Why are you more likely to be diagnosed with cancer if you've been in a car crash?

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Ahead of the men’s Football World Cup in a few days, former England captain Kevin Keegan shared the news that he had been diagnosed with cancer. While much of the focus on the reporting of the news has rightly been on how the footballing world has rallied around the former Liverpool and Newcastle great, there is an interesting lesson to be learned from the way in which Keegan’s cancer was discovered.

Keegan was in a car crash just a few weeks before his diagnosis. Multiple studies have shown that people who have the sorts of injuries caused by car crashes are also found to have a higher incidence of cancer. That is to say, there would be a larger number of cancer diagnoses in a group of people who’ve had car crashes than there would be in the same size group of people with similar backgrounds who haven’t had car crashes.

Most experts believe that there is little evidence to support the hypothesis that traumatic injuries sustained in car crashes can cause cancer. So if it isn’t the effect of traumatic injury sustained in the accident, what could explain this link? Maybe there is an underlying common cause? Perhaps people who drive more are likely to live more sedentary lifestyles which comes with increased cancer rates? Maybe frequent drivers are exposed to more cancer risk factors (such as obesity or UV exposure), while the increased driving time means they are also more likely to be involved in accidents? Maybe sleep deprivation leads both to more accidents and a higher cancer risk?

To correctly resolve the conundrum, we have to think about how we find 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. Often, people who are involved in car crashes end up in hospital. If we run a CT scan or an MRI on them to understand if there has been any internal damage as a result of the accident, then we may incidentally find a tumour. It’s not that being in a car crash has increased their chance of having cancer, but rather the chance that it is detected.

Indeed this is exactly how Keegan recently described the detection of his cancer. “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.” The underlying rate of cancer isn’t any higher in a group of people who’ve been in car crashes than in a similar group of people who haven’t. It’s just that we have a better chance of diagnosing it in car crash victims because of the increased medical scrutiny they undergo.

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That’s not to say car-crash victims are the only group of people with higher rates of cancer diagnoses. People who attend accident and emergency for almost any reason are subject to increased medical attention and, as a result, will also have higher rates of incidental diagnosis.

As I outline in my new book, You Don’t Know What You’re Missing, this is a classic example of a detection bias - increased monitoring of particular scenarios in comparison to another can induce the perception that one phenomenon has a higher probability than another. For example, despite shark attacks being incredibly rare (around eighty unprovoked attacks and only a handful of fatalities worldwide each year on average), the public are disproportionately worried about sharks.

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In large part, this is probably due to an availability bias – shark attacks are so graphic and feature prominently in popular culture (think Jaws, the Reef, the Shallows, etc.) 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 popular beaches, where many people like to go to swim.

While there is some evidence that some species of sharks might be attracted by splashing, as it sounds like struggling prey, other species are put off by it. There is no strong evidence to suggest that more humans 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. Gavin Naylor is the director of the Florida Program for Shark Research. His data show that the number of shark attacks in the US is strongly correlated with the numbers of people attending the beach. Naylor argues that ‘The more sharks and people there are in one place, the greater the chance of them bumping into each other’; so popular beaches may see more attacks simply because there are more people in the water, not because sharks have a preference for popular beaches.

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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 tell-tale 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.

Other detection biases can mean that interventions can seem to have the opposite impact to the one we might naively have expected. When policing is increased in an area, you might expect that key crime measures would go down, but instead they often go up. Why? Because more police usually means that more crime is detected and recorded even if the underlying crime rates remain the same or even, in fact, go down. Similarly, workplaces with better safety protocols can counterintuitively appear to have higher rates of workplace safety breaches. This can simply be a result of the fact that more breaches are detected and reported, rather than a result of the safety protocols making things less safe.

Multiple studies suggest that, similarly to Kevin Keegan’s case, many cancers are first spotted incidentally in accident and emergency in people who have attended for completely unrelated reasons. It’s important to remember that just because we see a higher incidence of cancer in people who attend accident and emergency, it doesn’t mean that being acutely unwell for other reasons is linked to having cancer. In fact, there is evidence to suggest that being unwell to such an extent that you require increased medical attention can actually prolong your life expectancy through the incidental finding of other diseases like cancer.