Mammography—Variables Associated with Accuracy
Several characteristics of women being screened that are associated with the accuracy of mammography include age, breast density, whether it is the first or subsequent exam, and time since last mammogram. Younger women have lower sensitivity and higher false-positive rates on screening mammography than do older women (refer to the Breast Cancer Surveillance Consortium performance measures by age for more information).
For women of all ages, high breast density is associated with 10% to 29% lower sensitivity. High breast density is an inherent trait, which can be familial [2,3] but also may be affected by age, endogenous  and exogenous [5,6] hormones, selective estrogen receptor modulators such as tamoxifen, and diet. Hormone therapy is associated with increased breast density and is associated not only with lower sensitivity but also with an increased rate of interval cancers.
The Million Women Study in the United Kingdom revealed three patient characteristics that were associated with decreased sensitivity and specificity of screening mammograms in women aged 50 to 64 years: use of postmenopausal hormone therapy, prior breast surgery, and body mass index below 25. In addition, a longer interval since the last mammogram increases sensitivity, recall rate, and cancer detection rate and decreases specificity.
Strategies have been proposed to improve mammographic sensitivity by altering diet, timing mammograms with menstrual cycles, interrupting hormone therapy before the examination, or using digital mammography machines. Obese women have more than a 20% increased risk of having false-positive mammography results compared with underweight and normal weight women, although sensitivity is unchanged.
Some cancers are more easily detected by mammography than other cancers are. In particular, mucinous, lobular, and rapidly growing cancers can be missed because their appearance on x-rays is similar to that of normal breast tissue. Medullary carcinomas may be similarly missed. Some cancers, particularly those associated with BRCA1/2 mutations, masquerade as benign tumors.[17,18]
Radiologist performance is critical to assessing mammographic interpretive performance, yet there is substantial, well-documented variability among radiologists. Factors that influence radiologists’ performance include their level of experience and the volume of mammograms they interpret. There is often a trade-off between sensitivity and specificity, such that higher sensitivity may be associated with lower specificity. Radiologists in academic settings have a higher positive predictive value (PPV) of their recommendations to undergo biopsy than do community radiologists. Fellowship training in breast imaging may lead to improved cancer detection, but it is associated with higher false-positive rates.
After controlling for patient and radiologist characteristics, screening mammography interpretive performance (specificity, PPV, area under the curve [AUC]) varies by facility and is associated with facility-level characteristics. Higher interpretive accuracy of screening mammography was seen at facilities that offered screening examinations alone, included a breast imaging specialist on staff, did single as opposed to double readings, and reviewed interpretive audits two or more times each year.
False-positive rates vary significantly between facilities performing diagnostic mammography and are higher at facilities where concern about malpractice is high. False-positive rates are also higher at facilities serving vulnerable women (women of racial or ethnic minorities and women with lower educational attainment, limited household income, or rural residence) than at facilities serving nonvulnerable women, perhaps because of poorer compliance with recommendations for follow-up examinations. Analyses that do not adjust for important patient characteristics may falsely conclude that there is more facility variation in overall accuracy than actually exists.
International comparisons of screening mammography have found higher specificity in countries with more highly centralized screening systems and national quality assurance programs.[24,25] For example, one study reported that the recall rate is twice as high in the United States as it is in the United Kingdom, yet there is no difference in the rate of cancers detected. Such comparisons may be confounded by social, cultural, and economic factors.
Prevalent Versus Subsequent Examination and the Interval Between Exams
The likelihood of diagnosing cancer is highest with the prevalent (first) screening examination, ranging from 9 to 26 cancers per 1,000 screens, depending on the woman’s age. The likelihood decreases for follow-up examinations, ranging from 1 to 3 cancers per 1,000 screens. The optimal interval between screening mammograms is unknown. In particular, the breast cancer mortality-focused, randomized, controlled trials used single screening intervals with little variability across the trials. A prospective United Kingdom trial randomly assigned women aged 50 to 62 years to receive mammograms annually or at the standard 3-year interval. Although the grade and node status were similar in both groups, more cancers of slightly smaller size were detected in the annual screening group, with a lead time of approximately 7 months in comparison with triennial screening.
A large observational study found a slightly increased risk of late-stage disease at diagnosis for women in their 40s who were adhering to a 2-year versus a 1-year schedule (28% vs. 21%; odds ratio (OR), 1.35; 95% confidence interval [CI], 1.01–1.81), but no difference was seen for women in their 50s or 60s.[28,29]
A Finnish study of 14,765 women aged 40 to 49 years assigned women born in even-numbered years to annual screens and women born in odd-numbered years to triennial screens. The study was small in terms of number of deaths, with low power to discriminate breast cancer mortality between the two groups. There were 18 deaths from breast cancer in 100,738 life-years in the triennial screening group and 18 deaths from breast cancer in 88,780 life-years in the annual screening group (hazard ratio, 0.88; 95% CI, 0.59–1.27).
