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The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study
Radiography xxx (xxxx) xxx
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The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study A.M. Asbeutah a, *, A. Brindhaban a, A.A. AlMajran b, S.A. Asbeutah c a
Department of Radiologic Sciences, Faculty of Allied Health Sciences, Kuwait University, P.O.Box 31470, Sulaibikhat, 90805, Kuwait Department of Community Medicine & Behavioral Sciences, Faculty of Medicine, Kuwait University, P.O. Box 24923, Safat, 13110, Kuwait c Health Sciences Center, Faculty of Medicine, Kuwait University, P.O.Box 24923, Safat, 13110, Kuwait b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2019 Received in revised form 21 November 2019 Accepted 5 December 2019 Available online xxx
Introduction: There are concerns regarding the increase in radiation dose among women undergoing both digital mammography (DM) and digital breast tomosynthesis (DBT). The aim of this study was to evaluate the effect of different exposure parameters on entrance skin dose (ESD) and average glandular dose (AGD) for DM and DBT using a phantom. Methods: The ESD and AGD of 30 DM and DBT (cranio-caudal projection) examinations using a tissue equivalent phantom where acquired using a GE Senographe Essential DM unit. Commercial phantoms were used to simulate three different breast thicknesses and compositions. Tube potential, tube load, and target/filter combinations were also varied with ESD and AGD recorded directly from the DM unit. Comparisons were made using the non-parametric Kruskal Wallis, ManneWhitney, and Wilcoxon signed rank tests. Results: The individual ESD values for 4 cm, 5 cm, and 6 cm thick phantoms for DM and DBT at Rh/Rh target/filter combination and 30e32 kV/56 mAs levels were 5.06 and 4.18 mGy; 5.82 and 5.08 mGy; and 7.26 and 11.4 mGy, respectively; while AGDs were 1.57 and 1.30 mGy, 1.33 and 1.39 mGy; and 1.29 and 3.60 mGy, respectively. The KruskaleWallis test showed a statistically significant difference in AGD for DM (P ¼ .029) but not for DBT (P ¼ 0.368). The ManneWhitney and Wilcoxon signed rank tests showed no statistically significant difference for ESD or AGD between both DM and DBT techniques (P ¼ .827 and .513). The percentage differences in ESD for phantom thicknesses of 4 cm, 5 cm, and 6 cm between DBT and DM ranged between 21% and 36%; while for AGD between 21% and 64.2%. Conclusions: The ESD and AGD for single view projection in DM and DBT showed differences at 4 and 6 cm breast thicknesses and compositions but not at 5 cm thickness with 30e32 kV and a Rh/Rh target/ filter combination. Implications for practice: A fibro-fatty breast results in less radiation dose variations in terms of ESD and AGD between DM and DBT techniques. © 2019 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Keywords: Breast imaging Dosimetry Digital breast tomosynthesis Digital mammography Average glandular dose Entrance skin dose
Introduction Digital mammography (DM) is the preferred breast imaging technique for diagnostic and/or screening purposes for the detection of breast cancer.1 Recent advances in digital imaging in general, and DM in particular, has led to digital breast tomosynthesis (DBT), which is emerging as an influential technology for threedimensional breast imaging. DBT works by acquiring multiple lowdose images along an arc over the breast and reconstructing
* Corresponding author. Fax: þ96524633836. E-mail address: [email protected] (A.M. Asbeutah).
images in slices.2 DBT has shown significant improvements detection of breast cancer compared to DM.3e5 In addition, DBT has shown a reduction of recall rate by approximately 40%, due to improvements in both sensitivity and specificity by 30% compared to DM.6e9 Although there are many advantages of DBT, there are also a few pitfalls. The most important of these pitfalls are that some of the malignances may be missed or misinterpreted10; there is limited capability for the visualisation of microcalcifications11; longer compression time, and the radiation dose is increased when DBT is combined with DM.12 Since the biological effects of radiation exposure are cumulative over time, it is prudent to limit exposures during DBT. There are concerns about the increase in radiation dose
https://doi.org/10.1016/j.radi.2019.12.004 1078-8174/© 2019 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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among women undergoing both DM and DBT. However, limited work has been published on radiation doses from DBT and a combination of DBT and DM when using a range of clinically relevant mammographic exposure parameters.13e16 Since DM and DBT are used in combination among many breast imaging facilities to diagnose breast cancer and the lack of radiation dose data available in the literature for combined DM and DBT processes, a comparison of doses levels between DM and DBT based on clinical imaging is difficult. The breast contains three types of tissue: glandular, fatty, and fibrous all covered by skin. Since the majority of breast cancers develop within glandular tissue average glandular dose (AGD) is used to estimate the radiation dose to the breast.17 The entrance skin dose (ESD) was considered in this study for comparison purposes. Hence, this study was undertaken to evaluate the radiation dose in terms of ESD and AGD for diagnostic DM and DBT using phantoms of different compressed breast thicknesses and compositions at different exposure factors and different target/filter material combinations commonly used in clinical practice. Methods Study design In this study, the ESD and AGD were recorded for different X-ray tube target/filter combinations and X-ray tube voltages (kV) during DM and DBT examinations, for three different breast phantoms of varying thicknesses and compositions. The ESD and AGD were recorded directly from the mammography unit. Statistical comparisons were made for radiation doses against changes in phantom thickness and composition, kV level and target/filter combination. Mammography phantoms The Computerized Imaging Reference Systems (CIRS Inc., Norfolk, VA, USA) mammography phantoms of breast equivalent material and composition were used to compare the performance of the DM and DBT techniques in terms of radiation dose. Phantoms are shaped in the form of compressed breast, are non-deformable and are made of epoxy resin material, equivalent to thicknesses of 4 cm made of 50% glandular and 50% fatty breast tissue; 5 cm made of 30% glandular and 70% fatty breast tissue; and 6 cm made of 20% glandular and 80% fatty breast tissue in terms of their X-ray attenuation properties (Fig. 1). Each phantom consists of different test objects representing microcalcifications, tumour masses, and simulated fibres. Each phantom contains twelve groups of calcium carbonate specks with particle sizes (mm) of 0.13, 0.165, 0.196, 0.23, 0.275, 0.40, 0.23, 0.196, 0.166, 0.23, 0.196 and 0.165, respectively. Also, each phantom contains five nylon fibres having diameters (mm) of 1.25, 0.83, 0.71, 0.53, and 0.3, respectively and seven hemispheric masses of 55% glandular and 45% adipose tissue with diameters (mm) of 4.76, 3.16, 2.38, 1.98, 1.59, 1.19, and 0.90, respectively. A schematic diagram of the phantom is shown in Fig. 2. Special attention was given to the placement of the phantom in the same position on the detector and the uniformity of the detector was measured according to the European guidelines.18 The compression device was used to hold the phantom still during the exposure.
