Overestimating Breast Cancer Risk

Whenever I hear that a woman has a 1 in 8 risk of being diagnosed with breast cancer, I cringe. First, because this is a scary statistic. And second, because it is misleading.

It does not mean that 1 in 8 women in the United States will actually get breast cancer. It is an estimation that a woman has a 1 in 8 (12.3%) chance of getting breast cancer during her entire lifetime -- if she lives to the age of 85. Of course, some women die from other causes before they reach 85. The leading cause of death in women is heart disease and the leading cause of cancer death in women is lung cancer, not breast cancer.

The 12.3% figure comes from adding up the average risk women face during each decade of their lives. Some women face a higher or lower risk than average, but according to the American Cancer Society, a woman’s chance of being diagnosed with breast cancer is:

Age
20-29...........0.05%..........1 in 1,837
30-39...........0.43%.............1 in 234
40-49...........1.43%...............1 in 70
50-59...........2.51%...............1 in 40
60-69...........3.51%...............1 in 28
70-79............3.88%..............1 in 26
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Lifetime......12.28%.................1 in 8

The lifetime risk is roughly the sum of the risk in each decade. (The math whizzes among us may have noticed that the percentages for each decade do not add up to 12.28%, but to 11.81%. These numbers were taken directly from the American Cancer Society’s Breast Cancer Facts and Figures 2007-2008, and do not include risk to age 85.) What is clear – though a bit mysterious mathematically – is that at no time during her life does a woman face a risk of getting breast cancer as high as 1 in 8. No one should be mislead to think that if she’s sitting in a room with 8 women, one of them is destined to get breast cancer.

I am not among the mathematically inclined, but I do have to question whether it’s valid to add up the risk during different periods to get a picture of overall risk. Suppose we were trying to predict the weather instead of the likelihood of getting breast cancer. If the forecast next week was for a 10% chance of rain each day, you’d expect pleasant weather. But if you added up the risk for each day and estimated that there was a 70% chance of rain next week, you’d be sure to take your umbrella.

Carrying around an umbrella is one thing, but when women are made unduly anxious about breast cancer, they may opt for overly aggressive treatment – prophylactic mastectomy -- when they are diagnosed with precancerous conditions, ductal carcinoma in situ (DCIS), or the breast cancer gene. Hearing you’re high risk for breast cancer becomes even more frightening when you have an inaccurate perception of what average risk is.