Breast Surface Radiation Dose During Coronary CT Angiography: Reduction by Breast Displacement and Lead Shielding

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1 Cardiopulmonary Imaging Original Research Foley et al. reast Dose During Coronary CT Cardiopulmonary Imaging Original Research Shane J. Foley 1 Mark F. McEntee 1 Stephan chenbach 2 Patrick C. rennan 3 Louise S. Rainford 1 Jonathan D. Dodd 4 Foley SJ, McEntee MF, chenbach S, rennan PC, Rainford LS, Dodd JD Keywords: breast neoplasms, CT, epidemiology, radiation dose, women DOI: /JR Received March 7, 2010; accepted after revision January 10, School of Medicine and Medical Science, University College Dublin, Dublin, Ireland. 2 Department of Cardiology, University of Erlangen, Erlangen, Germany. 3 Faculty of Health Sciences, University of Sydney, Lidcombe New South Wales, ustralia. 4 Department of Radiology, St Vincent s University Hospital, Elm Park, Dublin 4, Ireland. ddress correspondence to J. D. Dodd. JR 2011; 197: X/11/ merican Roentgen Ray Society reast Surface Radiation Dose During Coronary CT ngiography: Reduction by reast Displacement and Lead Shielding OJECTIVE. The purpose of this study was to prospectively evaluate the effect of cranial breast displacement and lead shielding on in vivo breast surface radiation dose in women undergoing coronary CT angiography. SUJECTS ND METHODS. Fifty-four women (mean age, 59.2 ± 9.8 years) prospectively underwent coronary 64-MDCT angiography for evaluation of chest pain. The patients were randomly assigned to a control group (n = 16), breast displacement group (n = 22), or breast displacement plus lead shielding group (n = 16). Thermoluminescent dosimeters (TLDs) were placed superficially on each breast quadrant and the areolar region of both breasts. reast surface radiation doses, the degree of breast displacement, and coronary image quality were compared between groups. phantom dose study was conducted to compare breast doses with z-axis positioning on the chest wall. RESULTS. total of 1620 TLD dose measurements were recorded. Compared with control values, the mean breast surface dose was reduced 23% in the breast displacement group (24.3 vs 18.6 mgy, p = 0.015) and 36% in the displacement plus lead shielding group (24.3 vs 15.6 mgy, p = ). Surface dose reductions were greatest in the upper outer (displacement alone, 66%; displacement plus shielding, 63%), upper inner (65%, 58%), and areolar quadrants (44%, 53%). The smallest surface dose reductions were recorded for -cup breasts: 7% for the displacement group and 3% for the displacement plus lead group (p = 0.741). Larger reductions in surface dose were recorded for -cup (25% and 56%, p = 0.273), C-cup (38% and 60%, p = 0.001), and D-cup (31% and 25%, p = 0.095) sizes. Most of the patients (79%) had either good (> 50% of breast above scan range) or excellent (> 75% of breast above the scan range) breast displacement. No significant difference in coronary image quality was detected between groups. The phantom dose study showed that surface TLD measurements were underestimates of absorbed tissue dose by a mean of 9% and that a strong negative correlation exists between the amount of cranial displacement and breast dose. CONCLUSION. Use of breast displacement during coronary CT substantially reduces the radiation dose to the breast surface. C linical acceptance of coronary CT angiography (CT) continues to increase. Two studies with results published in 2008 [1, 2] showed consistently high sensitivity and negative predictive value in the detection of obstructive coronary artery disease. Despite the rapidly evolving technology of coronary CT and the increasing applications, radiation dose continues to be a concern [3]. Depending on CT scanner capability and the protocol applied, total effective doses during coronary CT vary from 2.1 to 21.4 msv [4, 5]. These doses are among the highest delivered in diagnostic radiology and are of particular concern because three of the most ra- diosensitive organs lung, bone marrow, and breast are within the scan range [6]. Guidelines established in 2008 suggest higher breast radiosensitivity than previously thought [7]. The International Commission on Radiological Protection has recommended a 140% increase in tissue-weighting factors for the breast, from 0.05 to 0.12 [6]. n increased incidence of breast cancer has already been identified in populations exposed to radiation [8 10]. Therefore, techniques to minimize breast tissue radiation by optimizing CT technique would be beneficial [11, 12]. Coronary CT typically includes most of the breast tissue within its scan range, exposing the breasts to radiation even though the JR:197, ugust

