Most cancers are just down to ‘bad luck’, say scientists

How could WDDTY resist this one? For once, they are not the most misleading story about this study, Variation in cancer risk among tissues can be explained by the number of stem cell divisions, Tomasetti & Vogelstein, Science 2 January 2015: Vol. 347 no. 6217 pp. 78-81.

The abstract states:

Some tissue types give rise to human cancers millions of times more often than other tissue types. Although this has been recognized for more than a century, it has never been explained. Here, we show that the lifetime risk of cancers of many different types is strongly correlated (0.81) with the total number of divisions of the normal self-renewing cells maintaining that tissue’s homeostasis. These results suggest that only a third of the variation in cancer risk among tissues is attributable to environmental factors or inherited predispositions. The majority is due to “bad luck,” that is, random mutations arising during DNA replication in normal, noncancerous stem cells. This is important not only for understanding the disease but also for designing strategies to limit the mortality it causes.

The body of the paper contains considerable additional information. Luckily, Adam Jacobs has written a great blog post describing exactly what it does mean.

So, what does WDDTY say?

Around 60 per cent of cancers are just down to ‘bad luck’—and have little to do with genetics or lifestyle, researchers have claimed.  Instead, they happen because of random DNA mutations that occur when stem cells divide.

That’s not what the researchers say. All cancers are down to mutations in DNA, in the end; the point of the study is that these are most likely in those areas where cells are dividing most quickly. Which is hardly surprising. Or, as Adam Jacobs describes it:

They found a very strong correlation between those two quantities: tissues in which stem cells divided frequently (eg the colon) were more likely to develop cancer than tissues in which stem cell division was less frequent (eg the brain).

The correlation was so strong, in fact, that it explained two thirds of the variation among different tissue types in their cancer incidence. The authors argue that because mutations that can lead to cancer can occur during stem cell division purely by chance, that means that two thirds of the variation in cancer risk is due to bad luck.

Which is the source of the “two thirds” claim.

Only six major cancers were more likely to be caused by environmental and dietary factors, and these include skin, throat, thyroid, lung, liver and colon cancers, say researchers from the Johns Hopkins Kimmel Cancer Centre. But even some of these could be more down to ‘bad luck’, such as lung cancer in the non-smoker.

And as you wrote that, you should have realised that something was wrong in your understanding of the paper, because the correlation between smoking and lung cancer is vastly stronger than one in three. In fact, 80-90% of lung cancers occur in people who have smoked long-term, and of the remainder, most are in people exposed to radon, asbestos or other pollution.

This is quite easy to check. As is the fact that around 70% of cervical cancers are caused by just two strains of human papillomavirus (HPV).

In fact, the study is only explaining the variation in risk from one tissue to another. It doesn’t address the extent to which modifiable risk factors affect that risk. Essentially, it explains why some smokers get cancer and others don’t, but it absolutely does not prove that it’s just bad luck when smokers get cancer: it’s caused by smoking.

However, their discoveries don’t give people a licence to embrace unhealthy lifestyles, they say. A bad lifestyle can add to the ‘bad luck’ factor.

Correct – as the tobacco example shows, their study does not really address that question.

Nonetheless, a healthy diet will have less influence on two-thirds of cancers that are more likely to occur in tissues where there is more frequent division of stem cells in tissues.  Cancer arises when tissue-specific stem cells make random mistakes, the researchers say.

I am not sure you can draw that conclusion, but it’s reassuring to see that WDDTY now intends to drop its incessant stream of fatuous stories claiming that this or that food gives you cancer, and the other prevents it.

We’ll keep an eye on how well they do with that.

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