Breast cancer is the second leading cause of cancer death in women. It is estimated that about 40,970 women will die from breast cancer in the US in 2006. However, death rates from breast cancer have been declining. Early detection and improved treatment of breast cancer are believed to be largely responsible for this trend.

Screening for cancer uses available tools to identify cancer before any detectable symptoms are present. Mammography , which is an x-ray exam of the breast, is the most commonly used method for early detection of breast cancer. Mammography has been shown to save lives. Although estimates vary considerably based on age and other factors, research suggests that for roughly every 250 women regularly screened with mammography, one woman will be saved from a breast cancer death.

Screening, however, is far from perfect—an existing cancer may not be detected or the test may indicate a cancer when one does not exist. Another possible problem is over-diagnosis. This occurs when a correctly detected cancer will never become symptomatic in a person’s lifetime, leading to anguish and unnecessary treatment. Studies have found that the rate of over-diagnosis with mammography is anywhere from 5% to 50%. A study published online in the March 2006 issue of the British Medical Journal evaluated the rate of over-diagnosis with mammography.

About the Study

The study used data from a large breast cancer screening trial conducted in Sweden, which included more than 40,000 women aged 45-69 and lasted 10 years. The women, grouped as aged 45-54 and 55-69, were randomized to receive screening with mammography or not.

Researchers followed participants for 15 years after the initial trial ended and used national registries to track breast cancer detection and survival. The research team found that the rate of over-diagnosis of breast cancer was 10% in women who were screened at age 55-69 years, compared with an unscreened control group. This means that 1 in 10 women in the study would never have been harmed by her breast cancer had it not been detected by screening.

How Does This Affect You?

The American Cancer Society recommends that women get a mammogram every year, starting at age 40. Do these study results suggest that you should forgo your annual mammogram? No. Screening is an important way to detect breast cancer before it can be felt, when it is more likely to be small and still confined to the breast. Mammography may not be perfect, but at the moment, it’s the best we’ve got. However, before undergoing a mammogram, or any screening test, you should know that it is not unusual for a result to be wrong or irrelevant to your health.

Another important message from this study is that a sizable fraction of women with breast cancer live many years with no ill effects, even without treatment. While breast cancer is a scary disease that all women and their families would rightly wish to avoid, it may be comforting to know that some breast cancers cause little harm. The difficulty arises in distinguishing which cancers are harmless and which are not.