The impact of colorectal screening remains unclear, according to study results presented today at Digestive Diseases Week in Chicago, USA.
The results of a seven-year analysis published on bmj.com today suggest that it may be too early to say whether screening reduces deaths from colorectal cancer.
With more than 500,000 deaths each year, colorectal cancer is the third most common cause of deaths from cancer worldwide. Yet, despite a lack of evidence from randomised trials, many countries have launched large-scale colonoscopy screening programmes for the general population.
So researchers in Norway analysed the risk of colorectal cancer after screening as part of the ongoing Norwegian Colorectal Cancer Prevention Trial (NORCCAP).
The study included 55,736 men and women aged 55-64 years living in two screening areas in Norway (city of Oslo and Telemark county).
Participants were split into two groups. The screening group was invited to attend one flexible sigmoidoscopy screening examination and the control group received no screening. Individuals in both groups were tracked through national registries for an average of seven years.
In this first report from the trial, the researchers found that there was no significant difference in the levels of colorectal cancer between the screening and control groups, suggesting that any reduction may be too early to observe after seven years of follow-up.
In the screening group as a whole (all those invited to screening), total colorectal cancer mortality was reduced by 27% compared to the control group, but this was not statistically significant.
For those who actually underwent screening, total colorectal cancer mortality was significantly reduced by more than half (59%) compared to controls.
Although this is a promising result, the authors stress that this finding may be prone to bias.
Two main possibilities could explain the limited effect of flexible sigmoidoscopy screening in this study: either the method is not effective in reducing colorectal cancer or the lag period for the development of cancer from precursor lesions is considerably longer than is commonly assumed, say the authors.
However, in an accompanying editorial, Professor Thomas Imperiale from Indiana University Medical Center, said evidence to date strongly suggests that one time screening sigmoidoscopy can reduce incidence and mortality from colorectal cancer and may be a legitimate strategy.
The magnitude and duration of benefit have yet to be ascertained, he says, along with acceptability in different populations and the effort and cost needed for implementation. But, in the meantime, he suggests that data on risk reduction may be useful for clinicians as they discuss screening options with their patients, for people estimating cost effectiveness, and for policy makers.