Most cancers not just ‘bad luck’


January 26, 2015 — Although a recent article in the journal Science and a subsequent press release about the article led to a spate of headlines implying that most cancer is due to “bad luck,” David Hunter, Vincent L. Gregory Professor in Cancer Prevention, wrote in a January 15, 2015 Boston Globe op-ed that, in fact, most cancers are preventable.

How do we know that most cancers are not just bad luck?

Cancer rates in populations vary substantially over time within countries and between countries. We know that most of these differences are not due to genetic differences in cancer susceptibility between races. Multiple studies have shown that when people move from a country with a low incidence of cancer to one with high incidence, their offspring acquire the rate of the higher incidence country within a generation or two. So that suggests that it’s the environment and lifestyle within the new country that determines the majority of cancer risk. We usually cannot be completely sure why any individual gets a specific cancer, so there is a chance component, but the international differences show that cancer is mostly due to environment and lifestyle.

If cancer rates are mostly affected by lifestyle and environment, what about people who live a healthy lifestyle and never smoke and still get cancer?

For any individual—even a heavy smoker—it’s possible that their lung cancer was caused by something else. There are other risk factors for lung cancer—certain chemical exposures, radon, etc.—so it’s very difficult for an individual to know exactly why they got their form of cancer and because there are few exposures that cause cancer in 100% of people exposed, there is a chance component for the individual. That’s the reason we have to focus on the preventable fraction of cancer from a population perspective.

You note in your Boston Globe op-ed that we currently have the means to prevent many cancers. What are some examples of preventable cancers and what progress has been made in reducing the rates of these cancers?

Tobacco-associated cancers are largely preventable. There are also a variety of infectious causes of cancer—for instance, hepatitis B causes liver cancer, and HPV (human papilloma virus) causes cervical cancer—for which there are vaccines. Also, in the last ten years, we have established that obesity is associated with an increased risk for a wide variety of cancers. If you add all of these together, plus some others like occupational exposures to chemicals or exposure to radiation, it’s pretty easy as a thought experiment to prevent more than half of the cancer cases in the world.

Unfortunately, it’s a tough road to actually achieve this level of prevention. The amount of cigarettes smoked in the U.S. has fallen by about half since the mid-1960s—and lung cancer rates have fallen—but globally there were more cigarettes smoked in the world in 2012 than in any other year in human history. So while as a nation we have made progress, that hasn’t translated globally. In fact, tobacco companies have expanded their markets to many new countries.

As for the hepatitis B vaccine, a lot of countries, particularly in Asia, have successfully introduced the vaccine early in life—which is when it needs to be given—but there are still many countries where it’s not available. Similarly, we’ve had the HPV vaccine for four or five years, and there is some progress making it affordable in middle-income countries, but there’s still a long way to go to vaccinate all girls in the world.