Tuesday 25th February 2020

Mammography Benefits and Risks

Mammography system

Mammography Machine

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Assessing benefits and risks of mammography: figures

The value of screening by mammography has been questioned repeatedly for at least 2 decades by various scientists and authors, including people responsible for breast screening programmes, like Dr Maureen Roberts, clinical director of the Edinburgh Breast Screening Project, and Dr Cornelia J Baines, deputy director of the Canadian National Breast Screening Study.

The most effective way to assess how successful breast screening is in achieving its goal of saving lives is by randomised trials. In each of these a large number of women are randomly assigned to either: 1) a group attending breast screening at regular intervals as recommended by many health authorities, being submitted to further tests and biopsies in case of a positive mammogram, and receiving any subsequent treatment advised by doctors, or 2) a control group not participating in breast screening.

After a predetermined period of several years, mortality in the two groups is compared. Since the groups are randomly formed, any difference in mortality is attributed to breast screening.

Several randomised trials in the USA, Canada, Sweden and Scotland have been conducted, and these trials have been subjected to meta-analyses and systematic reviews, due to the importance of the subject and its uncertain, controversial nature. A 2001 Cochrane systematic review updated in 2008 of 8 randomised trials involving 600,000 women.

The Cochrane review found that the three trials with adequate randomisation - the most reliable - did not show an effect of screening on cancer mortality, including breast cancer, after 10 years or on all-cause mortality after 13 years, while the trials with suboptimal randomisation showed a significant reduction in breast cancer mortality.

It also found that both the numbers of lumpectomies and mastectomies (respectively, partial or total removal of a breast) and the use of radiotherapy were significantly higher in the groups of women attending screening in the two adequately randomised trials that measured this result.

In conclusion, the jury is out:

"The review found that screening for breast cancer likely reduces breast cancer mortality, but the magnitude of the effect is uncertain. Screening will also result in some women getting a cancer diagnosis even though their cancer would not have led to death or sickness. Currently, it is not possible to tell which women these are, and they are therefore likely to have breasts or lumps removed and to receive radiotherapy unnecessarily. The review estimated that screening leads to a reduction in breast cancer mortality of 15% and to 30% overdiagnosis and overtreatment. This means that for every 2000 women invited for screening throughout 10 years, one will have her life prolonged. In addition, 10 healthy women, who would not have been diagnosed if there had not been screening, will be diagnosed as breast cancer patients and will be treated unnecessarily. Furthermore, more than 200 women will experience important psychological distress for many months because of false positive findings. It is thus not clear whether screening does more good than harm." [Emphasis added]

In a word, it's a bit like playing the lottery: will you be the lucky winner whose life may be prolonged or one of the many unlucky losers? I don't play lottery, literally or metaphorically.

An author who studied this subject is Gerd Gigerenzer, former Professor of Psychology at the University of Chicago and a great popularizer of statistics, who writes in his Reckoning with Risk:

"An important result in these trials is that the total mortality - from breast cancer and other causes - was the same in the screening group as in the control group. That is, screening seems to reduce the number of women who die from breast cancer, but not the numbers of women who die."

This is an intriguing result: could the reason for it be the adverse effects of unnecessary invasive treatment on healthy women misdiagnosed with cancer, balancing the mortality reduction from breast cancer in women correctly diagnosed?

All in all, an easy way to look at the outcomes of these scientific studies is that women attending breast screening from the age of 50 to 69 raise their life expectancy on average by 12 days. Or another simple visualization of the results is that undergoing annual mammograms offering 25% risk reduction has approximately the same effect on life expectancy as driving 300 less miles every year.

If we further analyze the trial results in terms of the age of participants, for women aged under 50 there was no detectable reduction in breast cancer mortality even after 10-14 years. So for women in their 40s and younger there is no evidence that breast screening through mammography decreases mortality from breast cancer. For these women there are no proven benefits of mass mammography, only harms.

For women aged 50 and over there appears to be a benefit: 1 woman out of 270 undergoing mammography screening is estimated to be saved from dying from breast cancer. There are doubts, however, about this finding: according to two reviews published in the influential The Lancet, only two of the trials for women aged 50-69 have been adequately randomised, and these found no effect of screening on breast-cancer mortality or on total mortality. The other trials, the reviewers say, contained various imbalances between the experimental and control groups which could have confounded the results, e.g. in the Edinburgh trial the screening group had a double number of women from the highest socioeconomic class compared to the control group, and in the New York trial the screening group included fewer women who already had cancer than the control group. All this makes the authors conclude that screening for breast cancer with mammography is unjustified.

