- Email: [email protected]
Assessment of the effectiveness of attenuation of Pb aprons by using TLD dosimetry and Monte Carlo calculations
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Assessment of the effectiveness of attenuation of Pb aprons by using TLD dosimetry and Monte Carlo calculations H. Olaya Dávilaa, J.A. Díaz Merchána,b, H.R. Vega Carrilloc, S.A. Martínez Ovallea,b, a b c
⁎
Grupo de Física Nuclear Aplicada y Simulación, Universidad Pedagógica y Tecnológica de Colombia, Tunja, Colombia Clínica Cancerológica de Boyacá, Tunja, Colombia Unidad Académica de Estudios Nucleares de la Universidad Autónoma de Zacatecas, C. Ciprés 10, Fracc. La Peñuela, 98068 Zacatecas, Zac, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords: TLDs Monte Carlo X-rays Linear attenuation coefficients
We developed an experimental set-up by using a continuous emission X-ray (Pantak DXT-3000) and three types of Pb aprons, with thicknesses of 0.25, 0.5, and 0.75 mm, coated with Mylar fiber on their surface. Aprons were placed at a distance of 2.5 m from the focus. Aluminum filtration was performed at the beam output to reproduce the qualities of narrow beams, N40 (Eeffective =33 keV), N80 (Eeffective =65 keV), and N100 (Eeffective =83 keV), according to the ISO standard 4037 (1–3). Each apron was fixed with 10 thermoluminescent dosimeters (TLDs) over its surface, five dosimeters before and five dosimeters after irradiation with X-rays. Dosimeter readings were noted, and the attenuation coefficients for each effective energy were calculated. To confirm the method of effective energy of ISO-4037 and evaluate the effectiveness of aprons according to the energy range required for different medical practices, a Monte Carlo simulation using GEANT4 code was performed. Thus, the fluence and the absorbed dose in each of the dosimeters were determined, and then the coefficients of linear attenuation were calculated and compared with the experimental data and with those reported by the National Institute of Standards and Technology. Results were consistent between theoretical calculations and experimental measures. This work will serve to make assessments for other personalized radiation protectors made of Pb.
1. Introduction Pb-based materials for radiation protection are important in radiodiagnostic procedures to reduce the radiation exposure of workers and, in some cases, in patients aiming to fulfill the ALARA (as low as reasonably achievable) principle (Kazempour et al., 2015). Custommade radiation protectors are commonly made of Pb of less thickness and are used in diagnostic radiology at all medical facilities. Monte Carlo methods are widely used to design protectors against ionizing radiation with two purposes: to reduce the radiation exposure, as part of Occupational Health Program and to allow comfort in wearing the protectors. In the design, long useful life and decreased Pb toxicity are the two desirable goals (Kazempour et al., 2015; Zehtabian et al., 2015). Many studies of X-ray exposure in different medical practices have been reported. Santos et al. (2015) studied the exposure of workers in interventional radiology department by using the MCNPX code and dummy anthropomorphic phantom. Another study used the RANDO phantom where the dose in the skin, with and without Pb apron, was evaluated by digital panoramic X-ray
⁎
radiography, where a significantly increased level was found in the mammary region when Pb apron was not used because the radiations were mainly significantly absorbed by the lungs (Schulze et al., 2016). Pb apron protection is improperly used in Latin American countries because there is no awareness by medical staff about the degree of importance that apron use has from the radiation protection point of view (Valentin, 2007). Pb aprons are not used regularly by radiation workers during their work. In some hospitals, Pb aprons are not used because of their lack of portability, discomfort, and unavailability. The most common case where radiation protection items are not used is in the interventional radiology department. Here, the medical staff rarely uses Pb aprons, thyroid and gonad protectors, and glass sinkers because of the discomfort and limitation in maneuverability during medical intervention. However, conclusive studies have confirmed the importance of using radiation protection items to reduce the radiation exposure of sensitive structures such as the lens of the eye, which decreases the probability of inducing cataractogenesis. (Seals et al., 2016). There are several validation methods to establish the reliability of radiation protectors used by workers. Experimental measurements by
Corresponding author at: Grupo de Física Nuclear Aplicada y Simulación, Universidad Pedagógica y Tecnológica de Colombia, Tunja, Colombia. E-mail address: [email protected] (S.A. Martínez Ovalle).
http://dx.doi.org/10.1016/j.apradiso.2017.05.012 Received 6 December 2016; Received in revised form 5 May 2017; Accepted 9 May 2017 0969-8043/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Dávila, H.O., Applied Radiation and Isotopes (2017), http://dx.doi.org/10.1016/j.apradiso.2017.05.012
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
H.O. Dávila et al.
