The lifesaving benefits of mammographic screening clearly outweigh the potential drawbacks of overdiagnosis and false-positives, European researchers affirmed.
For every 1,000 women who undergo biennial screening for breast cancer between the ages of 50 and 70, an estimated seven to nine lives would be saved, according to Eugenio Paci, MD, of the ISPO Cancer Prevention and Research Unit in Florence, Italy, and colleagues.
This would be accompanied by four cases of overdiagnosis -- detection of a cancer that would otherwise have gone undetected throughout a woman's lifetime -- giving a ratio of lives saved to overdiagnosis of 1 to 0.4-0.6, the researchers reported online in the Journal of Medical Screening.
Two decades ago a meta-analysis of randomized studies determined that breast cancer screening led to lower mortality rates from the disease, and various European countries began implementing population-based screening, in which all women in the target age group were individually invited to be screened.
However, 10 years later a Cochrane review raised questions about the findings in those earlier trials, leading to concerns about the value of screening. And most recently, the U.S. Preventive Services Task Force touched off a firestorm when it said routine screening mammography should be put off until after age 50, and even then should be done just every other year.
To sort out these conflicting findings, the cooperative Euroscreen network was formed to assess the evidence and provide a "balance sheet" that could be used to aid women in deciding whether to undergo screening.
They reviewed the effects of screening in both incidence-based mortality and case-control studies, and found that in the incidence-based studies, there was a 25% reduction in deaths (RR 0.75, 95% CI 0.69 to 0.81) among women invited for screening and a 38% decrease (RR 0.62, 95% CI 0.56 to 0.69) among those who actually underwent screening.
In the case-control studies, the mortality reduction for screening invitation was 31% (OR 0.69, 95% CI 0.57 to 0.83) and 48% (OR 0.52, 95% CI 0.42 to 0.65) with participation in screening.
When they reviewed the evidence on false-positive screening results, they estimated a cumulative incidence of approximately 17% for noninvasive additional testing and 3% for invasive tests such as needle or surgical biopsies.
To create their balance sheet, they first determined the cumulative risks among women ages 50 to 80 during the years 1985 and 1986, before screening was routinely implemented, and found an average incidence of 6.7% and mortality of 3%.
This translated to 17 deaths per 1,000 for women between the ages of 50 and 70 and 13 between ages 70 and 80.
Out of these 30 deaths, 19 could have been prevented by screening, Paci and colleagues calculated.
Their data also showed that 14 women would need to be screened to diagnose one case of breast cancer, and 111 to 143 would need to be screened to save one life.
And for every 1,000 women screened biennially from age 50–51 until age 68–69 and followed up to age 79, an estimated seven to nine lives would be saved, four cases overdiagnosed, 170 women would have at least one recall followed by noninvasive assessment with a negative result, and 30 women would have at least one recall followed by invasive procedures yielding a negative result.
One difficulty they noted with regard to overdiagnosis was determining the contribution of ductal carcinoma in situ, which is more commonly being diagnosed with widespread screening.
Some studies have suggested that the increase in cases of these in situ carcinomas was balanced by a decrease in invasive cancers, but others found that recurrences are common and the tumors may be aggressive.
These concerns have yet to be clarified, and further work will be needed to more fully understand the impact of ductal carcinoma in situ on screening and outcomes, the researchers noted.
They also pointed out that their analysis did not attempt to consider larger issues such as the economic and social effects of screening.
Rather, their intention in this work was "to ensure that women are fully aware of the chief benefit and harms when they decide whether or not they wish to attend screening."
But the analysis is far from complete, and only after longer follow-up can firmer estimates be made, they said.
Future research also will have to consider the improvements in technology, such as digital mammography, and changes in practice such as the appearance of specialized breast diagnostic and treatment centers.
The researchers concluded that, although their estimates are only approximations at this time, "they clearly indicate ... that the relationship between benefit and harm of mammographic screening is much more favorable than some recent publications suggest."
This work was supported by the National Monitoring Italian Center and the National Expert and Training Center for Breast Cancer Screening in Nijmegen, the Netherlands.
The authors reported no financial conflicts of interest.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner