Inherited Cancer Risk Statistics: Absolute Risk Versus Relative Risk

TON - December 2012, Vol 5, No 11 — December 20, 2012

Genetics as it pertains to cancer risk tends to be a hot topic in the news lately. Patients may see a story that states, for example, “Researchers have found that the risk of developing cancer in women with a mutation in gene Q is 500% higher,” but what does that risk actually mean? One important component of genetic counseling is the ability to convey risk to patients.

What Is Risk?
Simply stated, risk is a probability. It is the chance that a particular event may occur. In the case of inherited cancer risk, it is the chance that a person with a mutation in a particular gene will develop cancer. For the purpose of this article, the hypothetical gene “Q” will be used. There are 2 main categories of risk: absolute risk and relative risk. Each category can produce very different-looking numbers.

Absolute Risk
Absolute risk is the chance of developing cancer over a specified time period. This type of risk is not compared with anything. It is simply the probability of an event, such as cancer, occurring. One commonly referenced absolute risk is lifetime risk. For example, the lifetime risk of developing ovarian cancer is approximately 1 in 70. It can also be stated as a percentage, 1.4%, or as a decimal, 0.014. The lifetime risk of developing ovarian cancer with a BRCA mutation is 10% to 60%. Absolute risk also may be expressed using any time interval, such as a 1-year risk, a 5-year risk, and so on. The lifetime risk of ovarian cancer is obviously much higher than a 1-year risk or a 5-year risk, because the lifetime risk adds up all the absolute risk over a woman’s lifetime.

In addition, incidence and prevalence are absolute risks. Incidence is the number of new cancer cases in a particular population over a specified period of time. Prevalence is the proportion of individuals in a population who have a specific characteristic (such as a mutation in gene Q) or cancer at a specific point in time.

Relative Risk
Relative risk is a “risk ratio.” It is the ratio of the probabilities of 2 absolute risks. Two groups are compared to show the relationship between a particular factor, exposure, or the like. A relative risk of 1 means that there is no difference between the 2 groups. A relative risk of 2 means that one group has twice as great a chance of developing a disease as the other group. It allows increase or decrease in risk due to a particular factor to be portrayed. In the case of inherited cancer risk, the number of cancers in a group of people with a mutation in gene Q is compared with the number of cancers in people without a mutation in gene Q. Sometimes this type of risk is referred to as “fold” or as a percentage. Risk often seems much higher when explained in terms of relative risk.

For example, women with a mutation in gene Q have a relative risk of developing cancer of 5, compared with women without a mutation. This also means that women with a mutation in gene Q have 5 times the risk of women without a mutation in gene Q. Sometimes this is stated as a 5-fold risk. Yet another way of stating this statistic is to say that the risk of developing cancer in women with a mutation in gene Q is 500% higher than the risk in women without a mutation in gene Q. As relative risk is a comparison between 2 groups, there is no upper limit to percentage of risk.

Therefore, although at first glance a 500% risk may seem huge, it is important to put risk into perspective. What is the underlying absolute risk of cancer? How common are mutations in gene Q? Also, keep in mind that relative risk does not give one any information about a person’s actual risk.

If a particular cancer is rare, a high relative risk may lead to only a few additional cancer cases. On the other hand, if a cancer is more common, a small relative risk may lead to quite a few additional cases of cancer. The same is true for a gene mutation. For example, let us apply a radio broadcaster’s statement “A mutation in gene Q raises the risk of cancer in women by 5-fold” to 2 different absolute risks of female cancer: 0.005 (.5%) and 0.125 (12.5%). If you multiply both by 5, you now have 0.025 (2.5%) and 0.625 (62.5%). If you had a population of 10,000 women, this would be an increase from 50 to 250 (200 more women affected) and 1250 to 6250 (5000 more women affected). Many women might be comfortable with a 2.5% risk of female cancer; however, few would probably be comfortable with a 62.5% risk of cancer.

Another way to think of this is to use pennies. If you have 1 penny (it is rare) and multiply it by 5, you now have 5 pennies. You cannot do much with 5 pennies. However, if you start with 50 pennies (now they are common) and multiply them by 5, you have 250 pennies. Now you can buy something.

Take-Home Points

  • Relative risk statistics may be conveyed several ways, including the following:
    – Women with a mutation in gene Q have a relative risk of developing cancer of 5, compared with women without a mutation.
    – Women with a mutation in gene Q have a 5-fold increase in risk, compared with women without a mutation.
    – Women with a mutation in gene Q have a 500% higher risk, compared with women without a mutation.
  • Relative risk is a comparison between 2 groups. Therefore, risk often seems much higher.
  • Relative risk provides us very little information on an individual’s actual risk of developing cancer. It is important to evaluate relative risk in the context of the study from which it was derived.

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