Screening Breast Sonography in Dense Breasts

Background
Despite its ability to reduce breast cancer mortality, screening mammography is an imperfect tool that misses breast cancers in some women, particularly those with dense breasts. This limitation has resulted in the suggestion that other imaging technologies be added to mammography screening to improve the early detection of breast cancer. One of these technologies is sonography.

In published results from 42,838 examinations at six centers it has been shown that screening breast sonography is capable of detecting some cancers that are undetected by mammography and physical examination. On the first prevalent screen, 3.5 cancers were detected only by sonography for every 1000 women screened. Over 90% of cancers seen only by sonography were in women with dense or heterogeneously dense parenchyma. Over 94% of cancers seen only on ultrasound were invasive tumors with average size of 9-11 mm, and in the studies where staging was detailed, 91% were node negative. Across three series, 2.2 cancers per 1000 were missed by sonography and seen only by mammography; 75% of cancers missed by sonography were due to ductal carcinoma in situ and 25% were invasive. Women at increased risk of breast cancer because of personal or family history of breast cancer were two to three times more likely to have breast cancer seen only by sonography, with prevalence of 0.48-1.3% compared to 0.18-0.25% in women of average risk. Standardized technique, descriptive, and interpretive criteria (www.acrin.org) have been proposed for breast ultrasound.

Pros and Cons
Advantages to the incorporation of sonography into a breast cancer screening program include the fact that breast ultrasound is well tolerated, noninvasive, and relatively inexpensive. Also, it is easy to biopsy lesions seen only by sonography. However, limitations are also evident. The examination is operator dependent, and the skills of individual technologists and radiologists are variable. In one study, technologists performed screening sonography. In the other series to date, the radiologist performed the scanning. There is a shortage of qualified personnel, both sonographers and radiologists, who can perform screening breast sonography with the necessary expertise. In view of this shortage, if screening breast sonography were to become standard practice, the workforce would face a marked increase in workload that might result in mammography access issues.
While the results of single center studies have been encouraging, it is not clear that the results are generalizable. In prior studies, the results of mammography were known at the time of sonography. Ultrasound was used as complement to the mammogram, and patient selection was performed with knowledge of the mammogram findings. This may have biased conclusions in favor of sonography in that vague areas of asymmetry may have been targeted.

Additionally, there is risk to the patient of increased cost and intervention. In published data, approximately 3% of women required an aspiration or biopsy as a result of screening sonography. Of those procedures, 11% showed cancer. Another 6% of women required short interval follow-up, and the risk of malignancy among lesions followed has not been well established. Importantly, while cancers detected with screening sonography have generally been stage I invasive breast cancers, there are no data to assess the ability of sonographic screening to decrease breast cancer mortality.

Summary
Published experience has demonstrated that in some women, sonography is capable of detecting cancers that cannot be found by mammography. The detection benefit of annual incidence screening sonography has not been established. The generalizability of these results and the role of annual supplemental sonography are being evaluated in an ongoing multicenter trial in high-risk women with dense breasts, blinded to the mammographic results (ACRIN 6666). Practitioners who are currently offering screening breast sonography, or who are considering offering it, should carefully review, and ideally meet, the eligibility criteria required for participation in ACRIN 6666, including the prior-experience requirement, maintenance of quality control procedures, appropriate completeness and consistency of performing screening sonographic examinations, rigorous recording and labeling of sonographic images, and appropriate record keeping and follow-up required. If offered, screening sonography should only be contemplated as a supplement to, not a replacement for, mammography.

In summary, it has not been established that women will benefit from the incorporation of sonography into routine breast cancer screening programs. Although there may be an appropriate subpopulation of women for whom the benefits from supplemental screening sonography may outweigh the risks, further studies, some underway, are needed to better define the appropriate target population and to see its effects on patient outcomes. Unlike screening mammography, screening breast sonography has not been shown to decrease mortality from breast cancer. At the present time, it is not the standard of care to offer or perform this examination.