with amorphous silicon/cesium iodide (Si/CsI) detector of 24 cm x 31 cm, a pixel pitch of 100 mm, and 5:1 anti-scatter grid ratio. The DBT component of the system acquired images using tube rotation through the angular range of ±25 with nine projections. Image acquisition was performed with a step-and-shoot method, with an acquisition time of less than 10 s for each breast. Images of each phantom were acquired using different target/ filter combinations of Molybdenum (Mo) and Rhodium (Rh) (Mo/ Mo, Mo/Rh, Rh/Rh), and varying kV levels (28, 30, and 32) at a fixed source image-receptor distance (SID) of 65 cm. All DM and DBT exposures were performed in the cranio-caudal (CC) projection. The CC projection was considered only as a way of a simple comparison and ease of positioning the phantom during the procedure compared to the medio-lateral oblique projection. The selection of target/filter combination for DM is a user selectable parameter. An initial DM exposure was made using the automatic exposure control (AEC) system, which is typically used in clinical practice. This is in line with the manufacturer's recommendation to adhere to the optimization principle of radiation protection, compensating for breast thickness and composition and target/filter combinations used.19 For DBT, target/filter combination is a machine selected parameter and could not be changed. DBT images were acquired using 30e32 kV and Rh/Rh target/filter combination at each phantom thickness. The exposure for each breast thickness and composition at three levels of kV were repeated three times for each target/filter combination. Three repeated measurements of ESD and AGD were taken in consideration during analysis. Image reconstruction for DBT was performed immediately after image acquisition with a slice thickness of 0.5 mm. Preliminary image quality analysis was performed to establish if the images were of diagnostic quality, as would be done by the technologists performing regular mammography examinations. The accuracy of the ESD and AGD values reported by the mammography unit was checked during regular quality assurance testing, prior to the study. The reproducibility and linearity of the X-ray tube output were also tested. Study acquisitions were performed by one technologist and two authors (AA and AB) with more than 20 years of combined experience in breast imaging. Statistical analysis All statistical analyses were performed using Statistical Package for Social Sciences (SPSS) version 25 for Windows (SPSS Inc., Chicago, IL, USA). The ESD and AGD recorded from each modality for each phantom for the same CC projection using different breast phantom thickness and compositions, different exposure factors, and different target/filter combinations. A ShapiroeWilk test was performed to confirm the normality of the variables and was considered normal if the P > .05. The non-parametric KruskaleWallis test was performed to test for any significant difference between different radiation doses in DM and DBT for different exposure parameters. A ManneWhitney test was used to test if there was any difference between DM and DBT techniques in terms of radiation doses. Moreover, the Wilcoxon signed rank test was used to test if there were any statistically significant differences between the radiation doses for both techniques. Statistical significance was considered at P < .05 level. Results
Image acquisition and radiation dose recording A General Electric Senographe Essential (GE Healthcare, Buc, France) DM unit with DBT capability was used for imaging the phantom in DM and DBT acquisitions. The DM system is equipped
The ShapiroeWilk test showed that ESD and AGD, for both DM and DBT techniques, were not normally distributed (P < .05). The individual values of ESD and AGD for both techniques, acquired at different breast thicknesses and compositions, different exposure
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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Figure 1. The different CIRS breast phantoms used in the study. (A) 4 cm thickness fibroglandular breast phantom; (B) A 5 cm fibrofatty breast phantom; and (C) A 6 cm fatty breast phantom.
