ISSSD 2014 April 13 to 16th, 2014. Cusco, Peru
VALIDATION OF MEAN GLANDULAR DOSE VALUES PROVIDED BY A DIGITAL BREAST TOMOSYNTHESIS SYSTEM IN BRAZIL
Bruno Beraldo Oliveira1, Lucas Paixão1, Sabrina Donato da Silva1 Maria Helena Araújo Teixeira2, Maria do Socorro Nogueira1/3
1 Post-graduation in Sciences and Technology of Radiations Minerals and Materials - CDTN/CNEN Presidente Antônio Carlos, 6627 31270-901, Belo Horizonte, Brazil [email protected]
2 Dr Maria Helena Araújo Teixeira Clinic Guajajaras, 40 30180-100, Belo Horizonte, Brazil [email protected]
3 Development Center of Nuclear Technology - CDTN/CNEN Presidente Antônio Carlos, 6627 31270-901, Belo Horizonte, Brazil [email protected]
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Abstract
Digital breast tomosynthesis (DBT) is an emerging imaging modality that provides quasi-three-dimensional structural information of the breast and has strong promise to improve the differentiation of normal tissue and suspicious masses reducing the tissue overlaps. DBT images are reconstructed from a sequence of low-dose X-ray projections of the breast acquired at a small number of angles over a limited angular range. The Hologic Selenia Dimensions system is equipped with an amorphous Selenium (a-Se) detector layer of 250 μm thickness and a 70 μm pixel pitch. Studies are needed to determine the radiation dose of patients that are undergoing this emerging procedure to compare with the results obtained in DBT images. The mean glandular dose (DG) is the dosimetric quantity used in quality control of the mammographic systems. The aim of this work is to validate DG values for different breast thicknesses provided by a Hologic Selenia Dimensions system using a DBT mode in comparison with the same results obtained by a calibrated 90X5-6M-model
Radcal ionization chamber. DG values were derived from the incident air kerma (Ki) measurements and tabulated conversion coefficients that are dependent on the half- value layer (HVL) of the X-ray spectrum. Voltage and tube loading values were recorded in irradiations using W/Al anode/filter combination, automatic exposure control mode and polymethyl methacrylate (PMMA) slabs which simulate different breast thicknesses. For Ki measurements, the ionization chamber was positioned at 655 mm from the focus and the same radiographic technique values were selected with the manual mode. DG values for a complete procedure ranged from 0.9 ± 0.1 to 3.7 ± 0.4 mGy. The results for different breast thicknesses are in accordance with values obtained by DBT images and with acceptable levels established by the Commission of the European Communities (CEC) and the International Atomic Energy Agency (IAEA). This work contributes to determine the reliable radiation dose received by the patients and validate the values provided by this DBT system.
Keywords: Digital breast tomosynthesis; mean glandular dose; incident air kerma
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1.- INTRODUCTION
Two-dimensional (2D) mammography currently is the gold standard for detecting breast cancer in an early stage and is used in screening programs in many countries [Van Schie et al., 2013]. It is a low cost, fast, noninvasive technique that involves low doses of ionizing radiation. However, 2D images results in tissue superposition [Sechopoulos 2013]. Digital breast tomosynthesis (DBT) is an emerging imaging modality that provides quasi- three-dimensional (3D) structural information of the breast and has strong promise to improve the differentiation of normal tissue and suspicious masses reducing the tissue overlaps [Lu et al., 2013]. DBT can reduce false positive findings and allow 3D localization of a lesion in the breast. The possibility of achieving these results at a dose level roughly equivalent to that used in conventional mammography has generated interest in the application of tomosynthesis for breast imaging [Wu et al., 2013]. The Hologic Selenia Dimensions system is equipped with an amorphous Selenium (a-Se) detector layer of 250 μm thickness and a 70 μm pixel pitch. For planar mammography, the system uses W/Rh and W/Ag anode/filter combinations while in DBT mode, the system changes the filtration to 700 μm of Al thickness. A Hologic proprietary filtered back projection (FBP) algorithm is used to reconstruct images from 15 low-dose X-ray projections of the breast acquired over 15º [Cockmartin et al., 2013]. DBT technique did not have widespread clinical use for many years but large-area digital detectors became available, and DBT gained a resurgence of research and clinical interest in the last decade [Li et al., 2013a]. The imaging performance of DBT is complicated mainly by the detector efficiency, the need for precise geometrical alignment, and the detection of X-ray scatter. None of DBT systems available incorporates the anti-scatter grid or a correction technique as a solution for reduction of the X-ray scatter signal that can result in loss of contrast and accuracy [Sechopoulos 2013; Sechopoulos et al., 2013]. The problem of a limited number of projections acquired is one of the challenges in attempting to reconstruct a 3D breast volume from 2D projection images [Wu et al., 2013]. DBT is characterized by anisotropic spatial resolution, with high spatial resolution in the planes parallel to the detector, and a considerably lower resolution in the perpendicular
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ISSSD 2014 April 13 to 16th, 2014. Cusco, Peru direction. However, this low spatial resolution is deemed to be enough to substantially reduce the issue of tissue superposition [Sechopoulos 2013]. The Food and Drug Administration (FDA) approved in 2011 the DBT systems using a 2D imaging alone or a combination of 2D and 3D imaging (Combo mode) [Gur et al., 2009; Philpotts et al., 2012]. The selection of Combo mode may result in more radiation exposures than with conventional mammography [Li et al., 2013b]. Radiation dose is an important concern in screening mammography.
