The Bowel Cancer Screening Programme in England is on track to cut bowel cancer deaths by its target of 16%, reveals an analysis of the first one million test results, published in Gut.
But the results also show a much higher proportion of cancers detected were left sided, suggesting that different strategies may need to be deployed to pick up disease on both sides of the body, as right sided cancers are thought to be more aggressive, say the authors.
Bowel cancer kills 16,000 people every year in the UK and is second only to lung cancer as the leading cause of cancer death in both the UK and Europe.
Survival from bowel cancer is still only around 50% in the UK, and significantly lower than in other comparable countries.
The authors base their findings on an analysis of the first 1.08 million faecal occult blood tests, returned by patients invited for the first round of bowel cancer screening in England.
The programme started in 2006 and was rolled out nationwide by the end of 2009 for those between the ages of 60 and 69. It involves three tests every two years, and has since been extended to those over 70.
Up to October 2008, over two million people had been invited to participate, with around half of those invitations accepted - just under 50% for men and almost 55% for women.
Returns were higher in the four provincial hubs, averaging 55-60%, but were lower in London, where they averaged 40%.
In all, 2.5% of men and 1.5% of women (21,106 people in all) had an abnormal test result out of the 1.08 million returns, and 17,518 people were investigated further.
Most underwent colonoscopy - a procedure involving a long tube with a camera on the end that is inserted into the back passage.
Men were more likely to have cancer and its immediate precursor ("higher risk adenomas") than women.
These were found in around one in eight (11.6%) and around four out of 10 (43%) men, respectively, compared with just under 8% and 29%, respectively, of the women.
Seven out of 10 cancers (71%) picked up by the screen were early stage disease, and as expected, right sided cancer was more common in women than in men.
But left sided cancers were considerably more common than expected, based on figures drawn from cancer registries.
These figures indicated that around two thirds of bowel cancers (67%) picked up by the screening programme would be left sided and around one in four (24%) would be right sided.
Instead, more than three out of four (77%) cancers detected by screening were left sided, compared with 14% that were right sided.
The authors point to research suggesting that right sided cancers are more aggressive and less likely to be picked up by screening. "Different screening strategies may be required to effectively screen for right-sided bowel cancer," they conclude.