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Short Communication |
Divisions of 1 Medical Oncology, 2 Imaging Research, 3 Clinical Trials and Epidemiology, and 4 Preventive Oncology and Departments of 5 Medical Imaging and 6 Medical Biophysics, Sunnybrook Health Sciences Centre and 7 Centre for Research in Women's Health, Women's College Hospital, University of Toronto, Toronto, Ontario, Canada
Requests for reprints: Ellen Warner, Toronto-Sunnybrook Regional Cancer Center, Toronto, Ontario M4N 3M5, Canada. Phone: 416-480-4617; Fax: 416-480-6002. E-mail: ellen.warner{at}sunnybrook.ca
| Abstract |
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Methods: Breast density was evaluated for all patients on a high-risk screening study who were diagnosed with breast cancer between November 1997 and July 2006. Density was measured in two ways: qualitatively using the four categories characterized by the Breast Imaging Reporting and Data System and quantitatively using a computer-aided technique and classified as (a)
10%, (b) 11% to 25%, (c) 26% to 50%, and (d) >50% density. Comparison of sensitivity of mammography (and MRI) for each individual density category and after combining the highest two and lowest two density categories was done using Fisher's exact test.
Results: A total of 46 breast cancers [15 ductal carcinoma in situ (DCIS) and 31 invasive] were diagnosed in 45 women (42 with BRCA mutations). Mean age was 48.3 (range, 32-68) years. Overall, sensitivity of mammography versus MRI was 20% versus 87% for DCIS and 26% versus 90% for invasive cancer. There was a trend towards greater mammographic sensitivity for invasive cancer in women with fattier breasts compared with those with greater breast density (37-43% versus 8-12%; P = 0.1), but this trend was not seen for DCIS.
Conclusion: It is necessary to add MRI to mammography for screening women with BRCA mutations even if their breast density is low. (Cancer Epidemiol Biomarkers Prev 2008;17(3):706–11)
| Introduction |
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Several prospective single and multicenter observational studies have shown that for screening women at very high risk for breast cancer based on genetic testing or family history, breast magnetic resonance imaging (MRI) is significantly more sensitive than mammography (71-96% versus 28-43%; refs. 5-11). However, MRI is
10 times more costly than mammography and has significantly lower specificity with accompanying higher rates of recalls, additional imaging, and biopsies for resolving ambiguous screening results (5-7, 9, 10).
Identifying specific subpopulations of very high risk women for whom mammography alone might be an adequate screening tool would be highly desirable. For screening the general population, the amount and percentage of radiodense breast parenchyma relative to fat is inversely correlated with mammographic sensitivity. The reported sensitivity of mammography for fatty breasts ranges from 80% to 92% compared to 30% to 69% for dense breasts (12-15). Based on these data, we sought to determine whether mammography would have acceptable sensitivity for screening very high risk women with low breast density.
| Materials and Methods |
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Screening Protocol
An in-depth description of study methodology can be found in previous publications (5, 16). Eligible women were screened annually with film-screen mammography, MRI, and ultrasound and semiannually with clinical breast examination. All three annual imaging modalities were done successively on the same day at Sunnybrook Health Sciences Centre. Ultrasound screening was discontinued in May 2005 due to inadequate sensitivity and specificity.
Mammograms, ultrasound, and MRI examinations were classified using the American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) as follows: 0, needs further workup; 1, negative; 2, benign finding; 3, probably benign finding, short-term follow-up; 4, suspicious abnormality, biopsy should be considered; and 5, highly suggestive of malignancy (17). Biopsy was suggested if at least one screening modality was suspicious for malignancy (5).
Mammographic Density Measurement
The mammographic density of the ipsilateral breast of all women diagnosed with breast cancer in the study was categorized (a) qualitatively and (b) quantitatively. The ipsilateral breast was chosen for consistency across the patient population; the contralateral breast mammogram was not available/appropriate for patients with previous contralateral breast cancer and a previous ipsilateral mammogram was not available for all patients. Qualitative categorization determined by one of seven radiologists, who were experienced in breast imaging, was one of four BI-RADS categories: 1, mostly fatty (<25% dense); 2, scattered fibroglandular tissue (25-50% dense); 3, heterogeneously dense (51-75% dense); and 4, extremely dense (>75% dense; ref. 17). Interobserver variability was not formally assessed.
Quantitative breast density categorization has been described in detail in a publication by Byng et al. (18) and is shown in Fig. 1 . In this study, the cranio-caudal view of the ipsilateral mammogram from the most recent round of screening was digitized using a Lumisys 85 digitizer at 260 µm pixel size and 12 bits precision. All mammograms were presented to an observer blinded to clinical data, including method of detection. Using the computer program Cumulus, the observer selected two threshold gray-level values: one to identify the overall breast area and the other for classifying the dense area (18, 19). Cumulus calculated only those pixels above the threshold values chosen. Percentage of mammographic density was defined as the area of dense tissue divided by the overall breast area multiplied by 100 (Fig. 1).
