Mammography Every Other Year Has Same Benefits of Yearly Screening, But Less Harm

A comprehensive analysis of various mammography-screening schedules suggests that biennial screening of average risk women between the ages of 50 and 74 achieves most of the benefits of yearly screening, but with less harm. The study's findings represent a unanimous consensus of six independent research groups from various academic institutions.

The study's findings were published in the November 17, 2009, in the journal Annals of Internal Medicine. Researchers from CISNET, the U.S. National Cancer Institute (NCI; Bethesda, MD, USA)-funded Cancer Intervention and Surveillance Modeling Network, utilized independent models to examine 20 screening strategies with different starting and stopping ages and intervals. Modeling estimates the lifetime impact (outcomes including benefits and harms) of breast cancer screening mammography. The CISNET models link known data across the course of life and include national data on age-specific breast cancer incidence, mortality, mammography characteristics, and treatment effects.

"It's reassuring that all CISNET modeling groups came to the same conclusion even when applying different models to these data,” said the study's lead author, Jeanne S. Mandelblatt, M.D., MPH, of Georgetown [University] Lombardi Comprehensive Cancer Center (Washington DC, USA), a CISNET member. "While the findings represent a comprehensive review of existing data, decisions about the best screening strategy depend on individual and public health goals, resources, and tolerance for false-positive mammograms, unnecessary biopsies, and over-diagnosis.”

The CISNET analysis revealed that screening every other year maintains nearly all of the benefit (an average of 81%) of annual screening with almost half the number of false-positives. Compared with no screening, mammography screening every other year from ages 50 to 69 achieves a median reduction in breast cancer mortality of 16.5% over a lifetime. If screening is started at age 40 versus 50 and performed every other year, there is a median mortality reduction of 19.5% (an additional one woman per 1,000), but an increase in false-positives, unnecessary biopsies, and anxiety.

False-positives represent mammograms read as abnormal that often require additional follow-up in women who are found to not have cancer. An unnecessary biopsy occurs after a false-positive mammogram when the biopsy is normal. Over-diagnosis is the detection of a cancer through screening that otherwise never would have produced symptoms or affected the woman's health. Since typically it is not possible to determine which tumors will progress, almost all tumors detected during screening are treated.

According to Dr. Mandelblatt, the benefits of biennial screening are consistent with what is known about the breast cancer's biology. In most women, most tumors are slow growing and this proportion increases with age, so that there is little loss in survival benefit across the population for screening every year versus every other year. For women with aggressive, faster growing tumors, annual screening is not likely to make a difference in survival. For these women, different approaches may be needed and is an important area of on-going research.

While the model results validated that mammography saves lives, Dr. Mandelblatt explained that there are smaller overall benefits from starting screening earlier than age 50 because few women develop breast cancer in the younger age groups, and screening younger women is accompanied by a large number of false-positive mammograms. "This can lead to stress for women and unnecessary biopsies. We need more research to understand how to tailor screening by individual risk,” she said. "These modeling data represent an average finding regarding the population of women so it can't be emphasized enough that women need to talk to their healthcare provider for a screening program that is best for them,” Dr. Mandelblatt concluded.

The CISNET analysis was one of many sources of evidence that the U.S. Preventive Services Task Force (an independent scientific panel convened by the Agency for Healthcare Research and Quality) relied upon in developing the new mammography screening guidelines just announced.

Related Links:
U.S. National Cancer Institute
Georgetown Lombardi Comprehensive Cancer Center