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Mammography for Breast Cancer Screening: Harm/Benefit Analysis

Updated July 2021

Position
Universal mammography screening of women is a public health intervention in a healthy population. The National Breast Cancer Coalition (NBCC) believes that, from a public health perspective, screening mammography of all women has demonstrated only a modest, if any, benefit in reducing breast cancer mortality and that the harms associated with screening outweigh those benefits. No individual woman can be assured that screening mammography will prevent her from dying of breast cancer.

Mammography does not prevent or cure breast cancer and has many limitations. Therefore, a woman’s decision to undergo a screening mammogram must be made on an individual level and should take into consideration her specific risk factors and her personal preferences. Women who have symptoms of breast cancer such as a lump, pain, or nipple discharge should seek a diagnostic mammogram. Ultimately, resources must be devoted to finding effective prevention and treatment interventions for breast cancer and tools that detect breast cancers that are or will become metastatic and will eventually cause disease and death.

 

Educational Videos
Screening mammography is a complex topic surrounded by heated controversy. NBCC has prepared these educational videos to educate the public about the primary issues involved in examining the value of screening mammography as a public health intervention.

  • About screening – why we do it
  • Measures of screening test performance
  • Biases associated with screening tests
  • The evidence behind screening mammography

Watch them here.

What is screening?

Screening a population to find disease is a public health measure. Screening tests are conducted in healthy people who do not feel ill or display any symptoms of disease, with the goal of identifying diseases at an earlier stage in order to reduce mortality. However, screening for a disease in asymptomatic individuals only makes sense if it leads to a better health outcome (e.g., reduced mortality) than treating a disease after symptoms occur.

Because screening is conducted in a healthy population the benefits (e.g., reduction in mortality) must significantly outweigh the risks to healthy individuals who undergo screening.

What is a Mammogram?

Although there are three general tests—mammogram, clinical breast exam, and magnetic resonance imaging (MRI)—used by health care providers to screen for breast cancer, the mammogram is the most common screening test for breast cancer. A mammogram is an x-ray of the breast that can reveal abnormalities (benign or malignant). The procedure involves compressing the breast between two plates and then applying a small dose of radiation to produce an x-ray image of the breast tissue. Mammograms can be used for screening and for diagnosis.

  • Screening Mammogram: Performed in a healthy population to detect a tumor before symptoms occur. The goal of screening mammography programs is to decrease mortality from breast cancer.
  • Diagnostic Mammogram: Performed to help detect breast cancer in a patient who has symptoms, such as a lump that can be felt in the breast, breast pain, or nipple discharge. Diagnostic mammograms are also used after suspicious results from a screening mammogram.

This position paper focuses specifically on screening mammography in asymptomatic women who are at average risk of developing breast cancer. 

What is the Scientific Evidence for Mammography Screening?

It is important to note that well-designed and conducted randomized trials are the most reliable way to assess the effectiveness of any medical intervention and control for confounding and bias. For breast cancer screening, the goal is to decrease the death rate from the disease without increasing the death rate due to the downstream effects of the screening; therefore, overall mortality is the most appropriate outcome to assess the public health value of mammography screening.

The various studies of mammography screening have been subject to meta-analyses and systematic reviews by the research community. As of 2020, a total of eight large, randomized, clinical trials involving over 600,000 women have examined the impact of screening mammography on breast cancer mortality. Seven of these were analyzed to establish the 2016 United States Preventive Services Task Force (USPSTF)[1] guidelines [1-3] and the Cochrane Collaboration positions [4-5]. The eighth trial (i.e., the Edinburgh trial conducted in the United Kingdom) was excluded from the analyses due to significant concerns over inadequate randomization. All of the included studies were rated as having “fair” quality of evidence with varying risks of bias based on the process used to randomize study participants (refer to the Appendix for the rating schemes). None of the trials examined women younger than 39 years or older than 70 years, and only the Age Trial was specifically designed to look at the impact of mammography screening in women during their 40s. Refer to the Appendix for details on each of the included trials and the results of the USPSTF and the Cochrane analysis. The most recent analysis conducted by the Cochrane Collaboration,[2] was in 2013, and analyzed the trials in two subgroups – those with adequate randomization (least susceptible to bias) and those with suboptimal randomization. Cochrane reported that among trials with adequate randomization, screening did not reduce breast cancer mortality, whereas among studies that were potentially more biased (less carefully done) screening reduced breast cancer mortality by about 25 percent. Since poor trials tend to overestimate the effect, the mortality reduction was estimated to be 15 percent by Cochrane.

The USPSTF analysis shows that the absolute number of deaths prevented by screening is very low and any benefit from screening is greatest in women between 60 and 69. According to the Cochrane analysis, mammography decreases the risk of death from breast cancer by about 15% in relative terms, or 0.05% in absolute terms.

Collectively, the analyses to date of all randomized controlled trials for mammography have concluded either no or only marginal benefit and significant harms for breast cancer screening of healthy women [1-5]. It is important to note that while very few in this healthy population may see benefit from screening, all are subjected to the harms.

Limitations and Potential Harms of Mammography Screening
Determining the benefits and harms of mammography screening is challenging. Not only do a woman’s individual risk factors need to be considered, but screening mammography can produce inaccurate results. Consequently, many women receive reports of either false-positive or false-negative results. A false-positive result occurs when a woman’s mammogram shows an abnormality or a suspicious area on the image, but she does not have breast cancer. A false-negative result occurs when a woman’s mammography results are normal, but she actually has breast cancer.

