TWO thirds of adult cancer cases were the result of genetic bad luck rather than unhealthy living, according to groundbreaking new research from the US.
Johns Hopkins University School of Medicine scientist Dr Bert Vogelstein said random mutations in DNA were the most common cause of cancer, with the rest caused by environment or inherited genes.
But he warned the finding should not be taken as a licence to drink or smoke to excess.
"This study shows that you can add to your risk of getting cancers by smoking or other poor lifestyle factors," Dr Vogelstein said.
"However, many forms of cancer are due largely to the bad luck of acquiring a mutation in a cancer driver gene regardless of lifestyle and heredity factors."
Researchers compared the number of times organ stem cells divided with the risk of cancer in the tissues.
Those with the most divisions were generally more prone to tumours.
They found 22 of 31 cancer types were caused by random cell mutations - really just genetic misfortune which scientists could not otherwise explain.
The remainder, including smoking-related lung cancer and skin cancer, were related to heredity and environmental factors like exposure to harmful chemicals.
"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," Dr Vogelstein said.