What do we mean when we talk about "cancer risk"?
People make decisions about risk all the time. Someone walking home at night might decide that it would be safer to take a longer route down well-lit streets than to use a dark shortcut. They cannot know what will happen with any certainty, but they can take steps to influence the odds in their favour. Weighing up cancer risk is a similar exercise.
Absolute versus relative risk
Reports of risk can be confusing because there are two types of risk.
A person’s absolute risk refers to their actual odds of getting cancer. For example, we know from statistics collected across the UK that an average woman’s absolute lifetime risk of breast cancer is one in nine or 11%.
Each person’s absolute risk varies throughout their life. A woman’s risk of breast cancer increases with age, as she accumulates more damage to her DNA. On average, her risk at age 30 is one in 1,900 but her risk at age 70 is one in 15.
Statements which say that something increases or reduces the risk of cancer by, say, 50%, refer to relative risk. This tells us one person’s chances of getting cancer compared to someone else’s (e.g. a smoker compared to a non-smoker).
For example:
- in the UK, a woman’s absolute risk of breast cancer is 11% or one in nine.
- if drinking two daily units of alcohol increases a woman’s risk by 13%, this means 13% of 11%, which comes to 1.4%.
- so on average, women who drink two units of alcohol a day have an absolute lifetime risk of 12.4% (11 + 1.4%, not 11 + 13%).
What is a ‘risk factor’?
Things that increase our risk of getting cancer are called ‘risk factors’. These include:
- our age
- our lifestyle choices, such as whether we smoke or what foods we eat
- the genes we inherit from our parents
- things in our environment, such as passive smoke or radiation.
It’s important to remember that risk, by its nature, is not definite. There is no guarantee that a person with a ‘low’ risk of cancer will not get it or that someone at ‘high’ risk definitely will.
Risk factors, and the way we deal with them, just shift the odds of getting cancer in one direction or the other.
For example, we know that smoking causes cancer because the poisons in tobacco smoke damage DNA. But not all smokers get cancer because:
- Some people may inhale more of these chemicals than others.
- Some people may be very good at repairing DNA damage because of their genes or other lifestyle choices, while others may be vulnerable to it.
- Damage to certain sections of DNA are much more likely to lead to cancer than damage to others.
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