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To compute the sensitivity of mammography and MRI for different density categories, the following quantitative categories were defined:
10%, 11% to 25%, 26% to 50%, and >50% dense. Although different from the BI-RADS quartiles, these categories were created based on our data set in which there were only eight women with a quantitative breast density of >50%.
For both qualitative and quantitative categories, the two lower and two higher density categories were collapsed due to very low numbers in the highest and lowest categories. Sensitivity was defined as the number of cancers detected by an imaging modality divided by the total number of cancers detected in the study. Using Fisher's exact tests, the sensitivities of mammography and MRI in the higher and the lower density categories were compared as were the overall sensitivities of both modalities.
| Results |
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Overall, as reported by others, mammography was significantly less sensitive than MRI for detecting breast cancer (24% versus 89%; P < 0.001; refs. 5-11). For women with BRCA1 versus BRCA2 mutations, the relative sensitivities of mammography and MRI were very similar (25% versus 25% for mammography and 84% versus 92% for MRI).
As illustrated in Table 2 , the sensitivity of mammography was 33%, 33%, 16%, and 33% for BI-RADS categories 1 to 4, respectively. Using quantitative categorization, sensitivity was 31%, 27%, 20%, and 12.5% for lowest to highest density categories, respectively.
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After collapsing the four categories into two (low density and high density) for both density measurement methods, comparisons were made between the mammographic sensitivities in the low density and high density categories (Fig. 3 ). The sensitivity of mammography for women with low density was not significantly greater than for women with high density as measured by either quantitative analysis (P = 0.49) or qualitative analysis (P = 0.30).
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| Discussion |
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One might argue that our study overestimates the benefits of MRI, as over time these DCIS lesions might have developed calcifications, enabling them to be detected by mammography before the development of invasion on subsequent screening. However, this seems unlikely given the universally low prevalence of DCIS without invasion reported in women with BRCA (particularly BRCA1) mutations not screened with MRI (3, 4, 20, 21).
The sensitivity of mammography for women in this study was generally poor (24%) compared with the
50% sensitivity reported when screening women with BRCA mutations using mammography without MRI. This is due, in part, to the much smaller size (and earlier stage) of the invasive cancers detected by MRI in this study (mean, 1.0 cm) compared with the size reported with mammography alone (mean, 1.3-1.7 cm; refs. 3, 20, 21). Some of the cancers detected by MRI, while still mammographically occult, would likely have been detected by mammography on a subsequent screen but at a larger size.
In our study, there was only a moderate correlation between qualitative and quantitative methods of breast density assessment with generally lower density values found with the quantitative technique. For example, using qualitative assessment, 27 of 45 cancers were rated as having >50% breast density, whereas the quantitative method identified only 8 cancers with this degree of density. This is not surprising in that the quantitative method is a simple binary threshold technique, whereas with the qualitative method the radiologist can take into account the degree of intensity of the densities and areas with density that is just below the quantitative threshold. On the other hand, the BI-RADS method of density assessment is limited, except at absolute extremes (that is, completely fatty or breasts composed entirely of dense fibroglandular tissue) due to its inherent nonquantitative techniques. In a recent report by Martin et al., although there was good correlation between quantitative estimates of breast density by trained radiologists and by an automated mammography density estimation program, there was poor correlation between the quantitative estimates and the qualitative assessments using BI-RADS categories (22).
One potential criticism of our study is that because the ipsilateral breast was used for density measurements, the presence of a tumor might have falsely elevated the measured density. However, because the mean tumor size was only 1 cm, such an effect was unlikely to have been significant. Moreover, similar results were obtained upon repetition of our analysis using the density of the contralateral breast for those patients without a history of contralateral breast cancer.
Corroborating our findings is a recent study by Lehman et al., investigating the sensitivity of MRI for detecting clinically and mammographically occult cancer in the contralateral breast at the time of a breast cancer diagnosis (23). In that study, MRI was just as likely to detect mammographically occult cancer in women with fatty breasts (9 of 299 = 3%) as in women with dense breasts (20 of 666 = 3%).
In conclusion, although mammography may be somewhat more sensitive for detecting invasive cancers in very high risk women with fatty breasts than in those with greater breast density, even in women with low breast density sensitivity was <50%, which is clearly inadequate. It is, therefore, appropriate to recommend MRI screening for all women with BRCA mutations regardless of their breast density.
| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: Some of the cases in this article have been reported previously with regard to relative sensitivity of MRI versus mammography (5, 16); however, breast density was not described or included as a variable.
Received 6/ 4/07; revised 12/12/07; accepted 12/18/07.
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