Figure 2. Schematic diagram of the 4 cm CIRS breast phantom (50/50) used during the study demonstrating the distribution of the masses, fibres, and microcalcifications within the target slab.
factors and different target/filter combinations are summarised in Table 1. For the DM technique, the lowest ESD of 4.04 mGy was achieved using 28 kV/50 mAs for the 4 cm phantom using a Rh/Rh target/ filter combination. The highest ESD of 9.65 mGy was achieved using 32 kV/56 mAs with the 6 cm phantom and a Mo/Mo target/filter combination. Moreover, the lowest AGD of 0.84 mGy was obtained using 28 kV/56 mAs for 6 cm phantom thickness (Rh/Rh target/ filter combination). The highest AGD (2.47 mGy) was obtained for the 4 cm phantom with 32 kV/56 mAs and a Mo/Mo target/filter combination. For the DBT technique, the ESD at 30e32 kV/56 mAs and a Rh/Rh target/filter combination for the 4 cm, 5 cm, and 6 cm phantom were 4.18, 5.08 and 11.4 mGy, respectively. However, the AGD at 30e32 kV/56 mAs (Rh/Rh target/filter combination) for the 4 cm, 5 cm, and 6 cm phantom was 1.3, 1.39 and 3.6 mGy, respectively.
The Kruskal Wallis test showed that there was no statistically significant differences in ESD (P ¼ .758 for DM and P ¼ .368 for DBT) for the different breast thicknesses and compositions, regardless of exposure factors or target/filter combinations. The trend was that the lowest ESD was achieved for the thin breast (28 kV/50 mAs; Rh/ Rh target/filter combination). The highest ESD was achieved with a thick breast (32 kV/56 mAs; Mo/Mo target/filter combination). The Kruskal Wallis test showed that there was a significant statistical difference in AGD between DM projections (P ¼ .029) but not for DBT (P ¼ .368) projections. The trend was that the lowest AGD was achieved with a thicker breast (28 kV/56 mAs; Rh/Rh target/filter combination). The highest AGD was achieved with a thin breast at (32 kV/56 mAs; Mo/Mo target/filter combination). The ManneWhitney test showed no significant statistical difference for ESD (P ¼ .827) and for AGD (P ¼ .513) between DM and DBT techniques. The median values for ESD for DM and DBT were
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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Table 1 Comparison between the individual DM and DBT dose values (ESD and AGD) at different phantom thicknesses and compositions, different kV/mAs settings, and different target/filter combinations. Phantom Thickness Composition
kV Setting mAs Setting Target/filter DM ESD (mGy) DM AGD (mGY) DBT ESD (mGY) DBT AGD (mGy) Difference ESDa Difference AGDb combination
4 cm (50/50)
28 28 28 30 30 30 32 32 32 28 28 28 30 30 30 32 32 32 28 28 28 30 30 30 32 32 32
5 cm (30/70)
6 cm (20/80)
a b
50 50 50 56 56 56 56 56 56 50 50 56 56 56 56 56 56 56 50 50 56 56 56 56 56 56 56
Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh
5.39 4.51 4.04 7.48 6.28 5.06 9.02 7.58 6.76 5.57 4.67 4.20 7.71 6.50 5.82 9.32 7.86 7.01 5.74 4.83 4.33 7.95 6.74 6.00 9.65 8.11 7.26
1.23 1.13 1.09 1.87 1.68 1.57 2.47 2.15 2.02 1.06 1.01 0.95 1.62 1.49 1.33 1.95 1.64 1.55 0.94 0.90 0.84 1.43 1.33 1.02 1.44 1.30 1.29
4.18
1.30
21.1%
20.8%
5.08
1.39
14.6%
4.3%
11.4
3.6
36.3%
64.2%
(ESD DBT - ESD DM/ESD DBT)*100%. (AGD DBT -AGD DM/AGD DBT)*100%.
5.82 and 5.08 mGy, respectively; while the median values for AGD for DM and DBT were 1.33 and 1.39 mGy, respectively. The percentage differences in ESD between DBT and DM for 4 cm, 5 cm, and 6 cm phantom thickness were 21.1%, 14.6%, and 36.3%, respectively. However, the percentage differences in AGD between both DBT and DM for 4 cm, 5 cm, and 6 cm phantom thickness were 20.8%, 4.3%, and 64.2%, respectively. The Wilcoxon signed rank test showed that there was no significant statistical difference in ESD (P ¼ 1.00) and for AGD (P ¼ .593) for both DM and DBT techniques. Discussion This study was undertaken to evaluate the radiation dose values in DM and DBT as there was limited work in the literature about the radiation doses for DBT. Also, DBT has been implemented in hospitals across Kuwait over the past 5 years. There was no data available on the radiation doses for the women who underwent both DM and DBT procedures. In the current study, we used the radiation exposure of a commercially available, Senographe Essential (GE Healthcare, Buc, France) where the main discerning features of this system are the step-and-shoot technique, in which the X-ray tube stops at each angular position, combined with a dedicated anti-scatter grid, with septa-oriented parallel to the chest wall, positioned on a fixed detector. Vendors in their quest for optimal DBT acquisition protocol made different choices regarding X-ray tube movement, range of angles, number of projections, dose distribution among projections, and the use of anti-scatter grids.20 Several studies have investigated patient radiation dose on a clinical system (Hologic Selenia Dimensions; Hologic Inc, Bedford, MA) that uses continuous tube movement and concluded that radiation exposure was found to be higher for DBT compared with DM.21e24 The tissue-equivalent phantoms of different thicknesses and compositions (4, 5, and
