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Understanding Breast Cancer Risk

A single person’s risk of developing breast cancer may be higher or lower than the U.S. national average, depending on specific risk factors.
 

Everyone has some risk of developing breast cancer.

According to the National Cancer Institute, 12.9% of women born in the United States — or one in eight women — develop breast cancer at some point in their lives. For men born in the United States, the current lifetime risk of developing breast cancer is 0.13%. This means about one in 800 men develop breast cancer at some point in their lives.

But it’s important to know that these numbers are averages for all people in the United States. A single person’s risk of developing breast cancer may be higher or lower than the U.S. national average, depending on specific risk factors.

Older age, for example, increases a person’s risk of developing breast cancer. According to statistics from the American Cancer Society, a woman’s risk of developing breast cancer in the next 10 years depending on age is as follows:

  • 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% or one in 33

If you and your doctor have not discussed your personal breast cancer risk, it’s a good idea to bring it up at your next appointment. Your doctor needs information 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 might have had.

 
 
 

Absolute risk and relative risk

Understanding the terms absolute risk and relative risk can help people better understand their own breast cancer risk.

Absolute risk refers to a person’s risk of developing a disease, such as breast cancer, over a defined period of time. For example, the National Cancer Institute is referring to absolute risk when it reports that the average U.S. woman has a 12.9% risk of developing breast cancer in her lifetime.

Relative risk compares risk in two different groups of people. Relative risk also shows how someone’s behavior can change their risk compared with their absolute risk.

For example, a 2017 study found that women who started smoking before age 17 had a 24% higher risk of developing 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 in their lifetime. It means the 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 × .129 = .03). So a woman who starts 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.

An increase of 24% sounds alarming. But it’s important to remember that this increase is relative to the average risk of developing breast cancer.

Breast cancer risk can also decrease.

For example, a 2019 study found that women who did at least 2.7 hours of moderate exercise a week, such as walking, or 1.5 hours a week of strenuous exercise, such as running, had a 20% lower risk of developing breast cancer than women who exercised less than those amounts. This 20% decrease in risk is relative risk. It means the risk of developing breast cancer is 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 × .129 = .026). So a woman who does 2.7 hours of moderate exercise a week or 1.5 hours of strenuous exercise a week has a 2.6% decrease in absolute risk, which means her lifetime risk of developing breast cancer is 10.3% (12.9% - 2.6%).

 

Hazard ratios

Researchers often use the term hazard ratio to talk about risk in scientific papers. Hazard ratios are a way of talking about risk between two groups of people: one group, which is exposed to something, and another group, which hasn’t been exposed to the same thing (also called a control group). When calculating a hazard ratio, absolute risk is always one.

For example, in the previously mentioned 2017 study, smoking before age 17 increased the absolute risk of developing 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.

Additionally, in the previously mentioned 2019 study, exercising moderately at least 2.7 hours a week or strenuously at least 1.5 hours a 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.

Written by: Jamie DePolo, senior editor

Reviewed by 1 medical adviser
 
Jenni Sheng, MD
Johns Hopkins University School of Medicine, Baltimore, MD
Learn more about our advisory board

— Last updated on August 5, 2022, 8:12 PM