The big science/health news story this week is about cancer rates, with news outlets splashing headlines like “Two-thirds of adult cancers largely ‘down to bad luck’ rather than genes” or “Most cancer types just bad luck.” But these headlines, and the stories, are just nonsense. The work, which is very interesting, showed no such thing, says an article in the Guardian, which ironically, also carried the headlines.
The stories are about a paper just published in Science under the title “Variation in cancer risk among tissues can be explained by the number of stem cell divisions.” The authors, Bert Vogelstein and Cristian Tomasetti, straight up tell you that their study explains variation in cancer risk, but it does not explain absolute cancer risk.
Basically, the authors had a neat idea. Cancer is caused by mutations in DNA, and mutations can be caused by, amongst other things, DNA replication. So, the more rounds of replication that DNA undergoes (i.e. the more cell divisions that occur in a particular tissue), the greater the chance of cancer.
To test this hypothesis, the authors analyzed data on lifetime cancer risk for 31 types of cancer, and for each one they estimated the rate of cell division. They then plotted these data against each other.
These data suggest there is a relationship between risk of cancer and number of cell divisions. But it says nothing about the proportion of cancers due to cell division.
What Proportion?
So, what proportion of cancers is due to bad luck? Unfortunately it’s difficult to tell from the paper. The figure from the paper is on the log scale, and “if we extrapolate the model to zero (no cell divisions), we’d see it assumes there is no risk of cancer,” the writer says.
“How could we decide how much of the cancer risk is due to bad luck? Well, first, we have to decide what is bad luck, which is an entirely different argument. But after we’ve done that, the only real way to figure out how much of the cancer risk is due to bad luck is to either estimate the rates at which people get cancer through bad luck, or (perhaps easier) to estimate the non-bad luck rates.”
If we stare at the figure from the paper, we can see that this second strategy is used to examine one effect: that of smoking on lung cancer rates. The cancer risk is 18 times higher in smokers than non-smokers. Since roughly 18 percent of the adult US population are smokers, this suggests that for lung cancer, about 75 percent of the risk is due to smoking (i.e. the extra risk due to smoking is multiplied by the proportion of smokers divided by the total lung cancer risk).
“I feel like I need to bemoan the lack of scientific literacy amongst health journalists. I would simply suggest that the next editor of this newspaper insists that all of its health journalists (and perhaps all journalists of whatever stripe) read Ben’s Goldacre’s Bad Science, which is one of the best explanations of how to think statistically,” the writer added.
“Please, journalists, get a clue before you write about science.”
