A doctor talks about: Screening mammography

Published: February, 2010

Celeste Robb-Nicholson, M.D.

The release of updated breast cancer screening guidelines from the United States Preventive Services Task Force (USPSTF) in November 2009 sparked a heated national debate about how best to use screening mammography. Many of my patients are still asking questions about their own breast cancer screening strategies.

The USPSTF asked an expert panel to re-examine the scientific evidence on the effectiveness of breast cancer screening in women ages 40 and older who are at average risk for developing breast cancer. The panel reviewed clinical studies published since guidelines were last updated, in 2002, and it applied new methods for analyzing all the data. It concluded that the use of mammography, clinical breast exam, and breast self-exam should be scaled back. The most controversial recommendations were to delay routine screening mammography until age 50 (previous guidelines recommended mammography screening every one to two years for women in their 40s) and to screen women in their 50s and 60s only once every two years, rather than annually, as the earlier guidelines had advised.

Many physicians, researchers, health organizations, advocacy groups, politicians, and individual women reacted with concern to the guidelines, and for good reason. Breast cancer is the second leading cause of cancer death among women in the United States. According to the most recent calculations, 192,370 cases of invasive breast cancer were diagnosed in 2009, and 40,170 women died from the disease. Mammography screening has been the foundation of breast cancer prevention in this country since the 1980s. Since 1990, breast cancer mortality has been decreasing at a fairly steady 2% per year, and many experts attribute that decrease to a combination of regular screening mammography and improved treatment.

Summary of the USPSTF recommendations on screening for breast cancer

  • The USPSTF recommends against routine screening mammography in women in their 40s who aren't at increased risk for breast cancer. The decision to start mammography before age 50 should be based on a woman's risk for breast cancer and personal preferences about the benefits and harms.

  • The USPSTF recommends mammography every two years for women ages 50 to 74.

  • Current evidence is not sufficient to assess the effectiveness of screening mammography in women ages 75 and over.

  • Current evidence is not sufficient to assess the effectiveness of clinical breast exam in addition to screening mammography.

  • The USPSTF recommends against clinicians teaching women breast self-examination.

  • Current evidence is not sufficient to assess the additional effectiveness of digital mammography or breast MRI instead of film mammography for breast cancer screening.

Source: Annals of Internal Medicine, Nov. 17, 2009, pp. 716–726. The recommendations may also be viewed online, at www.health.harvard.edu/brcascreen.

Why review an effective screening program?

The USPSTF is an independent group of experts appointed by the government to review the evidence for clinical practices such as screening tests and treatments. A doctor makes decisions based on what's best for an individual patient, while decisions about a general screening program are made on statistical grounds: does it pay off in saved lives? When the USPSTF reviewed breast cancer screening in 2002, there already were questions about screening mammography for women in their 40s, which has been a subject of debate for decades. The task force was prompted to take another look by new clinical studies and new statistical methods developed since 2002.

The panel reviewed all clinical trials of screening mammography with more than 10 years of follow-up. They found that starting at age 40, screening mammography reduces breast cancer mortality overall by 15% — about the same as in 2002. But this time, the panel also tried to determine how the benefits of screening (lives saved) stacked up against the harms, such as false-positive results, which can lead to unnecessary testing, biopsies, and psychological distress.

Lacking enough clinical data to assess harms, the panel used statistical models, which suggested that screening women starting at age 40 would reduce breast cancer deaths by only a modest amount, compared with screening starting at a later age. The net benefit for women in their 40s is small because breast cancer is less common in this age group. These women also show more false positives, because their breasts are denser, making cancers harder to spot. To avoid one breast cancer death, 1,904 women in their 40s would need to be screened. In women in their 50s, one life would be saved for every 1,339 women screened, and for those in their 60s, screening just 377 women would yield one life saved. According to the USPSTF's model, screening women in their 40s for breast cancer does not make good public health sense because there is no net benefit: the number of deaths prevented is too small, and the potential harm from false positive test results is too great.