using thermoluminescent dosimeters (TLDs) are widely used. Häusler et al. (2009) studied the dose received by 39 Medical Physicists at 14 hospitals in Germany during interventional radiology procedures. They found that if protectors are properly used, the radiation exposure does not exceed the annual dose limit. Although the use of TLD is an excellent choice to evaluate the effectiveness of Pb protectors, it is important to note that if experimental measurements could be substantiated with calculations, it is possible to develop theoretical models that underpin any experiment (Kazempour et al., 2015; Zehtabian et al., 2015; Santos, 2015; Matyaginand and Collins, 2016; Tekin and Kara, 2016; Tekin, 2016; Tekin et al., 2017). The purpose of the present study is to evaluate the effectiveness of Pb aprons, on the basis of their thickness, to fulfill the radiation protection needs against X-rays. The work was performed using calculations by using the Geant 4 Monte Carlo code, and the measurements were obtained using TLDs. 1.1. Theoretical basis When a monoenergetic beam of electromagnetic radiation was incident on a homogeneous material of known density and thickness, the relative intensity of the beam subsequent to the processes of absorption and transmission is given as follows:
I = BIo e−μx
(1) −1
where µ is the linear attenuation coefficient expressed in cm , x is the thickness of the material expressed in cm, and B is the buildup factor. Implicitly, it has attenuation coefficient mass µm, given by the relationship:
μm =
μ ρ
(2)
where ρ is the density of the material expressed in g/cm . In monoenergetic bundles, for each kind of energy and material, a corresponding coefficient μm is present. In the case of polyenergetic beams of Xrays, for each spectrum, there is an effective power that corresponds to the attenuation coefficient mass of a given material (Zehtabian et al., 2015). The linear attenuation coefficient µ is directly related to the effective cross-section. The total interaction σ is defined as the probability of related interaction between a collision and the effective area expressed in barn (1 barn =10–24 cm2). The relationship between the linear attenuation coefficient and the effective cross-section is given as follows: 3
μ N = Aσ ρ A
Fig. 1. Geometry of aprons for Monte Carlo calculations using GEANT4 and locations of the TLDs on the surface of aprons.
(Eeffective =33 keV), N-80 (Eeffective =65 keV), and N-100 (Eeffective =83 keV), which were surveyed using a plane parallel ionization chamber with calibration factors for five X-ray qualities having halfvalue layers (HVLs) from 0.20 mm of Al to 1.5 mm of Cu. At the output of radiation beam, porous Al sheets of thicknesses from 0.1 mm were used to establish the first HVL and the second HVL that guarantees the uniformity of the radiation beam. Irradiations in the laboratory were controlled for buildup factor to be close to 1, which was not considered in the simulation. Ten TLD-100 were allocated to different sites on the aprons, as shown in Fig. 1. The TLDs were calibrated using a 137Cs radioactive source, and the TLDs were read using a Harshaw 4500 reader. At each site on the apron, five TLDs were placed on the outer apron surface and five on the inner apron surface to evaluate the apron's X-ray attenuation. Previously, the TLDs were erased on the Harshaw reader. During irradiation, another two TLD-100 were used for background measurements. For each experimental set-up, the aprons were irradiated with Xrays for 5 min. Once the aprons were irradiated, the TLDs were read and corrected by the mean readout value of TLDs that were used for background measurements. According to the ISO 4037 standard, the effective energy of each Xray quality used in this study are 33, 65, and 83 keV for N-40, N-80, and
(3)
The main interaction mechanisms between X-ray photons with matter in medical diagnosis are the photoelectric effect and the Compton scattering. The total cross-section is the sum of cross-sections of both interaction mechanisms:
σTotal = σPhotoelectric + σCompton
(4)
2. Materials and methods 2.1. Measurements For the experimental study, we used three types of Pb aprons having thicknesses of 0.25, 0.50, and 0.75 mm that are currently used in radiodiagnostic procedures. Each Pb apron was located on an Aldersson type anthropomorphic phantom that was sited at 2.5 m from the focus. Phantom was irradiated using a continuous emission X-ray (Pantak DXT-3000) at the Secondary Standard Dosimetry Laboratory in BogotaColombia. Irradiations were performed using X-ray beams N-40 2
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
H.O. Dávila et al.
N-100, respectively. For each beam, the linear attenuation coefficient was calculated using Eq. (5).
1 ⎛I⎞ μ = − ln ⎜ ⎟ x ⎝ Io ⎠
Table 2 TLD readouts, I/Io, and µ-values for Apron 1 irradiated with 83 keV effective energy Xrays.
