Key Messages:

• Risk describes the chance that a person will develop cancer or have a recurrence (return of cancer after treatment).
  
  
• Risk factors increase your chance of cancer; understanding your risk factors can help you and your doctor make decisions about your health, such as being screened for cancer.
  
  
• Absolute risk and relative risk are statistics that help doctors understand your risk of cancer compared with the general population.

When you read about cancer, you may often see the term “risk” used. What exactly does this mean? In general terms, risk is the chance that an event will happen. When talking about cancer, risk is most often used to describe the chance that a person will develop cancer or have a recurrence. Using this information helps researchers and oncologists (doctors who treat people with cancer) improve the health of large numbers of people.

Example: Learning that people who smoke have a higher risk of lung cancer than people who don’t smoke began a worldwide campaign to encourage people to quit smoking or not start smoking.

Risk factors

A risk factor is anything that increases a person's chance of developing cancer. Although risk factors, such as smoking or a family history of cancer, can influence the development of cancer, most do not directly cause cancer. Some people with several risk factors never develop cancer, while others with no known risk factors do. However, knowing your risk factors and communicating them to your doctor may help you make more informed lifestyle and health care choices.

General risk factors for cancer include older age, a personal and/or family history of cancer, using tobacco, some types of viral infections (such as human papillomavirus or HPV), specific chemicals (such as benzene), and exposure to radiation. People can avoid some of these by not smoking or avoiding multiple sunburns, for example. But other risk factors cannot be controlled, such as getting older. Learn about the risk factors for specific types of cancer.

Understanding your risk for cancer is important because it can help your doctor determine whether you could benefit from additional care, such as:

• Receiving a cancer screening test
  
  
• Receiving a screening test at an earlier age and/or more often than the general population
  
  
• Having another intervention, such as surgery or medication to lower your cancer risk

Example: A woman whose mother or sister had breast cancer is twice as likely to develop breast cancer as a similar woman who does not have this same family history. Some women with especially strong family histories of breast cancer may consider a prophylactic mastectomy (preventive removal of the breasts). This appears to reduce the risk of developing breast cancer by at least 95%. Or a woman may wish to take a medication to lower her risk of breast cancer.

People with a known genetic syndrome in the family may consider genetic testing. Your doctor or genetic counselor can help you analyze your risk of developing cancer.

Understanding the difference between absolute and relative risk

Absolute risk is the chance that a person will develop a disease during a given time. It is helpful for determining how many people are at risk for a disease. You may often see this expressed as a percentage.

Example: The statement one out of eight women (12.5%) will develop breast cancer in her lifetime tells the absolute risk for the general population of women. This number describes the risk only for the general population and is not specific to any individual woman.

Absolute risk estimates may also be different for various groups of people.

Example: Women older than 70 have a higher absolute risk of breast cancer than younger women because breast cancer risk increases with age.

Relative risk compares the risk between a group of people who have a particular risk factor and those who don’t.

Example: One way to think about relative risk is to think of two similar groups of 100 people (in terms of age, gender, smoking history, and any other characteristics). For this example, let’s say that one group consists of people who have a particular risk factor and the other group is people who don’t have that risk factor. During a specific period of time, researchers keep track of how many people from each group develop cancer. If two people in the first group and one person in the second group develops cancer, those in the first group have two times or twice the risk of the second group. This is a 100% increase in relative risk. The absolute risk, however, would be 2% or 2 out of 100 people.

Both relative risk and absolute risk can help doctors understand whether an individual's risk is higher or lower than the general population. These measurements also provide important information when making decisions about lifestyle or cancer screening. Knowing that a risk factor increases a person’s relative risk of developing cancer by 100% might sound high. But looking at the absolute risk (one person in 100 compared to two people in 100) provides a more complete picture. When applying studies you hear in the news to your own situation, it is important to find out the absolute risk; most studies report relative risks, which can make something sound more important than it is. Read more about evaluating medical news.

Questions to ask the doctor about risk

Risk estimates and all other statistics apply to populations, not to individuals, and should be interpreted with caution. The best way to understand your own risk is to talk with your doctor. To help find out and understand your risk of cancer, consider asking your doctor the following questions.

• What risk factors do I have and how do they affect my risk of cancer?
  
  
• What is my chance of developing cancer in the next five years? In my lifetime?
  
  
• What can I do to lower my risk of cancer?
  
  
• If I change my behavior to eliminate a risk factor (for example, quit smoking), what are my chances of developing cancer in the next five years? In my lifetime?
  
  
• If I develop a new risk factor (for example, if a close relative develops cancer), how much does my risk increase compared to the general population?
  
  
• What screening tests do you recommend to help detect cancer?

More Information

Understanding Statistics Used to Estimate Risk and Recommend Screening

Risk Factors and Prevention

Additional Resources

National Cancer Institute: Understanding Risk

Last Updated: May 03, 2010