Understanding Breast Cancer Risk
Everyone has some risk of developing breast cancer.
Based on current information, 12.9% of women born in the United States — or one in eight women — will develop breast cancer at some point during their lives. For men born in the United States, the current lifetime risk of breast cancer is 0.13%. This means about one in 800 men will develop breast cancer at some point during their lives.
But it’s important to know that these numbers are averages for all women and men in the United States. An individual person’s breast cancer risk may be higher or lower, depending on specific risk factors.
Older age, for example, increases an individual person’s risk of developing breast cancer. According to statistics from the American Cancer Society, the risk that a woman will be diagnosed with breast cancer during the next 10 years, starting at the following ages, is:
age 20: 0.1% or one in 1,479
age 30: 0.5% or one in 209
age 40: 1.5% or one in 65
age 50: 2.4% or one in 42
age 60: 3.5% or one in 28
age 70: 4.1% or one in 25
age 80: 3.0% or one in 33
If you and your doctor have not discussed your personal risk of breast cancer, it’s a good idea to bring it up at your next appointment. Your doctor will ask you about a number of factors that can affect your personal risk, including your personal and family history of cancer and any prior radiation therapy you’ve had. Learn more about risk factors.
Absolute risk and relative risk
Understanding the terms absolute risk and relative risk can help you better understand your own risk of breast cancer.
The absolute risk of a disease describes a person’s risk of developing the disease — in this case, breast cancer — over a defined period of time. The National Cancer Institute and American Cancer Society report the absolute risk of breast cancer in the general population over a lifetime. So, for the average U.S. woman (without taking other personal risk factors into account), 12.9% is the absolute risk of developing breast cancer at some point during her life.
Relative risk compares the risk of two different groups of people. Relative risk also can tell you how much something you do can change your risk compared to your absolute risk.
Some examples may help explain the difference.
We know the absolute risk of breast cancer in U.S. women is 12.9%. A 2017 study found that women who started smoking before age 17 had a 24% higher risk of breast cancer.
This 24% increase in risk is relative risk. It doesn’t mean that women who start smoking before age 17 have a 24% risk of developing breast cancer some time in their lives. It means their risk is 24% higher than the average risk of 12.9%.
To figure out the increase in absolute risk, we have to do some math: 24% of 12.9% is 3% (.24 x .129 = .03). So a woman who started smoking before age 17 has a 3% increase in absolute risk, which means her lifetime risk of developing breast cancer is 15.9% (12.9% + 3%).
Relative risk can be confusing because many people focus on the increase in risk and assume it’s an increase in absolute risk.
In the example above, the increase is 24%, which sounds alarming. But it’s important to remember that this increase is relative risk. So it’s relative to the average risk of breast cancer.
Now let’s look at an example of breast cancer risk decreasing.
A 2019 study found that women who did at least 2.7 hours of moderate exercise per week, such as walking, or 1.5 hours per week of strenuous exercise, such as running, had a 20% lower risk of breast cancer compared to women who exercised less than those amounts.
This 20% decrease in risk is relative risk. It means the women who did at least 2.7 hours of moderate exercise had a risk of breast cancer that was 20% lower than the average risk of 12.9%.
To figure out the decrease in absolute risk, we have to do math again: 20% of 12.9% is 2.6% (.2 x .129 = .026). So a woman who did 2.7 hours of moderate exercise per week has a 2.6% decrease in absolute risk, which means her lifetime risk of breast cancer is 10.3% (12.9%-2.6%).
Researchers often use the term “hazard ratio” to talk about risk in scientific papers. A hazard ratio compares the negative effect of being exposed to something in one group of people to a group of people who haven’t been exposed to the same thing. When using hazard ratios, the absolute risk is one.
In the first example above, starting smoking before age 17 increased the absolute risk of breast cancer by 24%. This is expressed as a hazard ratio of 1.24. It means the risk is 24% higher than the absolute risk.
In the second example, exercising moderately at least 2.7 hours per week decreased absolute risk by 20%. This is expressed as a hazard ratio of 0.80. It means the risk is 80% of the absolute risk, or 20% lower.
— Last updated on January 19, 2022, 4:15 PM