That was the blaring headline of a front page article in Tuesday's New York Times. A new study concluded that "annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care." So are mammograms useless?
But wait, didn't I just post on a study that demonstrated the importance of breast cancer screening and how it saved lives (i.e. a disproportionate number of women who died of breast cancer were not being screened), suggesting that more women should get mammograms earlier?
What is going on?
The new study was a randomized control trial (the gold-standard of clinical trials) in which approximately 90,000 Canadian women aged 40 to 59 were randomly divided into 2 groups in 1980. One group received annual mammograms, and the other (control) group did not for a screening period of 5 years (1980 - 1985). All women received an annual clinical breast exam (CBE) and were taught how to perform a breast self-exam. During the five year screening period, 666 invasive breast cancers were diagnosed in the mammogram group and 524 in the control group, and of these, 180 women in the mammogram group and 171 women in the control group died of breast cancer during the 25 year follow-up period (i.e. from 1980 to 2005). The authors concluded that there was no benefit to the mammography (i.e. mortality rate was the same with our without annual mammograms) and that there was an overdiagnosis of 106 tumors in the mammogram group (i.e. more cancers were diagnosed).
By comparison, the previous work was a retrospective observational study in which researchers found of 609 breast cancer deaths in the period from 1990-1999 in the Boston area, 29% were in screened women (who were undergoing mammogram screening), and 71% were in unscreened women. Approximately 80% of all women were being screened, and all had access to similar levels of healthcare. Thus, a disproportionate number of women died who were not receiving mammogram screening, strongly suggesting that mammograms were saving lives.
What can explain the differences between the two studies?
1. In the Canadian study (study #1), the non-mammogram women were still receiving clinical breast exams (CBE), which could have been effective at detecting breast cancer in lieu of mammograms. In the Boston study (study #2), it is possible (likely) that many of the unscreened women were not undergoing clinical breast exams (along with not having mammograms). Thus, CBEs could be an important difference-maker (esp. if you are not getting mammograms).
2. The mammograms in the Boston study could have been better than in the Canadian study. It is known that there can be significant variability in the quality of mammograms.
3. Random statistical differences in the two studies. If you perform the same study twice, the results will not be identical because of random differences in the participants, in the screening, in the treatments, etc. Statistics are used to identify significant differences beyond random variations but they are not perfect.
One important tidbit is the potential benefit from clinical breast exams by medical professionals. Interestingly there is some disagreement over the need for CBEs. Some think that annual CBEs do not provide any added benefit over annual mammograms. Nevertheless, the American Cancer Society (ACS) recommends clinical breast exams about every 3 years for women in their 20s and 30s, and every year for women 40 and over.
So are mammograms useless?
Probably not. If you average the results from a large number of studies, the weight of evidence tilts in favor of mammograms saving lives. For example, 6 out of 8 randomized controlled trials showed some benefit for mammograms. But there is still a good deal of uncertainty; published estimates of the proportional contribution of screening mammography to declining mortality vary from none to an estimated 75 to 80%.
Clearly, more studies need to be, and they will be done. In the meantime, be diligent about performing breast self-exams (every month), get regular clinical breast exams (every year if you are 40 or over), and start mammograms at age 40 (as recommended by the American Cancer Society).
Figure 1. The weight of current medical evidence favors at least some efficacy for mammograms.
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