The main reason we get cancer is just plain 'bad luck' and not lifestyle choices, according to a new study.
Researchers found random mutations play a major role in cancer for two-thirds of adults who contract the disease.
They claim changing our lifestyle dramatically may not be as effective as we thought because of the influence 'bad luck' has on the overall outcome.
But doctors still said poor health and lifestyle choices still contributed to the development of cancer and should not be ignored.
Lead researcher Professor Bert Vogelstein, of the Johns Hopkins University School of Medicine in the United States, said: "All cancers are caused by a combination of bad luck, the environment and heredity, and we've created a model that may help quantify how much of these three factors contribute to cancer development.
"Cancer-free longevity in people exposed to cancer-causing agents, such as tobacco, is often attributed to their 'good genes,' but the truth is that most of them simply had good luck.
"It was well-known that cancer arises when tissue-specific stem cells make random mistakes, or mutations, when one chemical letter in DNA is incorrectly swapped for another during the replication process in cell division.
"The more these mutations accumulate, the higher the risk that cells will grow unchecked, a hallmark of cancer.
"The actual contribution of these random mistakes to cancer incidence, in comparison to the contribution of hereditary or environmental factors, was not previously known."
Co-researcher Doctor Cristian Tomasetti, also of Johns Hopkins University School of Medicine, added: "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.
"We should focus more resources on finding ways to detect such cancers at early, curable stages."
To sort out the role of such random mutations in cancer risk, the scientists charted the number of stem cell divisions in 31 tissues and compared these rates with the lifetime risks of cancer in the same tissues among Americans.
Prof Vogelstein said: "Our study shows, in general, that a change in the number of stem cell divisions in a tissue type is highly correlated with a change in the incidence of cancer in that same tissue.
"One example, is in colon tissue, which undergoes four times more stem cell divisions than small intestine tissue in humans.
"Likewise, colon cancer is much more prevalent than small intestinal cancer."
Dr Tomasetti said: "You could argue that the colon is exposed to more environmental factors than the small intestine, which increases the potential rate of acquired mutations.
"However, we saw the opposite finding in mouse colons, which had a lower number of stem cell divisions than in their small intestines, and, in mice, cancer incidence is lower in the colon than in the small intestine."
The research duo classified the types of cancers they studied into two groups.
They statistically calculated which cancer types had an incidence predicted by the number of stem cell divisions and which had higher incidence.
They found that 22 cancer types could be largely explained by the "bad luck" factor of random DNA mutations during cell division.
The other nine cancer types had incidences higher than predicted by "bad luck" and were presumably due to a combination of bad luck plus environmental or inherited factors.
Prof Vogelstein added: "We found that the types of cancer that had higher risk than predicted by the number of stem cell divisions were precisely the ones you'd expect, including lung cancer, which is linked to smoking; skin cancer, linked to sun exposure; and forms of cancers associated with hereditary syndromes."