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Americans seem very afraid of cancer. Much of this fear is legitimate. Cancer is a significant cause of death. Unlike other causes, it often seems to come out of nowhere. It's the "silent killer".

But evidence increasingly argues that cancer may be preventable, too. Unfortunately, this has inflamed as much as it has assuaged people's fears.

Cancer prevention is the topic of this week's Healthcare Triage.

This episode was adapted from a column Aaron wrote for The Upshot. Links to references and further reading can be found there: http://www.nytimes.com/2016/07/06/upshot/helpless-to-prevent-cancer-actually-a-lot-is-in-your-control.html?_r=0

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Americans seem very afraid of cancer.  Much of this fear is legitimate.  Cancer is a significant cause of death and unlike other causes, it often seems to come out of nowhere.  It's the silent killer, but evidence increasingly argues that cancer may be preventable too.  Unfortunately, this has inflamed as much as it has assuaged peoples' fears.  Cancer prevention is the topic of this week's Healthcare Triage.

(Intro)

As a doctor, I've encountered many people who believe that heart disease, which is the single biggest cause of death in Americans, is largely controllable.  After all, if people just ate better, were physically active, and stopped smoking, then lots of them would get better.  This ignores the fact that many aspects of heart disease like age, race, family history, aren't modifiable.  People don't often seem to feel the same way about cancer.  They think it's out of their control.  This belief was reinforced by a study published in Science in January of 2015 which attempted to explain why some tissues seemed to lead to cancer more often than others.

They found a strong correlation between the number of times a cell divides in the course of a lifetime and the risk of developing cancer.  In other words, this study argued that the more times we ask DNA to replicate, the more often something can go wrong.  Some took this to mean that cancer is much more due to bad luck than to other factors that people could control.  Unfortunately, this single explanation is not really what the study showed.  

Lung cells, for instance, divide quite rarely and still account for a significant amount of cancer.  Cells in the gastrointestinal tract divide all the time and account for many fewer cancers.  Some cancers, like melanoma, were argued to be due to intrinsic factors when we clearly know that extrinsic factors like sun exposure are a major cause of that type of cancer. 

Further, this study was focused more on the relative risks of cancer in one type of tissue versus another.  What we really care about is how much we can reduce the risk of cancer in one person versus another, by changing our behavior.

A more recent study published in Nature argues that there's a lot we can do.  Many studies have shown that environmental risk factors and exposures contribute greatly to many cancers.  Diet's related to colorectal cancer, alcohol & tobacco are related to esophageal cancer, HPV is related to cervical cancer, and Heptatitis C is related to liver cancer, and you'd have to be living under a rock not to know that smoking causes lung cancer and that too much sun can lead to skin cancer.

Using sophisticated modeling techniques, researchers argued that less than 10-30% of a lifetime risk of getting cancer was due to intrinsic risk factors, or the bad luck.  The rest was things you can change.  In JAMA Oncology, researchers sought to quantify how a healthy lifestyle might actually impact the risk of getting cancer.  They identified four domains that are often noted to be related to disease prevention: smoking, drinking, obesity, and exercise.  They defined people who engaged in healthy levels of all of these activities as a low-risk group.  They then compared their risk of getting cancer to people who weren't in this group.  They included two very studied cohorts from the Nurses' Health Study and the Health Professionals Follow-up Study as well as national cancer statistics.

Of the nearly 90,000 women and more than 46,000 men, 16,531 of the women and 11,731 of the men fell into the low risk group.  For each type of cancer, they calculated a population-attributable risk, which is the percentage of people who got cancer who wouldn't have if they had adopted low-risk behaviors.  In other words, it's the percent of people who got cancer who might have been able to avoid it. 

About 82% of women and 78% of men who got lung cancer might have prevented it through healthy behaviors.  About 29% of women and 20% of men might have prevented colon and rectal cancer.  About 30% of both might have prevented pancreatic cancer.  Breast cancer was much less preventable, about 4%.  Overall, though, about 25% of cancer in women and 33% in men could be prevented by these lifestyle behaviors.  Close to half of all cancer deaths could be prevented in people as well.

No study is perfect and this one is no exception.  These cohorts are overwhelmingly white and consist of health professionals who are not completely generalizable to the population at large, but the checks against national data showed that if anything, these results might be underestimating how much cancer is preventable by healthy behaviors.  This also wasn't a randomized controlled trial, and if people want to, they can certainly claim that it doesn't prove causation.

A bigger concern to me is that people might interpret these findings assigning fault to people who get cancer.  You can't blame people for getting cancer, especially children.  Much of it is still out of peoples' control, and you don't want to get into situations where you don't feel like people deserve help because they didn't try hard enough to stay healthy.  Again, lots of cancer is still out of peoples' control.

I was also concerned because in this study, low risk status required all four healthy lifestyles.  Failing in any one domain put you into the high risk category, and that seemed like a lot to ask of people.  On further reading, though, I discovered that the requirements weren't overly burdensome.  Not smoking was defined as never having smoke or having quit in the last five years.  That's clearly good for health.  Moderate alcohol consumption was defined as no more than one drink a day on average for women and no more than two for men.  That's pretty much what we argued for in our episode on alcohol and in no way requires abstinence.  Adequate weight was defined as a BMI of at least 18.5 and no more than 27.5.  The cut-off for overweight is 25, meaning you don't have to be thin., you just have to be less than obese, which is a BMI of 30.  Finally, exercise was defined as 150 minutes a week of moderate intensity activity or 75 minutes of vigorous intensity activity.  That's the benchmark we talked about in our episode on exercise.

Turns out that I already meet these four goals.  I don't smoke.  I drink, but at levels that are considered moderate.  My BMI is currently 26.7, and I hit my 150 minutes each week.  I'm low risk, and I bet more of you are than you might think.  As we talk about cancer moon shots, which will likely cost billions of dollars and might not achieve results, it's worth considering that, as in many cases, prevention is not only the cheapest course, but the most effective.  Simple changes to peoples' behaviors have the potential to make many cancers never occur.  They have a side benefit of preventing health problems in many other areas, too.  Investment in these efforts may not be as exciting, but they may yield greater results.

(?~6:24)