Two thirds of cancers are unavoidable even if you live a healthy life, a study has shown.
Scientists in the US found cancers are caused by random mistakes in the genetic code that occur when cells divide.
The findings challenge the widespread view that cancer mutations are generally inherited or triggered by environmental factors.
Instead, the vast majority of cancers are probably down to unlucky defects in replicating DNA that occur out of the blue, they suggest.
Lead scientist Dr Cristian Tomasetti, from Johns Hopkins Kimmel Cancer Center in the US, said: "It is well-known that we must avoid environmental factors such as smoking to decrease our risk of getting cancer.
"But it is not as well-known that each time a normal cell divides and copies its DNA to produce two new cells, it makes multiple mistakes.
"These copying mistakes are a potent source of cancer mutations that historically have been scientifically undervalued, and this new work provides the first estimate of the fraction of mutations caused by these mistakes."
The research, published in the journal Science, indicates that almost two-thirds of cancer-causing mutations are due to DNA copying errors.
The discovery helps explain why cancer often strikes people who follow all the rules of healthy living and have no family history of the disease.
Cancers triggered by copying errors could occur "no matter how perfect the environment", according to co-author Dr Bert Vogelstein, also from the Johns Hopkins Kimmel Cancer Center.
He said: "We need to continue to encourage people to avoid environmental agents and lifestyles that increase their risk of developing cancer mutations.
"However, many people will still develop cancers due to these random DNA copying errors, and better methods to detect all cancers earlier, while they are still curable, are urgently needed."
The team studied mutations that drive abnormal cell growth in 32 different types of cancer.
Using DNA sequencing and epidemiological data, they developed a mathematical system of assessing the role of genetic copying errors in cancer.
The results showed that it generally took two or more critical gene mutations to trigger cancer.
In some cancer types, such as those affecting the prostate, brain and bone, more than 95% of the harmful mutations were due to random DNA copying errors.
Copying mistakes were linked to 77 per cent of pancreatic cancers but only 35 per cent of lung cancers, which were mostly triggered by smoking and other environmental factors.
Overall, 66 per cent of cancer mutations resulted from copying errors, 29 per cent from lifestyle or environmental factors, and just 5 per cent from inherited faulty genes.
The researchers compared DNA copying errors to "typos" in a written manuscript.
"You can reduce your chance of typographical errors by making sure you're not drowsy while typing and that your keyboard isn't missing some keys," said Dr Vogelstein.
"But typos will still occur because no-one can type perfectly."
Professor Mel Greaves, director of the Centre for Evolution and Cancer at The Institute of Cancer Research in London, said: "Even if, as this study suggests, most individual cancer mutations are due to random chance, the researchers admit that the cancers they cause may still be preventable.
"We have good evidence to show that cancer is caused by a complex mix of environmental exposures, inherited risk, and random chance.
"And while the genes we inherit from our parents are unreturnable and many chance events are non-negotiable, fortunately for us, exposures are major contributors to our risk of cancer and offer a route to risk reduction or prevention."
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