Lifestyle choices and genetics are big risk factors for certain cancers, but a new study concludes that the majority of cancer incidence is due mostly to bad luck when our cells divide.

The study comes from scientists at the Johns Hopkins Kimmel Cancer Center who created a statistical model to measure the proportion of cancer cases that are caused mainly by random DNA mutations during stem cell division.

By their calculations, two-thirds of adult cancer incidents can be explained by “bad luck” when stem cells divide.

All cancers are caused by a combination of bad luck, the environment and heredity, says lead researcher Dr. Bert Vogelstein, a professor of oncology at the Johns Hopkins University School of Medicine.

"We’ve created a model that may help quantify how much of these three factors contribute to cancer development,” he said in a statement.

Cancer occurs when stem cells in tissues make random mistakes, or mutations, during the replication process in cell division. The more that these mutations accumulate, the higher the risk that cells will begin to grow, unchecked, into tumours.

But Vogelstein says it's never been clearly understood how much of a contribution these random mistakes made to cancer incidence, compared to genetic inheritance, lifestyle, or environmental factors.

So they focused on 31 tissue types, looking at the number of stem cell divisions in each cancer. They then compared these rates with lifetime cancer risk among the same cancer types in the American population.

Significantly, they did not include breast cancer and prostate cancer in their study, even though these are two of the most commonly diagnosed cancers among adults. The researchers explained that they could not find reliable stem cell division rates on these cancer types.

Of the 31 cancer types they did look at, they found that 22 could be largely explained by the “bad luck” factor of random DNA mutations during cell division.

The other nine cancer types had incidences that were due more to a combination of bad luck plus environmental or inherited factors. Those nine included lung cancer, skin cancer, colon cancer, liver cancer from hepatitis, and head and neck cancer caused by HPV.

In general, tissue types that saw a high number of stem cell divisions were linked to higher incidences of cancer. For example, colon tissue undergoes four times more stem cell divisions than small intestine tissue in humans. Likewise, colon cancer is much more prevalent than small intestinal cancer.

One of the paper’s authors, biomathematician Dr. Cristian Tomasetti, says that while "bad luck" is a major factor in cancer occurrence, that doesn't mean that there's no point in avoiding known cancer risks such as sun exposure or smoking or heavy drinking.

“Absolutely not. I really want to make sure that is not the message,” Tomasetti told The Wall St. Journal.

Lung and skin cancer and several other cancers are clearly tied to lifestyle factors, and people should avoid dangerous behaviors known to increase their risk, he said.

But Tomasetti said there should also be more of a focus on detecting cancers early, when they are still in the curable stages.

The full study appears in the journal Science.