Understanding the Risk of Recurrence
One of the goals of breast cancer treatment is to prevent a future recurrence of the cancer (the cancer coming back). Your doctors may recommend one course of action over another because it will lower your risk of recurrence. Understanding what that actually means to your future will help you to be an active participant in choosing the best treatment plan for you.
How much does your risk of recurrence change?
If you hear that a certain treatment can reduce your risk by 25%, what does that mean?
To understand what the numbers mean about YOUR risk for a breast cancer recurrence, the key terms to know are Relative Risk and Absolute Risk. Relative risk is used to compare risks between two groups, whereas absolute risk stands on its own.
Relative Risk is the number that tells you how much something you do, like taking a pill, can change your risk, compared to your risk without taking a pill.
Absolute Risk 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 depends on what your risk is to begin with.
Example 1: Say there is a clinical trial evaluating a new drug that will prevent breast cancer, and 200 women have signed up. In the control group, 100 women received a placebo pill and two developed breast cancer. In the treatment group, 100 people received the drug and only one person developed breast cancer. The two groups are compared - two developed breast cancer in the control group vs one in the treatment group. A 50% reduction in breast cancer! That sounds pretty good. People who want to avoid breast cancer might consider taking this drug, even if there are side effects. But the reality is that the absolute risk reduction was much smaller. If the risk of developing breast cancer at all was 2%, taking the drug may lower the risk to 1%. So a 1% change in absolute risk of breast cancer might not seem worth it if there are side effects to the drug.
Suppose your risk of breast cancer is 12%, and then you decide to take Drug A, which can lower the risk of breast cancer by 25%.
That means your risk of breast cancer with Drug A could be 25% lower than without Drug A. That's the Relative Risk decrease with Drug A.
But how big a difference does a 25% decrease really mean for you? Lowering your 12% risk by 25% drops your risk by 3%.
That 3% is the Absolute Risk decrease for YOU, leaving you with a risk of 9%, if you take Drug A.
Tips for Understanding Studies.
Absolute vs. Relative Risk
Radio commentator Paul Harvey has a feature called "The Rest of the Story." Researchers, clinicians or journalists who report only on relative differences in making claims about a new idea should tell the rest of the story. It is absolute differences that probably matter most to most people trying to make sense out of such claims.
Consider the risk for blindness in a patient with diabetes over a 5-year period. If the risk for blindness is 2 in 100 (2%) in a group of patients treated conventionally and 1 in 100 (1%) in patients treated with a new drug, the absolute difference is derived by simply subtracting the two risks: 2% - 1% = 1%.
Expressed as an absolute difference, the new drug reduces the 5-year risk for blindness by 1%.
The relative difference is the ratio of the two risks. Given the data above, the relative difference is:
1% ÷ 2% = 50%
Expressed as a relative difference, the new drug reduces the risk for blindness by half.
Each is accurate. But if your job is marketing manager for the new drug, you are likely to only use the relative risk reduction. If your job is journalist, you would serve your readers and viewers better by citing the raw data and pointing out the absolute risk reduction. That's the "rest of the story" often missing in news releases and direct-to-consumer prescription drug ads.