How to Understand Relative vs Absolute Risk of Cancer


Relative versus absolute risk is a vital concept for you to understand in a discussion with your doctor.

Any recommendation from your doctor will probably have some expected benefit. Usually, expected benefit is framed as a percentage. For example, “This Wonder Pill will reduce your risk of cancer recurrence by 50%.”

That sounds awesome.

(Cool tip: You know what else can reduce your risk of breast cancer recurrence by 50%? Exercise! Specifically, three to five hours of moderate exercise such as walking per week. More info from Dana Farber at this link.)

It’s easy to hear this and imagine 100 people who have had breast cancer, 50 of whom will be saved by taking the Wonder Pill (or by exercising). That would be a 50% absolute risk reduction.

But that’s not true. It’s a trick your brain plays on you when it hears the words, “50 percent.”

The Idea of Relative Risk

What does that 50% risk reduction really mean? Well, it depends on your starting point — what your risk of recurrence is to begin with, at the moment before you begin taking the Wonder Pill (or exercising!).

For cancer patients, that varies based a huge number of variables, including the type of cancer, the stage of the cancer, the aggressiveness of the cancer, what prior therapy you’ve had (e.g., surgery), and so on. So you need to know your current starting point in order to understand what that 50% means for you.

The Key Is Absolute Risk

Let’s look at two examples.

Patient A has an aggressive cancer. Therefore, Patient A has a 30% risk of recurrence even after surgery, chemotherapy and radiation. This is called the residual risk. The Wonder Pill will reduce Patient A’s risk of recurrence by 50%. That means after taking the Wonder Pill, Patient A will have a 15% risk of recurrence.

Fifty percent is the relative benefit (the relative risk reduction) that Patient A will get from taking the Wonder Pill. The absolute benefit to Patient A is 15% (30% – 15%).

Now let’s turn to Patient B.

Patient B has a less aggressive cancer. Therefore, Patient B has a 10% risk of recurrence after surgery and radiation (no chemotherapy was prescribed, due to the cancer’s low aggressiveness). This is Patient B’s residual risk. The Wonder Pill will reduce Patient B’s risk of recurrence by 50% — the same relative benefit Patient A received. However, the absolute benefit is very different.

After taking the Wonder Pill, Patient B will have a 5% risk of recurrence. The absolute benefit to Patient B is 5% (10% – 5%).

As you can see, the difference between relative risk (or relative benefit) and absolute risk (or absolute benefit) is very important.

Make sure you understand which one you’re talking about when your doctor presents risk reduction figures.

Case Study: Absolute Benefit of Statins

For a real-life discussion of relative versus absolute risk, you can read this article about statins for heart disease:

This article focuses on a medium-risk population (with a 25% risk of heart attack, stroke or death in the next 10 years). Even given those risk factors, the authors question the absolute benefits of statins. Here’s a key quote from the article (the 1.6% and 0.37% risk reductions referenced below are absolute risk reductions):

“It takes 5 years of daily statin therapy to achieve a 1.6% chance of avoiding a heart attack, and a 0.37% chance of avoiding a stroke. Most disappointing, statins seem unable to prevent death in this group. And most concerning, the drugs may increase diabetes, a serious and life-altering disease.”

How to Ask Your Doctor About Relative vs Absolute Risk

The complexities of relative versus absolute risk can get intricate. But you can understand the basics enough to have an informed discussion with your doctor. In fact, it’s vital that you understand what you are actually gaining from any medical (or lifestyle!) decision.

Here’s how to phrase the question:

“Is that a relative or absolute risk reduction?” or, “Is that a relative or absolute benefit?”

If you don’t understand the implications of the answer you hear right away, don’t worry. Write it down, research it later, and then ask any follow-up questions to your doctor. You have time to become informed, and you deserve to understand the decisions you’re making.


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