Your risk of getting cancer may depend more on luck than you might think.
A newstudy published today in Science reports that your lifetime risk of getting a certain type of cancer is highly dependent on what cells those cancers might originate in. The researchers found that tissues with stem cells which divided more frequently—such as in the colon—were more likely to develop cancer than tissues in which stem cell division occurred less often—such as in the brain.
That’s not to say that exposure to carcinogens won’t lead to cancer—smoking is still a leading cause of the disease. But for 22 cancer types, random mutations could explain more of the risk for contracting the disease, as opposed to nine other cancer types, for which it was more likely that genes or lifestyle play a role.
Here’s Sarah Boseley, writing for The Guardian:
The scientists looked at how often stem cell division, the normal process of cell renewal, takes place in 31 different tissue types, to find out whether the sheer number of divisions can lead to more mistakes – or DNA mutations – occurring. They did not look at tissues from two of the commonest forms of cancer – breast and prostate – which are known to have particular environmental triggers, such as obesity. These were not included because they could not find reliable data on the normal division rate of stem cells in these tissues.
“Our study shows, in general, that a change in the number of stem cell divisions in a tissue type is highly correlated with a change in the incidence of cancer in that same tissue,” said Vogelstein. One example, he says, is in colon tissue, which undergoes four times more stem cell divisions than small intestine tissue in humans. Likewise, colon cancer is much more prevalent than small intestinal cancer.
Now, this does not mean that two-thirds of cancers are purely up to chance, as you might think from skimming many headlines circulating today. Rather, two-thirds of the variation in cancer risk for different tissues can be explained by bad luck—a mistake made when one cell divided into two that led to the condition. Medical statistician Adam Jacobs breaks it down further in a critique of coverage of the study on his blog, The Stats Guy:
The problem is that [the “two-thirds” figure] applies only to explaining the variation in cancer risk from one tissue to another. It tells us nothing about how much of the risk within a given tissue is due to modifiable factors. You could potentially see exactly the same results whether each specific type of cancer struck completely at random or whether each specific type were hugely influenced by environmental risk factors.
Despite the headlines, people are still at risk from cancer caused by environmental factors like smoking or living near a road choked with sooty diesel exhaust. But it also means that, even if you live a life relatively free from carcinogens, that bad luck could eventually strike. Cancer is a complicated disease, but it is not so mysterious that scientists attribute two-thirds of all cancers are simply the result of fortune.