Other findings

Besides false positives, the USPSTF panel also considered another sort of harm: the possibility of detecting and treating early breast cancer that wouldn't adversely affect a woman during her lifetime. In older women, breast cancer may not shorten life span — as, for example, in many cases where a woman has another life-threatening condition, such as cardiovascular disease. These women could suffer the harms of undergoing unnecessary breast cancer treatment. The USPSTF recommended that the decision about mammography in an older woman should be based on her overall health, other illnesses, and her willingness to have more tests and treatments for a breast cancer that may not shorten her life.

The panel also re-evaluated the recommended interval for breast cancer screening, which was previously set at one year. The panel found that mammography every two years in women in their 50s and 60s would avert 70% to 90% of breast cancer deaths averted with annual screening, which is twice as expensive and causes twice as many harms. So the USPSTF concluded that mammography every two years in this age group maximizes the potential benefits and minimizes the harms.

The panel concluded that there weren't enough data to determine whether clinical breast exam (examination of the breasts by a clinician) is useful. It also determined that breast self-examination is not worth teaching to patients, because it's not standardized and leads to many false-positive tests. Digital mammography is gradually replacing film mammography, and breast MRI is increasingly used to screen high-risk women. However, the panel concluded that there weren't enough studies to justify a recommendation for more general use of these methods in breast cancer screening.

Debate not over

The 2009 guidelines were released without much warning in the middle of a public health care debate. To many people, that made questioning the wisdom of yearly mammograms in women in their 40s seem like a political decision. It created a sense of mistrust and fears of rationing. But this is a medical issue, not a political one. Researchers and scientists constantly re-evaluate clinical practices as evidence accumulates. That's how science is done, and we need scientific evidence to determine how best to use technology to improve women's health.

Breast cancer and imaging experts have weighed in for and against the revised guidelines. Advocates welcome a more cost-effective approach to breast cancer screening and emphasize the potential to reduce unnecessary biopsies and psychological stress. Opponents point to a falling breast cancer death rate that's due, in part, to annual screening mammography. Some reject the statistical models used by the USPSTF. Others worry that increasing the screening interval will cause some women to neglect routine screening altogether. For some, the harms of screening simply do not outweigh the fears of missing a breast cancer. The debate will surely continue.

What now?

For now, this is what I'm telling my patients:

  • Talk to your clinician about the implications of the recommendations for you. Physicians are not obliged to discontinue or cut back on screening.

  • The revised guidelines apply to women at average risk of developing breast cancer. Know your own risks and work with your clinician to develop a screening strategy that's right for you.

  • If you're in your 40s, you're not prohibited from having screening mammography. The USPSTF recommends that you make an informed decision based on your personal preferences and medical history as well as the increased risk of a false-positive result and further testing.

  • If you're in your 70s, have a frank discussion with your clinician about the benefits and harms of breast cancer screening. If you're healthy and active, it may be in your best interest to continue screening. But if you have a limiting chronic illness, it may not be worth your time, energy, and resources to find or treat a cancer that may well not affect how long you live.

  • I think breast self-examination is still a good idea. You should be familiar with the architecture of your breasts, since you may be the first person to detect a change.

What's next?

The USPSTF guidelines remind us that where breast cancer screening is concerned, one size (or frequency) doesn't fit all. As we learn more about the molecular and genetic basis of breast cancer, we will improve our ability to identify the women most likely to benefit from screening as well as those for whom the risks outweigh the benefits. The balance between benefits and harms may also shift as digital mammography takes the place of film.

The practice of annual mammography in women starting at age 40 is unlikely to change overnight, and many organizations will continue to recommend it. But we should be prepared for an ongoing debate that includes public discussion about what we expect from breast cancer screening, how much disease we want to detect and treat, what we think about disease that "may not matter" in terms of longevity — and how much, individually and as a society, we are willing to pay for screening.

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