The risk of colorectal cancer is greatly reduced for up to 10 years following even nonscreening colonoscopies, such as those performed in the setting of a positive fecal occult blood test or abdominal symptoms, a study showed.
Cancer risk was also lowered in the right colon, regardless of the indication for the colonoscopy, reported Dr. Hermann Brenner and his colleagues in the March issue of Gastroenterology (doi:10.1053/j.gastro.2013.09.001).
The finding is mostly good news for patients and practitioners. However, the investigators pointed out that as the use of diagnostic colonoscopy becomes more widespread in the United States and beyond, future researchers risk "contamination by taking diagnostic colonoscopies into account in the design and analysis of results from screening endoscopy randomized controlled trials."
Dr. Brenner of the University of Heidelberg (Germany) looked at 2,516 patients with a first diagnosis of colorectal cancer and 2,284 healthy controls culled from the DACHS study, a population-based case-control study conducted in the Rhine-Neckar region of Germany since 2003.
Patients were excluded if they were younger than 50 years old (in which case screening colonoscopy is not generally recommended) and if they had any history of inflammatory bowel disease, which requires frequent colonoscopies.
The median age was 70 years; 59% of all participants were men.
Overall, reported the authors, slightly more than a third (38.3%) of controls had a previous colonoscopy, most commonly for screening (12.0%) followed by abdominal symptoms (9.1%).
The other possible indications were screening, a positive fecal occult blood test result, surveillance, rectal bleeding, or "other."
Looking at both cases and controls, the investigators said that although only a small fraction of colonoscopies conducted until 2002 were performed for screening, screening was the most common indication for colonoscopies conducted from 2003 on.
The authors found that even though there was a stronger risk reduction in colorectal cancer among patients who underwent screening colonoscopy between 1 and 10 years prior (and this reduction was profound, with an odds ratio of 0.09 and a 95% confidence interval of 0.07-0.13), there was still a significant reduction after colonoscopies performed for other indications, including a positive fecal occult blood test result (OR, 0.33; 95% CI, 0.19-0.57), surveillance after a preceding colonoscopy (OR, 0.33; 95% CI, 0.24-0.45), rectal bleeding (OR, 0.28; 95% CI, 0.20-0.40), and abdominal symptoms (OR, 0.15; 95% CI, 0.10-0.21).
Unspecified other indications were associated with an odds ratio of 0.21 for cancer (95% CI, 0.14-0.30).
"All risk reductions were substantially less pronounced (and partly not statistically significant) for cancer in the right colon compared with cancer in the left colon and rectum," wrote the authors.
Nevertheless, colonoscopy was also associated with a reduced risk of cancer in the right colon regardless of the indication, although the effect was more pronounced specifically after a screening exam (OR for screening colonoscopy, 0.22; 95% CI, 0.14-0.33).
"To our knowledge, only two case-control studies from Canada and the United States have previously provided separate estimates of colorectal cancer risk after diagnostic and screening colonoscopy," wrote the authors.
"However, with ORs of 0.69 and 0.81, risk reduction was much less pronounced in the Canadian study than in the U.S. study and our study."
The authors disclosed no conflicts of interest related to this study; they stated that funding was provided by the German Research Council as well as the German Federal Ministry of Education and Research.