Digital mammography is more expensive than screen-film mammography (SFM) but is more amenable to data storage and sharing. The net impact of screening with digital mammography versus film mammography, in terms of health outcomes and the net difference in rates of overdiagnosis, is unknown. Performance of both SFM and digital mammography for measures such as cancer detection rate, sensitivity, specificity, and PPV have been compared directly in several trials, and the trials yielded similar results.
A large cohort of women (n = 42,760) who underwent both digital and film mammography was evaluated at 33 U.S. centers in the Digital Mammographic Imaging Screening Trial (DMIST). No differences in breast cancer detection were observed (AUC of 0.78 +/- 0.02 for digital and AUC of 0.74 +/- 0.02 for film; P = .18). Digital mammography was better at cancer detection in women younger than 50 years (AUC of 0.84 +/- 0.03 for digital; AUC of 0.69 +/- 0.05 for film; P = .002).
A second DMIST report found that film mammography had a higher AUC in women aged 65 years and older (AUC 0.88 for film; AUC 0.70 for digital; P = .025); however, this finding was not statistically significant when multiple comparisons were considered.
In a large U.S. cohort study, sensitivity for women younger than 50 years was 75.7% (95% CI, 71.7–79.3) for film mammography and 82.4% (95% CI, 76.3–87.5) for digital mammography; specificity was 89.7% (95% CI, 89.6–89.8) for film mammography and 88.0% (95% CI, 88.2–87.8) for digital mammography. A comparison of the findings from 1.5 million digital mammography screens and 4.5 million screen-film mammogram (SFM) screens that were performed in the Netherlands from 2004 to 2010 indicated higher recall and detection rates for the digital mammography screens. Among radiologists who read both digital and SFM exams (n = 1.5 million), the recall rates were 2.0% for digital mammography (95% CI, 2.0–2.1) versus 1.6% for SFM (95% CI, 1.6–1.6); the detection rates were 5.9 per 1,000 (95% CI, 5.7–6.0) for digital mammography and 5.1 per 1,000 (95% CI, 5.0–5.2) for SFM. The PPV was statistically significantly lower in the digital mammography group (PPV, 31.2%; 95% CI, 30.6–31.7) than in the screen-film group (PPV, 34.4%; 95% CI, 33.8%–35.0%). For women aged 49 to 54 years, the recall rates for digital screens versus film screens were 2.7% versus 2.0%, respectively; the detection rates were 5.1 versus 4.0 per 1,000 screens, respectively; and the PPV was 21.4% and 22.1%, respectively. For women aged 55 to 74 years, the recall rates for digital screens versus film screens were 1.7% versus 1.4%, respectively; the detection rates were 6.2 versus 5.6 per 1,000 screens, respectively; and the PPV was 35.7% versus 40.1%, respectively.
A meta-analysis  of 10 studies, including the DMIST [13,31] and the aforementioned U.S. cohort study, compared digital mammography with film mammography in 82,573 women who underwent both types of the exam. In a random-effects model, there was no statistically significant difference in cancer detection between the two types of mammography (AUC of 0.92 for film and AUC of 0.91 for digital). For women younger than 50 years, all studies found that sensitivity was higher for digital mammography but that specificity was either the same or higher for film mammography. The meta-analysis found no other differences based on age.
Computed radiography (CR) utilizes a cassette-based removable detector and external reading device to generate a digital image. A large concurrent cohort study compared 254,758 full-field digital mammography (FFDM) screens with 487,334 SFM screens and 74,190 CR screens. Again, the cancer detection rate was not different between FFDM (4.9 per 1,000) and SFM (4.8 per 1,000), although the recall rate was higher for FFDM. Importantly, cancer detection was lower for CR at 3.4 per 1,000, adjusted OR 0.79 (95% CI, 0.68–0.93). Two prior studies of noncontemporaneous cohorts showed no difference between CR and SFM or higher cancer detection rate from CR.[36,37]
Mammography and Computer-Aided Detection (CAD)
CAD systems are designed to help radiologists read mammograms by highlighting suspicious regions such as clustered microcalcifications and masses. Generally, CAD systems increase sensitivity and decrease specificity  and increase detection of ductal carcinoma in situ (DCIS). Several CAD systems are in use. One large population-based study comparing recall rates and breast cancer detection rates before and after the introduction of CAD systems found no change in either rate.[38,41] Another large study noted an increase in recall rate and increased DCIS detection but no improvement in invasive cancer detection rate.[40,42]
Population-Based Evaluation of Digital Mammography and CAD
Using a Surveillance, Epidemiology, and End Results–Medicare linked database, the authors examined the use of new screening mammography modalities by more than 270,000 women aged 65 years and older in two time periods: 2001 to 2002 and 2008 to 2009. Digital mammography increased from 2% to 30%, CAD increased from 3% to 33%, and spending increased from $660 million to $962 million. There was no difference in detection rates of early-stage (DCIS or stage I) or late-stage (stage IV) tumors.
