.(25) for eye lens dosimetry. FUNDING This study was partially supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES—Brasília, Brazil (process number BEX 0961/13-2 to Silva E.H.). Ueno S. and Koguchi Y. are employees at Chiyoda Technol Corporation. All the other authors have nothing to disclosure. REFERENCES 1 International Commission on Radiological Protection . Statement on tissue reactions ( 2011 ). 2 International Atomic Energy Agency . Radiation protection and safety of radiation sources: international basic safety standards ( 2014 ). 3 Chodick , G. et al. . Risk of cataract after exposure to low doses of ionizing radiation: a 20-year prospective cohort study among US radiologic technologists . Am. J. Epidemiol. 168 , 620 – 631 ( 2008 ). Google Scholar CrossRef Search ADS PubMed 4 Vano , E. , Kleiman , N. J. , Duran , A. , Rehani , M. M. , Echeverri , D. and Cabrera , M. Radiation cataract risk in interventional cardiology personnel . Radiat. Res. 174 ( 4 ), 490 – 495 ( 2010 ). Google Scholar CrossRef Search ADS PubMed 5 Koukorava , C. , Carinou , E. , Ferrari , P. , Krim , S. and Struelens , L. Study of the parameters affecting operator doses in interventional radiology using Monte Carlo simulations . Radiat. Meas. 46 ( 11 ), 1216 – 1222 ( 2011 ). Google Scholar CrossRef Search ADS 6 Efstathopoulos , E. P. , Pantos , I. , Andreou , M. , Gkatzis , A. , Carinou , E. , Koukorava , C. , Kelekis , N. L. and Brountzos , E. Occupational radiation doses to the extremities and the eyes in interventional radiology and cardiology procedures . Br. J. Radiol. 84 , 70 – 77 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 7 Principi , S. , Soler , C. D. , Ginjaume , M. , Vilagrasa , M. B. , Escutia , J. J. R. and Duch , M. A. Eye lens dose in interventional cardiology . Radiat. Prot. Dosim. 165 ( 1–4 ), 289 – 293 ( 2015 ). 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MCNPX™ user's manual, version 2.7.0 ( 2011 ). 12 Gualdrini , G. et al. . A new cylindrical phantom for eye lens dosimetry development . Radiat. Meas. 46 , 1231 – 1234 ( 2011 ). Google Scholar CrossRef Search ADS 13 International Organization for Standardization . X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—Part 1: Radiation characteristics and production methods. ISO4037-1, 58 ( 1996 ). 14 Kadoya , N. et al. . Dosimetric properties of radiophotoluminescent glass detector in low-energy photon beams . Med. Phys. 39 ( 10 ), 5910 – 5916 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 15 Behrens , R. Air kerma to Hp(3) conversion coefficients for a new cylinder phantom for photon reference radiation qualities . Radiat. Prot. Dosim. 151 ( 3 ), 450 – 455 ( 2012 ). Google Scholar CrossRef Search ADS 16 Principi , S. , Guardiola , C. , Duch , M. A. and Ginjaume , M. Air kerma to Hp(3) conversion coefficients for IEC 61267 RQR X-RAY radiation qualitites: application to dose monitoring of the lens of the eye in medical diagnostics . Radiat. Prot. Dosim. 170 ( 1–4 ), 45 – 48 ( 2016 ). Google Scholar CrossRef Search ADS 17 Wesolowska , P. E. , Cole , A. , Santos , T. , Bokulic , T. , Kazantsev , P. and Izewska , J. Characterization of three solid state dosimetry systems for use in high energy photon dosimetry audits in radiotherapy . Radiat. Meas. 106 , 556 – 562 ( 2017 ). Google Scholar CrossRef Search ADS 18 International Electrotechnical Commission . Medical diagnostic X-ray equipment—radiation conditions for use in the determination of characteristics ( 2005 ). 19 AGC Techno Glass . RPL glass dosemeter Dose Ace manual ( 2001 ). 20 Yasuda , H. and Fujitaka , K. Responses of TLD Mg2SiO4:Tb and radiophotoluminescent glass to heavy charged particles and space radiation . Radiat. Prot. Dosim. 