(5)
Here, µ is the linear attenuation coefficient, I and Io are the background-corrected mean values of TLD readouts of TLDs sited on the outer apron surface (Io) and those allocated on the inner apron surface (I), x is the Pb thickness that was increased by 0.005 cm to consider the Mylar coating present on both the apron surfaces. 2.2. Calculations
3.1. Experimental 3.1.1. Apron 1 The mean readout value of TLDs used for background measurements was 32 nC. For Apron 1 having 0.25-mm-thick Pb when irradiated with N-80 quality X-ray beam (65 keV effective energy), the individual background-corrected TLD readouts for those sited on the frontal (outer) and dorsal (inner) surfaces of the apron, the ratio between I and Io, and the linear attenuation coefficient are presented in Table 1. The average value of the attenuation coefficient was 57.91 ± 2.10 cm−1. According to the NIST, the mass attenuation coefficient (µ/ρ) of Pb for 65 keV photons is 5.02 cm2/g; thus, the linear attenuation coefficient is µ =(4.8 cm2/g).(11.4 g/cm3) =57.57 cm−1. Therefore, the measured µ values are approximately the same as those reported by the NIST. The measured value is 0.6% greater than the NIST µ value, which is less than the relative uncertainty of the measured value (3.6%). Table 2 presents the experimental results for apron 1, having 0.25mm-thick Pb, when irradiated with N-100 quality X-ray beam (83 keV effective energy). The average attenuation coefficient was 26.97 ± 1.0 cm−1. The NIST tables indicate that the mass attenuation coefficient of Pb for 0.083 MeV photons is µ/ρ=2.419 cm2/g. Thus, the linear attenuation
Attenuation coefficient µ (cm−1)
225 246 235 241 230
51 57 54 55 60
22.6 23.1 22.9 22.8 26.1
59.37 58.49 58.82 59.10 53.75
Attenuation coefficient µ (cm−1)
355 360 327 371 362
178 181 172 183 190
50.1 50.2 52.5 49.3 52.4
27.61 27.50 25.70 28.27 25.79
Outer surface of the apron Io (nC)
Inner surface of the apron I (nC)
(I/I0) ratio (%)
Attenuation coefficient µ (cm−1)
354 362 329 375 363
94 94 96 97 95
26.5 25.9 29.1 25.8 26.1
24.11 24.52 22.39 24.59 24.37
Outer surface of the apron Io (nC)
Inner surface of the apron I (nC)
(I/I0) ratio (%)
Attenuation coefficient µ (cm−1)
354 362 329 375 363
48 50 38 45 40
13.5 13.8 11.0 12.0 11.0
25.64 26.39 29.40 28.27 29.40
coefficient is µ =27.576 cm−1, which is 2.2% higher than the measured value. However, this difference is smaller than the uncertainty of the measured value (3.7%). 3.1.2. Apron 2 The experimental results for Apron 2 are presented in Table 2. This apron has 0.55-mm-thick Pb and was irradiated with N-100 quality Xray quality beam (83 keV effective energy). The average value of the attenuation coefficient was 23.99 ± 0.91 cm−1. For 83 keV photons, Pb has a mass attenuation coefficient of 2.228 cm2/g. Therefore, the linear attenuation coefficient is 25.40 cm−1. This value is 5.8% greater than the measured attenuation coefficient, whose uncertainty is 3.7%. The difference between the NIST and the measured µ values is significant and the probable explanation is attributed to the density of Pb used in the experiment (less than 11.40 g/cm3) and loss of Pb thickness because of the fluidity of the material due to frequent use (Table 3). 3.1.3. Apron 3 The experimental results for Apron 3 are presented in Table 4. This apron has 0.75-mm-thick Pb and was irradiated with N-100 quality Xray beam (83 keV effective energy). The average value of the attenuation coefficient was 27.82 ± 1.73 cm−1. For 83 keV photons, Pb has a mass attenuation coefficient of 2.228 cm2/g. Therefore, the linear attenuation coefficient is 25.40 cm−1. This value is 8.6% greater than the measured attenuation coefficient, whose uncertainty is 6.20%. The difference between the NIST and the measured µ values is small. The high uncertainty is because intensity values recorded by the TLDs after the attenuation are slightly higher than the background values.
Table 1 TLD readouts, I/Io, and µ-values for Apron 1 irradiated with 65 keV effective energy Xrays. (I/I0) ratio (%)
(I/I0) ratio (%)
Table 4 TLD readouts, I/Io, and µ-values for Apron 3 irradiated with 83 keV effective energy Xrays.
3. Results and discussion
Inner surface of the apron I (nC)
Inner surface of the apron I (nC)
Table 3 TLD readouts, I/Io, and µ-values for Apron 2 irradiated with 83 keV effective energy Xrays.
By using the Monte Carlo code GEANT4 (Agostinelli et al., 2003), a model including each TLD, the apron, and their coating was designed. The TLDs were modeled at the same positions where TLDs were sited during the measurements, as shown in Fig. 1. The source was set 2.5 m from the apron, and it was modeled as a cone beam with the vertex at the focus. The X-ray beam was modeled as a monoenergetic beam with the same energy as that of the effective energy of the X-ray tube, i.e., 33, 65, and 83 keV. To have good Monte Carlo statistics, 2.0×109 stories in each case were cluster parallelized, and information was analyzed using ROOT C++ toolkit. X-ray fluence on the outer and inner apron surfaces was used to estimate the linear attenuation coefficients for each dosimeter with different Pb thicknesses, which were then compared with the values reported by the National Institute of Standards and Technology (NIST (Hubbell and Seltzer, 1995)).
Outer surface of the apron Io (nC)
Outer surface of the apron Io (nC)
3
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
H.O. Dávila et al.
Table 5 Linear attenuation coefficients according to the measured (Meas), NIST, and Monte Carlo (MC) simulation values. Pb thickness of the apron (mm)
0.25 0.55 0.75
N-40
N-80
N-100
Meas.
NIST
MC
Meas.
NIST
MC
Meas.
NIST
MC
269.15 N.C N.C
291.06 291.06 291.06
268.35 274.22 N.C
57.98 N.C N.C
57.57 57.57 57.57
56.36 45.70 45.65
26.97 23.99 28.35
27.57 27.57 27.57
27.44 21.15 22.80
N.C (Not calculated): The measurements (Meas) of intensities I in the TLDs are of order of background, and it was not possible to estimate the µ-values. In the case of Monte Carlo simulation (0.75-mm-thick lead apron irradiated with N-40), the I/Io ratio was of the order of 0.001.
reviewed because the Monte Carlo calculations have uncertainties < 1%.
For larger Pb thicknesses and lower effective energies, the linear attenuation coefficients were not measured because of the readouts of inner surface. For other X-ray qualities, N-40 (33 keV) and N-80 (65 keV), and the 0.55- and 0.75-mm-thick Pb aprons, it was not possible to measure the linear attenuation coefficients because of readouts of TLDs located on inner surface of the aprons, which were the same as the background values.
