Every day, we take steps to reduce the risks in our lives. We wear seat belts to reduce the chance that we’ll get hurt in a car accident and we brush our teeth to prevent cavities, but most of us don't spend too much time thinking about risk. However, it's important to understand risk as it relates to our health. "Risk" in the health and medical fields can have special meanings. Knowing the basic types of risk can help you understand your chances of getting breast cancer and the steps you can take to lower your risk.
The most basic type of risk is absolute risk. Absolute risk is a person's chance of developing a certain disease over a certain period of time. It is estimated by looking at a large group of people who are similar in some way (the same age, for example) and counting how many people in the group develop the disease in question over a certain time period. Knowing the absolute risk of a disease can help you understand the health risks in your life.
For example, if we followed 100,000 women ages 30 to 34 for one year, about 25 women would develop breast cancer . This means the one-year absolute risk of breast cancer for a 30- to 34-year-old woman is 25 per 100,000 women, or 1 per 4,000 women. Another way to say this is that the chances of developing breast cancer in the next year are 25 in 100,000 (or 1 in 4,000) for the average 30- to 34-year-old woman.
In another example, if we followed 100,000 women ages 70 to 74 for one year, about 425 of them would develop breast cancer . This means the one-year absolute risk of breast cancer for a 70- to 74-year-old woman is 425 in 100,000 women, or about 1 in 235 women.
The examples above show the absolute risk of breast cancer is low in young women and much higher in older women.
One absolute risk you may see is the lifetime risk of breast cancer. Women in the United States have a "1 in 8” (or about 12 percent) lifetime risk of getting breast cancer . This means that for every eight women in the U.S. who live to be age 85, one will be diagnosed with breast cancer during her lifetime.
The lifetime risk of breast cancer is much higher than the one-year risk of breast cancer. This is because the lifetime risk adds up all the one-year absolute risks over a woman's life span, up to age 85.
Learn how lifetime risk of breast cancer varies worldwide.
Anything that increases or decreases a person's absolute risk of developing a disease is called a risk factor. A risk factor can be related to lifestyle (such as lack of exercise), genetics (such as family history) or the environment (such as radiation exposure).
For example, women who are not active have a higher chance of getting breast cancer than women who are active (learn more). So, lack of exercise is a risk factor for breast cancer.
Though the term "relative risk" may not sound familiar, we often see or hear about relative risks in news stories about health. A relative risk is a way to present the increase or decrease in risk due to a certain factor.
A relative risk compares two absolute risks. The numerator (the top number in a fraction) is the absolute risk among people with the risk factor. The denominator (the bottom number) is the absolute risk among those without the risk factor. The absolute risk of those with the factor divided by the absolute risk of those without the factor gives you the relative risk.
A relative risk that is greater than one (1.25, for example) shows a factor increases risk. A relative risk that is less than one (0.75, for example) shows a factor decreases risk. And, a relative risk of one shows the factor neither increases nor decreases risk (this means the factor is not likely related to risk of the disease).
Understanding relative risks
Say a study shows women who don't exercise have a 25 percent increase in breast cancer risk compared to women who do exercise. This statistic is a relative risk. It means women who don't exercise are 25 percent more likely to develop breast cancer than women who do exercise.
It’s important to remember that relative risks depend on the underlying absolute risks of the disease.
- When a condition is rare (such as breast cancer among young women), a high relative risk leads to only a few extra cases among those with the risk factor.
- When a condition is more common (such as breast cancer among older women), even a small relative risk can lead to many more cases among those with the risk factor.
We can think about relative risk in terms of money. If you have a single dollar, this makes dollars "rare”. If you double your money, you only gain one extra dollar. But, if you have a million dollars, this makes dollars "common" and a doubling your money means you gain a million extra dollars. In both cases, you double your money, but the real increase in dollars is quite different. The same is true with disease risk. The higher the absolute risk of getting a disease, the greater the number of extra cases that will occur for a given relative risk.
Using our example of the exercise study, we can also show how absolute risks affect the number of extra cases. Inactive women have a 25 percent greater risk of breast cancer than active women (a relative risk of 1.25). Since older women are more likely to get breast cancer, lack of exercise has a greater impact on breast cancer risk in older women than in younger women. Let’s first look at the women in the study ages 70 to 74 years. The study finds that 500 women per 100,000 who are inactive develop breast cancer during one year (this is the absolute risk for women with the risk factor, lack of exercise). The study also shows that 400 women per 100,000 who are active develop breast cancer (this is the absolute risk for women without the risk factor).
We see the relative risk is 1.25 for women who are inactive compared to those who are active.
Among women ages 70 to 74, being inactive led to 100 more cases of breast cancer per 100,000 women in one year (500 cases – 400 cases = 100 cases).
However, in women ages 20 to 29, the study finds 5 per 100,000 who were inactive developed breast cancer in the next year. And, 4 women per 100,000 who were active got breast cancer.
Here too, the relative risk is 1.25. However, in women ages 20 to 29, being inactive caused only one extra case of breast cancer per 100,000 women (5 cases – 4 cases = 1 case).
Thus, the same relative risk of 1.25 led to many more extra cases of breast cancer in the older women (100 extra cases) than in the younger women (one extra case).
Reading relative risks
Relative risks are presented in many ways. This brief guide may help you recognize a relative risk when you hear it on the news or read it on the Internet.
When a relative risk is between 1 and 1.99 it may be presented in several ways. Say, from the example of inactivity above, the relative risk is 1.25. You may see:
- "Inactive women have a relative risk of 1.25 compared to active women."
- "Inactive women have a 25 percent higher risk of breast cancer compared to active women."
- "Inactive women have a 1.25-fold increased risk of breast cancer compared to active women."
When a relative risk is 2 or more, it is often presented as how many times risk is increased. For example, women with atypical hyperplasia, a benign breast condition, have a relative risk of about 4 compared to women without atypical hyperplasia. You may see:
- "Women with atypical hyperplasia have a relative risk of 4 compared to women without atypical hyperplasia.”
- “Women with atypical hyperplasia have 4 times the risk of women without atypical hyperplasia."
- "There is a 4-fold increase in risk among women with atypical hyperplasia compared to women without atypical hyperplasia."
When a relative risk is below 1, it means that the risk factor lowers the risk of disease. For example, women who breastfeed (for a lifetime total of one year or more) have a relative risk of breast cancer of about 0.75 compared to women who do not breastfeed. You may see:
- "Women who breastfeed have a 25 percent lower risk of breast cancer compared to women who do not breastfeed."
Making healthy choices
Understanding absolute risk and relative risk can help you be a well-informed consumer of health information. You can also use this knowledge to make informed health choices.
No matter what your underlying risk of breast cancer is, a healthy lifestyle is always important. Learn more about healthy behaviors and breast cancer risk.