Cancer risk: What the numbers mean

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http://www.mayoclinic.com/health/cancer/CA00053

Take the time to understand what cancer risk is and how it's measured. This can help you put your own cancer risk into perspective.

You might wonder what your chances are of developing cancer. But cancer statistics can be confusing. News reports make it sound as if nearly every day something is found to dramatically raise your risk. Sorting through all the information and figuring out what's valid and what isn't can be tricky.

What is risk?

When scientists talk about risk, they're referring to a probability — the chance that something may occur, but not a guarantee that it will. As an example of probability, if you flip a coin, there is a one in two chance, or a 50 percent chance, that the coin will land heads up.

Risk estimates for cancer and other diseases are determined by studying large groups of people to discover the probability that any given person or category of people will develop the disease over a certain period of time, and to see what characteristics or behaviors are associated with increased or decreased risk.

How is risk expressed?

Risk is generally divided into two categories:

Absolute risk

Absolute risk refers to the actual numeric chance or probability of developing cancer during a specified time period — for example, within the year, within the next five years, by age 50, by age 70, or over the course of a lifetime.

One type of absolute risk is lifetime risk, which is the probability that an individual will develop cancer over the course of a lifetime. For instance, an American man's absolute risk of developing prostate cancer in his lifetime is about 17 percent. Put another way, about 17 out of every 100 men will develop prostate cancer at some time in their lives.

Keep in mind that lifetime risk isn't the risk that a person will develop cancer in the next year or next five years. An individual's cancer risk has a lot to do with other factors, such as his or her age. For instance, a woman's lifetime risk of developing colon and rectal cancer is just over 5 percent, or about 537 cases per 10,000 women. But her risk of developing colon and rectal cancer before the age of 40 is .07 percent, or about seven cases per 10,000 women.

Relative risk

Relative risk gives you a comparison or ratio rather than an absolute value. It shows the strength of the relationship between a risk factor and a particular type of cancer by comparing the number of cancers in a group of people who have a particular exposure trait with the number of cancers in a group of people who don't have that trait.

For instance, relative risk might compare the lung cancer risk for people who smoke with the lung cancer risk in a similar group of people who don't smoke. You might hear relative risk being expressed like this: The risk of lung cancer for men who smoke is 23 times higher than the risk for men who don't smoke. So the relative risk of lung cancer for men who smoke is 23.

Relative risk is also given as a percentage. For example, the risk of lung cancer for men who smoke is more than 2,000 percent higher than it is for men who don't smoke. Keep in mind that when you hear about relative risk, there's no upper limit to the percentage increase in risk. Most people think 100 percent is the highest possible risk, but that isn't true when talking about relative risk.

A relative risk of 100 percent means your risk is twice as high as that of someone without that risk factor. A 200 percent relative risk means that you are three times as likely to develop that condition.

Where do cancer risk statistics come from?

Most information about cancer risk and risk factors come from epidemiologic studies, which are studies that focus on large, well-defined groups of people. Over the last 50 years, cancer researchers have identified many of the major environmental factors that contribute to cancer, including smoking for lung cancer and sunlight for skin cancer. But uncovering more subtle cancer risks has proved more difficult.

Many studies of cancer risk factors rely on observational approaches. In these studies, researchers keep track of a group of people for several years without trying to change their lives or provide special treatment. This can help scientists find out who develops a disease, what those people have in common and how they differ from those who didn't get sick.

How do cancer risk statistics relate to you?

Risk statistics can be frustrating because they can't tell you your own specific risk for cancer. Just because studies have found that men have a nearly 50 percent chance of developing cancer in their lifetimes doesn't mean that your risk, if you're a man, is 50 percent. Your individual risk is based on many different factors, such as your habits, your family history of cancer and the environment in which you live.

Even then, the combination of risk factors might not apply directly to you. Cancer is highly individualistic. You can have two people with the same age, sex, race, socioeconomic status and comparative lifestyles and still have totally different disease experiences. Risk statistics are helpful in general statements such as "exercising regularly tends to coincide with a reduced risk of chronic diseases, such as cancer." But exercising regularly won't guarantee that you won't get cancer.

Talk with your doctor about your risk of cancer. He or she can review what elements in your life may increase your risk. You can then discuss what you can do to help lower this risk.

Keep cancer risk statistics in perspective

You might hear a news report about a study that seems to indicate you may be at increased risk of a particular type of cancer. Don't jump to conclusions based on this one report. Take a step back and think about what the risk really means.

Observational studies aren't foolproof. Researchers agree that one study by itself isn't authoritative. This is why you sometimes see studies with seemingly contradictory results. Scientists tend to weigh the evidence over time and many research studies, to better determine whether a finding is true. News reports, though, often focus on each new study in isolation, rather than as a part of an evolving picture. This can sometimes cause unnecessary alarm or confusion.

When reading about cancer risk statistics, pay attention to these details:

Who's being observed? A news report may say a certain activity increases the risk of cancer for a group of people. But who was being observed in the study? Pay attention to the ages of the people and their characteristics. For example, some people are genetically predisposed to certain types of cancer.

How many people were studied? In general, studies involving thousands of people are more accurate than are those that examine a small group of people.

Have similar studies been done? The findings of one study are more reliable if they're similar to findings by other studies.

News reports that focus on alarming statistics, such as a 300 percent increase in risk, but don't give you much context aren't very helpful. If you're concerned about the risk, gather more information.