2 Foley et al. breast is not the target organ. Coronary CT has a unique protocol with limited z-axis coverage in which only the mid to lower portion of the thorax usually is imaged. Displacing the breasts cranially outside the direct x-ray beam therefore has potential as a method of reducing radiation exposure to this organ. Furthermore, removing breast tissue from the scan range may also improve image quality. Previous studies of CT have shown that lead shields are effective in reducing the amount of external scatter reaching superficial tissues outside the scanning plane [13 15]. The aims of this study were to evaluate the effects of breast displacement and breast displacement plus lead shielding on breast surface radiation dose and coronary image quality in women undergoing coronary CT. phantom study was performed to correlate breast surface dose and absorbed dose. Subjects and Methods Patients The institutional review board approved the study, and written informed consent was obtained from all patients. Fifty-four women (mean age, 59.2 ± 9.8 [SD] years; range, years) prospectively underwent coronary CT for the evaluation of chest pain between January and December Patients were excluded if they had a history of mastectomy, allergy to iodinated contrast agents, or renal failure. Computer-generated randomization ( was used to randomly assign patients to three groups: a control group (n = 16), a breast displacement group (n = 22), and a breast displacement plus lead shielding group (n = 16). Coronary CT ngiography Protocol ll patients were examined with a retrospective ECG-gated coronary CT protocol with a single-source 64-MDCT scanner (Somatom Sensation 64, Siemens Healthcare). CT technical parameters included 120 kvp, 800 ms effective, 0.6-mm slice thickness, 0.6-mm collimation, 50% slice overlap, 330-ms gantry rotation time, and pitch of 0.2. ll scans were obtained with ECG tube current modulation (mindose mode was not available on the scanner at the time of this study). The craniocaudal scan range was based on the initial topogram, extending from the carina to the base of the heart, obtained in one breathhold. n oral β-blocker ( mg metoprolol, Fig. 1 reast displacement device., Photograph shows breast displacement device and lead shield. Side extensions are available to cover larger breasts, and lead shield can be attached with hook-and-loop fastener., Photograph shows breast displacement device and lead shield in position on model. Fig year-old woman with atypical chest pain. Control group, C-cup breast., CT topogram shows considerable portion of breasts lies within scan range (dashed lines)., xial CT image at level of left main coronary artery shows considerable glandular breast tissue (arrows) in scan range. strazeneca) was administered to all patients, who also received two sprays of sublingual nitroglycerin immediately before coronary CT acquisition to induce coronary artery dilation. fter administration of a timing bolus to ascertain the optimum time for the start of scan acquisition (20 ml of contrast agent injected at 6 ml/s with a region of interest placed over the ascending aorta), ml of iohexol 350 mg/ml (Omnipaque, GE Healthcare) was injected into an antecubital vein at 6 ml/s and followed by a 40-mL saline bolus chaser at 6 ml/s. reast Displacement reast cup size was self-reported by each patient and recorded. reast displacement was achieved with a breast displacement system (Chrysalis, Meta Imaging Solutions) (Fig. 1) applied before scanning. The control group had no breast displacement device applied (Fig. 2). The patients in the breast displacement manually displaced their breasts, and the displacement system was wrapped around the chest wall, retaining the breasts in a cephalic position (Fig. 3). The device is radiolucent and has adjustable straps and extension pieces to accommodate a variety of patient and breast sizes. For patients in the third group, a lead rubber shield (0.5 mm lead equivalence, cm) was attached anterior to the displacement device with a strip of hook-and-loop fastener and was placed to cover as much of the breast tissue as possible without impinging on the scan range. The topogram was reviewed to ensure correct application of the device, which has a fiducial marker embedded into its superior edge to indicate the upper border of the displacement device, thus signifying the lower margin of the breast tissue. The breasts were displaced as far as possible in a cranial direction with maintenance of patient comfort. One reader blinded to breast radiation results and patient group subjectively assessed the effectiveness of the device in displacing the breasts. reast displacement was qualitatively rated on a four-point scale. Topograms and axial images were used to assess the location of the breasts compared with the prescribed z-axis coverage. Poor was defined as minimal displacement, % of the breast tissue within the scan range; moderate, 50 74% of breast tissue within the scan range; good, 25 49% of breast tissue within the scan range; and excellent, 0 24% of breast tissue within the scan range. We quantitatively scored breast displacement by summing the number of axial slices that contained breast tissue. reast Surface Dose Measurements Radiation doses to the breast surface were recorded with thermoluminescent dosimeters (TLDs). 368 JR:197, ugust 2011

