The issue in breast cancer to spark the most controversy has by far been the issue of mammography screening.   The debate has been over whether the scientific evidence supports widespread screening mammography for healthy women who have no symptoms.  This is not about screening women who have a family history of breast cancer, or a prior diagnosis of DCIS, or a genetic mutation that puts them at high risk, but rather screening healthy, low-risk, asymptomatic women.  Does it save lives?  

Research on Mammography Screening

Well-designed and carried out clinical trials are the best way to determine if any medical intervention actually works.  People are randomly assigned to either receive the intervention or not, and the two groups are compared to each other using statistics.  In the case of screening for breast cancer, the goal is to decrease the death rate from the disease. The best way to answer the question, "Does Mammography Save Lives?” is to look at breast cancer survival rates from clinical trials with large groups of women, half who received mammography screening and half who didn't.

Mammography Screening Studies

There are eight published, large, randomized, clinical trials that looked at the benefits of screening mammography for survival. Four of the trials were conducted in Sweden, one was conducted in Canada, two were conducted in the United Kingdom, and one was conducted in the United States. The eight trials are known as:

• The New York trial or HIP trial (1963) - enrolled 60,495 women ages 40-64
• The Malmo trial (1976) - enrolled 42,283 women ages 45-69
• The Two-County trial (1977) - enrolled 133,065 women over age 40
• The Edinburgh trial (1978) - enrolled 44,268 women ages 45-64
• The Canadian trial (parts 1 and 2; 1980) - enrolled 89,835 women ages 40-59
• The Stockholm trial (1981) - enrolled 60,117 women ages 40-64
• The Gothenburg trial (1982) - enrolled 49,924 women ages 39-59
• The Age trial (2006) – enrolled 160,921 women ages 39 - 41

These eight trials included few women over the age of 70 and very few women of color. None of the trials examined women younger than 39 years old, and only one trial -- the Age trial -- was specifically designed to look at the impact of mammography screening in women during their 40s.

The trials vary in reliability and quality of the results.  Some were poorly designed or carried out.  Two of the trials are generally considered to be the most reliable – the Malmo and Canadian trials.  In both trials, the women who got mammography screening had the same breast cancer mortality or death rate as the women who did not.  The Age trial found a reduction in breast cancer mortality from screening during the 40s, but it was a weak finding and not statistically significant.  Results from the other five studies were also weak and not statistically significant, though when the results are pooled, there is a 15% relative risk reduction in breast cancer mortality from mammography.  

It is important to keep in mind that looking at relative risk reduction ignores the actual or absolute risk of developing and dying from breast cancer.  The absolute risk of a woman dying from breast cancer is less than 1% without any screening.  Looking at this another way, the chance that an average 50 year old woman will not die of breast cancer over the next 15 years is 99.12%.  Mammography screening increases those odds by 0.07% to 99.29%.  (Keen, 2009)

Women tend to overestimate both their risk of developing breast cancer and the actual benefit mammography has in reducing the chance of dying from breast cancer.  The absolute benefit of mammography is modest at best, but so is the risk of getting breast cancer in the first place.

Analyses of the Mammography Screening Studies by the Scientific Community

The most thorough and reliable evaluations of the research to date have come from reviews done by the independent research community, including  those conducted by researchers affiliated with the Cochrane Collaboration (Olsen, 2001; Gotzsche, 2006), the U.S. Preventive Services Task Force (Nelson, 2009), and the American College of Physicians (Armstrong et al. in 2007). These scientists reviewed and evaluated the evidence on benefits and harms of mammography screening and assessed the quality of the trials.

For the 2006 Cochrane review, Gotzsche and Nielson used standard criteria to rate the quality of each mammography clinical trial.  The review included the Malmo, Canadian, New York, Two-County, Stockholm, and Goteborg trials. The Edinburgh trial was deemed biased and not included in the 2006 review.   The researchers concluded that only two of the trials were adequately randomized, meaning the women were assigned to mammography or not in a fair and unbiased way – the Malmo and Canadian trials – and these trials did not show that mammography screening decreased mortality from breast cancer.  The women who did undergo mammography screening had the same breast cancer mortality (death rate) as the women who did not.

