A study has revisited this issue, and the answer seems to be ‘very few’. Women must be given accurate information outlining the risks and benefits of mammography so that they can make an informed decision about whether to be screened.
A study of 40,000 women in Norway aged 50-69, published in the New England Journal of Medicine, investigated the effects of screening mammography on breast cancer mortality. Some counties in Norway conduct mammography screenings, while others do not. Four groups of women were studied: a screening counties group and a nonscreening counties group followed from 1996 to 2005; and also ‘historical’ screening and nonscreening groups, who had been followed from 1986 to 1995. The goal of the study was to find out how much of the reduction in breast cancer mortality that has been observed over time was due specifically to mammography screening. The reduction in breast cancer mortality over time was 10% greater in the screening groups than the nonscreening groups. 
What are the risks and benefits of screening mammography? The Nordic Cochrane Centre, an independent research group that conducts extensive and thorough reviews of the medical literature, assessed the potential benefits and harms of mammography in 2009. These were their conclusions: For every 2000 women that are screened regularly for ten years, one will have her life prolonged. However, 10 healthy women will be unnecessarily treated for breast cancer, either by having a lumpectomy, mastectomy, chemotherapy, or radiotherapy. Also, 200 healthy women will experience a false alarm, leading to substantial psychological and emotional strain. In their analysis, the Cochrane group stated that it is “not clear whether screening does more good than harm.” In women under the age of 50, false positive results are very common.  In 2009, the U.S. Preventive Services Task Force began recommending against routine screening mammography in women between the ages of 40 and 49.
Mammograms are not nearly as life-saving as we are led to believe. The main problem with mammograms is over-diagnosis. Eighty percent of biopsies initiated by a mammogram result are negative. Furthermore, many slow-growing, non-threatening tumors are being detected and treated; at the same time, the more dangerous and aggressive cancers may be missed because they can grow and become lethal in the time interval between screenings, and by then treatment will not work. [3, 5]
Whether or not to undergo mammography is a personal choice, but it is important to know the true risks and benefits of the screening in order to make a sound decision. Regardless of their decision on this matter, women should not rely solely on detection by mammography to protect them against breast cancer. The take home message is that mammograms can’t be counted on as the sole intervention to save women’s lives—they just don’t do enough. Taking steps to prevent breast cancer from developing in the first place – for example, exercising regularly, maintaining a slim, healthy weight, eating plenty of mushrooms, onions, and cruciferous vegetables, minimizing processed foods and animal products, maintaining adequate vitamin D levels, and limiting alcohol consumption – is a much more effective approach than detecting and treating breast cancer after it has begun to develop.
A pamphlet on the potential harms and benefits of mammography screening is available on the Cochrane group’s website.
Read more about diet and lifestyle methods for cancer prevention at DrFuhrman.com.
1. Kalager, M., et al., Effect of screening mammography on breast-cancer mortality in Norway. N Engl J Med, 2010. 363(13): p. 1203-10.
2. Gotzsche, P.C. and M. Nielsen, Screening for breast cancer with mammography. Cochrane Database Syst Rev, 2009(4): p. CD001877.
3. Wright, C.J. and C.B. Mueller, Screening mammography and public health policy: the need for perspective. Lancet, 1995. 346(8966): p. 29-32.
4. Screening for breast cancer: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med, 2009. 151(10): p. 716-26, W-236.
5. Esserman, L., Y. Shieh, and I. Thompson, Rethinking Screening for Breast Cancer and Prostate Cancer. JAMA, 2009. 302(15): p. 1685-1692.