Regulations require medical physicists to evaluate the Mean Glandular Dose (DG) as a part of quality control of services [Li et al., 2013a]. Studies are needed to determine the radiation dose of patients that are undergoing this emerging procedure to compare with the results obtained in DBT images.
The aim of this work was to validate DG values for different breast thicknesses provided by a Hologic Selenia Dimensions system using a DBT mode in comparison with the same results obtained by a calibrated 90X5-6M-model Radcal ionization chamber.
2.- MATERIALS AND METHODS
DG values were estimated from the incident air kerma (Ki) for exposure of patients using tabulated conversion coefficients that are dependent on the half-value layer (HVL) of the X-ray beam [IAEA 2007]. Measurements were carried using a Hologic Selenia Dimensions system located in a mammography clinic of Belo Horizonte city, Brazil. The medical physicist’s quality control tests of the DBT system used was in according to the quality assurance program for digital mammography established by the International Atomic Energy Agency (IAEA) [IAEA 2011]. The distance between the X-ray focal spot and the breast support was 700 mm. Irradiations were carried out using the automatic exposure control (AEC) and the DBT mode for polymethyl methacrylate (PMMA) slabs ranging from 20 to 70 mm of thickness. PMMA spacers of 1, 2, 5, 10 and 20 mm of thickness were used over the slabs to represent breasts of different attenuations (Figure 1). The W/Al anode/filter combination was used
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ISSSD 2014 April 13 to 16th, 2014. Cusco, Peru and the values of tube voltage (kV) and tube loading (mAsAEC) were recorded for each exposure.
Figure 1.- Setup for exposure parameters determination.
2.1.- Ki and HVL measurements
The Radcal 90X5-6M ionization chamber was centred laterally at 60 mm from the chest wall and 45 mm from the breast support. The Ki was measured using the manual mode for the same parameters selected to expose different thickness of PMMA slabs and spacers. For different values of mAs, the kerma (MAEC) values were estimated by extrapolation from a measurement taken at the closest available manual mAs setting (Mmanual) according to Equation 1 [IAEA 2011].
mAs M M AEC AEC manual (1) mAsmanual
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For Ki calculations, the Equation 2 were used, where Ki is the incident air kerma measured without backscatter at the surface of the slab of PMMA, Nmammo is the value of the calibration factor for beam quality, and kTP is the dosimeter correction factor for temperature and pressure [IAEA 2011].
Ki M AEC Nmammo kTP (2)
In these calculations, for correction of Ki values to the location of the entrance surface, the
Equation 3 were used, where Ki,n is Ki value obtained at a determined thickness, d is the distance between focus and breast support and n is the total thickness of PMMA slabs and spacers [IAEA 2011].
2 d 45 Ki,n Ki,45 d n (3)
The same setup was maintained and HVL values of the beam were calculated by adding high purity (99.999%) aluminum filtrations of 0.25, 0.35 and 0.5 mm of thickness positioned on the compression plate. The Equation 4 was used to obtain the acceptable HVL values depending of the tube voltage, where C is 0.25 for W/Al combination [IAEA 2011].
kV kV 0.03 HVL C 100 100 (4)
2.2.- DG measurements
In DBT systems, the DG is the sum of the doses received from individual projections. DG calculations were carried out using the Equation 5, where g, c, s and t() are conversion factors [Dance et al., 2011; IAEA 2011].
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DG Ki g c s t (5)
The factor g gives the DG for a breast having a 50% fibroglandular and 50% fat composition and is tabulated against breast thickness and HVL. The factor c allows the calculation for breast of different glandularity and is tabulated against HVL and thickness for typical breast compositions. The factor s gives a correction that depends on the anode/filter combination [Dance 1990; Dance et al., 2000]. The factor t() is the tomo factor for projection angle [Dance et al., 2011; IAEA 2011]. DG calculations were also carried out using the Equation 6, where the factor T gives a correction for a complete DBT examination [Dance et al., 2011].
DG Ki g c s T (6)
3.- RESULTS
Exposure parameters for different thickness of PMMA using the AEC mode and W/Al combination are shown in Table 1. The thickness and the fibroglandular proportion of equivalent breast were obtained by adding the respective thickness of PMMA spacers on the PMMA slabs.