3 reast Dose During Coronary CT at the level of the origin of the left main coronary artery and in air anterior to the chest wall [16]. Standard TLD 100 lithium fluoride chips ( mm) and a TLD reader (Harshaw 5500, Thermo Fisher Scientific) were used. The TLDs were first calibrated in air with an ionization chamber. Calibration was performed at 120 kvp at the center of rotation in the gantry of the CT scanner. Ten TLDs were irradiated to known doses and measured. This step was used to calculate a conversion factor for converting the TLD readout to a dose in milligrays. Three TLDs were placed on each breast at the center of each of the standard anatomic quadrants (upper outer, upper inner, lower outer, lower inner) halfway between the nipple and the outer breast margin; an additional three TLDs were placed directly on the areola. efore breast displacement and scanning, a technologist placed the TLDs in protective polyvinyl chloride pockets, labeled them, and taped them on the designated quadrants of each breast. fter coronary CT, the TLDs were removed and read in the TLD reader within 24 hours. For each patient, a surface dose for each quadrant and mean breast radiation surface dose (average of the five measured sectors) were recorded. The mean surface dose for each displacement group also was calculated. CT dose index and dose-length product were recorded directly from the CT console at completion of the examinations. To assess whether breast displacement resulted in reduction in absorbed dose to the breast, a phantom study was performed. Five groups of three TLDs were placed on the surface and 4 cm deep within a C-cup sectional anthropomorphic breast phantom (Rando, lderson Research Laboratories) at the aforementioned locations. The phantom was scanned with a coronary CT protocol identical to the patient protocol. Scanning Fig year-old woman with chest pain. reast displacement alone group, C-cup breast., CT topogram shows that with addition of breast displacement device, most breast tissue has been displaced cranially outside scan range (dashed lines)., xial CT image at level of origin of left main coronary artery shows breast displacement device around chest wall (arrows) and resultant absence of glandular breast tissue from scan range. was repeated with the breast phantom displaced in a cranial direction at 3-cm increments up to 12 cm with new TLDs each time. Qualitative Coronary nalysis Multiplanar reformatting was used for optimal evaluation of each coronary artery. Coronary artery image quality was qualitatively assessed by an expert reader not involved in TLD placement or analysis and blinded to patient demographics and patient group. The overall quality of images of the coronary vessels was scored with the mean score for the four main coronary arteries (left main, left anterior descending, left circumflex, and right coronary artery) on the basis of a 4-point grading system described previously [11]: 1, nondiagnostic, indicating low image quality that precluded appropriate evaluation of the coronary arteries; 2, adequate, indicating image quality that because of motion, noise, or low contrast, obviously affected assessment of the vessel but still was sufficient to rule out significant stenosis; 3, good, indicating nonlimiting artifacts, reduced attenuation of vessel lumen, or calcification with preserved evaluability of luminal stenosis and plaque characteristics; 4, excellent, indicating complete absence of motion artifacts, good attenuation of the vessel lumen, and clear delineation of vessel walls with ability to assess luminal stenosis and plaque characteristics. Quantitative nalysis We quantitatively assessed image noise by recording the SD of attenuation in Hounsfield units in two homogeneous regions of interest. This assessment was performed by a single reader, who placed a circular regions of interest (2 cm 2 ) in the aortic root Patient Satisfaction On completion of imaging, a random sample of 20 patients from both displacement groups were asked to complete a short questionnaire regarding wearing the displacement device. Each patient rated comfort and symptoms of pain and breathlessness on a 5-point Likert scale. They were also asked whether they would be willing to wear the device in the future. Statistical nalysis Statistical analysis was performed with