Knowing what you can do to decrease your risk of cancer or recurrence is important. But it’s also good to know how much something like eating more broccoli or drinking less alcohol can actually lower your risk. Likewise, if you are considering treatment, it’s good to know by how much a certain chemotherapy can reduce your risk of recurrence. So what do these risk terms mean?
Risk is the probability that something will occur. In relation to cancer, risk most often describes the chance that a person will develop cancer or the chance that their cancer will recur.
Oftentimes research studies will talk about “absolute risk” and “relative risk,” with the relative risk being referenced more frequently. In the media, relative risk is almost always quoted. But this can be quite misleading. There is a big difference between absolute risk and relative risk, and understanding the difference can influence your healthcare decisions.
So what is the difference?
In the context of health, relative risk is a comparison between groups of people who have either different risk factors or are receiving different treatment. For example, you might ask yourself: how does my maintaining a healthy weight change my risk for cancer compared to being overweight, or if I take this chemotherapy, how much does it reduce my risk of my cancer recurring compared to a different chemo or no chemo? Relative risk is expressed as a percentage increase or a percentage decrease. If you do something and it doesn’t affect your risk,the relative risk reduction is 0% (no difference). If you take an action that reduces your risk by 30% compared to someone who doesn’t take that action, then that action reduces your relative risk by 30%. If something you do triples your risk, then your relative risk increases by 300%.
Absolute risk is the chance that a person will develop a disease during a given time. For example “One out of eight women will develop breast cancer in their lifetime.” This describes the absolute risk for the general population of women. An important point to note is that it cannot identify the risk for a specific person – only the general population.
It can get a bit technical, so let’s look at a made up every day example to help illustrate this.
Let’s suppose you just bought a new dishwasher. The manual says that your dishwasher is covered by a warranty for the first year. It also says that on average, 10% of dishwashers will need a service during the first year. The absolute risk that your machine will need a service in the first year is 10%.
The manual also recommends some things you can do that can affect the likelihood of your dishwasher needing a service. The risk of needing a service decreases to 8% if you use a tablet detergent instead of a liquid detergent.
So what are your risks?
You use tablet detergent:
The absolute risk of needing a service in the first year is decreased by 2%. That 2% is the difference between the 10% absolute risk of all machines needing a service and the 8% absolute risk if you use only tablet detergent (10%-8%=2%). Thus, using tablet detergent lowers your absolute risk by 2%.
To calculate your relative risk, you’ll compare the standard absolute risk (10%) to the reduction in absolute risk from using tablet detergent (2%). The arithmetic is:
(2% divided by 10%) x 100 = 20% (expressed another way, its 2/10 x 100 = 20%.)
So you can see that using tablet detergent results in a 20% reduction in relative risk.
Using tablet detergent reduced your absolute risk from 10% to 8% — 2% absolute risk reduction.
Using tablet detergent reduced your relative risk by 20%.
A 20% reduction in relative risk sounds pretty good, doesn’t it? But when you put it in the context of the absolute risk, using tablet detergent decreased the absolute risk by only 2%.
This might start you thinking about other factors, such as the price difference between tablet and liquid detergents. If tablets are much more expensive than liquids, is it really worth that extra expense for just a 2% (absolute) risk reduction in needing to pay for a service in the first year?
Although this illustration might seem a bit frivolous, it demonstrates how you need to know the absolute risk number before you can make a considered choice. The same holds true when it comes to your health — when you are trying to determine what lifestyle changes you can make, or when you are considering which chemotherapy drug to use. Studies might show that a given drug produces a 50% relative risk reduction in cancer recurrence, but that could have been the result of the absolute risk being a reduction from 4% to 2%. That change is 50% (relative) but only 2% (absolute). Armed with this knowledge, you can then decided if it is worth taking that drug with its side effects just for a 2% absolute risk reduction?
I know it seems complicated, but the bottom line is: don’t be afraid to ask your oncologist/healthcare provider for the absolute risk values before you decide on a treatment plan. It’s all part of your informed consent, so you can make a balanced knowledgeable decision.