References Agostinelli, S., Allison, J., Amako, K.A., Apostolakis, J., Araujo, H., Arce, P., Behner, F., 2003. GEANT4—a simulation toolkit. Nucl. Instrum. Methods Phys. Res. Sect. A: Accel., Spectrom. Detect. Assoc. Equip. 506 (3), 250–303. Häusler, U., Czarwinski, R., Brix, G., 2009. Radiation exposure of medical staff from interventional x-ray procedures: a multicentre study. Eur. Radiol. 19 (8), 2000–2008. Hubbell, J.H., Seltzer, S.M., 1995. Tables of X-ray mass attenuation coefficients and mass energy-absorption coefficients 1 keV to 20 MeV for elements Z=1 to 92 and 48 additional substances of dosimetric interest (No. PB–95-220539/XAB; NISTIR–5632). National Inst. of Standards and Technology-PL. Ionizing Radiation Div, Gaithersburg, MD (United States). International Organization for Standardization, 1995. X and Gamma Reference Radiations for Calibrating Dosemeters and Doserate Meters and for Determining Their Response as a Function of Photon Energy: Rayonnements X Et Gamma de Référence Pour L′étalonnage Des Dosimètres Et Des Débitmètres, Et Pour la Détermination de Leur Réponse en Fonction de L′énergie Des Photons. Dosimetry Fo X and Gamma Reference Radiations for Radiation Protection Over the Energy Range from 8 KeV to 1, 3 MeV and from 4 MeV to 9 MeV. Dosimétrie Des Rayonnements X Et Gamma de Référence …. ISO. Kazempour, M., Saeedimoghadam, M., Shooli, F.S., Shokrpour, N., 2015. Assessment of the radiation attenuation properties of several lead free composites by Monte Carlo simulation. J. Biomed. Phys. Eng. 5 (2), 67. Matyagin, Y.V., Collins, P.J., 2016. Effectiveness of abdominal shields in chest radiography: a Monte Carlo evaluation. Br. J. Radiol. 89 (1066) (20160465). Santos, W.S., Neves, L.P., Perini, A.P., Belinato, W., Caldas, L.V., Carvalho, A.B., Maia, A.F., 2015. Exposures in interventional radiology using Monte Carlo simulation coupled with virtual anthropomorphic phantoms. Phys. Med. 31 (8), 929–933. Schulze, R.K.W., Cremers, C., Karle, H., de las Heras Gala, H., 2016. Skin entrance dose with and without lead apron in digital panoramic radiography for selected sensitive body regions. Clin. Oral Investig. 1–7. Seals, K.F., Lee, E.W., Cagnon, C.H., Al-Hakim, R.A., Kee, S.T., 2016. Radiation-induced cataractogenesis: a critical literature review for the interventional radiologist. Cardiovasc. Interv. Radiol. 39 (2), 151–160. Tekin, H.O., 2016. MCNP-X Monte Carlo code application for mass attenuation coefficients of concrete at different energies by modeling 3×3 in. NaI (Tl) detector and comparison with XCOM and Monte Carlo data. Sci. Technol. Nucl. Install. 2016. Tekin, H.O., Kara, U., 2016. Monte Carlo simulation for distance and absorbed dose calculations in a PET-CT Facility by using MCNP-X. J. Commun. Comput. 13, 32–35. Tekin, H.O., Singh, V.P., Manici, T., 2017. Effects of micro-sized and nano-sized WO 3 on mass attenauation coefficients of concrete by using MCNPX code. Appl. Radiat. Isot. 121, 122–125. Valentin, J., 2007. The 2007 Recommendations of the International Commission on Radiological Protection. Elsevier, Oxford, UK, pp. 1–333. Zehtabian, M., Piruzan, E., Molaiemanesh, Z., Sina, S., 2015. Design of light multi-layered shields for use in diagnostic radiology and nuclear medicine via MCNP5 Monte Carlo code. Iran. J. Med. Phys. 12 (3), 223–228.
