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Bad Luck? How and Why We Get Cancer

By Malcolm Thaler, MD ; Updated February 02, 2018

Most people chalk up cancer to two main causes: an unhealthy lifestyle (you smoked, you drank too much alcohol, you didn't exercise enough, you ate processed food) or genetics (it's your family's fault).

While it sometimes does come down to one of these factors, researchers have found that most of the time it is something else entirely: a simple matter of (bad) luck. According to a mathematical analysis by researchers at Johns Hopkins School of Medicine, 65 percent of the cancer risk is due to random mistakes in a cell's DNA that can occur every time it divides (i.e., bad luck).

Cancer Biology in a Very Small Nutshell

The cells in our body are constantly dividing, replenishing our tissues with new, healthy cells. Among all these cells, the stem cells in our organs divide most abundantly. Every time our cells divide, however, mistakes can be made in the DNA molecules that govern the cell's subsequent growth and behavior. Usually, our bodies repair these mistakes, but sometimes one slips by. As a result, the cell and its progeny, having now inherited the genetic mistake, grow uncontrollably. That's when they become cancers.

Rate of Cancer Is Just a Matter of Math

The stem cells throughout our bodies grow and divide at different rates, depending on the tissue they reside in. Colon cells, for example, are replenished at a much faster rate than cells in our stomach or small intestine. According to the research, the more a particular stem-cell type divides, the greater the chance that a genetic mistake will occur and the higher the likelihood of cancer.

That's why colon cancer is more common than cancer of the stomach. The correlation between stem-cell divisions and cancer incidence is striking. The investigators found that approximately two-thirds of our cancer risk is a result of these random mistakes.

This is not to say that lifestyle, environment and genetics don't play an important role. Hereditary factors appear to determine about 10 to 15 percent of cancers. Lifestyle and environment determine a slightly higher percentage. For some cancers, these non-random factors can be particularly important (for example, the BRCA gene for many familial breast cancers and smoking for lung cancer). But for the majority of people who get cancer, it simply isn't their fault or the fault of their genes -- it's just something that happens.

What This All Means to You

In many cases it's not what we do but rather the luck of the draw that's responsible. Some people may find this comforting (you can stop blaming yourself if you get cancer) or disturbing (the loss of control may seem frightening).

However, we do still have some control: Screening, early detection and treatment are our greatest weapons against cancer. And for some cancers, some preventative actions can make a difference: Quitting smoking and avoiding excess sun exposure are two obvious examples.

But cancer prevention steps have also been found in measures we should all be taking to improve our cardiovascular health: more physical activity, maintaining a healthy weight, limiting alcohol intake and eating a Mediterranean-style diet.

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