Understanding Breast Cancer Risk

All women are at risk for getting breast cancer. As you get older, your risk increases. Assuming you live to age 90, your risk of getting breast cancer over your lifetime is about 14%. That might sound scary, because it means that an average of about one out of every seven women will get breast cancer over a 90-year life span.

You can also look at it another way: A 14% risk means there's an 86% chance that you WON'T get breast cancer.

How Much Do Risk Factors and Preventive Factors Change Your Risk?

Knowing what factors can increase or decrease your risk for breast cancer is important. But you probably want to know just HOW MUCH those factors change your risk.

If you hear that a certain treatment can reduce your risk by 40%, what does that mean?

To understand what the numbers mean about YOUR risk for breast cancer, the key terms to know are relative risk and absolute risk.

Relative risk is the number that tells you how much something you do, such as taking a pill, can change your risk, compared to your risk without taking that pill. Relative risk can be expressed in percentages and in "hazard ratios." If you do nothing new, your hazard ratio is 1.0—this means that your risk doesn't change. If you do something and your risk decreases by half, or goes down to 0.5, then you are half as likely to have the risk. But if your risk goes up, from 1.0 to 1.88, then you are 88% more likely to encounter the risk. If your risk goes up to 3.0, then you have a threefold (300%) increased risk of having the problem.

Absolute risk is the size of your own risk. Absolute risk reduction is the number of percentage points by which your own risk changes if you do something, like taking a pill. The size of your absolute risk reduction depends on what your risk is to begin with.

Example of Risk Going Up for a Woman with No History of Breast Cancer

Smoking is associated with an increased risk of breast cancer as well as other diseases.

Suppose your risk of breast cancer is 14%. If you smoke, your risk goes up by 32% (about a third higher risk). That means your risk of breast cancer from smoking is 32% higher than the 14% risk if you don't smoke. That's the relative risk increase from smoking.

But how big a difference does a 32% increase really make for you? To figure out the change to your absolute risk, see what happens when you make your risk of 14% go up by a third:

• Multiply your risk of 14% by the relative risk increase of 32%. You get 4% (14% x 32% = 4.48%, or 4%). 4% is the size of the increase in risk.
• Add the 4% increase to the 14% risk you started out with, and you end up with 18% risk (14% + 4% = 18%),

That means your absolute risk for breast cancer is 18% if you have no history of the disease and you smoke.

Example of Risk Going Down for a Woman with Breast Cancer History

Suppose you have had breast cancer and had lumpectomy with clear margins (meaning no cancer was found between the tumor and the edge of the surrounding tissue that was removed along with it).

After lumpectomy with clear margins, your risk of the breast cancer coming back in the same breast is about 30%. But if you choose to have radiation therapy after your lumpectomy, you can reduce your risk of the cancer coming back by two-thirds or 66%. This is the relative risk decrease.

But how much of a difference does radiation's 66% drop really make? To figure out the change to your absolute risk, take two-thirds off your risk of 30%:

• Multiply your risk of the cancer coming back (30%) by the relative risk decrease from radiation therapy (66%), and you get a decrease of 20% (30% x 66% = 19.80% or 20%).
• To figure out your remaining risk of recurrence after radiation, subtract the 20% from the 30% risk of recurrence that you started out with (30% - 20% = 10%). So your absolute risk of the cancer coming back falls to 10% if you have radiation therapy.

Now, after lumpectomy and radiation, is there something else you can do to knock down the 10% risk further? You may also choose to take hormonal therapy (for example, tamoxifen or an aromatase inhibitor) for five years. Learn more about hormonal therapy choices. If you do that, you can reduce your risk by another 50%. By taking hormonal therapy for five years, you lower your relative risk of the cancer coming back in the same breast by half or 50%. To see how big a difference hormonal therapy makes in your absolute risk, take half off your risk:

• Multiply your risk by the relative risk decrease from tamoxifen (10% x 50% = 5%).
• Then subtract that 5% from your risk (10%–5% = 5%).

Now your absolute risk of the cancer coming back is 5%. So by having radiation therapy and taking hormonal therapy for five years, you have reduced your risk of the breast cancer recurring from 30% to 5%.

Knowing how much your breast cancer risk changes with lifestyle changes and treatment options can help you and your doctor make the best decisions for YOU.