Table 1.- Exposure parameters for each thickness of PMMA slabs. Thickness of Thickness of Fibroglandular Tube voltage Tube loading PMMA [mm] equivalent breast* proportion of [kV] [mAs] [mm] equivalent breast* [%] 20 21 97 26 41 30 32 67 29 42 40 45 41 30 57 50 60 20 33 77 60 75 9 36 86 70 90 4 42 69 * Data provided by IAEA [2011].
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3.1.- Ki and HVL measurements
Ki and HVL values for different thickness of PMMA, besides tolerances and HVL values provided by the DBT system are shown in Table 2. The relative expanded uncertainty (k =
2) of the Ki and HVL measurements was calculated as 6.7%.
Table 2.- Ki and HVL values for each thickness of PMMA slabs.
Thickness of Ki [mGy] HVL [mm Al] PMMA [mm] Provided Calculated Tolerance 20 1.87 ± 0.13 0.45 0.46 ± 0.03 0.29 - 0.51 30 2.88 ± 0.19 0.51 0.53 ± 0.04 0.32 - 0.54 40 4.26 ± 0.29 0.53 0.55 ± 0.04 0.33 - 0.55 50 7.25 ± 0.49 0.59 0.60 ± 0.04 0.36 - 0.58 60 10.20 ± 0.68 0.64 0.64 ± 0.04 0.39 - 0.61 70 12.07 ± 0.81 0.74 0.70 ± 0.05 0.45 - 0.67
3.2.- DG measurements
Calculated DG values with the respective uncertainties and different thickness of PMMA for individual projections and for a complete DBT examination are shown in Figures 2 and
3 respectively. The relative expanded uncertainty (k = 2) of the DG measurements was calculated as 10%. Figure 3 also indicates values provided by the DBT system and by Oliveira et al., [2011] using a computed radiography (CR) system beyond the acceptable and desired levels established by the Commission of the European Communities (CEC) and the International Atomic Energy Agency (IAEA) [CEC 2006; IAEA 2011].
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Figure 2.- DG values for each thickness of PMMA slabs and projection angles.
Figure 3.- DG values for each thickness of PMMA slabs.
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4.- DISCUSSION
Considering the uncertainties, HVL results are in accordance with the tolerance established by IAEA [2011] and with values provided by the DBT system. The validation of HVL values provided by the Brazilian DBT system images is important to obtain reliable DG values. In Figure 2, there is no statistical difference in DG values for the same PMMA thickness and different projections of the X-ray tube. The relative error between the minimum and maximum DG values was approximately 76%. In Figure 3, DG values ranging from 0.90 ± 0.09 to 3.75 ± 0.37 mGy. Considering the uncertainties, DG results are in accordance with the values provided by the DBT system images and the acceptable levels established by CEC [2006] and IAEA [2011]. Moreover, for larger thickness of PMMA, the calculated DG values are under the desired levels established. In both figures, the minimum DG values was registered at 20 mm of PMMA thickness and the results increases with the thickness of PMMA, due to higher X-ray attenuation in larger thickness of this material. In accordance with Oliveira et al., [2011] there are around 600 mammography equipments with CR systems in Brazil and according to data obtained by the manufacturer’s representatives approximately 15 Hologic Selenia Dimensions systems. Therefore, it is important to perform a comparison of the radiation dose of patients that are undergoing a CR procedure with this new radiographic technique.
Oliveira et al., [2011] have published information on assessment of DG and image quality in mammography using a CR system located in a Brazilian laboratory. Considering the uncertainties, their DG values are higher than the results obtained in this work only for the last two thickness of PMMA.
5.- CONCLUSIONS
In this work, DG values were calculated to compare with values provided by the DBT system and the reference levels established internationally. Considering the uncertainties, the results are according to DG values provided by the equipment. Moreover, the results for
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ISSSD 2014 April 13 to 16th, 2014. Cusco, Peru each thickness of PMMA are in accordance with acceptable levels established by IAEA [2011] and CEC [2006]. The dosimetry is an important part of quality control and crucial for the optimization of DBT procedures. Taking into consideration the fact that this DBT system showed previous results of medical physicist’s quality control tests in according to the quality assurance program for digital mammography established by IAEA [2011], this system has proved to be calibrated and optimized. Therefore, the aim of this work was fulfilled by consolidating the dosimetry in DBT systems in addition to validate DG values provided by this equipment.
The results also showed that DG increases with the thickness of PMMA because of the higher X-ray attenuation in larger thickness of equivalent breasts. There is no statistical difference in DG values for each PMMA thickness and individual projections with the X- ray tube ranging from -7.5º to +7.5º.
For thick and fat equivalent breasts, DG values are under the desired levels established. This work also shows that the adoption of a DBT system does not imply an increase in DG values when compared with results obtained by Oliveira et al. [2011] in a CR system.
Acknowledgments
The authors are thankful to CNEN and CAPES for the incentive with PhD’s fellowship and Dr. Maria Helena Araújo Teixeira for providing the mammography unit. This work was supported by FAPEMIG and Ministry of Science and Technology - MCT/Brazil, through the Brazilian Institute of Science and Technology (INCT) for Radiation Metrology in Medicine.
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