87 ( 2 ), 115 – 119 ( 2000 ). Google Scholar CrossRef Search ADS 21 Jahn , A. , Sommer , M. and Henniger , J. OSL efficiency for BeO OSL dosimeters . Radiat. Meas. 71 , 104 – 107 ( 2014 ). Google Scholar CrossRef Search ADS 22 Hranitzky , C. , Stadtmann , H. and Olko , P. Determination of LiF:Mg,Ti and LiF:Mg,Cu,P TL efficiency for X-rays and their application to Monte Carlo simulations of dosemeter response . Radiat. Prot. Dosim. 119 ( 1–4 ), 483 – 486 ( 2006 ). Google Scholar CrossRef Search ADS 23 Davis , S. D. , Ross , C. K. , Mobit , P. N. , Zwan , L. , Van der, Chase , W. J. and Shortt , K. R. The response of lif thermoluminescence dosemeters to photon beams in the energy range from 30 kV x rays to 60Co gamma rays . Radiat. Prot. Dosimetry 106 ( 1 ), 33 – 43 ( 2003 ). Google Scholar CrossRef Search ADS PubMed 24 International Electrotechnical Commission . Radiation protection instrumentation—passive integrating dosimetry systems for personal and environmental monitoring of photon and beta radiation ( 2012 ). 25 Bordy , J. M. , Daures , J. , Denozière , M. , Gualdrini , G. , Ginjaume , M. , Carinou , E. and Vanhavere , F. Proposals for the type tests criteria and calibration conditions of passive eye lens dosemeters to be used in interventional cardiology and radiology workplaces . Radiat. Meas. 46 ( 11 ), 1235 – 1238 ( 2011 ). Google Scholar CrossRef Search ADS 26 Eakins , J. S. , Gilvin , P. J. , Hager , L. G. and Tanner , R. J. Development of an improved dosemeter for assessments of risk to the eye . Radiat. Meas. 71 , 430 – 433 ( 2014 ). Google Scholar CrossRef Search ADS 27 Bilski , P. et al. . The new EYE-D™ dosemeter for measurements of Hp(3) for medical staff . Radiat. Meas. 46 ( 11 ), 1239 – 1242 ( 2011 ). Google Scholar CrossRef Search ADS 28 Borges , F. L. S. , Guimarães , M. C. , Da Silva , T. A. and Nogueira Tavares , M. S. Calibration of an eye lens dosemeter in terms of Hp(3) to be used in interventional radiology . Radiat. Phys. Chem. 104 , 45 – 47 ( 2014 ). Google Scholar CrossRef Search ADS 29 Fetterly , K. A. , Schueler , B. A. , Grams , M. P. and Sturchio , G. M. Estimating head and neck tissue dose from x-ray scatter to physicians performing x-ray guided cardiovascular procedures: a phantom study . J. Radiol. Prot. 37 , 43 – 58 ( 2017 ). Google Scholar CrossRef Search ADS PubMed 30 Cornetto , A. P. , Aimonetto , S. , Pisano , F. , Giudice , M. , Sicuro , M. , Meloni , T. and Tofani , S. The contribution of interventional cardiology procedures to the population radiation dose in a ‘health-care level i’ representative region . Radiat. Prot. Dosim. 168 ( 2 ), 261 – 270 ( 2016 ). 31 Gilvin , P. J. , Baker , S. T. , Gibbens , N. J. , Roberts , G. H. , Tanner , R. J. , Eakins , J. S. , Hager , L. G. and Daniels , T. J. Type testing of a head band dosemeter for measuring eye lens dose in terms of Hp(3) . Radiat. Prot. Dosim. 157 ( 3 ), 430 – 436 ( 2013 ). Google Scholar CrossRef Search ADS 32 Silva , E. H. , da, Struelens , L. , Covens , P. , Ueno , S. , Ube , M. , Vanhavere , F. and Buls , N. Where is the best position to place a dosemeter in order to assess the eye lens dose when lead glasses are used? Radiat. Meas. 106 , 257 – 261 ( 2017 ). Google Scholar CrossRef Search ADS 33 Mori , H. 2015 . AGC Techno Glass, private communication. APPENDIX DIMENSIONS OF GD-352M AND OF THE IMPROVED DOSEMETER Figure A.1. View largeDownload slide Dimensions of GD-352M, in mm(33). Figure A.1. View largeDownload slide Dimensions of GD-352M, in mm(33). Figure A.2. View largeDownload slide MCNPX model of GD-352M. Figure A.2. View largeDownload slide MCNPX model of GD-352M. Figure A.3. View largeDownload slide Dimensions of the improved dosemeter, in mm. Figure A.3. View largeDownload slide Dimensions of the improved dosemeter, in mm. Figure A.4. View largeDownload slide MCNPX model of the improved dosemeter. Figure A.4. View largeDownload slide MCNPX model of the improved dosemeter. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: Mar 23, 2018

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