Screening mammography's impact on breast cancer mortality: The debate continues
Despite years of being an accepted form of screening, the debate about the benefits and disadvantages of mammograms continues on.
By Maria Perno Goldie, RDH, MS
The risks and benefits of screening mammography continue to warrant considerable debate and attention. Randomized controlled trials (RCTs) conducted from the 1960s to 1980s showed that screening mammography reduced breast cancer mortality.(1) But these studies were conducted in a period when breast cancer treatments were less effective and women were not as aware of breast cancer and its causes and treatments. This study looked at RCTs comparing mammographic screening with no mammographic screening.
The authors concluded that if the assumption is that screening reduces breast cancermortality by 15%, and that overdiagnosis and overtreatment is at 30%, for every 2,000 women screened throughout 10 years, one will avoid dying of breast cancer, and 10 healthy women who would not have been diagnosed if there had not been screening will be needlessly treated. Also, more than 200 women will experience significant psychological distress, such as anxiety and uncertainty, due to false positive findings. To help ensure that the women are fully informed before they decide whether to have a screening, Cochrane has written an evidence-based leaflet for lay people that is available in several languages.(2) Due to substantial advances in treatment and better breast cancer awareness since the trials were conducted, it is likely that the absolute effect of screening today is smaller than in the trials. Recent observational studies show more overdiagnosis than in the trials, and very little or no reduction in the incidence of advanced cancers with screening.
Investigators from Canada recently reported conclusions from 25 years of follow-up in a screening mammography trial that was initiated in 1980 and involved some 90,000 women ages 40 to 59.(3) All participants experienced baseline clinical breast exams by trained clinicians. Women ages 40 to 49 were randomized to five annual mammograms, in addition to annual breast exams, compared to usual care. Women ages 50 to 59 were randomized to five annual mammograms, in addition to annual breast exams, or to only annual breast exams. Outcomes were tracked during the five-year screening period and subsequent follow-up through 2005.
The conclusion was that the 25-year collective mortality from breast cancer was comparable among women in the mammography and no-mammography groups, and these findings did not differ with age. Within 15 years of baseline, over 106 cases of breast cancer were identified with screening mammography. Therefore, 22% of screen-detected cancers (106/484) represented overdiagnosed breast tumors.(3) An editorial in the British Medical Journal (BMJ) suggests that long-term follow-up does not support breast cancer screening in women under the age of 60.(4) The authors of the editorial agree with Miller and colleagues that the justification for screening by mammography should be reassessed by policymakers.
The outcomes of the Canadian study support those of other studies of breast cancer screening.(5,6) These studies propose that screening's influence on the decline in breast cancer mortality is exceeded by improvements in treatment. Also, the benefits of screening mammography are smaller and the harms associated with overdiagnosis are greater than have been previously appreciated. Mammograms are expensive and have high rates of false-positive findings.(7,8) For years, we’ve been told that mammograms can save lives. The feeling of getting an annual mammogram is comforting. So for many women this new information will be confusing, and others will be very suspicious. Some experts suggest encouraging patients to follow the 2009 U.S. Preventive Services Task Force guidelines which says that average-risk women should be screened every two years beginning at age 50.(9)
Maria Perno Goldie, RDH, MS
1. Gøtzsche PC, Jørgensen KJ. Screening for breast cancer with mammography. Cochrane Database of Systematic Reviews, 2013, ID: CD001877.
3. Miller AB, et al. Twenty-five-year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: Randomized screening trial. BMJ 2014 Feb 11; 348:g366.
4. Kalager M, Adami HO, Bretthauer M. Editorial. Too much mammography. BMJ 2014; 348:g1403.
5. Bleyer A, Welch HG. Effect of three decades of screening mammography on breast-cancer incidence. N Engl J Med 2012 Nov 22; 367:1998.
6. Autier P, et al. Breast cancer mortality in neighbouring European countries with different levels of screening but similar access to treatment: Trend analysis of WHO mortality database. BMJ 2011 Jul 28; 343:d4411.
7. O'Donoghue C, Eklund M, Ozanne EM, Esserman LJ . Aggregate cost of mammography screening in the United States: Comparison of current practice and advocated guidelines. Ann Intern Med 2014 Feb 4; 160:145.
8. Welch HG, Passow HJ. Quantifying the benefits and harms of screening mammography. JAMA Intern Med 2013 Dec 30; [e-pub ahead of print].
9. U.S. Preventive Services Task Force (USPSTF). Screening for Breast Cancer. www.uspreventiveservicestaskforce.org/uspstf/uspsbrca.htm