SPSS software (version 15.0, SPSS). Quantitative variables are expressed as mean ± SD. Dosimetry results were checked for normality and analyzed by one-way analysis of variance and post hoc onferroni testing when appropriate. Pearson correlation analysis was used to investigate the relation between amount of displacement and dose for both the patient and the phantom studies. The nonparametric image quality results were analyzed with Kruskal-Wallis statistics. prospective power calculation was performed with G*Power software [17]. power of greater than 80% would be achieved to detect a 10% difference in dosimetry between groups with a minimum of 16 patients in each group. Statistical significance was defined as p Results The patient demographics are shown in Table 1. There were no significant between-group differences in mean body mass index (p = 0.1), heart rate (p = 0.755), vessel enhancement (525 ± 95 vs 515 ± 77 vs 476 ± 80, p = 0.214), dose-length product (607.1 ± 76.2 vs ± 79.2 vs ± 70.5, p = 0.593), or z-axis coverage (138 ± 21 vs 138 ± 16 vs 141 ± 17, p = 0.774). Ten patients had at least one obstructive coronary lesion (> 50% luminal narrowing), six in the breast displacement group, two in the control group, and two in the breast displacement plus lead group. total of 1620 TLD measurements were recorded. Compared with the dose in controls, the mean breast surface radiation dose in the breast displacement group was reduced 23% (24.3 vs 18.6 mgy, p = 0.015) and in the displacement plus lead shielding group, 36% (24.3 vs 15.6 mgy, p = ). The upper quadrants and areolar region had statistically significant reductions (Table 2) of 66% (p = 0.004), 65% (p = 0.002), and 44% (p = 0.001) in the displacement alone group and 63% (p = 0.01), 58% (p = 0.011), and 53% (p = ) in the displacement JR:197, ugust

4 Foley et al. TLE 1: aseline Characteristics of Study Group Characteristic Control reast Displacement reast Displacement Plus Lead No. of patients ge (y) 61 ± ± ± 10 Height (m) 1.63 ± ± ± 6.4 Weight (kg) 70.4 ± ± ± 11.6 ody mass index a 26.6 ± ± ± 4.9 Heart rate (beats/min) 55.5 ± ± ± 8.8 a Weight in kilograms divided by the square of height in meters. TLE 2: Mean reast Radiation Surface Dose for Each Patient Group Location Control reast Displacement reast Displacement Plus Lead Upper outer quadrant ( 66) a 4.3 ( 63) a Upper inner quadrant ( 65) a 7.4 ( 58) a Lower outer quadrant (+12) 23.1 ( 14) Lower inner quadrant (+5) 27.6 ( 17) reolar ( 44) a 15.6 ( 53) a Mean ( 23) a 15.6 ( 36) a Note Results are in milligrays. Numbers in brackets are percentage change compared with controls. a p < 0.05 relative to the control group. plus lead shielding group. The lower outer and lower inner breast quadrants exhibited no statistically significant differences in any group. Dosimetry results were evaluated by breast cup size (Fig. 4). The smallest surface dose reductions were recorded for the patients with -cup breasts with reductions of 7% in the displacement group and 3% in the displacement plus lead group (p = 0.741). Larger surface dose reductions were recorded for the larger breast sizes in the displacement and displacement plus lead groups. The patients with -cup breasts had reductions of 25% and 56% (p = 0.273); C-cup breasts, 38% and 60% (p = 0.001); and D-cup breasts 31% and 25% (p = 0.095). nalysis of breast displacement achieved for various breast sizes showed that most of the patients (79%, 30/38) had either good or excellent displacement. Patients with -cup breasts had the least successful displacement (83%, 5/6, rated poor or moderate), and those with D-cup breasts had the most successful displacement (100%, 15/15, rated good or excellent). reast tissue was evident in more than 56% of the mean z-axis coverage of the control group (77 of 138 mm) but with displacement and displacement plus lead, this coverage decreased to 22% (30 of 138 mm) and 16% (23 of 141 mm) (p = 0.001) (Table 3). positive correlation (r = 0.57, p = 0.001) was found between amount of breast tissue included in the scan range and mean dose. Results of the phantom study (Table 4) showed that the average absorbed dose to the breast was 9% greater than the skin dose. Significant negative correlations were found between amount of cephalic displacement and mean surface dose (r = 0.943, p = 0.016) and absorbed dose (r = 0.98, p = 0.003). mean dose reduction of 90% was achieved when the breasts were maximally (12 cm) displaced in the cephalic direction from their original position. Significant dose reductions were achieved for the upper quadrants and areolar region at 3 6 cm of cephalic displacement and for the lower quadrants at 6 9 cm. The mean qualitative coronary image quality scores for the control, displacement, Fig. 4 Graph shows comparison of mean breast surface