What causes cancer? It’s a simple question to ask, and – in so many ways – one of the hardest to answer.
And last week, as 2015 slowly got into gear and many of us put the finishing touches to our new year’s resolutions, some eyebrow-raising headlines appeared claiming that scientists had ‘proved’ that the answer is ‘mainly bad luck’.
This story came from a research paper from scientists at Johns Hopkins University in the US, (press released here). And it appeared to contradict the message that many organisations have been trying to hammer home (including us): that although there are no guarantees, we can stack the odds of avoiding cancer in our favour if we embrace a healthy lifestyle.
(In fact, just a week ago we published new stats showing how, if the UK’s population had been healthier overall, an estimated 600,000 cases would have been avoided over the last five years.)
We weren’t the only ones with raised eyebrows – a whole host of blogs and opinion pieces appeared over the weekend, scrutinising the claims. We’ll look at some of these criticisms below, and discuss why the research DOESN’T mean that two-thirds of cases of cancer are ‘caused by bad luck’ (whatever that means).
But before we do, let’s have a very quick recap of what the researchers did, and what they found.
The root of the story
The researchers set out to answer a simple but fascinating question: why do cancers arise more often in some tissues (e.g. the bowel) than others (e.g the brain)?
To try to answer this, they looked at what was already known about cell division rates in a given tissue (specifically, specialised cells called ‘stem cells’, which renew and replenish the tissues in our bodies) and how this related to the overall risk of cancer in that tissue.
‘The Stats Guy’ blog summarised it thus:
[T]he authors … looked at the published literature on 31 different types of cancer (eg lung cancer, thyroid cancer, colorectal cancer, etc) and estimated 2 quantities for each type of cancer. They estimated the lifetime risk of getting the cancer, and how often stem cells divide in those tissues.
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).
In other words, they found a mathematical relationship between the rate stem cells divide in a tissue, and the rate of cancer in that tissue (although several blogs questioned the way the researchers had used the statistics).
According to the researchers, the maths could explain two-thirds of the variation between different tissues.
It’s an interesting finding, which casts new light on an old mystery. But as Professor David Spiegelhalter noted on his blog:
“[This] may be a fairly reasonable statement to make about population rates in different tissues, but of course says nothing about variation in risks between individuals, and certainly does not say that two-thirds of cases are just luck.” [emphasis ours]
And that’s the key criticism of the way this paper has been interpreted. The media coverage has inadvertently jumped from talking about cancer rates in different tissues to speculating about cancer rates in the population. (Although it’s worth noting that the authors themselves still support the fact that certain cancers can be prevented by lifestyle changes.)
The study also had another notable weakness: it looked only looked at cancer types where there was hard data about the rates of stem cell division.
As a result, they missed two of the most common cancers – breast cancer (which is influenced by lifestyle factors like a woman’s reproductive health, her weight after the menopause, and how alcohol much she drinks) and prostate cancer (which is only weakly influenced by lifestyle).
So although their finding is intriguing, there’s a way to go before we can say for sure it applies to all cancer types.
So where does ‘luck’ come in?
Let’s recap. Every time a cell in your body divides to create two new ones – something that happens billions of times a day – there’s a very (very) small chance a mistake could creep into its DNA, and one of the resulting cells could begin the long deadly journey towards developing into cancer.
And, as these researchers have shown, the more often a cell divides, the more chance there is of something untoward eventually happening.
So, in this sense, there’s an element of ‘bad luck’ about cancer. And it’s true that the disease can affect any of us – from the most avid gym bunny to the most determined couch potato.
But we also know there are a whole host of things that affect the chances of DNA damage developing in a dividing cell.
The chemicals in tobacco smoke, for instance, can modify DNA molecules, resulting in errors when they’re copied. The same is true of ultraviolet light.
And, similarly, things that change the rates of cell division over our lives will also influence the chance of a ‘bad’ cell division. For example, fluctuating hormone levels through puberty, periods, pregnancy and the menopause, can affect cell division rates in the breasts, womb and ovaries. Obesity too can influence our hormone levels, and thus cancer risk.
And long-term (‘chronic’) inflammation, as caused by certain infections, asbestos, and a host of other external influences, is thought to speed up cell division in other tissues.
(This is not an exhaustive list, just a flavour of the different things that can influence the development of cancer).
So to ascribe a particular patient’s cancer to ‘bad luck’ is essentially impossible. It’s a combination of myriad influences, some of which we can control, others which we can’t. And it’s certainly not about ‘blame’, as we’ve pointed out before.
In his response to the media stories, Professor Spiegelhalter had this excellent analogy:
[Imagine] there are tickets in a bucket marked cancers of different types, and a lot of blank tickets (and some marked ‘run over by bus’ etc). Smoking means you might get 20 times as many ‘lung-cancer’ tickets, but you still may be lucky and not draw one: many smokers don’t get lung cancer.
So chance plays a very strong role, even in so-called preventable cancers. This leads to the apparently paradoxical observation that most lung cancers are ’caused’ by smoking, while all lung cancers are also a matter of bad luck.
What about the media coverage?
But this isn’t about pointing fingers. It just seems a huge shame that something that is, as far as these things go, established scientific fact – that cancer rates are affected by lifestyle – has been called into question.
For those that doubt the effect these stories can have, just read this tweet:
THANKYOU BBC!! Smoking doesn’t cause cancer. It’s just bad luck
— Kevin Unwin (@kevounwin) January 2, 2015
One of the gratifying things about this story was the speed with which other voices all over the Internet shone a spotlight on the story even before the day was out. So we’ll leave the final word to Adam over at The Stats Guy blog:
We often see medical research badly reported in the newspapers. Often it doesn’t matter very much. But here, I think real harm could be done. The message that comes across from the media is that cancer is just a matter of luck, so changing your lifestyle won’t make much difference anyway. We know that lifestyle is hugely important not only for cancer, but for many other diseases as well.
For the media to give the impression that lifestyle isn’t important, based on a misunderstanding of what the research shows, is highly irresponsible.
- Tomasetti C. & Vogelstein B. Cancer etiology. Variation in cancer risk among tissues can be explained by the number of stem cell divisions., Science (New York, N.Y.), PMID: http://www.ncbi.nlm.nih.gov/pubmed/25554788
- Black cat image via Istolethetv/Flickr, used under a CC-BY 2.0 license