Staying Abreast of Screening Mammography

Published - Written by Rena Xu

Should women of a certain age be screened regularly for breast cancer?  It’s a seemingly simple question that has proven controversial to answer, as evidenced by the uproar last November over a revised recommendation from the US Preventive Services Task Force to start mammographic screening at age 50 instead of 40.

Supporters of routine screening mammography have argued that it significantly reduces breast cancer mortality. According to estimates by the US Preventive Health Services Task Force, screening mammography is associated with a 15 to 23% reduction in breast cancer mortality.  Meanwhile, the WHO has stated on the basis of several randomized trials that screening women between the ages of 50 and 69 years potentially reduces the mortality rate by 25%.

Critics, on the other hand, have challenged the validity of these findings.  Many previous trials studying the effectiveness of screening mammography used historical comparison groups, looking at mortality among current cohorts of women as compared with unscreened cohorts from the past.  Such a comparison introduces the possibility of confounding.  Were the observed reductions in mortality due to screening mammography?  Or were they due to other factors influencing breast cancer mortality that have changed over time, such as advances in treatment?

A recent study conducted by Kalager et al in Norway addressed this shortcoming by using a unique cohort design.  Between 1996 and 2005, Norway implemented a national breast cancer screening program, rolling it out in a staggered fashion to the 15 counties.  Before 1996, no counties had a screening program in place; by 2005, all counties were enrolled.  This phased implementation allowed Kalager et al. to define cohorts of women who between 1996 and 2005 either had access to screening (a “screening group”) or did not yet have access to screening (“non-screening group”).  Moreover, it allowed them to compare these contemporary cohorts with their historical pre-screening counterparts – cohorts of women who lived in those particular counties between 1986 and 1995.

The authors report that in the screening group, there was a 28% relative reduction in mortality as compared with the group’s historical counterpart.  Meanwhile, the non-screening group showed an 18% relative reduction in mortality as compared with its historical counterpart.  Taken together, these data suggest a relative mortality reduction of 10% among the screening group as compared with the non-screening group. The current 10-year risk of breast-cancer mortality for a 50-year-old woman is estimated to be about 4 in 1000. Assuming the benefits of screening are already accounted for, this means that screening avoids 0.4 deaths per 1000 women.  Put another way, 2500 women would need to be screened over 10 years for a single breast cancer death to be avoided.

What’s more, the 10% relative reduction may not be entirely attributable to screening alone.  Before enrollment in the program, each county had to establish multidisciplinary breast cancer management teams. As a result, the benefits of integrated, comprehensive care may have contributed to the observed relative reduction in mortality.

In an accompanying editorial, Dr. H. Gilbert Welch of The Dartmouth Institute highlights some important implications of these findings.  He writes, “Screening mammography has become one of the most prominent measures of health performance.  Since their inception, health care report cards focused on ensuring that all women undergo the test…. But by highlighting that the mortality benefit is modest, Kalager et al. help confirm that the decision is, in fact, a close call.”

NEJM deputy editor Dr. Dan Longo states:

It is important that the decision to embark upon screening mammography be an informed decision.  We do not want women to be misled about either the potential for benefit or the potential for harm. This paper describing a carefully designed screening program suggests that the benefit for the screened group may be considerably lower than other estimates. Women and their physicians will need to take these results into consideration along with an individual’s risk profile in deciding on when to initiate a screening mammography program.

Given these findings and the uproar that occurred in November of 2009 when a federal panel suggested limiting mammographic screening, how do you approach screening with your patients?  Do you advise screening for women under age 50 with no family history of breast cancer?  How much influence over the decision of whether – and when – to get screened should lie with the individual patient, versus the physician, versus the health care system as a whole?