3.2. Monte Carlo calculations Photon fluence was used on the cells to define the TLDs on the outer and inner surfaces, and the same procedure as that used in the experiments was applied to estimate the linear attenuation coefficient. The measured and calculated results and their comparison with the NIST µ values are shown in Table 5. Theoretically, the linear attenuation coefficients must be the same for a given energy and a defined material; for such a reason, these values as reported in the NIST are presented in Table 5 for each case. 4. Conclusions Linear attenuation coefficient of Pb aprons for X-rays was measured and calculated using the Monte Carlo method. The X-ray effective energies of narrow qualities [ISO 4037-1, 1995] were experimentally reproduced in the Secondary Standard Dosimetry Laboratory having a very good approach to monoenergetic values reported by the (NIST). The approach is based on the intercomparison of linear attenuation coefficients for different beam qualities (narrow), where the maximum peak does not have a considerable effect on the effective energy when a filter is added; however, the beam intensity is affected. The obtained Monte Carlo results are generally consistent with the measured values, except for Pb aprons of 0.75 and 0.50 mm thickness irradiated with X-rays of effective energies of 33 and 65 keV because Xrays are attenuated in more than 99%. The experiment reproduced in GEANT4 in this work is reliable to evaluate any custom accessory shielding. The ratios between intensities with and without Pb aprons could be used as characteristic coefficients to each apron according to the effective energy that is being used in the radiology service. The measured and Monte Carlo calculations have good agreement with NIST µ values. Pb aprons of 0.55- and 0.75-mm thickness irradiated with N-40 and N-80 quality beams need to be further
4
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Assessment of the effectiveness of attenuation of Pb aprons by using TLD dosimetry and Monte Carlo calculations H. Olaya Dávilaa, J.A. Díaz Merchána,b, H.R. Vega Carrilloc, S.A. Martínez Ovallea,b, a b c
⁎
Grupo de Física Nuclear Aplicada y Simulación, Universidad Pedagógica y Tecnológica de Colombia, Tunja, Colombia Clínica Cancerológica de Boyacá, Tunja, Colombia Unidad Académica de Estudios Nucleares de la Universidad Autónoma de Zacatecas, C. Ciprés 10, Fracc. La Peñuela, 98068 Zacatecas, Zac, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords: TLDs Monte Carlo X-rays Linear attenuation coefficients
We developed an experimental set-up by using a continuous emission X-ray (Pantak DXT-3000) and three types of Pb aprons, with thicknesses of 0.25, 0.5, and 0.75 mm, coated with Mylar fiber on their surface. Aprons were placed at a distance of 2.5 m from the focus. Aluminum filtration was performed at the beam output to reproduce the qualities of narrow beams, N40 (Eeffective =33 keV), N80 (Eeffective =65 keV), and N100 (Eeffective =83 keV), according to the ISO standard 4037 (1–3). Each apron was fixed with 10 thermoluminescent dosimeters (TLDs) over its surface, five dosimeters before and five dosimeters after irradiation with X-rays. Dosimeter readings were noted, and the attenuation coefficients for each effective energy were calculated. To confirm the method of effective energy of ISO-4037 and evaluate the effectiveness of aprons according to the energy range required for different medical practices, a Monte Carlo simulation using GEANT4 code was performed. Thus, the fluence and the absorbed dose in each of the dosimeters were determined, and then the coefficients of linear attenuation were calculated and compared with the experimental data and with those reported by the National Institute of Standards and Technology. Results were consistent between theoretical calculations and experimental measures. This work will serve to make assessments for other personalized radiation protectors made of Pb.
1. Introduction Pb-based materials for radiation protection are important in radiodiagnostic procedures to reduce the radiation exposure of workers and, in some cases, in patients aiming to fulfill the ALARA (as low as reasonably achievable) principle (Kazempour et al., 2015). Custommade radiation protectors are commonly made of Pb of less thickness and are used in diagnostic radiology at all medical facilities. Monte Carlo methods are widely used to design protectors against ionizing radiation with two purposes: to reduce the radiation exposure, as part of Occupational Health Program and to allow comfort in wearing the protectors. In the design, long useful life and decreased Pb toxicity are the two desirable goals (Kazempour et al., 2015; Zehtabian et al., 2015). Many studies of X-ray exposure in different medical practices have been reported. Santos et al. (2015) studied the exposure of workers in interventional radiology department by using the MCNPX code and dummy anthropomorphic phantom. Another study used the RANDO phantom where the dose in the skin, with and without Pb apron, was evaluated by digital panoramic X-ray
⁎
radiography, where a significantly increased level was found in the mammary region when Pb apron was not used because the radiations were mainly significantly absorbed by the lungs (Schulze et al., 2016). Pb apron protection is improperly used in Latin American countries because there is no awareness by medical staff about the degree of importance that apron use has from the radiation protection point of view (Valentin, 2007). Pb aprons are not used regularly by radiation workers during their work. In some hospitals, Pb aprons are not used because of their lack of portability, discomfort, and unavailability. The most common case where radiation protection items are not used is in the interventional radiology department. Here, the medical staff rarely uses Pb aprons, thyroid and gonad protectors, and glass sinkers because of the discomfort and limitation in maneuverability during medical intervention. However, conclusive studies have confirmed the importance of using radiation protection items to reduce the radiation exposure of sensitive structures such as the lens of the eye, which decreases the probability of inducing cataractogenesis. (Seals et al., 2016). There are several validation methods to establish the reliability of radiation protectors used by workers. Experimental measurements by
Corresponding author at: Grupo de Física Nuclear Aplicada y Simulación, Universidad Pedagógica y Tecnológica de Colombia, Tunja, Colombia. E-mail address: [email protected] (S.A. Martínez Ovalle).
http://dx.doi.org/10.1016/j.apradiso.2017.05.012 Received 6 December 2016; Received in revised form 5 May 2017; Accepted 9 May 2017 0969-8043/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Dávila, H.O., Applied Radiation and Isotopes (2017), http://dx.doi.org/10.1016/j.apradiso.2017.05.012
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
H.O. Dávila et al.
