Brem et al33 conducted a multicenter, prospective study of 15,318 participants with dense breasts, comparing the results of screening mammography combined with automated breast ultrasonography with screening mammography alone. Adding automated breast ultrasonography yielded an additional 1.9 cancers detected per 1,000 women screened.33 In a study that included data from 6,425 sessions of automated whole-breast ultrasonography and mammography performed in asymptomatic women, Kelly et al34 concluded that automated whole-breast ultrasonography improved callback rates, confidence in callbacks among women with dense breasts, and improved the accuracy rate for detecting breast cancer. In women with dense breasts, automated whole-breast ultrasonography alone detected 65% of the cancers compared with 39% of cancers using mammography alone — an overall 2-fold increase in the rate of cancer detection.34
Multiple studies substantiate that supplemental breast ultrasonography — whether 2D hand-held or 3D automated whole-breast ultrasonography — can improve rates of cancer detection.27-30,33,34 However, the limitations of ultrasonography must be considered. Low positive predictive values are generalizable to nearly all of the studies. Results from the trial conducted by Berg et al27 showed a reasonable recall rate for ultrasonography of 5.4%, but the positive predictive value for recall was 6.5% and the positive predictive value for engendered biopsies was 8.9%. Hooley et al29 and Weigert et al30 also reported low positive predictive values of 6.5% and 6.7%, respectively. Brem et al33 showed a decreased positive predictive value with ultrasonography (performed with automated whole-breast ultrasonography) as compared with mammography alone, which had a specificity rate of 13.4%. As comparison studies become available over years of sequential screening, it has been suggested that the number of biopsies prompted by false-positive findings on ultrasonography may improve.28
Ultrasonography is a workhorse for diagnostic breast imaging, but its role in screening remains unclear. Studies utilizing screening ultrasonography demonstrate its capability for detecting invasive malignancies in dense breasts at small sizes and localized stages that could potentiate an increase in breast cancer survival rate; however, more studies are needed to determine the impact on mortality.27 The best indications for screening ultrasonography in dense breasts may be for women with intermediate risk or in those women at high risk but with a contraindication to magnetic resonance imaging (MRI).28 Magnetic Resonance Imaging MRI is a cross-sectional imaging modality that provides soft-tissue contrast between fat, fibroglandular tissue, and lesions. MRI of the breast was approved by the FDA as an adjunct to mammography for the detection of breast cancer in 1991.35 Since that time, MRI of the breast has been further refined and is now considered the most sensitive imaging tool available for the diagnosis of invasive breast cancer.36 Limitations of MRI of the breast must be weighed against the potential benefits when selecting candidates for screening MRI.36 As such, the role of screening MRI in patients with dense breasts has not been well defined.
Fibroglandular tissue does not mask lesions on MRI, unlike mammography. Administration of intravascular contrast (gadolinium) helps distinguish suspicious lesions from normal breast tissue due to preferential contrast uptake by malignant tumors (Figure 4). However, normal fibroglandular tissue is enhanced to varying degrees, which is an imaging characteristic termed background parenchymal enhancement. Similar to breast density, 4 categories of background parenchymal enhancement are included in the BI-RADS criteria (Figure 5)1 :
(To view a larger version of Figure 5, click here.)