The risks associated with a false-positive result include unnecessary biopsy as well as psychological effects, such as anxiety and distress, that can last years [6-8]. The 10-year cumulative rates of having a false-positive result from two U.S. studies were higher for women participating in annual screening compared to biennial (or every other year) screening (61% vs. 42%, respectively). Likewise, the rates of negative biopsies were greater with annual vs. biennial screening (7% vs. 5%, respectively).6 Although biopsies are relatively simple surgeries, they are still surgeries, and can cause distress, scarring, disfigurement, and/or infectious complications. Furthermore, unnecessary biopsies and subsequent tests add to health care costs.

In terms of absolute numbers based on the USPSTF analysis, for every 1000 women aged 50-74 years screened every two years for 10 years, there will be 953 false positives and 146 unnecessary biopsies will be conducted. When one includes women 40 and up, the number of false positives climbs to 1,529 with 213 unnecessary biopsies.

Another important harm of screening is overdiagnosis and, consequently, overtreatment. The results from randomized clinical trials reveal that between 11 and 22% of all identified breast cancers, including ductal carcinoma in situ (DCIS), are overdiagnosed [6]. Detecting, removing, and treating breast cancers that would never have spread to other parts of the body does not save any lives but does impose harm. Until researchers are able to determine which breast cancers will eventually spread and reduce quality of life or result in death, overdiagnosis will continue to be a concern of the current screening technologies.

In terms of absolute numbers of individuals overdiagnosed by screening mammography based on the USPSTF analysis, for every 1000 women aged 50-74 years screened every two years for 10 years, a total of 19 women will be overdiagnosed, and this number climbs to 21 for comparable screening for women 40-74 years of age.

Another risk of routine mammography is associated with exposure to radiation. Computational modeling studies predict that between 2 and 11 women out of every 100,000 undergoing routine mammography screening will die as a result of radiation-induced cancer with the incidence highest in women who begin getting annual mammograms in their 40s and lowest in the women who begin getting mammograms biennially in their 50s [6].6

Conclusion

Screening is an intervention in a healthy population. We should not intervene in a healthy population if the benefits do not significantly outweigh the harms. Screening must reduce mortality in order to be effective. We must not do more harm than good.

Evidence from both randomized controlled trials and observational studies of varied quality indicate that, overall, mammography screening has a modest effect on breast cancer mortality. When analyzed in absolute terms, the death rate is reduced by just 0.03% in women under 50 and 0.2% in women between 60 and 69 who receive the most benefit. Like with all medical interventions, there are harms associated with screening mammography such as misdiagnosis and overtreatment. Two comprehensive reviews of the evidence conclude that the overall impact on mortality is small and biases in the trials could either “erase or create it.” Women should discuss with their doctors their own risk profile, the potential benefits, harms, and complexities of screening mammography, and make informed decisions about screening. Mammography may provide benefits for some women, but it may also harm others.

NBCC believes in evidence-based health care and has long raised questions about the value of mammography screening and other interventions. Women need honest and complete information regarding the value of all medical interventions. Public health resources need to be used with certainty to improve the public’s health. The reality is that screening has not been effective. While screening programs have substantially increased the incidence of DCIS and localized invasive breast cancer, the incidence of regional or distant late-stage disease has largely remained unchanged.

NBCC believes that in order to make true progress in breast cancer we need to better understand what causes this disease and how to prevent it, what puts individual women at risk beyond the known risk factors, how different types of breast cancer behave, which treatments are appropriate and effective for each type of breast cancer, and how to prevent and cure metastasis.


[1] The USPSTF is an independent, volunteer group of national experts in prevention and evidence-based medicine that makes recommendations about clinical preventive services such as screening tests, counseling services, and preventive medications

[2] Cochrane is a global independent network of researchers, professionals, patients, carers and people interested in health. Cochrane produces systematic reviews of research and evidence in healthcare and health policy which are published in The Cochrane Library.

1Clinical Guideline: Screening for breast cancer: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med. 2016;164:279-296.

2 Nelson HD, Fu R, Cantor A, et al. Effectiveness of Breast Cancer Screening: Systematic Review and Meta-analysis to Update the 2009 U.S. Preventive Services Task Force Recommendation. Ann Intern Med 2016;164:244-55.

3 Nelson HD, Cantor A, Humphrey L, et al. Screening for Breast Cancer: A Systematic Review to Update the 2009 U.S. Preventive Services Task Force Recommendation. Evidence Synthesis No. 124. AHRQ Publication No. 14-05201-EF-1. Rockville, MD: Agency for Healthcare Research and Quality; 2016.

4 Gotzsche, P. and M. Nielsen. Screening for breast cancer with mammography. Cochrane Database Syst Rev., 2011. 1 p. CD001877.

5 Gotzsche, P.C. and K.J. Jorgensen. Screening for breast cancer with mammography. Cochrane Database of Systematic Reviews, 2013. 6(10.1002/14651858.CD001877.pub5). 

6 Nelson HD, Pappas M, Cantor A, et al. Harms of breast cancer screening: systematic review to update the 2009 U.S. Preventive Services Task Force Recommendation. Ann Intern Med 2016; 164:256-67.

7 H. Gilbert Welch, Philip C. Prorok, A. James O’Malley, Barnett S. Kramer, Breast-cancer tumor size, overdiagnosis, and mammography screening effectiveness. N. Engl. J. Med.2016; 375:1438-1447.

8 Bond M, Pavey T, Welch K, Cooper C, Garside R. Systematic review of the psychological consequences of false-positive screening mammograms. Health Technol. Assess. April 2013;17(13).