using thermoluminescent dosimeters (TLDs) are widely used. Häusler et al. (2009) studied the dose received by 39 Medical Physicists at 14 hospitals in Germany during interventional radiology procedures. They found that if protectors are properly used, the radiation exposure does not exceed the annual dose limit. Although the use of TLD is an excellent choice to evaluate the effectiveness of Pb protectors, it is important to note that if experimental measurements could be substantiated with calculations, it is possible to develop theoretical models that underpin any experiment (Kazempour et al., 2015; Zehtabian et al., 2015; Santos, 2015; Matyaginand and Collins, 2016; Tekin and Kara, 2016; Tekin, 2016; Tekin et al., 2017). The purpose of the present study is to evaluate the effectiveness of Pb aprons, on the basis of their thickness, to fulfill the radiation protection needs against X-rays. The work was performed using calculations by using the Geant 4 Monte Carlo code, and the measurements were obtained using TLDs. 1.1. Theoretical basis When a monoenergetic beam of electromagnetic radiation was incident on a homogeneous material of known density and thickness, the relative intensity of the beam subsequent to the processes of absorption and transmission is given as follows:
I = BIo e−μx
(1) −1
where µ is the linear attenuation coefficient expressed in cm , x is the thickness of the material expressed in cm, and B is the buildup factor. Implicitly, it has attenuation coefficient mass µm, given by the relationship:
μm =
μ ρ
(2)
where ρ is the density of the material expressed in g/cm . In monoenergetic bundles, for each kind of energy and material, a corresponding coefficient μm is present. In the case of polyenergetic beams of Xrays, for each spectrum, there is an effective power that corresponds to the attenuation coefficient mass of a given material (Zehtabian et al., 2015). The linear attenuation coefficient µ is directly related to the effective cross-section. The total interaction σ is defined as the probability of related interaction between a collision and the effective area expressed in barn (1 barn =10–24 cm2). The relationship between the linear attenuation coefficient and the effective cross-section is given as follows: 3
μ N = Aσ ρ A
Fig. 1. Geometry of aprons for Monte Carlo calculations using GEANT4 and locations of the TLDs on the surface of aprons.
(Eeffective =33 keV), N-80 (Eeffective =65 keV), and N-100 (Eeffective =83 keV), which were surveyed using a plane parallel ionization chamber with calibration factors for five X-ray qualities having halfvalue layers (HVLs) from 0.20 mm of Al to 1.5 mm of Cu. At the output of radiation beam, porous Al sheets of thicknesses from 0.1 mm were used to establish the first HVL and the second HVL that guarantees the uniformity of the radiation beam. Irradiations in the laboratory were controlled for buildup factor to be close to 1, which was not considered in the simulation. Ten TLD-100 were allocated to different sites on the aprons, as shown in Fig. 1. The TLDs were calibrated using a 137Cs radioactive source, and the TLDs were read using a Harshaw 4500 reader. At each site on the apron, five TLDs were placed on the outer apron surface and five on the inner apron surface to evaluate the apron's X-ray attenuation. Previously, the TLDs were erased on the Harshaw reader. During irradiation, another two TLD-100 were used for background measurements. For each experimental set-up, the aprons were irradiated with Xrays for 5 min. Once the aprons were irradiated, the TLDs were read and corrected by the mean readout value of TLDs that were used for background measurements. According to the ISO 4037 standard, the effective energy of each Xray quality used in this study are 33, 65, and 83 keV for N-40, N-80, and
(3)
The main interaction mechanisms between X-ray photons with matter in medical diagnosis are the photoelectric effect and the Compton scattering. The total cross-section is the sum of cross-sections of both interaction mechanisms:
σTotal = σPhotoelectric + σCompton
(4)
2. Materials and methods 2.1. Measurements For the experimental study, we used three types of Pb aprons having thicknesses of 0.25, 0.50, and 0.75 mm that are currently used in radiodiagnostic procedures. Each Pb apron was located on an Aldersson type anthropomorphic phantom that was sited at 2.5 m from the focus. Phantom was irradiated using a continuous emission X-ray (Pantak DXT-3000) at the Secondary Standard Dosimetry Laboratory in BogotaColombia. Irradiations were performed using X-ray beams N-40 2
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N-100, respectively. For each beam, the linear attenuation coefficient was calculated using Eq. (5).
1 ⎛I⎞ μ = − ln ⎜ ⎟ x ⎝ Io ⎠
Table 2 TLD readouts, I/Io, and µ-values for Apron 1 irradiated with 83 keV effective energy Xrays.
(5)
Here, µ is the linear attenuation coefficient, I and Io are the background-corrected mean values of TLD readouts of TLDs sited on the outer apron surface (Io) and those allocated on the inner apron surface (I), x is the Pb thickness that was increased by 0.005 cm to consider the Mylar coating present on both the apron surfaces. 2.2. Calculations
