Screening people with a family history of bowel cancer could cut rates of the disease by about 80%, a study suggests.
Bowel cancer kills around 14,000 each year
But the British Medical Journal research shows most do not need to be screened before the age of 45, when they should receive five-yearly checks.
The Cancer Research UK team who carried out the study say this is true even for those who have two or three relatives with the disease.
Around 34,000 people are diagnosed with bowel cancer in the UK each year.
Up to a third occur in someone with a family history of the disease.
'No genetic fault'
Cancer Research UK carried out the study at the family cancer clinic at St Mark's Hospital, Middlesex.
Over 1,600 people with at least one close relative diagnosed with bowel cancer took part.
They were screened regularly and monitored for up to 16 years.
The number of cancers found was compared to the number of cancers expected in a similar, unscreened population.
A small proportion of people with bowel cancer have a fault in a specific gene which leads to a condition called hereditary nonpolyposis colorectal cancer (HNPCC), but the majority of cases have no known genetic fault. HNPCC families are currently screened with colonoscopy every two years from the age of 25.
The new research says this should continue, as screening reduces cancer deaths in this group by an estimated 70%.
Until now, there had been no strong evidence on how to deal with the remaining majority of people with a family history of bowel cancer.
However, the Middlesex study shows an 80% decrease of tumours by screening this group. Screening using colonoscopy means pre-cancerous growths can be detected and removed before they progress to cancer.
'Less intensive screening'
Professor Peter Sasieni of Cancer Research UK, who led the study, said: "We now know that screening with colonoscopy prevents the majority of bowel cancers in people with a family history.
"The study also shows that screening isn't necessary before the age of 45 and, even then, it only needs to be performed every five years or so."
He added: "This is good news for people with a family history because it means they can dramatically reduce their risk of cancer by going for occasional screening.
"It's also good news for hospitals because, in most cases, this will mean less intensive screening and less of a drain on resources."
The government is introducing a national bowel screening programme in across England from April 2006. Men and women aged between 60 - 69 years old will be invited for stool sample screening every two years.
Professor John Toy, Cancer Research UK's Medical Director, says: "With the introduction of a national screening programme for bowel cancer due to begin next year, we anticipate a major increase in the number of people referred for colonoscopy.
"This research is important because it shows how most cancers can be prevented in people with a family history.
"It also indicates how resources can best be used and help to minimise unnecessary colonoscopies for people."
A spokesman for the charity Bowel Cancer UK said: "Bowel Cancer UK actively encourages anyone with a family history of the disease to inform their GP and ask to have a colonoscopy if they haven't already done so in the last one to two years.
"We will also be encouraging as many eligible people to take part in the national bowel cancer screening programme, when it rolls out next year, as this will also significantly increase the numbers of people diagnosed with the disease."