Screening the general population of women for breast cancer with mammography is a very appealing idea. Breast cancer is the commonest cause of cancer death in women worldwide. The great majority of women who develop breast cancer have no major identifiable risk factors. There is a strong correlation between breast cancer size at diagnosis and death from the disease. And, most importantly, mammography can often detect a breast cancer years before it is large enough to be detected by a patient or health care provider.
One would, therefore, have expected markedly fewer breast cancer deaths in the screened women compared to the unscreened women in the many studies in which women were randomly assigned to either receive or not receive screening mammography. However, the results of these studies were not nearly as dramatic as expected; even in women aged 50 to 69, the age group in whom the benefit seems to be greatest, there was only a 15% to 20% decrease in the risk of death.
The pro-mammography camp argues that these randomized studies greatly underestimated the benefits of screening mammography because, in the decades since these studies were done, mammography technology has greatly improved. Also, because about 25% of the women randomized to mammography didn’t have all the screenings, while a similar number in the control groups had at least one mammogram during the study period, the observed differences between the groups were blunted.
On the other hand, the anti-mammography camp argue that the benefits in the studies are exaggerated because breast cancer treatment has improved so much; the cancers found by mammography would have been cured today even without screening. Moreover, screening has lots of disadvantages:
- False positive tests which generate unnecessary anxiety, additional expensive tests, and painful biopsies;
- The radiation from mammography itself can sometimes cause cancer.
And here’s perhaps the strongest argument against mammography .
- Mammography has a high rate of overdiagnosis. This means that some cancers will be found which, in the absence of screening, would never have been detected in the woman’s lifetime, either because the cancers would have gone away on their own, because they would have grown extremely slowly, or because the women would have died for some other reason before the cancers became symptomatic. While overdiagnosis is bad in itself (who wants to hear that they have cancer?) it leads to overtreatment with surgery, radiation, hormone therapy and often chemotherapy, with all the associated side effects.
There is no doubt that screening mammography leads to some overdiagnosis. This is especially true of ductal carcinoma in situ (DCIS), an entity that was rarely diagnosed before screening mammography became widespread. Today DCIS accounts for up to 25% of cancers found by screening. Since there is still no accurate way to determine which cases of DCIS will become invasive cancer if left untreated and which will remain dormant or slowly grow for many years, all cases are treated with surgery, usually with radiation, and often with hormonal therapy. Nobody knows how to estimate what percentage of screen-detected cancers are overdiagnosed. This is because since screening finds cancers before they would otherwise be detected, at any point in time a group of screened women will always have more breast cancers diagnosed than a concurrent unscreened group of similar women, even if none of the cancers have been overdiagnosed by screening. Therefore, estimates of the extent of overdiagnosis depend on assumptions and statistical manipulations that are generally beyond the comprehension of the non-statistician reader. Unfortunately, these estimates may be influenced by the biases of the authors.
In the recent paper by Jorgensen et al. the authors compared the change in cancer incidence over time in a region of Denmark that offered screening mammography to women aged 50 to 69, to the change in incidence in regions without screening services, and used two methods to estimate the percentage of overdiagnosed cancers due to screening mammography. With the first method, they estimated that 15% of invasive cancers and 24% of all cancers (including DCIS) were overdiagnosed by screening. The second method, which ignored the probability that cancer incidence increased over time, produced estimates of 39% and 48% respectively. It must be kept in mind that the authors of this study have been leaders of the anti-mammography crusade for many years, which explains why only the higher estimate was included in the conclusion of their abstract.
An American study by Bleyer et al. published five years earlier in the New England Journal concluded that about 30% of screen-detected cancers were overdiagnosed. Here the comparison was done between the tumour incidence in screened women and the incidence in a ‘matched’ group of women diagnosed before screening became available. Again these results are based on the assumption, which has been challenged, that breast cancer incidence did not increase between the two time periods.
In order to avoid some of the biases mentioned in these previous studies other authors have estimated overdiagnosis rates from the randomized trials. In these studies overdiagnosis was estimated to be in the range of only 1% to 10%. However, one could argue that these authors have tended to come from the pro-mammography camp. Also with today’s greater sensitivity of mammography, overdiagnosis rates may be higher. In fact, newer imaging techniques such as breast tomosynthesis ( 3-D mammography) will likely lower false positive rates but may further increase the rate of overdiagnosis.
What is the solution? One extremely active area of research is the search for a molecular test, that can be done on a breast core biopsy, that will distinguish between a clinically aggressive cancer and one that can be safely left untreated and simply observed. An equally important research goal is to determine which women are at such low risk of developing breast cancer that they can avoid screening altogether. Until then, the best we can do is inform our patients about the Canadian guidelines recommending screening mammography every 2 to 3 years for women aged 50 to 74 not known to be high risk, and discuss the risks and benefits of screening with them so that they can make an informed choice.
Click here to read the Table of Contents for the series: Breast Cancer Screening, Mammography and “Alternative Facts”
The opinions expressed in this article are the author’s own and do not reflect the view of Cancer Knowledge Network or Multimed Inc.
Dr. Warner is a medical oncologist and Professor of Medicine at the University of Toronto who has been at the Sunnybrook Odette Cancer Centre since 1993, where her practice and research have been devoted to breast cancer. In 1994, she created a program for hereditary breast and ovarian cancer patients, which introduced genetic counseling and testing to the Odette Cancer Centre. Since 1997, she has led a study to explore the role of magnetic resonance imaging (MRI) in screening women with an inherited predisposition to develop breast cancer which has helped make annual MRI surveillance the standard of care for this very high risk population.
Dr. Warner is also the creator and director of PYNK: Breast Cancer Program for Young Women, an interdisciplinary clinical and research program for young breast cancer patients which was officially launched at Sunnybrook in 2008. This program, the only one of its kind in Canada, addresses the special medical and psychosocial needs of this population and has a major focus related research and knowledge transfer. Dr. Warner is a co-investigator of RUBY, a Canada-wide research program focusing on newly diagnosed breast cancer patients aged 40 and younger, and principal investigator of the RUBY fertility sub studies.