Population‐based screening for breast cancer became established within the United States almost 30 years ago, following the Two County Swedish Study, which showed a benefit from mammographic screening.

Mammography has conclusively been shown to not only detect breast cancer, but save lives, especially in women over 50 years of age. As a result, numerous medical organizations, including the American Cancer Society, recommend routine mammographic screening beginning at age 35.

The principal benefit of mammographic screening is not the detection of breast cancer, per se, but the mortality benefit that accrues within the screened population. Much of this benefit is attributable to finding breast cancer in its early stages, when it is most treatable. Breast cancers that are not detected early have a significantly worse prognosis.

Despite continuing advancement in technology and required training for mammography readers, a certain number of breast cancers are missed by mammography.(5) Computer aided detection studies have shown that most missed cancer lesions can, in retrospect, be detected by expert readers, but many cannot.

Approximately 10‐15% of breast cancers appear to be fundamentally undetectable using screening mammography.

One factor that significantly increases the risk of missing a breast cancer on mammography is overall mammographic breast density. Women with increased breast density have been shown not only to have a higher risk of having a missed breast cancer, but a higher risk of developing a breast cancer in the first place.

The causative problem here is that breast cancers, and other suspicious lesions, appear on mammography as white densities. If the overall background density is also white, as occurs in women with dense breasts, suspicious lesions are much harder to detect.

Radiologists refer to the ability of a diagnostic test to detect the difference between a breast cancer, and background breast tissue as contrast resolution. The contrast resolution of mammography depends upon the level of background density. Women with fatty replaced breasts have high contrast resolution on mammography, and accordingly breast cancer in this population is easy to detect. Women who have dense breasts have a low contrast resolution on mammography and are more likely to have missed breast cancers.

This fundamental limitation of mammography has led to the development of alternative technologies that can improve contrast resolution in women who have mammograms that are difficult to interpret. One of these technologies is MRI. MRI has been shown to greatly improve contrast resolution in womenwho have mammographically dense breasts. MRI however, has the disadvantage of being expensive and requiring an intravenous contrast agent. The false positive rate of MRI is also significant. Additionally, lesions seen on MRI can be difficult to biopsy.

Sonographic Screening

Breast cancer screening using sonography has also been studied as a method of improving contrast resolution in women with mammographically dense breasts. Generally, sonographic screening has been performed using special hand‐held ultrasound probes that are used to scan the entirety of both breasts. The technique requires practice and is generally performed by a physician. A study can require over 45 minutes,especially if there are sonographically visible findings. A series of studies on hand‐held ultrasound screening were performed in the 1990s, but failed to demonstrate significant improvements in cancer detection rates.

Further evaluation of hand‐held sonographic screening is currently underway as part of the ACRIN 6666 study. Preliminary results presented at the 2007 RSNA conference show an increase in cancer detection following mammography of 1.2‐7.6 per 1000 women screened. This is a significant improvement over studies performed the 1990s, probably as a result of technological advancement in the development of improved ultrasound probes and display devices.

Recently, continuing technological advancement has made available whole‐breast sonographic scanning using an automated probe which images wide field of view of breast tissue in three dimension reconstructed images.