A cancer-causing mutation is only found in smokers, and giving up may not be enough to stop the disease developing, according to researchers.

People with the K-ras mutation are four times more likely to die than other sufferers of lung cancer and smoking at any time in their lives makes people prone to the condition.

Researchers at Harvard University and the University of California found the mutation in ten per cent of lung cancer cases - and they found it was three times more common in women smokers than men.

This difference between the sexes is due, they suggest, to the effect of the hormone oestrogen, which may spur the growth of tumours.

The most crucial behavioural decision would be to never take up smoking, rather than quitting

John Wiencke, University of California

They add that K-ras helps tumours proliferate, and suggest a screening programme for it could help people survive non-small cell andenocarcinoma, which accounts for 40% of all lung cancers.

The scientists studied 365 patients about to undergo surgery and tracked them for four years afterwards. They found that one time smokers who had quit were just as prone to the mutation as long-term smokers.

"The most crucial behavioural decision would be to never take up smoking, rather than quitting," said John Wiencke, professor of epidemiology and biostatistics at the University of California.

"The striking thing is how early the K-ras mutation seems to act on tumours, how aggressive tumours are when under its influence, and how vulnerable women are to this mutation.

Detect early

"It seems appropriate to try to detect the mutation as early as possible in patients with adenocarcinoma and then treat those patients quickly and with many of the armaments we now have against cancer."

A report, published in the Journal of the National Cancer Institute in America, says it is likely tobacco carcinogens cause almost all K-ras mutations.

Professor Gordon McVie, of the Cancer Research Campaign, said the role of K-ras mutations was well established.

He disputed the suggestion that giving up smoking had no impact. "That is not entirely true. The link is lessened if you give up before the age of about 55."

He added: "What is particularly important is the epidemic in women - they seem particularly to be getting it and they are dying." But he could not be sure the hormone oestrogen was to blame.