When the former England and Newcastle manager Kevin Keegan recently revealed that he had stage 4 cancer, the footballing world responded with an overwhelming show of support. But hidden within his story is a surprising lesson about how cancer is often discovered – not through symptoms, but by chance.
Keegan was in a car crash just weeks before his diagnosis. Studies have found that people who suffer car crash injuries are more likely to be diagnosed with cancer than similar people who haven’t.
Most experts agree that car crash injuries don’t actually cause cancer. So what explains the link?
One possibility is a shared underlying cause. People who drive a lot, simply by spending more time on the road, are more likely to have accidents. They may also lead more sedentary lives, a factor linked to higher cancer rates. Frequent drivers may be more likely to be overweight or to spend long hours in the sun (exposed to harmful UV radiation), both of which raise cancer risk. Sleep deprivation is another candidate: it raises the risk of both crashes and cancer.
To resolve the conundrum, we have to consider how we detect cancer. Typically, we diagnose cancer in people who undergo some type of medical examination – either because their cancer has caused them to feel unwell or for some other reason.
When people are involved in car crashes, they often end up in hospital, where CT scans and MRIs are routinely used to check for internal injuries. In the process, doctors may stumble across a tumour that would otherwise have gone unnoticed. The crash hasn’t made them more likely to have cancer – it has just made them more likely to have it found.
Keegan described it himself: “I was in a car accident and, through that, I had to have an operation. Whilst having the scan for the operation, they found out I had cancer.” People who have been in car crashes are no more likely to have cancer than anyone else – they’re just more likely to have it discovered, because the accident brings them into contact with the medical system.
Car-crash victims aren’t the only ones affected. Anyone who ends up in accident and emergency – for whatever reason – faces the same increased medical scrutiny, and is therefore more likely to have an unrelated cancer picked up in the process.
Detection bias
As I outline in my new book, You Don’t Know What You’re Missing, this is a classic example of a detection bias – the idea that increased monitoring of one situation compared with another can make a phenomenon appear more common than it really is. Take sharks. Despite, on average, 80 attacks and only a handful of deaths worldwide each year, people are disproportionately afraid of them.
In large part, this is probably due to an availability bias. Shark attacks are so graphic and feature so prominently in popular culture – including in films like Jaws, The Reef and The Shallows – that they occupy a disproportionate amount of space in our imaginations. However, in part, this may also be due to the misconception that sharks are attracted to crowded beaches.
Vaclav Sebek/Shutterstock.com
While there is evidence that some species of sharks might be attracted by splashing, because it sounds like struggling prey, other species are put off by it. There is no strong evidence to suggest that more people in the water will lead to a higher probability of attracting a shark.
It is true, however, that there are more shark attacks in places where lots of people swim, but this isn’t primarily because sharks are disproportionately attracted to these popular areas. Popular beaches may see more attacks simply because there are more people in the water, not because sharks have a preference for popular beaches.
It’s also true that busy beaches are disproportionately more likely to report shark sightings. But again, this is purely a function of the fact that more people are around to spot their telltale fins poking out of the water – another detection bias.
Indeed, well-frequented beaches are more likely to have lifeguards who may be on the lookout for sharks or even to employ drones to help reassure beach users that it’s safe to go in the water.
Detection bias turns up in all sorts of unexpected places. When policing is increased in an area, you might expect recorded crime to fall. Often it rises – not because the area has become more dangerous, but because more officers means more crimes are spotted and logged. The underlying crime rate may not have changed at all.
The same thing happens in workplaces. Organisations with rigorous safety protocols can appear to have more safety breaches than those without, simply because they’re better at catching and recording them.
And medicine is no different. Many cancers are first spotted incidentally in A&E in patients who have attended for completely unrelated reasons. This doesn’t mean that being ill or injured causes cancer. If anything, there is evidence that people who end up requiring urgent medical care sometimes live longer as a result because conditions like cancer get caught earlier than they otherwise would have.
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