6 cm) were used because these represent the common breast thicknesses in clinical settings. Also, three common kV levels of 28, 30 and 32 with three different target/filter combinations (Mo/Mo, Mo/Rh, and Rh/Rh) were used for each CC projection for DM while the system allowed Rh/Rh for DBT. The CC projection was performed because it is easier to position a phantom compared to the medio-lateral oblique projection and to compare between the two techniques in terms of radiation dose. The procedure was performed by one technologist and two authors with more than 20 years of combined experience in the field of breast imaging. All the above-mentioned factors we believe allowed us to achieve consistent outcomes. Findings from our study are consistent with those reported in other studies16 except at 4 and 6 cm breast thicknesses and compositions. The percentage differences in ESD or AGD between the DM and DBT was apparent in 4 and 6 cm thicknesses than at 5 cm breast thickness regardless of the kV level or target/filter combination.16 However, the previous study,16 reported differences in ESD and AGD proportional to breast thickness. This may be due to the use of three different target/filter combinations (Mo/Mo, Mo/ Rh, and Rh/Rh) for DM and the use of only Rh/Rh for DBT in our study compared to W/Rh for DM and W/Al for DBT in the study by Alakhras et al.16 In addition, they used in their study one phantom tissue type with different thicknesses (1e6 cm) while in our study we used the three different common breast thicknesses and compositions (fibroglandular, fibrofatty, and fatty). Our results conclude that the highest radiation dose was achieved in using low kV, Mo/Mo target/filter combination, for thicker breast (6 cm). The lowest radiation dose was demonstrated in using a high kV setting, Rh/Rh target/filter combination, and thicker breast. Many studies supported the use of different target/filter combination for the purpose of reducing the radiation dose with accepted image quality and one of these target/filter materials was Rh/Rh.25e28
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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In our study, the percentage differences in AGD for 4 cm, 5 cm, and 6 cm phantom thicknesses were 21%, 4%, and 64%, respectively between the two techniques. However, in the study by Alakhras et a.l16 the difference in AGD between the two techniques was approximately 158% for 1 cm thickness while it was only 4% for 6 cm thickness. They concluded that the AGD increases with increasing the thickness for both techniques. Also, they noticed that the DBT appeared to be less affected by the breast thickness. We attributed this to their use of the Hologic Selenia Dimensions system which was documented by several researchers that produce higher radiation doses for DBT compared with DM.21e24 Several limitations exist in our study. One of them is that we could not perform all the exposure attempts using DBT as the system reverted to automatic mode. The radiation dose in terms of ESD and AGD were recorded directly from the mammographic image and we did not undertake measurement using TLDs or solidstate dosimeters. It should also be noted that this study only compared the dose to breast tissue and did not consider whole body dose or effective dose for DM or DBT. Further studies are needed to explore the relationship between the different mammographic parameters such as breast thickness/composition, exposure factors, and target/filter combination in a clinical setting and help understand which settings produce the lowest dose with acceptable diagnostic image quality. Conclusions The ESD and AGD for single view projection in DM and DBT showed differences at 4 and 6 cm breast thickness and composition but not at 5 cm thickness with 30e32 kV and a Rh/Rh target/filter combination. Conflict of interest statement The authors declare no conflicts of interest. Acknowledgements The authors would like to thank all of the staff members especially Dr. Nouralhuda Karmani and Mrs. Yasmin Echreshzadeh from the Breast Imaging Clinic at Al-Sabah Hospital, Ministry of Health, Kuwait for their help with data collection. No Funding was requested or granted for this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.radi.2019.12.004. References 1. Cheung YC, Lin YC, Wan YL, Yeow KM, Huang PC, Lo YF, et al. Diagnostic performance of dual-energy contrast-enhanced subtracted mammography in dense breasts compared to mammography alone: interobserver blind-reading analysis. Eur Radiol 2014;24(10):2394e403. 2. Alakhras M, Bourne R, Rickard M, Ng KH, Pietrzyk M, Brennan PC. Digital tomosynthesis: a new future for breast imaging? Clin Radiol 2013;68(5): e225e36. 3. Skaane P, Bandos AI, Gullien R, Eben EB, Ekseth U, Haakenaasen U, et al. Comparison of digital mammography alone and digital mammography plus tomosynthesis in a population-based screening program. Radiology 2013;267(1):47e56. 4. Ciatto S, Houssami N, Bernardi D, Caumo F, Pellegrini M, Brunelli S, et al. Integration of JD digital mammograph y with tomosynthesis for population breast-cancer screening (STORM): a prospective comparison study. Lancet Oncol 2013;14(7):583e9.
