'Bad luck' is questionable explanation for cancer

Q: I thought cancer was the result of risky behaviors like smoking and spending too much time in the sun. But then I read an article about cancer and bad luck. So which is it - behaviors or luck?

A: I know the article you are referring to, and I wasn't very happy with the way it was presented by the media. Let me start with the bottom line: Cancer is caused by (1) our genes, and by (2) our lifestyle (risky behaviors) and environment. It's not just one or the other.

Sometimes genes that we inherit from our parents cause cancer. An example is the BRCA1 gene that causes some cases of breast cancer.

Many cancer-causing genetic changes, however, occur after we are born. Sometimes, risky behaviors produce changes in our genes. An example is how cigarette smoke affects certain genes in the lungs and other organs to cause cancer.

Factors in the environment that may or may not be influenced by our behavior can cause cancer. Examples are cancers of the uterus (cervix), liver and stomach caused by various viruses and bacteria. Or skin cancers as the result of unprotected exposure to sunlight.

The study you refer to involved a complex analysis of cancers and stem cells in multiple different organs. Each of our organs has its own stem cells. Organ stem cells are there to replace older cells in the organ when those older cells die. Stem cells divide frequently.

The study was asking whether some cancers occur simply because an organ's stem cells divide frequently. That could cause cancer because each time a cell divides, there is a risk of developing genetic changes ("bad luck"). If so, organs that have the most frequently dividing stem cells might have the greatest risk of cancer.

That's what the researchers found. And they estimated that about two-thirds of all cancers might result from such bad luck.

In my opinion, the authors' conclusion that bad luck accounts for most cancers is questionable on two grounds. First, we know that the rates of some cancers (like skin cancer, or nose and throat cancer) are a hundred times more common in some parts of the world than others. That implies a huge effect of the environment.

Second, using the phrase "bad luck" implies that there is nothing to be done about random cancer-causing genetic changes. That may be true currently, but it is entirely possible that a deeper understanding of the biology of cancer will reveal ways to reduce the risk of such random changes.

I think it's unwise to say that most cancer is caused by bad luck - particularly when the argument is quite speculative. That's because it could discourage us from pursuing a healthy lifestyle, and from getting cancer screening tests that can catch cancers at a curable stage. (I've put a table of recommended cancer screening tests on my website,

We can control our fate, though not completely. That's true of minimizing our risk of cancer, as well. Cancer is not just a matter of bad luck.

• Dr. Anthony Komaroff is a physician and professor at Harvard Medical School. To send questions, go to, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.

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