What Causes Cancer?

Here is the short answer: most of the time, we don’t know. Almost everyone diagnosed with cancer wonders, even obsesses, about this question. How did this happen? Why did it happen? What did I do, eat, smoke, or breath? What didn’t I do, eat, smoke, or breath? The best estimate is that about 40% of all cancers can be explained by known factors. The two biggies here are smoking and obesity, but other things like sun exposure (think melanoma) or environmental factors (think asbestos exposure or Agent Orange) are also counted.

The best estimate is that about 40% of all cancers can be explained by known factors.

When I have worked with someone, this question almost always is discussed more than once. Sometimes it is associated with painful self-blame. Lung cancer patients who did smoke are especially vulnerable to this as well as to shaming questions from others. One of my favorite quotes came from a woman I knew who had lung cancer and had been a sometimes smoker. When she was asked, usually in a judgy way, “Did you smoke?” she smiled and responded: Only after sex.

Other people come up with all sorts of explanations as to the cause of their cancer. It makes sense that we are trying to understand what has happened to us, knowing a reason would give us some control. We could then stop whatever the offending habit had been and, hopefully, improve our odds of staying well in the future. One woman with breast cancer was sure it had been caused by her large dog stepping on her breast.

Many believe that stress caused their cancer, and some are especially haunted by a belief that an abortion or long-ago drug use is responsible. Of course, there is plenty of support for some of these worries in the popular press. We are surrounded by suggestions that poor anger management, too few green vegetables, or a daily cocktail caused the cancer.

A new study published in the Journal of the National Cancer Institute reports that some of the other 60% of cancer cases may have been caused by lifestyle or environmental factors; we don’t understand them nor can we identify all of them. And, of course, many cancers are due to chance and bad luck. In my experience, people who have been tested and know that they carry a BRCA1 or BRCA2 or another less common genetic mutation are certainly unhappy about this fact, but often express almost relief at having an understanding of the likely cause of their cancer.

The article attempts to summarize what we know about the causes of cancer. One of the few absolutes is that smoking can and does cause lung and head and neck cancers, and it may contribute to others in ways not yet understood. However, not all smokers ever develop cancer.

Next on the list are obesity and air pollution; these factors are more complicated and less clear. Being overweight is associated with a higher risk of at least 13 cancers, and the best estimate is that obesity causes about 4% of all cancers worldwide. But obesity is measured by BMI, and that measurement does not take into account the balance of fat and muscle, nor a person’s diet and level of exercise. We also now know that the changes in our gut microbes and blood insulin levels, often measured in people who are obese, may increase cancer risk.

Since nothing is ever clear, it is worth noting here that a widely anticipated study of the use of a common anti-diabetes drug in the treatment of early breast cancer returned no positive results. That is, the addition of metformin to adjuvant treatment did not improve outcomes for these women.

We all suspect that various environmental factors contribute to cancer. These include the air we breathe, processed foods we eat, and toxins in many substances that are part of our daily lives. It, however, is impossible to isolate them, so there is little specific data available. We know that the incidence of cancer is greater than it was hundreds of years ago, but we must remember that the average life span is much longer. Since cancer is more often a disease of older age, many more people are living long enough to develop cancer than in the days before antibiotics and most medical treatments.

Finally, many cancers are plain old bad luck, the product of the proverbial crap shoot of life. This can be the hardest etiology to accept because we are forced to acknowledge our limited control over our bodies and our lives. However, at least for me and my own two breast cancers, it seems the most likely.