Why are cancer rates increasing?

Category: Science blog February 4, 2015 Greg Jones 41 comments

Our new figures today demonstrate the impact of cancer on society with more accuracy.

Half of us will hear the words “you have cancer” at some point in our lives.

That’s a stark figure. And it’s a reminder, if ever one were needed, of the challenge we face in beating this disease.

This isn’t to say we’re not making progress: more people are beating cancer today than ever before. Survival has doubled in the last 40 years. And half of people diagnosed will survive their cancer for more than 10 years, an all-time high.

But alongside the undeniable improvements in care, we need to talk about the scale of the problem.

Previously we’ve calculated that more than one person in three would develop cancer at some point during their life – in other words, the so-called ‘lifetime risk’ was more than 1 in 3.

But our latest estimate, which uses the most accurate calculation method to date, now puts our chances of developing the disease at 1 in 2.

Let’s be clear – this isn’t a sudden increase in risk. The different numbers are down to a change in the method used to make the calculation. But they reveal a gradual increase in risk, that’s taken place over decades.

So this begs the obvious question: just why is our risk increasing?

The simple answer is, as the animation above shows: most of us are living longer.

Growing problem

By far the biggest risk factor for most cancers is simply getting older. More than three-quarters of all people diagnosed with cancer in the UK are over the age of 60.

And this is because cancer is a disease of our genes – the bits of DNA code that hold the instructions for all of the microscopic machinery inside our cells. Over time, mistakes accumulate in this code – scientists can now see them stamped in cancer’s DNA. And it’s these mistakes that can kick start a cell’s journey towards becoming cancerous.

The longer we live, the more time we have for errors to build up. And so, as time passes, our risk of developing cancer goes up, as we accumulate more of these faults in our genes.

In the graph below, you can see how UK life expectancy has increased over time and the number of people living into old age is higher than ever before.

This means there are now more people than ever living to an age where they have a higher risk of developing cancer.

But we can stack the odds of avoiding cancer in our favour. Things that happen throughout our lives can speed up – or slow down – the rate at which errors occur in our genes. These include things we can control, and some we can’t.

They include our lifestyle, our genetics & family history, our exposure to viruses, the job we do, the air we breathe – and they can all play different roles in our overall risk of developing the disease.

Specific cancers, specific reasons

The main reason cancer risk overall is rising is because of our increasing lifespan. And the researchers behind these new statistics reckon that about two-thirds of the increase is due to longevity.

The rest, they think, is caused by changes in cancer rates across different age groups. And when you look at these changes in detail, you can see patterns reflected in how we live our lives, clearly showing how important an impact our lifestyle can have.

For example, diets high in red and processed meats have contributed to the rise in bowel cancer cases. And more and more people are becoming overweight and obese in the UK, which raises the risk of developing a number of cancers. And our culture of sunbathing and using sunbeds is contributing to rising rates of melanoma skin cancer.

Changes in alcohol consumption play an important role too.

In women, breast screening has meant we’re detecting more cancers and finding them at a younger age (although some of this may also be because of ‘overdiagnosis’ – something we discuss at length in this blog post). But the increase in breast cancer rates is also down to changes in our lifestyles: women have fewer babies later, and breastfeed less.

In men, things are changing too: the introduction of Prostate Specific Antigen (PSA) testing has led to an increase in the number of prostate cancers diagnosed, many of which might have previously gone undetected and never have caused harm in these men’s lifetimes.

But these increases need to be set against one, dramatic, decrease. Smoking remains the largest preventable cause of cancer in the world, responsible for more than one in four UK cancer deaths, and nearly a fifth of all cancer cases. But fewer men are now smoking tobacco.

So, overall, lung cancer rates are falling (but again, it’s not a simple picture: the later rise in smoking among women compared to men means that their rates of smoking-related cancers are still increasing, as the graphic below shows).

Rates of certain other cancers have fallen too – notably those linked to certain infections. For example, stomach cancer is much less common than it used to be, most likely because of changes in the way food is prepared, and falling rates of infection with H. pylori (a bacterium that increases risk).

And the NHS screening programme has almost certainly prevented an epidemic of cervical cancer – rates of which are expected to fall further as the effects of the vaccine against human papillomavirus (HPV) – the virus that causes it – kick in.

So how do we turn our understanding of these complex, interlocking risk factors into a simple ‘1 in 2’ figure?

