Breast cancer takes a tremendous toll in the United States. After lung cancer, breast cancer is the second leading cause of death from cancer among women in the United States and is the most common non-skin-related malignancy among U.S. women. Each year, more than 180,000 new cases of invasive breast cancer are diagnosed and more than 40,000 women die from the disease.
Until research uncovers a way to prevent breast cancer or to cure all women regardless of when their tumors are found, early detection will be looked upon as the best hope for reducing the burden of this disease. The hope is that early detection of breast cancer by screening could be as effective at saving lives as the Papanicolaou smear (Pap smear) used for cervical cancer screening.
Early detection is widely believed to reduce breast cancer mortality by allowing intervention at an earlier stage of cancer progression. Clinical data show that women diagnosed with early-stage breast cancers are less likely to die of the disease than those diagnosed with more advanced stages of breast cancer. A thorough annual physical breast examination and monthly breast self-examination can often detect tumors that are smaller than those found in the absence of such examinations, but data on the ability of physical examinations alone to reduce breast cancer mortality are limited.
X-ray mammography, with or without a clinical examination, has been shown in randomized clinical trials both to detect cancer at an earlier stage and to reduce disease-specific mortality. As a result, screening mammography has secured a place as part of routine health maintenance procedures for women in the United States. The mortality rate from breast cancer has been decreasing in the United States by about 2 percent per year over the last decade, suggesting that early detection and improved therapy are both having an impact on the disease.
Mammography is not perfect, however. Routine screening in clinical trials resulted in a 25 to 30 percent decrease in breast cancer mortality among women between the ages of 50 and 70. A lesser benefit was seen among women ages 40 to 49. The benefit of screening mammography for women over age 70 is more difficult to assess because of a lack of data for this age group from randomized clinical trials. Screening mammography cannot eliminate all deaths from breast cancer because it does not detect all cancers, including some that are detected by physical examination.
Some tumors may also develop too quickly to be identified at an early, “curable” stage using the standard screening intervals. Furthermore, it is technically difficult to consistently produce mammograms of high quality, and interpretation is subjective and can be variable among radiologists. Mammograms are particularly difficult to interpret for women with dense breast tissue, which is especially common in young women.
The dense tissue interferes with the identification of abnormalities associated with tumors, leading to a higher rate of false-positive and false-negative test results among these women. These difficulties associated with dense tissue are especially problematic for young women with heritable mutations who wish to begin screening at a younger age than what is recommended for the general population.
Mammography can also have deleterious effects on some women, in the form of false-positive results and overdiagnosis and overtreatment.