radiation dose per cup size. Dose reduction increases with increasing cup size. Reduction is statistically significant for C cup only. Mean reast Surface Radiation Dose (mgy) Cup and displacement plus lead groups were 3.4 ± 0.8, 3.6 ± 0.6, and 3.6 ± 0.7 (p = 0.442, χ 2 = 1.63) (Fig. 5). The mean noise measurements in the aorta for each group were 9.4 ± 2, 9.7 ± 3, and 10.5 ± 2 (p = 0.898, χ 2 = 0.216) and in air were 20.5 ± 7, 21.7 ± 11, and 22.9 ± 7 (p = 0.142, χ 2 = 3.908). No patient reported pain, discomfort, or shortness of breath, and 95% of patients (19/20) replied that they would wear the device again. ecause of anxiety, one patient with a history of claustrophobia requested the device be removed. Discussion Strategies for reduction of radiation dose to the breast during CT of the chest have been evaluated in numerous studies [18 21]. n important attribute of coronary CT is its short scan range in the z direction (typically from the level of the carina to the base of the heart) in comparison with that of chest CT. This property affords a distinct opportunity to specifically reduce the amount of breast tissue directly in the path of the x-ray beam by displacing the breasts cranially in relation to the scan range. To our knowledge, this study is the first to prospectively evaluate this technique in coronary CT and to evaluate the in vivo effect on breast surface radiation dose. Compared with the control group, the breast displacement group had a mean 24% reduction in breast surface dose. The greatest reductions were found in the upper outer (66%) and inner (65%) quadrants and in the areolar region (44%). ecause these areas account for most of the glandular breast tissue (83%) [22], this reduction is important. lthough displacement significantly reduced the mean amount of breast tissue included in the z-axis coverage from 77 mm to 30 mm, significant dose reductions were not achieved in the Control Displacement Displacement + Lead Cup C Cup D Cup 370 JR:197, ugust 2011

5 reast Dose During Coronary CT TLE 3: Mean mount of reast Tissue (mm) Within Scan Range Categorized by Cup Size Cup Size Control Displacement Displacement + Lead 69 ± 35 (3) 30 ± 13 (4) 43 ± 23 (2) 72 ± 14 (4) 22 ± 13 (5) a 29 ± 5 (2) a C 81 ± 15 (2) 40 ± 11 (5) a 12 ± 13 (5) a D 80 ± 22 (7) 28 ± 18 (8) a 23 ± 15 (8) a Mean 77 ± ± 15 a 23 ± 17 a Note Values in parentheses are numbers of patients in each group. a p < 0.05 relative to the control group. TLE 4: Mean reast Phantom Dose (mgy) With Incremental Cephalic Displacement Displacement (cm) Location Upper outer quadrant Skin a 4.8 a 2.5 a 1.4 a Deep a 3.7 a 2.1 a Upper inner quadrant Skin a 3.3 a 2.3 a Deep a 8.5 a 4.0 a 2.1 a Lower inner quadrant Skin a 25.5 a 7.5 a 5.2 a Deep a 4.3 a Lower outer quadrant Skin a 2.8 a Deep a 3.3 a reolar Skin a 15.6 a 2.4 a 1.8 a Deep a 3.6 a Mean Skin Deep a p < lower quadrants, likely because of their continued presence in the z-axis coverage. Two skin dose measurements in the phantom study reinforce this finding: doses in the lower outer quadrant (displacement, 3 cm) and lower inner quadrant (displacement, 6 cm) also increased slightly. More important, absorbed dose in the phantom consistently decreased at all degrees of displacement. reast cup size clearly influenced the results. Patients with -cup breasts were the only group not to have a significant reduction in the amount of breast tissue in the scan range and consequently had minimal dose reduction. This finding is supported by the positive correlation (r = 0.57, p < 0.01) between breast surface dose and amount of breast tissue in the scan range. We postulated that adding lead shielding would further improve the radioprotective effects of the displacement technique by reducing the quantity of scattered radiation to the breasts. n additional mean dose reduction of 16% was recorded compared with the findings in the displacement alone group. The lower quadrants and areolar regions benefited most from displacement plus shielding, exhibiting statistically significant reductions compared with the control and displacement alone groups. In other quadrants, we found no decrease and in some quadrants, slight dose increases with displacement, but the difference was not statistically significant and was likely to be related to the relatively small patient numbers in each subgroup. The combination of displacement and lead shielding proved most effective for patients with -cup and C-cup breasts, almost doubling the mean dose reduction achieved when this method was used. numeric increase was found in the -cup patients with displacement plus lead shielding. This finding is related to the minimal displacement achieved with this cup size while avoiding placing the lead shield directly in the scan range. Thus shielding of the