Start as you mean to go on

In order to understand the 1 in 2 figure, you need to wrap your head around a concept called ‘lifetime risk’ – something that seems simple at first glance, but is actually difficult to communicate.

The idea of ‘lifetime cancer risk’ answers the question ‘what is the likelihood of a child eventually being diagnosed with cancer at any point in their life?’

But, in order to calculate this for a child born today, we have to make some assumptions about their cancer risk in the future.

Traditionally, that was done using the latest available cancer diagnosis rates, and assuming they would stay the same for the entire life of our theoretical newborn child. For example, let’s imagine we want to estimate the lifetime cancer risk for someone born on the 1st January 2015.

In the year 2075, this person will be celebrating their 60th birthday. Now we obviously don’t have rates for cancer cases for a 60-year-old in the year 2075. So our best estimate was to use the incidence rates from 2015.

The same is true when this person reaches 70 or 80. At each age, we’ll previously have assumed their risk of being diagnosed with cancer is the same is it would have been for someone the same age in the year 2015.

Using this method, it was previously predicted that for every ten people born today, at least four would be diagnosed with cancer in their lifetime.

Time to update

The problem with this method is that incidence rates are unlikely to stay the same through our lives. In reality, incidence rates 50 or 60 years in the future may be very different, and our lifestyles may have changed significantly (for example, smoking rates will – hopefully – continue to fall).

But imagine if you had a group of people who you could follow for their entire lives. You could then study all sorts of things: the average age they live to, how many of them develop cancer and at what different ages they’re likely to be diagnosed.

And, thanks to the UK’s world-class cancer registration data, we have the information required to allow us to make these predictions.

By trying to predict cancer rates in the future, and looking back at the data we already have, we can calculate lifetime risk far more precisely. Cancer Research UK scientist Professor Peter Sasieni is behind our new stats, and his work is published today in the British Journal of Cancer.

Sasieni and his team started by looking at the year a person was born, and the estimates for cancer rates for a newborn child in that year.  Let’s take people born in 1930 as an example.

Instead of using the rates from 1930 to work out the lifetime risk of these people as they aged, the researchers took the cancer rates from later years – for example 1990 – to work out their cancer risk as a, say, sixty-year old. And, for all other years of life, they take the new estimated rates into account.

This different method gives a far more accurate picture of how risk changes over the course of a person’s life, and now we know that the previous figure was an underestimate.

Sasieni’s calculations show that the lifetime risk of developing cancer for someone who was born in 1930 is around 1 in 3. But, by 1960, for those born in that year, lifetime risk had risen to the new figure of 1 in 2.

Now you might well be asking ‘what if I was born after 1960?’

For those born after 1960, we have to predict how incidence rates will change much further into the future. This means there’s a greater level of uncertainty but, if trends remain the same, we can say that the lifetime cancer risk for someone born after 1960 will be at least 1 in 2.

Collective effort

As we said at the beginning, it’s important to emphasise that there hasn’t been a sudden jump in lifetime cancer risk overnight. The new method shows that the rise has been gradual, and follows the same trend of increasing life expectancy.

Importantly, cancer survival is showing the same pattern. Forty years ago, 1 in 4 people survived the disease for at least 10 years. Today that figure is 2 in 4, and it’s our ambition to accelerate progress so that 3 in 4 people survive cancer within the next 20 years.

At Cancer Research UK, we know that far too many lives are still lost to cancer. That’s why we’re committed to this challenge.

But tackling this growing issue has to be a collective effort.

Research has shown us the impact that a healthy lifestyle can have – as well as the role chance plays in our risk of getting cancer. As individuals, we can stack the odds in our favour by not smoking, maintaining a healthy weight, being more active and drinking less alcohol. We can also get to know our bodies and what’s normal for us so we can spot any unusual or persistent changes early on.

But it’s not just about individuals. Politicians have an important role to play in supporting NHS cancer services, making sure patients have access to the best treatments, and supporting public health measures to keep smoking rates in decline, and tackle obesity.

We need to make sure doctors have the right tools and support to recognise the signs and symptoms of cancer, and refer the right patients for further tests as early as possible.

And last, but certainly not least, we need our researchers to continue asking questions, searching for answers and making the breakthroughs that will help more patients survive the disease.

That’s why, this World Cancer Day, we’re saying #wewillunite to beat cancer sooner.

– Greg