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Breast 2015;24(2):93e9. 13. Feng SS, Sechopoulos I. Clinical digital breast tomosynthesis system: dosimctric characterization. Radiology 2012;263(1):35e42. 14. Paulis LE, Lobbes MB, Lalji UC, Gelissen N, Bouwman RW, Wildberger JE, et al. Radiation exposure of digital breast tomosynthesis using an antiscatter grid compared with full-field digital mammography. lnvestig Radio 2015;50(10): 679e85. 15. Gennaro G, Bernardi D, Houssami N. Radiation dose with digital breast tomosynthesis compared to digital mammography: per-view analysis. Eur Radiol 2018;28(2):573e81. 16. Alakhras M, Mello-Thomas C, Bourne R, Rickard M, Diffey J, Brennan PC. Radiation dose differences between digital mammography and digital breast tomosynthesis are dependent on breast thickness? Proc of SPIE 2016;9783: 225e36. 17. Sechopoulos I, Sabol JM, Berglund J, Bolch WE, Brateman L, Christodoulou E, et al. Radiation dosimetry in digital breast tomosynthesis: report of AAPM Tomosynthesis Subcommittee Task Group 223. 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Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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Radiography journal homepage: www.elsevier.com/locate/radi
The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study A.M. Asbeutah a, *, A. Brindhaban a, A.A. AlMajran b, S.A. Asbeutah c a
Department of Radiologic Sciences, Faculty of Allied Health Sciences, Kuwait University, P.O.Box 31470, Sulaibikhat, 90805, Kuwait Department of Community Medicine & Behavioral Sciences, Faculty of Medicine, Kuwait University, P.O. Box 24923, Safat, 13110, Kuwait c Health Sciences Center, Faculty of Medicine, Kuwait University, P.O.Box 24923, Safat, 13110, Kuwait b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2019 Received in revised form 21 November 2019 Accepted 5 December 2019 Available online xxx
Introduction: There are concerns regarding the increase in radiation dose among women undergoing both digital mammography (DM) and digital breast tomosynthesis (DBT). The aim of this study was to evaluate the effect of different exposure parameters on entrance skin dose (ESD) and average glandular dose (AGD) for DM and DBT using a phantom. Methods: The ESD and AGD of 30 DM and DBT (cranio-caudal projection) examinations using a tissue equivalent phantom where acquired using a GE Senographe Essential DM unit. Commercial phantoms were used to simulate three different breast thicknesses and compositions. Tube potential, tube load, and target/filter combinations were also varied with ESD and AGD recorded directly from the DM unit. Comparisons were made using the non-parametric Kruskal Wallis, ManneWhitney, and Wilcoxon signed rank tests. Results: The individual ESD values for 4 cm, 5 cm, and 6 cm thick phantoms for DM and DBT at Rh/Rh target/filter combination and 30e32 kV/56 mAs levels were 5.06 and 4.18 mGy; 5.82 and 5.08 mGy; and 7.26 and 11.4 mGy, respectively; while AGDs were 1.57 and 1.30 mGy, 1.33 and 1.39 mGy; and 1.29 and 3.60 mGy, respectively. The KruskaleWallis test showed a statistically significant difference in AGD for DM (P ¼ .029) but not for DBT (P ¼ 0.368). The ManneWhitney and Wilcoxon signed rank tests showed no statistically significant difference for ESD or AGD between both DM and DBT techniques (P ¼ .827 and .513). The percentage differences in ESD for phantom thicknesses of 4 cm, 5 cm, and 6 cm between DBT and DM ranged between 21% and 36%; while for AGD between 21% and 64.2%. Conclusions: The ESD and AGD for single view projection in DM and DBT showed differences at 4 and 6 cm breast thicknesses and compositions but not at 5 cm thickness with 30e32 kV and a Rh/Rh target/ filter combination. Implications for practice: A fibro-fatty breast results in less radiation dose variations in terms of ESD and AGD between DM and DBT techniques. © 2019 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Keywords: Breast imaging Dosimetry Digital breast tomosynthesis Digital mammography Average glandular dose Entrance skin dose
Introduction Digital mammography (DM) is the preferred breast imaging technique for diagnostic and/or screening purposes for the detection of breast cancer.1 Recent advances in digital imaging in general, and DM in particular, has led to digital breast tomosynthesis (DBT), which is emerging as an influential technology for threedimensional breast imaging. DBT works by acquiring multiple lowdose images along an arc over the breast and reconstructing
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images in slices.2 DBT has shown significant improvements detection of breast cancer compared to DM.3e5 In addition, DBT has shown a reduction of recall rate by approximately 40%, due to improvements in both sensitivity and specificity by 30% compared to DM.6e9 Although there are many advantages of DBT, there are also a few pitfalls. The most important of these pitfalls are that some of the malignances may be missed or misinterpreted10; there is limited capability for the visualisation of microcalcifications11; longer compression time, and the radiation dose is increased when DBT is combined with DM.12 Since the biological effects of radiation exposure are cumulative over time, it is prudent to limit exposures during DBT. There are concerns about the increase in radiation dose
https://doi.org/10.1016/j.radi.2019.12.004 1078-8174/© 2019 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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among women undergoing both DM and DBT. However, limited work has been published on radiation doses from DBT and a combination of DBT and DM when using a range of clinically relevant mammographic exposure parameters.13e16 Since DM and DBT are used in combination among many breast imaging facilities to diagnose breast cancer and the lack of radiation dose data available in the literature for combined DM and DBT processes, a comparison of doses levels between DM and DBT based on clinical imaging is difficult. The breast contains three types of tissue: glandular, fatty, and fibrous all covered by skin. Since the majority of breast cancers develop within glandular tissue average glandular dose (AGD) is used to estimate the radiation dose to the breast.17 The entrance skin dose (ESD) was considered in this study for comparison purposes. Hence, this study was undertaken to evaluate the