Risk of Developing Breast Cancer

The term “risk” is used to refer to a number or percentage that describes how likely a certain event is to occur. When we talk about factors that can increase or decrease the risk of developing breast cancer, either for the first time or as a recurrence, we often talk about two different types of risk: absolute risk and relative risk.

Absolute risk

Absolute risk is used to describe an individual’s likelihood of developing breast cancer. It is based on the number of people who will develop breast cancer within a certain time period. Absolute risk also can be stated as a percentage.

When we say that 1 in 8 women in the United States, or 13%, will develop breast cancer over the course of a lifetime, we are talking about absolute risk. On average, an individual woman has a 1-in-8 chance of developing breast cancer over an 80-year lifespan.

The absolute risk of developing breast cancer during a particular decade of life is lower than 1 in 8. The younger you are, the lower the risk. For example:

• From age 30 to 39, absolute risk is 1 in 233, or 0.43%. This means that 1 in 233 women in this age group can expect to develop breast cancer. Put another way, your odds of developing breast cancer if you are in this age range are 1 in 233.
• From age 40 to 49, absolute risk is 1 in 69, or 1.4%.
• From age 50 to 59, absolute risk is 1 in 38, or 2.6%.
• From age 60 to 69, absolute risk is 1 in 27, or 3.7%.

As you can see, the older you are, the higher your absolute risk of breast cancer. Keep in mind that these numbers and percentages are averages for the whole population. Your individual breast cancer risk may be higher or lower, depending on a number of factors, including family history, reproductive history (such as menstrual and childbearing history), race/ethnicity, and other factors.

Take family history, for example. The absolute risk of breast cancer is much higher for women who have inherited mutations in the genes known as BRCA1 or BRCA2. Their absolute risk over the course of a lifetime ranges from 40-85%. This means that out of every 100 women who have these mutations, anywhere from 40 to 85 of them can expect to develop breast cancer should they live to age 80.

If you have breast cancer, absolute risk also can be used to describe the likelihood of a certain treatment outcome or the course of the disease. For example, suppose that, based on the characteristics of the breast cancer (stage, grade, other test results), your age and medical history, and the treatments you have, your doctor tells you that your likelihood of disease-free survival at 5 years — being alive with no evidence of breast cancer — is 90%. This means that your absolute risk of having the breast cancer come back within 5 years is 10%, or 1 in 10. In other words, 1 out of 10 women with similar characteristics and the same treatment plan can expect to have a recurrence within that time frame. Nine out of 10, or 90%, would not.

Relative risk

Relative risk is a number or percentage that compares one group’s risk of developing breast cancer to another’s. This is the type of risk frequently reported by research studies, which often compare groups of women with different characteristics or behaviors to determine whether one group has a higher or lower risk of breast cancer than the other (either as a first-time diagnosis or recurrence).

Understanding relative risk can help you answer an important question: If I make certain lifestyle choices or have certain treatments, how much will I increase or decrease my risk of developing breast cancer or having a recurrence?

Example of breast cancer risk going up

Many studies have shown that women who have two or more alcoholic drinks each day have a higher risk of developing breast cancer. (A drink is defined as 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of 80-proof liquor.) You may hear this relative risk described as a percentage or a number:

• Compared to women who do not drink, women who have two or more drinks per day have a 25% higher risk of breast cancer. Put another way, they are 25% more likely to develop breast cancer over the course of a lifetime than nondrinkers are. This doesn’t mean that their lifetime risk of getting breast cancer is 25% — it means that their risk of getting breast cancer is 25% higher relative to people who don’t drink. This percentage is how you are likely to see relative risk reported by television, the Internet, and newspapers.
• Compared to women who do not drink, women who have two or more drinks per day have a relative risk of 1.25. This number is how researchers and scientific papers would usually talk about relative risk. The number “1” is assigned to the baseline group (women who do not drink), since their risk remains the same. The .25 describes the relative increase in risk for the other group; it is another way of expressing the 25% higher lifetime risk (25% = .25).
  
  Another way of saying this is that women who drink two or more alcoholic drinks per day have 1.25 (1 + .25 = 1.25) times the risk of developing breast cancer than women who do not drink.

Relative risk can be a tricky concept, because most people tend to focus on the reported percentage — e.g., 25% higher risk — which sounds alarming. Yes, a 25% higher risk of developing breast cancer (relative to people who don’t drink) is significant, but it doesn’t tell a woman what her lifetime risk is if she drinks two or more alcoholic drinks per day for the rest of her life. Since women in this group have 1.25 times the risk of developing breast cancer, it’s necessary to multiply the absolute risk of breast cancer for women in the general population (13%, or .13) by relative risk (1.25):

• .13 x 1.25 = .1625, or 16.25%. This means that a woman’s absolute lifetime risk of developing breast cancer if she drinks two or more alcoholic drinks per day is just over 16%, or roughly 1 in 6, versus 13%, or 1 in 8, for women who do not drink.

Many different factors can increase and/or decrease your risk of developing breast cancer. Online tools such as the National Cancer Institute’s Breast Cancer Risk Assessment Tool allow you to input individual information to calculate your risk.

Example of breast cancer risk going down

Suppose you have had breast cancer and undergone lumpectomy (removal of the tumor itself and a margin of healthy surrounding tissue). The absolute risk of the breast cancer coming back in the same breast is about 25%. But if you have radiation therapy to the remaining breast tissue, you can reduce that risk by about 60%. To describe this relative risk decrease, your doctor might say:

• Compared to women who have lumpectomy alone, you have a 60% lower risk of developing breast cancer again in the same breast if you have radiation therapy after lumpectomy.

Medical researchers might express it this way:

• Compared to women who do not have radiation therapy, your relative risk of developing breast cancer is .40 (1 – .60 = .40). Again, the number “1” is assigned to the baseline group, which is not taking the extra action to decrease the risk. The .60 is subtracted from 1 because it represents a decrease in risk. In other words, you have about 40% of the risk of developing breast cancer again in the same breast as they do.

So in this scenario, what difference does radiation therapy really make for you in terms of reducing the absolute risk of cancer recurrence in the same breast? To know that, you have to multiply the risk of recurrence without radiation (25%, or .25) by the relative risk of .40:

• .25 X .40 =.10. In this hypothetical situation, your absolute risk of the cancer returning in the same breast is 10%, or 1 in 10, if you have radiation therapy, versus about 25%, or 1 in 4, if you don’t. Put another way, 1 in 10 women who have radiation therapy can expect to experience recurrence in the same breast, versus 1 in 4 women who do not have the treatment.

So, relative risk is the number that tells you how much something you do, such as a certain behavior or treatment, can change your risk for breast cancer compared to those who don’t do it. A relative risk of:

• .5 means that your risk decreases by half, or 50%
• 1.88 means that your risk increases by 88%
• 3.0 means that your risk triples, or goes up by 300% (you have three times the risk)

As the examples above show, knowing how much your breast cancer risk goes up or down with certain lifestyle factors and treatment options can help you and your doctor make the best decisions for you. These are hypothetical examples; you can find out more about breast cancer risk in the Lower Your Risk section.