breasts was minimal for this subgroup. bsorbed radiation dose to the breast tissue cannot be measured directly, but the phantom study showed that surface TLD measurements closely approximate the absorbed dose in the breast. s have previous findings [21], the results showed that surface measurements are underestimates of absorbed dose. This phenomenon is likely a result of x-ray beam hardening because the softer radiation is absorbed superficially, causing the more energetic beam to traverse deeper tissues. The phantom study also showed consistent reductions in absorbed dose with increasing displacement, the largest mean dose reductions of up to 90% occurring with increasing breast displacement. Interestingly, for significant reductions to occur in the lower breast quadrants, phantom displacement of up to 9 cm was required. This finding suggests that had we used more rigorous displacement of the breasts, we might have found more substantial reductions in the lower quadrants. lthough a surface dose of 24.3 mgy is reported for our control group, this value is within the range of mgy, the highest individual quadrant dose recorded being 54 mgy (lower inner quadrant). Such variation is attributable to the varying patient morphologic features and positioning of the breast tissue with the patient supine and arms extended and shows the value of using TLDs to record individual patient doses. wide variation of doses exists in the literature [23, 24], which several other factors may influence, including type of scanner, scanning parameters, breast thickness, and amount of glandular tissue within the breasts [25 27]. However, differences may also be ascribed to whether doses are calculated [28] or directly measured [29]. We used TLDs to record surface breast doses in clinical use and had the advantage of taking into account individual patient sizes and positions. We emphasize JR:197, ugust

6 Foley et al. Fig. 5 CT angiograms of similar image quality in depicting left anterior descending coronary artery., 48-year-old woman with atypical chest pain. Control group; C-cup breast; body mass index, 23.2., 43-year-old woman with atypical chest pain. reast displacement alone group; C-cup breast; body mass index, C, 62-year-old woman with atypical chest pain. reast displacement plus lead shielding group; C-cup breast; body mass index, 26. that our primary goal was to assess the relative changes in breast surface dose in comparison with control values. ecause patients were randomized into three different groups with similar demographics, we believe the effects of breast size and amount of glandular tissue should not have influenced our results. Many coronary CT techniques have been developed to reduce radiation dose [30]. ECG tube modulation, in which tube current is reduced during systole, results in radiation dose reductions of approximately 35 40% [31]. Greater reduction can be achieved with prospective ECG-triggered coronary CT [32] by decreasing the peak kilovoltage [11] or increasing the pitch [33]. We evaluated only one dose-saving technique (breast displacement plus lead shielding), but it seems likely that a combination of techniques would result in even greater reduction in breast surface dose. Two technical aspects of our study are worthy of further comment. First, we hypothesized that coronary image quality might improve with the removal of breast tissue from the scan range or be compromised by the lead shield. However, we found no statistical difference between groups in either the qualitative or quantitative analysis. Second, the technique we describe is less suitable for other coronary CT applications, such as coronary bypass graft evaluation and triple-rule-out protocols, in which the scan range typically begins above the aortic arch and would include the breasts. Our study had several limitations. The most important was that TLD dose measurements record surface rather than absorbed organ dose. The number of patients in our study was relatively low. lthough the power analysis showed good strength in discriminating between-group surface dose differences, cupsize analysis might have been limited by the small numbers, and further work on this topic may be beneficial. ecause of privacy concerns regarding breast manipulation, patients performed the breast displacement themselves under supervision. Patients might not have performed the procedure optimally, and consistently more successful displacement might have been achieved if a healthcare professional had performed the displacement. Conclusion reast displacement during coronary CT is a feasible technique that results in a significant reduction in mean breast surface radiation dose without affecting coronary image C 372 JR:197, ugust 2011

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