radiation dose in terms of ESD and AGD for diagnostic DM and DBT using phantoms of different compressed breast thicknesses and compositions at different exposure factors and different target/filter material combinations commonly used in clinical practice. Methods Study design In this study, the ESD and AGD were recorded for different X-ray tube target/filter combinations and X-ray tube voltages (kV) during DM and DBT examinations, for three different breast phantoms of varying thicknesses and compositions. The ESD and AGD were recorded directly from the mammography unit. Statistical comparisons were made for radiation doses against changes in phantom thickness and composition, kV level and target/filter combination. Mammography phantoms The Computerized Imaging Reference Systems (CIRS Inc., Norfolk, VA, USA) mammography phantoms of breast equivalent material and composition were used to compare the performance of the DM and DBT techniques in terms of radiation dose. Phantoms are shaped in the form of compressed breast, are non-deformable and are made of epoxy resin material, equivalent to thicknesses of 4 cm made of 50% glandular and 50% fatty breast tissue; 5 cm made of 30% glandular and 70% fatty breast tissue; and 6 cm made of 20% glandular and 80% fatty breast tissue in terms of their X-ray attenuation properties (Fig. 1). Each phantom consists of different test objects representing microcalcifications, tumour masses, and simulated fibres. Each phantom contains twelve groups of calcium carbonate specks with particle sizes (mm) of 0.13, 0.165, 0.196, 0.23, 0.275, 0.40, 0.23, 0.196, 0.166, 0.23, 0.196 and 0.165, respectively. Also, each phantom contains five nylon fibres having diameters (mm) of 1.25, 0.83, 0.71, 0.53, and 0.3, respectively and seven hemispheric masses of 55% glandular and 45% adipose tissue with diameters (mm) of 4.76, 3.16, 2.38, 1.98, 1.59, 1.19, and 0.90, respectively. A schematic diagram of the phantom is shown in Fig. 2. Special attention was given to the placement of the phantom in the same position on the detector and the uniformity of the detector was measured according to the European guidelines.18 The compression device was used to hold the phantom still during the exposure.
with amorphous silicon/cesium iodide (Si/CsI) detector of 24 cm x 31 cm, a pixel pitch of 100 mm, and 5:1 anti-scatter grid ratio. The DBT component of the system acquired images using tube rotation through the angular range of ±25 with nine projections. Image acquisition was performed with a step-and-shoot method, with an acquisition time of less than 10 s for each breast. Images of each phantom were acquired using different target/ filter combinations of Molybdenum (Mo) and Rhodium (Rh) (Mo/ Mo, Mo/Rh, Rh/Rh), and varying kV levels (28, 30, and 32) at a fixed source image-receptor distance (SID) of 65 cm. All DM and DBT exposures were performed in the cranio-caudal (CC) projection. The CC projection was considered only as a way of a simple comparison and ease of positioning the phantom during the procedure compared to the medio-lateral oblique projection. The selection of target/filter combination for DM is a user selectable parameter. An initial DM exposure was made using the automatic exposure control (AEC) system, which is typically used in clinical practice. This is in line with the manufacturer's recommendation to adhere to the optimization principle of radiation protection, compensating for breast thickness and composition and target/filter combinations used.19 For DBT, target/filter combination is a machine selected parameter and could not be changed. DBT images were acquired using 30e32 kV and Rh/Rh target/filter combination at each phantom thickness. The exposure for each breast thickness and composition at three levels of kV were repeated three times for each target/filter combination. Three repeated measurements of ESD and AGD were taken in consideration during analysis. Image reconstruction for DBT was performed immediately after image acquisition with a slice thickness of 0.5 mm. Preliminary image quality analysis was performed to establish if the images were of diagnostic quality, as would be done by the technologists performing regular mammography examinations. The accuracy of the ESD and AGD values reported by the mammography unit was checked during regular quality assurance testing, prior to the study. The reproducibility and linearity of the X-ray tube output were also tested. Study acquisitions were performed by one technologist and two authors (AA and AB) with more than 20 years of combined experience in breast imaging. Statistical analysis All statistical analyses were performed using Statistical Package for Social Sciences (SPSS) version 25 for Windows (SPSS Inc., Chicago, IL, USA). The ESD and AGD recorded from each modality for each phantom for the same CC projection using different breast phantom thickness and compositions, different exposure factors, and different target/filter combinations. A ShapiroeWilk test was performed to confirm the normality of the variables and was considered normal if the P > .05. The non-parametric KruskaleWallis test was performed to test for any significant difference between different radiation doses in DM and DBT for different exposure parameters. A ManneWhitney test was used to test if there was any difference between DM and DBT techniques in terms of radiation doses. Moreover, the Wilcoxon signed rank test was used to test if there were any statistically significant differences between the radiation doses for both techniques. Statistical significance was considered at P < .05 level. Results
Image acquisition and radiation dose recording A General Electric Senographe Essential (GE Healthcare, Buc, France) DM unit with DBT capability was used for imaging the phantom in DM and DBT acquisitions. The DM system is equipped
The ShapiroeWilk test showed that ESD and AGD, for both DM and DBT techniques, were not normally distributed (P < .05). The individual values of ESD and AGD for both techniques, acquired at different breast thicknesses and compositions, different exposure
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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Figure 1. The different CIRS breast phantoms used in the study. (A) 4 cm thickness fibroglandular breast phantom; (B) A 5 cm fibrofatty breast phantom; and (C) A 6 cm fatty breast phantom.
