A few weeks ago, a Science paper about cancer was relayed in all newspapers, as a study showing that most cancers are due to random factors rather than to either genes or environmental factors. I was highly skeptical. Many cancer scientists were apparently critical too. But I'm not a cancer scientist, and the reason for my skepticism was firstly epistemological: how do you prove that something is due to chance, rather than to any unknown factor? By definition, you cannot quantify what you don't know. So when I read: two cancers out of three are due to pure chance, I frowned. But again I'm not a cancer scientist; so maybe I was just missing some key element.

There was so much press coverage about this story that I finally decided to read the original paper. You do not need to be a cancer scientist to understand the study and its methodology. It is actually quite simple (conceptually). It is known that different types of tissues (e.g. brain, stomach) have widely different risks of cancer. Why? This could be for example because environmental factors target more some tissues than others. The authors' hypothesis was simply that cells (specifically stem cells) in different tissues have different division rates. Indeed, cancer-causing mutations are introduced during cell division, and so you would expect that there are more cancers in tissues where cells divide more. And so that's exactly what they found: 0.8 correlation between cell division rate and cancer risk across different types of tissues.

I suppose this is certainly quite an interesting result for cancer research. But how did journalists interpret that as cancers being mostly due to bad luck? Well this time, it seems that authors are to be blamed. I quote the ending sentence of the paper: “For [a subset of] tumors, primary prevention measures are not likely to be effective”. It is pretty clear what it means, and it goes way beyond the results of the study.

Let me restate: the study shows that differences in average risk between types of tissues is highly correlated with stem cell division rate. In other words: statistically, variations in average risk between different tissues can be explained essentially by different division rates. This has nothing to do with the variation in risk between different people! Let me just give you a simple example. Imagine that the risk of developing a cancer is the product of cell division rate, which is tissue-specific, and of an environmental factor, which impacts all tissues in the same way (to simplify). To make my point even simpler, imagine this factor is either 0 or some fixed quantity, depending on whether the person smokes or not. Then in this case, the average risk in a tissue is completely explained by the tissue's cell division rate (correlation is 1), since environmental factors affect all tissues equally. Does this mean that cancers are only due to chance? Of course not, since the risk for a person is either 0 for a non-smoker or a fixed value for a smoker. Now in the light of this example, I find the article's conclusion that “primary prevention measures are not likely to be effective” very worrying, since in this case primary prevention would entirely eradicate the disease.

A quick summary: the study shows that the risk of cancer, averaged across all individuals, varies between types of tissues mainly because those tissues have different average cell division rates (I'm being optimistic here since the study showed correlation and not causality). That's all. It says exactly zero about whether prevention is helpful or not, or about the influence of genes. I have to say the interpretation made by the authors is rather apalling.