3.1. Experimental 3.1.1. Apron 1 The mean readout value of TLDs used for background measurements was 32 nC. For Apron 1 having 0.25-mm-thick Pb when irradiated with N-80 quality X-ray beam (65 keV effective energy), the individual background-corrected TLD readouts for those sited on the frontal (outer) and dorsal (inner) surfaces of the apron, the ratio between I and Io, and the linear attenuation coefficient are presented in Table 1. The average value of the attenuation coefficient was 57.91 ± 2.10 cm−1. According to the NIST, the mass attenuation coefficient (µ/ρ) of Pb for 65 keV photons is 5.02 cm2/g; thus, the linear attenuation coefficient is µ =(4.8 cm2/g).(11.4 g/cm3) =57.57 cm−1. Therefore, the measured µ values are approximately the same as those reported by the NIST. The measured value is 0.6% greater than the NIST µ value, which is less than the relative uncertainty of the measured value (3.6%). Table 2 presents the experimental results for apron 1, having 0.25mm-thick Pb, when irradiated with N-100 quality X-ray beam (83 keV effective energy). The average attenuation coefficient was 26.97 ± 1.0 cm−1. The NIST tables indicate that the mass attenuation coefficient of Pb for 0.083 MeV photons is µ/ρ=2.419 cm2/g. Thus, the linear attenuation
Attenuation coefficient µ (cm−1)
225 246 235 241 230
51 57 54 55 60
22.6 23.1 22.9 22.8 26.1
59.37 58.49 58.82 59.10 53.75
Attenuation coefficient µ (cm−1)
355 360 327 371 362
178 181 172 183 190
50.1 50.2 52.5 49.3 52.4
27.61 27.50 25.70 28.27 25.79
Outer surface of the apron Io (nC)
Inner surface of the apron I (nC)
(I/I0) ratio (%)
Attenuation coefficient µ (cm−1)
354 362 329 375 363
94 94 96 97 95
26.5 25.9 29.1 25.8 26.1
24.11 24.52 22.39 24.59 24.37
Outer surface of the apron Io (nC)
Inner surface of the apron I (nC)
(I/I0) ratio (%)
Attenuation coefficient µ (cm−1)
354 362 329 375 363
48 50 38 45 40
13.5 13.8 11.0 12.0 11.0
25.64 26.39 29.40 28.27 29.40
coefficient is µ =27.576 cm−1, which is 2.2% higher than the measured value. However, this difference is smaller than the uncertainty of the measured value (3.7%). 3.1.2. Apron 2 The experimental results for Apron 2 are presented in Table 2. This apron has 0.55-mm-thick Pb and was irradiated with N-100 quality Xray quality beam (83 keV effective energy). The average value of the attenuation coefficient was 23.99 ± 0.91 cm−1. For 83 keV photons, Pb has a mass attenuation coefficient of 2.228 cm2/g. Therefore, the linear attenuation coefficient is 25.40 cm−1. This value is 5.8% greater than the measured attenuation coefficient, whose uncertainty is 3.7%. The difference between the NIST and the measured µ values is significant and the probable explanation is attributed to the density of Pb used in the experiment (less than 11.40 g/cm3) and loss of Pb thickness because of the fluidity of the material due to frequent use (Table 3). 3.1.3. Apron 3 The experimental results for Apron 3 are presented in Table 4. This apron has 0.75-mm-thick Pb and was irradiated with N-100 quality Xray beam (83 keV effective energy). The average value of the attenuation coefficient was 27.82 ± 1.73 cm−1. For 83 keV photons, Pb has a mass attenuation coefficient of 2.228 cm2/g. Therefore, the linear attenuation coefficient is 25.40 cm−1. This value is 8.6% greater than the measured attenuation coefficient, whose uncertainty is 6.20%. The difference between the NIST and the measured µ values is small. The high uncertainty is because intensity values recorded by the TLDs after the attenuation are slightly higher than the background values.
Table 1 TLD readouts, I/Io, and µ-values for Apron 1 irradiated with 65 keV effective energy Xrays. (I/I0) ratio (%)
(I/I0) ratio (%)
Table 4 TLD readouts, I/Io, and µ-values for Apron 3 irradiated with 83 keV effective energy Xrays.
3. Results and discussion
Inner surface of the apron I (nC)
Inner surface of the apron I (nC)
Table 3 TLD readouts, I/Io, and µ-values for Apron 2 irradiated with 83 keV effective energy Xrays.
By using the Monte Carlo code GEANT4 (Agostinelli et al., 2003), a model including each TLD, the apron, and their coating was designed. The TLDs were modeled at the same positions where TLDs were sited during the measurements, as shown in Fig. 1. The source was set 2.5 m from the apron, and it was modeled as a cone beam with the vertex at the focus. The X-ray beam was modeled as a monoenergetic beam with the same energy as that of the effective energy of the X-ray tube, i.e., 33, 65, and 83 keV. To have good Monte Carlo statistics, 2.0×109 stories in each case were cluster parallelized, and information was analyzed using ROOT C++ toolkit. X-ray fluence on the outer and inner apron surfaces was used to estimate the linear attenuation coefficients for each dosimeter with different Pb thicknesses, which were then compared with the values reported by the National Institute of Standards and Technology (NIST (Hubbell and Seltzer, 1995)).
Outer surface of the apron Io (nC)
Outer surface of the apron Io (nC)
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Table 5 Linear attenuation coefficients according to the measured (Meas), NIST, and Monte Carlo (MC) simulation values. Pb thickness of the apron (mm)
0.25 0.55 0.75
N-40
N-80
N-100
Meas.
NIST
MC
Meas.
NIST
MC
Meas.
NIST
MC
269.15 N.C N.C
291.06 291.06 291.06
268.35 274.22 N.C
57.98 N.C N.C
57.57 57.57 57.57
56.36 45.70 45.65
26.97 23.99 28.35
27.57 27.57 27.57
27.44 21.15 22.80
N.C (Not calculated): The measurements (Meas) of intensities I in the TLDs are of order of background, and it was not possible to estimate the µ-values. In the case of Monte Carlo simulation (0.75-mm-thick lead apron irradiated with N-40), the I/Io ratio was of the order of 0.001.
reviewed because the Monte Carlo calculations have uncertainties < 1%.
For larger Pb thicknesses and lower effective energies, the linear attenuation coefficients were not measured because of the readouts of inner surface. For other X-ray qualities, N-40 (33 keV) and N-80 (65 keV), and the 0.55- and 0.75-mm-thick Pb aprons, it was not possible to measure the linear attenuation coefficients because of readouts of TLDs located on inner surface of the aprons, which were the same as the background values.