Figure 2. Schematic diagram of the 4 cm CIRS breast phantom (50/50) used during the study demonstrating the distribution of the masses, fibres, and microcalcifications within the target slab.
factors and different target/filter combinations are summarised in Table 1. For the DM technique, the lowest ESD of 4.04 mGy was achieved using 28 kV/50 mAs for the 4 cm phantom using a Rh/Rh target/ filter combination. The highest ESD of 9.65 mGy was achieved using 32 kV/56 mAs with the 6 cm phantom and a Mo/Mo target/filter combination. Moreover, the lowest AGD of 0.84 mGy was obtained using 28 kV/56 mAs for 6 cm phantom thickness (Rh/Rh target/ filter combination). The highest AGD (2.47 mGy) was obtained for the 4 cm phantom with 32 kV/56 mAs and a Mo/Mo target/filter combination. For the DBT technique, the ESD at 30e32 kV/56 mAs and a Rh/Rh target/filter combination for the 4 cm, 5 cm, and 6 cm phantom were 4.18, 5.08 and 11.4 mGy, respectively. However, the AGD at 30e32 kV/56 mAs (Rh/Rh target/filter combination) for the 4 cm, 5 cm, and 6 cm phantom was 1.3, 1.39 and 3.6 mGy, respectively.
The Kruskal Wallis test showed that there was no statistically significant differences in ESD (P ¼ .758 for DM and P ¼ .368 for DBT) for the different breast thicknesses and compositions, regardless of exposure factors or target/filter combinations. The trend was that the lowest ESD was achieved for the thin breast (28 kV/50 mAs; Rh/ Rh target/filter combination). The highest ESD was achieved with a thick breast (32 kV/56 mAs; Mo/Mo target/filter combination). The Kruskal Wallis test showed that there was a significant statistical difference in AGD between DM projections (P ¼ .029) but not for DBT (P ¼ .368) projections. The trend was that the lowest AGD was achieved with a thicker breast (28 kV/56 mAs; Rh/Rh target/filter combination). The highest AGD was achieved with a thin breast at (32 kV/56 mAs; Mo/Mo target/filter combination). The ManneWhitney test showed no significant statistical difference for ESD (P ¼ .827) and for AGD (P ¼ .513) between DM and DBT techniques. The median values for ESD for DM and DBT were
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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Table 1 Comparison between the individual DM and DBT dose values (ESD and AGD) at different phantom thicknesses and compositions, different kV/mAs settings, and different target/filter combinations. Phantom Thickness Composition
kV Setting mAs Setting Target/filter DM ESD (mGy) DM AGD (mGY) DBT ESD (mGY) DBT AGD (mGy) Difference ESDa Difference AGDb combination
4 cm (50/50)
28 28 28 30 30 30 32 32 32 28 28 28 30 30 30 32 32 32 28 28 28 30 30 30 32 32 32
5 cm (30/70)
6 cm (20/80)
a b
50 50 50 56 56 56 56 56 56 50 50 56 56 56 56 56 56 56 50 50 56 56 56 56 56 56 56
Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh Mo/Mo Mo/Rh Rh/Rh
5.39 4.51 4.04 7.48 6.28 5.06 9.02 7.58 6.76 5.57 4.67 4.20 7.71 6.50 5.82 9.32 7.86 7.01 5.74 4.83 4.33 7.95 6.74 6.00 9.65 8.11 7.26
1.23 1.13 1.09 1.87 1.68 1.57 2.47 2.15 2.02 1.06 1.01 0.95 1.62 1.49 1.33 1.95 1.64 1.55 0.94 0.90 0.84 1.43 1.33 1.02 1.44 1.30 1.29
4.18
1.30
21.1%
20.8%
5.08
1.39
14.6%
4.3%
11.4
3.6
36.3%
64.2%
(ESD DBT - ESD DM/ESD DBT)*100%. (AGD DBT -AGD DM/AGD DBT)*100%.