The researchers then calculated an overall effect on mortality by taking into account the quality of all but the Edinburgh trial. They concluded that mammography decreased the risk of death from breast cancer by about 15% in relative terms, or 0.05% in absolute terms. Finally, Gotzsche and Nielsen found that mammography screening led to more false-positives, more unnecessary surgeries, and more use of aggressive breast cancer treatments. (Gotzsche, 2006)

A 2007 review for the American College of Physicians focused on screening mammography in women 40-49 years of age. It included publications from the original mammography trials as well as 117 other studies. The reviewers found that the studies have estimated a 7% to 23% relative risk reduction in breast cancer mortality rates with screening mammography in women in this age group. They also found rates of false-positive results as high as 20% to 56% after 10 mammograms leading to increases in unnecessary procedures and breast cancer-related anxiety.

These reviewers concluded that the evidence suggests that more women in the 40-49 years age range have risks that outweigh the benefits of screening mammography. Subsequently, the American College of Physicians issued detailed guidelines for screening mammography among younger women that encourage doctors to carefully assess an individual woman's risks for breast cancer, and to discuss with them the potential benefits and harms of screening mammography in order to make informed individual decisions about screening (Armstrong, 2007).

The most recent analysis was done for the US Preventive Services Task Force (USPSTF).  The Task Force reviewed the previous seven clinical trials, the new Age trial, and an update of the Swedish trial.  The authors pooled the results and determined that there was a 15% relative risk reduction in breast cancer mortality in favor of screening.  For women under 50, the addition of the Age trial did not markedly change the results of previous analyses – there was still a 15% decrease in mortality- but the results were not statistically significant or strong.  Since this was the only trial that specifically evaluated the effectiveness of screening women in their 40s, the results were given more weight in the analysis (Nelson, 2009).

Controversy Not New

This is not a new controversy.  Universal mammography screening, particularly for women under 50, has always been controversial.  A consensus panel convened by the National Cancer Institute conducted its own evaluation of the seven mammography trials to date in 1997 and concluded that there was insufficient evidence to show that mammography screening prevents breast cancer deaths in any age group of women, but particularly for those under 50 (NIH, 1997).  They recommended against routine screening for women in their 40s.  Congress got involved and the Senate passed a non-binding resolution in support of mammograms for women under 50 by a vote of 98-0 (Kassirer 1997).  NCI subsequently endorsed screenings for women in their 40s.

Could mammography be harmful?

As with any medical decision, a decision about mammography should take into account not only benefits, but any potential harms.  The National Cancer Institute's Physicians Data Query (PDQ) has identified five potentially harmful results of screening mammography:

• Unnecessary follow-up procedures

False-positive results:  A mammogram shows a suspicious image, but actually there is no breast cancer

• A false sense of security

A negative mammogram, or a mammogram that doesn't detect breast cancer when in fact it is present, leading a women to ignore any change or symptom she notices in her breasts

• Radiation

The effects of repeated exposure to radiation (such as annual screening mammograms) can build up over a lifetime

• Overdiagnosis

Finding and treating a cancer or pre-cancerous condition such as DCIS that would not have ever been life-threatening

• Anxiety

Current Guidelines

The US Preventive Services Task Force issued updated guidelines in November, 2009, taking into consideration the Nelson et al. review of mammography studies, and several statistical modeling studies done to assess both the benefits and harms of mammography.  Below are their 2009 guidelines, as well as the most recent recommendations of the American College of Physicians, and the American Cancer Society.

US Preventive Services Task Force (2009)

• Recommends against automatic, routine screening mammography in women aged 40-49.  The decision to start regular, biennial screening before the age of 50 years should be an individual one and take into account the patient's individual context and values.
• Recommends biennial screening mammography for women aged 50-74 years.
• There is insufficient evidence to assess the additional benefits and harms of screening mammography in women 75 years and older

American College of Physicians (2007)

In women 40 to 49 years of age, clinicians should:

• periodically perform individualized assessment of risk for breast cancer to help guide decisions about screening mammography
• inform women 40 to 49 years of age about the potential benefits and harms of screening mammography
• should base screening mammography decisions on benefits and harms of screening, as well as on a woman's preferences and breast cancer risk profile

American Cancer Society

• Yearly mammograms are recommended starting at age 40 and continuing for as long as a woman is in good health.