References Agostinelli, S., Allison, J., Amako, K.A., Apostolakis, J., Araujo, H., Arce, P., Behner, F., 2003. GEANT4—a simulation toolkit. Nucl. Instrum. Methods Phys. Res. Sect. A: Accel., Spectrom. Detect. Assoc. Equip. 506 (3), 250–303. Häusler, U., Czarwinski, R., Brix, G., 2009. Radiation exposure of medical staff from interventional x-ray procedures: a multicentre study. Eur. Radiol. 19 (8), 2000–2008. Hubbell, J.H., Seltzer, S.M., 1995. Tables of X-ray mass attenuation coefficients and mass energy-absorption coefficients 1 keV to 20 MeV for elements Z=1 to 92 and 48 additional substances of dosimetric interest (No. PB–95-220539/XAB; NISTIR–5632). National Inst. of Standards and Technology-PL. Ionizing Radiation Div, Gaithersburg, MD (United States). International Organization for Standardization, 1995. X and Gamma Reference Radiations for Calibrating Dosemeters and Doserate Meters and for Determining Their Response as a Function of Photon Energy: Rayonnements X Et Gamma de Référence Pour L′étalonnage Des Dosimètres Et Des Débitmètres, Et Pour la Détermination de Leur Réponse en Fonction de L′énergie Des Photons. Dosimetry Fo X and Gamma Reference Radiations for Radiation Protection Over the Energy Range from 8 KeV to 1, 3 MeV and from 4 MeV to 9 MeV. Dosimétrie Des Rayonnements X Et Gamma de Référence …. ISO. Kazempour, M., Saeedimoghadam, M., Shooli, F.S., Shokrpour, N., 2015. Assessment of the radiation attenuation properties of several lead free composites by Monte Carlo simulation. J. Biomed. Phys. Eng. 5 (2), 67. Matyagin, Y.V., Collins, P.J., 2016. Effectiveness of abdominal shields in chest radiography: a Monte Carlo evaluation. Br. J. Radiol. 89 (1066) (20160465). Santos, W.S., Neves, L.P., Perini, A.P., Belinato, W., Caldas, L.V., Carvalho, A.B., Maia, A.F., 2015. Exposures in interventional radiology using Monte Carlo simulation coupled with virtual anthropomorphic phantoms. Phys. Med. 31 (8), 929–933. Schulze, R.K.W., Cremers, C., Karle, H., de las Heras Gala, H., 2016. Skin entrance dose with and without lead apron in digital panoramic radiography for selected sensitive body regions. Clin. Oral Investig. 1–7. Seals, K.F., Lee, E.W., Cagnon, C.H., Al-Hakim, R.A., Kee, S.T., 2016. Radiation-induced cataractogenesis: a critical literature review for the interventional radiologist. Cardiovasc. Interv. Radiol. 39 (2), 151–160. Tekin, H.O., 2016. MCNP-X Monte Carlo code application for mass attenuation coefficients of concrete at different energies by modeling 3×3 in. NaI (Tl) detector and comparison with XCOM and Monte Carlo data. Sci. Technol. Nucl. Install. 2016. Tekin, H.O., Kara, U., 2016. Monte Carlo simulation for distance and absorbed dose calculations in a PET-CT Facility by using MCNP-X. J. Commun. Comput. 13, 32–35. Tekin, H.O., Singh, V.P., Manici, T., 2017. Effects of micro-sized and nano-sized WO 3 on mass attenauation coefficients of concrete by using MCNPX code. Appl. Radiat. Isot. 121, 122–125. Valentin, J., 2007. The 2007 Recommendations of the International Commission on Radiological Protection. Elsevier, Oxford, UK, pp. 1–333. Zehtabian, M., Piruzan, E., Molaiemanesh, Z., Sina, S., 2015. Design of light multi-layered shields for use in diagnostic radiology and nuclear medicine via MCNP5 Monte Carlo code. Iran. J. Med. Phys. 12 (3), 223–228.
3.2. Monte Carlo calculations Photon fluence was used on the cells to define the TLDs on the outer and inner surfaces, and the same procedure as that used in the experiments was applied to estimate the linear attenuation coefficient. The measured and calculated results and their comparison with the NIST µ values are shown in Table 5. Theoretically, the linear attenuation coefficients must be the same for a given energy and a defined material; for such a reason, these values as reported in the NIST are presented in Table 5 for each case. 4. Conclusions Linear attenuation coefficient of Pb aprons for X-rays was measured and calculated using the Monte Carlo method. The X-ray effective energies of narrow qualities [ISO 4037-1, 1995] were experimentally reproduced in the Secondary Standard Dosimetry Laboratory having a very good approach to monoenergetic values reported by the (NIST). The approach is based on the intercomparison of linear attenuation coefficients for different beam qualities (narrow), where the maximum peak does not have a considerable effect on the effective energy when a filter is added; however, the beam intensity is affected. The obtained Monte Carlo results are generally consistent with the measured values, except for Pb aprons of 0.75 and 0.50 mm thickness irradiated with X-rays of effective energies of 33 and 65 keV because Xrays are attenuated in more than 99%. The experiment reproduced in GEANT4 in this work is reliable to evaluate any custom accessory shielding. The ratios between intensities with and without Pb aprons could be used as characteristic coefficients to each apron according to the effective energy that is being used in the radiology service. The measured and Monte Carlo calculations have good agreement with NIST µ values. Pb aprons of 0.55- and 0.75-mm thickness irradiated with N-40 and N-80 quality beams need to be further
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