5.82 and 5.08 mGy, respectively; while the median values for AGD for DM and DBT were 1.33 and 1.39 mGy, respectively. The percentage differences in ESD between DBT and DM for 4 cm, 5 cm, and 6 cm phantom thickness were 21.1%, 14.6%, and 36.3%, respectively. However, the percentage differences in AGD between both DBT and DM for 4 cm, 5 cm, and 6 cm phantom thickness were 20.8%, 4.3%, and 64.2%, respectively. The Wilcoxon signed rank test showed that there was no significant statistical difference in ESD (P ¼ 1.00) and for AGD (P ¼ .593) for both DM and DBT techniques. Discussion This study was undertaken to evaluate the radiation dose values in DM and DBT as there was limited work in the literature about the radiation doses for DBT. Also, DBT has been implemented in hospitals across Kuwait over the past 5 years. There was no data available on the radiation doses for the women who underwent both DM and DBT procedures. In the current study, we used the radiation exposure of a commercially available, Senographe Essential (GE Healthcare, Buc, France) where the main discerning features of this system are the step-and-shoot technique, in which the X-ray tube stops at each angular position, combined with a dedicated anti-scatter grid, with septa-oriented parallel to the chest wall, positioned on a fixed detector. Vendors in their quest for optimal DBT acquisition protocol made different choices regarding X-ray tube movement, range of angles, number of projections, dose distribution among projections, and the use of anti-scatter grids.20 Several studies have investigated patient radiation dose on a clinical system (Hologic Selenia Dimensions; Hologic Inc, Bedford, MA) that uses continuous tube movement and concluded that radiation exposure was found to be higher for DBT compared with DM.21e24 The tissue-equivalent phantoms of different thicknesses and compositions (4, 5, and
6 cm) were used because these represent the common breast thicknesses in clinical settings. Also, three common kV levels of 28, 30 and 32 with three different target/filter combinations (Mo/Mo, Mo/Rh, and Rh/Rh) were used for each CC projection for DM while the system allowed Rh/Rh for DBT. The CC projection was performed because it is easier to position a phantom compared to the medio-lateral oblique projection and to compare between the two techniques in terms of radiation dose. The procedure was performed by one technologist and two authors with more than 20 years of combined experience in the field of breast imaging. All the above-mentioned factors we believe allowed us to achieve consistent outcomes. Findings from our study are consistent with those reported in other studies16 except at 4 and 6 cm breast thicknesses and compositions. The percentage differences in ESD or AGD between the DM and DBT was apparent in 4 and 6 cm thicknesses than at 5 cm breast thickness regardless of the kV level or target/filter combination.16 However, the previous study,16 reported differences in ESD and AGD proportional to breast thickness. This may be due to the use of three different target/filter combinations (Mo/Mo, Mo/ Rh, and Rh/Rh) for DM and the use of only Rh/Rh for DBT in our study compared to W/Rh for DM and W/Al for DBT in the study by Alakhras et al.16 In addition, they used in their study one phantom tissue type with different thicknesses (1e6 cm) while in our study we used the three different common breast thicknesses and compositions (fibroglandular, fibrofatty, and fatty). Our results conclude that the highest radiation dose was achieved in using low kV, Mo/Mo target/filter combination, for thicker breast (6 cm). The lowest radiation dose was demonstrated in using a high kV setting, Rh/Rh target/filter combination, and thicker breast. Many studies supported the use of different target/filter combination for the purpose of reducing the radiation dose with accepted image quality and one of these target/filter materials was Rh/Rh.25e28
Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004
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In our study, the percentage differences in AGD for 4 cm, 5 cm, and 6 cm phantom thicknesses were 21%, 4%, and 64%, respectively between the two techniques. However, in the study by Alakhras et a.l16 the difference in AGD between the two techniques was approximately 158% for 1 cm thickness while it was only 4% for 6 cm thickness. They concluded that the AGD increases with increasing the thickness for both techniques. Also, they noticed that the DBT appeared to be less affected by the breast thickness. We attributed this to their use of the Hologic Selenia Dimensions system which was documented by several researchers that produce higher radiation doses for DBT compared with DM.21e24 Several limitations exist in our study. One of them is that we could not perform all the exposure attempts using DBT as the system reverted to automatic mode. The radiation dose in terms of ESD and AGD were recorded directly from the mammographic image and we did not undertake measurement using TLDs or solidstate dosimeters. It should also be noted that this study only compared the dose to breast tissue and did not consider whole body dose or effective dose for DM or DBT. Further studies are needed to explore the relationship between the different mammographic parameters such as breast thickness/composition, exposure factors, and target/filter combination in a clinical setting and help understand which settings produce the lowest dose with acceptable diagnostic image quality. Conclusions The ESD and AGD for single view projection in DM and DBT showed differences at 4 and 6 cm breast thickness and composition but not at 5 cm thickness with 30e32 kV and a Rh/Rh target/filter combination. Conflict of interest statement The authors declare no conflicts of interest. Acknowledgements The authors would like to thank all of the staff members especially Dr. Nouralhuda Karmani and Mrs. Yasmin Echreshzadeh from the Breast Imaging Clinic at Al-Sabah Hospital, Ministry of Health, Kuwait for their help with data collection. No Funding was requested or granted for this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.radi.2019.12.004. References 1. Cheung YC, Lin YC, Wan YL, Yeow KM, Huang PC, Lo YF, et al. Diagnostic performance of dual-energy contrast-enhanced subtracted mammography in dense breasts compared to mammography alone: interobserver blind-reading analysis. Eur Radiol 2014;24(10):2394e403. 2. Alakhras M, Bourne R, Rickard M, Ng KH, Pietrzyk M, Brennan PC. Digital tomosynthesis: a new future for breast imaging? Clin Radiol 2013;68(5): e225e36. 3. Skaane P, Bandos AI, Gullien R, Eben EB, Ekseth U, Haakenaasen U, et al. Comparison of digital mammography alone and digital mammography plus tomosynthesis in a population-based screening program. Radiology 2013;267(1):47e56. 4. Ciatto S, Houssami N, Bernardi D, Caumo F, Pellegrini M, Brunelli S, et al. Integration of JD digital mammograph y with tomosynthesis for population breast-cancer screening (STORM): a prospective comparison study. Lancet Oncol 2013;14(7):583e9.
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Please cite this article as: Asbeutah AM et al., The effect of different exposure parameters on radiation dose in digital mammography and digital breast tomosynthesis: A phantom study, Radiography, https://doi.org/10.1016/j.radi.2019.12.004