So is cancer mostly 'bad luck' or not?
- 13 January 2015
- From the section Magazine
News reports that most cases of cancer are the result of bad luck dominated the headlines at the start of the year. But there has since been a lot of criticism of the reporting, and some of the science itself. So what should the reports have said?
Headline-writers and news bulletin editors around the world just couldn't get enough of a new study of cancer published on 2 January. "Two thirds of cancers are due to bad luck" reported one typical news story - and most other media outlets had similar headlines.
But there's been criticism of the way this statistic was reported, some of it directed at journalists, and some at the researchers themselves.
To understand the study, published in the journal Science, it helps to understand the scientific basics of cancer.
The disease occurs when cells in a specific part of the body begin to mutate and reproduce uncontrollably. The cancerous cells can invade and destroy surrounding tissues.
The researchers from Johns Hopkins School of Medicine in the US reported they had found a correlation between the number of cell divisions that take place in a given tissue and the likelihood that it would become cancerous. They looked at 31 tissue types (two common cancers, prostate and breast cancer, were not considered).
"Some tissues are fairly stable, so, for instance, muscle and brain tissue does not divide once it's done developing," explains P Z Myers, a biologist and associate professor at the University of Minnesota, Morris, in the US.
"So those tissues have a very low likelihood of coming down with cancer, whereas things like the lining of your intestine is constantly being regenerated and sloughed off and so those cells have a high proliferative output and they're much more likely to become cancerous."
The researchers weren't looking at what determines why some people get cancer and others don't. They were instead asking why some types of cancer are common and some are rare. Why lots of people get lung cancer, for example, but few people get leg cancer.
If you smoke, you greatly increase your chance of getting lung cancer, of course, and other behavioural and environmental factors are known to cause other types of cancer. But some people who don't smoke also get lung cancer and other behavioural, environmental and genetic factors do not account for all cases of cancer.
So how much cancer is down to random cell division error?
The researchers say they've calculated that two thirds (65%) of "the differences in cancer risk among different tissues" is down to cell division gone wrong - "bad luck". Now many media reports have simply concluded that this means that two thirds of cancer cases are just the result of random haywire cell division. That's not correct.
But on the other hand, a lot of people aren't quite sure exactly what the researchers mean. Statistical and scientific experts who have been blogging about the misreading of the research don't all agree about what the 65% figure refers to.
The most likely explanation seems to be that the researchers were referring to the correlation between cell divisions in different types of tissue, and the tendency of those tissues to develop cancer.
If you imagine plotting a graph listing all these different types of cancer, with the frequency of cell divisions on one axis, and the frequency of cancer on the other axis. If the dots on the graph were scattered all over the place you would say that there was no correlation between cell division - what these researchers call "bad luck" - and cancer.
And if all the dots lined up perfectly, there would be a 100% correlation between cell division and cancer. And the answer the researchers have found seems to be somewhere in between: the dots on the graph line up pretty well, so the rate of cell division is about 65% correlated with the rate of cancer.
But if that's the correct reading of the research, several commentators have also complained about the research method.
The authors weren't able to give More or Less an interview, but they did say they are writing a technical paper to clarify their first paper.
So who is to blame for the confusing headlines? Prof George Davey-Smith, a clinical epidemiologist at Bristol University, argues that it's not the journalists on the mainstream newspapers, TV bulletins and news websites.
The headline of Science's own editorial, he points out, was "the bad luck of cancer". The subheading added: "Analysis suggests most cases can't be prevented." And this a conclusion, he says, that the data cannot support.
"In the press release [from the Johns Hopkins School of Medicine] the authors say they've come up with a method to quantify the contribution of these stochastic or chance factors, which their method doesn't," he adds.
"It's both in the journal and in the press release so it's just not fair to attribute the mis-reporting of this to journalists - they've just copied what's in the journal and in the press release."
In the end, the amount of media attention given to this study was not justified by the findings, Davey-Smith says.
"Explaining the difference in risk between the leg and the lung is of no interest to anyone and says nothing about the contribution of chance to cancers in the population. It's like getting two statistics, two estimates, that bear no relationship to each other and because you've got a number, applying that number to some other domain."
But while the headlines may have been misleading, and the study itself has some serious critics, the research does contain an important message for people who have cancer and lead a healthy lifestyle, according to P Z Myers.
"What's important about the study is that it does say that if you have cancer - and I think this is something people who have cancer would like to hear - it's not something you should blame yourself for."
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