A team of U.S. researchers set out to learn why some tissues were millions of times more vulnerable to cancer than others.  And their results, published this week in the journal Science, shows that two-thirds of the cancer types analyzed were not caused by lifestyle, but by simple chance mutations.

In the United States, 6.8% of people develop lung cancer, 0.6% brain cancer and 0.00072% get tumors in their laryngeal (voice box) cartilage at some point in their lifetime. While toxins from regular tobacco use could explain why lung cancer is more common, the digestive system is exposed to more environmental toxins than the brain, and yet brain tumors are three-times more common as tumors of the small intestine. 

Researchers at Johns Hopkins University School of Medicine and Bloomberg School of Public Health believe the answer rests in the way tissues regenerate.  Old cells are constantly being replaced with new ones made by dividing stem cells.  With each division comes the risk of mutation that moves the stem cell one step closer to becoming cancer.  The turnover rates vary depending on the cells location with the lining of the digestive system being replaced at a faster pace compared to the brain.

Researchers compared how often stem cells divided in 31 tissues in the body over a lifetime with the odds of a developing cancer in those same tissues.  They concluded that two-thirds of cancer types were simply "due to bad luck", from the dividing cells picking up mutations and becoming cancerous.  These cancer types included brain cancers, small intestine cancers and pancreatic cancers.

Cristian Tomasetti, an assistant professor of oncology and one of the researchers, said a focus on prevention would not prevent such cancers.

"If two thirds of cancer incidence across tissues is explained by random DNA mutations that occur when stem cells divide, then changing our lifestyle and habits will be a huge help in preventing certain cancers, but this may not be as effective for a variety of others" Tomasetti says. "We should focus more resources on finding ways to detect such cancers at early, curable stages."

The remaining one-third of cancers are affected by lifestyle factors and heightened by genetic risk factors.  Some of the most common forms are Basal cell carcinoma, lung cancer and colon cancer. 

But research performed by Cancer Research UK also shows that more than four in ten of the total number of cancers come down to lifestyle choices.

Dr Emma Smith, senior science information officer at the charity, told the BBC: "We estimate that more than four in 10 cancers could be prevented by lifestyle changes, like not smoking, keeping a healthy weight, eating a healthy diet and cutting back on alcohol."

"Making these changes is not a guarantee against cancer, but it stacks the odds in our favor" Smith says. "It's vital that we continue making progress to detect cancer earlier and improve treatments, but helping people understand how they can reduce their risk of developing cancer in the first place remains crucial in tackling cancer."