What are the risks and benefits of mammograms?


In yesterday’s column, I answered a question from a 47-year-old woman who had never had a mammogram and wondered if she should have one. She had heard that one group of experts — the American Cancer Society (ACS) — had recently changed its recommendations on this issue.

The reader’s question raises a broader issue: What are the benefits of regular mammograms, and what are the risks for a woman like her? A screening test, such as a mammogram, looks for early signs of a disease. It aims to catch the disease in a curable stage. That’s the benefit.

But not everyone has the same benefit. A woman is more likely to benefit from a screening test if her risk of developing the disease is higher. For example, a woman whose mother or sister has had breast cancer is at higher risk for getting breast cancer herself. Such women often begin mammograms before age 40. The new guidelines we talked about in yesterday’s and today’s columns apply only to women at average risk.

Another important factor that affects a woman’s risk of breast cancer is her age. Breast cancer is fairly unusual in women age 40-49, but becomes more common after age 50. So the benefits of mammograms are less in women age 40-49 than in women age 50-59.

How great are the benefits in women age 40-49, like our reader who asked the question? My Harvard Medical School colleagues Drs. Nancy Keating and Lydia Pace estimate that if 10,000 women turning 40 were screened each year for 10 years, about 200 cases of breast cancer would be discovered. However, in about 100 of those 200 women, the cancer would be so slow-growing that it would never spread or cause suffering.

Unfortunately, the opposite also is true: Some of the 200 women in whom cancer is discovered will die anyway, because the cancer already has spread by the time it is discovered. My colleagues estimate that, of the 10,000 women who get annual mammograms, five will be saved from premature death caused by breast cancer. That’s the benefit of annual mammograms in women age 40-49.

Now let’s look at the risks in these 10,000 women receiving annual mammograms. In about 6,000, a suspicious spot will be seen on the mammogram. That is, 6,000 women will live for some time with the fear that they have cancer. They will have a repeat mammogram. About 1,000 of these repeat mammograms still will look suspicious. This will lead to a biopsy, which is stressful and uncomfortable. The biopsies will find that about 100 women have a cancer that is at risk to spread. That is, 100 of the 6,000 women who were told they might have cancer will prove to have a cancer that is truly fearful.

Now, to your question, dear reader. The new ACS recommendations say that a 47-year-old woman of average risk for breast cancer, and without a past mammogram, should get one. I’m hopeful that my description of the risks and benefits will help your decision.