We now come to the question of frequency of mammograms in a breast screening programme.

The health departments or other authorities of different countries have different recommendations on both frequency and age, but there have also been changes. In the USA the U.S. Preventive Services Task Force (USPSTF) until late 2009 recommended a mammography screening every 1 to 2 years for all women aged 40 or over; now it advises a mammogram every 2 years for all women between 50 and 74. In Britain, the National Health Service (NHS) recommendation is every 3 years for all women aged 50 to 70.

These policy changes reflect the results of research being carried out on the efficacy and desirability of breast screening programs at various intervals and ages. As we've seen above, repeated studies have shown no evidence of benefits for screening before the age of 40, which has determined the reversal of earlier recommendations policy in America.

Going back in time even further, until the 1990s health organizations advocated baseline, or first, mammograms for women aged 35 to 39, but now no reputable association advises screening for women in their 30s. If you look at the American Cancer Society history of recommendations, you can see, starting from the early 1990s, the change towards mammographic screening for older women only.

This recent trend in the direction of less screening is in line with the doubts and uncertainties about mammography screening expressed here and elsewhere. There are studies calling for even fewer mammograms for women at low risk of developing breast cancer, that is women with no previous breast biopsy from a cancer scare, no family history of breast cancer, and not very dense breasts. For these women, the studies say, the guidelines should recommend screening every 3 to 4 years.


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Over-diagnosis of breast cancer can be defined as the cases that would never have come to medical and clinical attention without screening, i.e. would not have caused any problems to the women concerned.

In a follow-up study 15 years after the end of the Malmö mammography screening trial in Sweden, the rate of overdiagnosis of both in situ and invasive breast cancers was found to be 10% for women aged 55-69. At the end of the Malmö trial, breast screening was offered to all women aged 45-54 but not to a group of women aged 55-69. The incidence of breast cancer was then calculated and compared in the invited group and unscreened control group. After 15 years, without overdiagnosing the two groups should have had a balanced number of breast cancer diagnoses, in the experimental group detected by screening and in the control group detected through clinical symptoms. A long follow-up is necessary to assess the magnitude of over-diagnosis, and analysis after just a few screening rounds is insufficient. 15 years after the trial ended the invited group showed a 10% significantly higher incidence of breast cancer.

A successive analysis of the same data by different authors concluded that the risk of overdiagnosis is 24%, that is 1 in 4 cancers detected by mammography are pseudocancers.

A third research, a cohort study of incidence of breast cancer during the introduction of nationwide screening programmes in Norway and Sweden, like the previous two published in the British Medical Journal, concludes:

"Without screening one third of all invasive breast cancers in the age group 50-69 years would not have been detected in the patients' lifetime. This level of overdiagnosis is larger than previously reported. " [Emphasis added]

The problem of breast cancer misdiagnosis, although particularly poignant in breast screening, is not limited to it, but it can involve biopsies and other diagnostic tools. Susan G. Komen for the Cure, a major breast cancer survivors' association, published the results of a study estimating that in the USA alone over 90,000 women diagnosed with breast cancer, either invasive or in situ, may have been given a wrong diagnosis and therefore inappropriately treated. It also says:

"Specific to breast cancer pathology, our interviewees who routinely provide second opinions estimate that 2% to 4% of breast cancer diagnoses in the United States may be incorrect.[personal communication, Drs. Kivitz, Page, Allred] Some diagnoses are more difficult to distinguish, and some are so rare that a general pathologist may never see a case in his or her lifetime. The misclassification rate for atypia, microinvasion, or in-situ disease is estimated to be 20%.[personal communication, Dr. Lagios] Predictive and prognostic factors are also at risk for error; ER status may be misclassified in as many as 20% of cases [Allred 2002; CAP 2004] and HER2 in 26% of cases.[CAP 2004] Published data support these findings.[Wells 1998; Collins 2004]. ...A paper published by Staradub and colleagues confirms the value of a second opinion.[Staradub 2002] Complete agreement was found in only 20% of the 340 cases in their study."


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Photo accompanying the article MammoLoRad by Kieranmaher made available under a CC0 1.0 Universal Public Domain Dedication.