Tomosynthesis, or 3-dimensional (3-D) mammography, is similar to standard 2-D mammography in how the examination is performed: the breasts are compressed in the same positions as for mammography, and the examination uses x-rays to create the image. In tomosynthesis, multiple short-exposure x-rays are obtained at different angles as the x-ray tube moves over the breast. This process takes a few seconds longer than a standard mammogram. Individual images are then reconstructed into a series of thin slices that can be viewed individually or like a movie. Cancers and other abnormalities are detected because of differences in density and shape compared to surrounding tissue, with some cancers and other findings causing architectural distortion. Overlapping tissues can be more easily recognized accurately as normal with tomosynthesis, and some cancers are better seen than on standard mammography. In some centers, tomosynthesis-guided biopsy may be available because some cancers seen only on tomosynthesis cannot be found with ultrasound.
The combination of 2-D and 3-D mammography has been reported to be more accurate than 2-D mammography alone, with respect to both improved detection of breast cancer (averaging added yield of 1.3/1,000, similar to CAD) and, importantly, reduction in recall rates. On average, 1.8% fewer women will be recalled for extra testing when tomosynthesis is performed in addition to standard 2-D digital mammography for screening. More than 80% of the cancers detected only with tomosynthesis are invasive and node negative.[44,45] In particular, tomosynthesis depicts architectural distortion better than standard digital mammography; in one series of 26 cases of architectural distortion in women who had both 2-D and 3-D mammography, 19 (73%) were seen only on tomosynthesis, and 4 (21%) of those 19 were malignant.
When tomosynthesis is performed in combination with 2-D mammography, the resulting radiation exposure to the patient is essentially doubled. This is expected to result in another 1.3 fatal cancers per 100,000 women screened at age 40 years (fewer with increasing age), compared with another 130 cancers detected (see Table 2).
The performance of tomosynthesis in isolation (with synthetic 2-D mammograms created) has not been adequately validated in practice, with only one reader study and one prospective clinical trial undertaken to date. The effect of annual tomosynthesis on breast cancer mortality has not been tested in a prospective clinical trial.
Tomosynthesis in the diagnostic setting (specifically, evaluation of mammographic abnormalities) has been shown to be at least as effective as spot compression views for workup of noncalcified abnormalities, including asymmetries and distortions.[48,49] Tomosynthesis is not worse than standard 2-D mammography at allowing suspicious microcalcifications to be identified, but magnification views are typically still needed to characterize suspicious calcifications.
The use of tomosynthesis in both screening and diagnosis may decrease the need for ultrasound and other additional testing (see Table 2). At this time, there are no data on the association of tomosynthesis and overall mortality reduction.
|CDR = cancer detection rate; DBT = digital breast tomosynthesis, also known as 3-D mammography; FFDM = full field digital mammography, also known as standard 2-D mammography; no. = number.|
|Study Design||Prospective; each patient had both exams||Prospective; each patient had both exams||Historical control with 2-D only||Historical control with 2-D only||Historical control with 2-D only|
|No. of DBT||12,631||7,292||9,499||173,663||23,149||226,234|
|No. of FFDM||12,631||7,292||13,856||281,187||54,684||365,293|
|CDR FFDM (2-D) Alone||6.1/1,000||5.3/1,000||4.04/1,000||4.2/1,000||4.9/1,000|
|Difference (No. of Women)||+1.9/1,000 (24)||+2.7/1,000 (20)||+1.3/1,000 (12)||+1.2/1,000 (208)||+1.4/1,000 (32)||+1.3/1,000 (296)|
|P-Value (Detection Rate)||.001||< .0001||0.18||< .001||.035|
|Absolute Recall Rate Difference||-0.8%||-2.0%||-3.2%||-1.6%||-2.6%||-1.8%|
|P-Value (Recall Rate)||< .001||< .0001||< .001||< .001||> .0001|
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