Colonoscopy Reduces Colorectal Cancer Risk But Only on Left Side
By Zosia Chustecka
Medscape Medical News
December 30, 2009 — Another study of colonoscopy screening in thecommunity setting has shown that it does reduce the risk for colorectalcancer, but this reduction is seen only for left-sided, notright-sided, colorectal cancer.
The latest findings come from Germany, from a study of 3287 participants, and are published online December 30 in the Journal of the National Cancer Institute.
Advanced colorectal cancer was detected by colonoscopy in 36 (6.1%) of586 participants who had undergone a previous colonoscopy in thepreceding 10 years vs 308 (11.4%) of 2701 participants with no previouscolonoscopy.
This is a "substantial risk reduction," comment the study authors,headed by Hermann Brenner, MD, MPH, from the German Cancer ResearchCenter in Heidelberg, Germany.
However, the risk reduction was seen only in left-sided advancedneoplasms and not right-sided ones, the researchers point out. Aprevious colonoscopy was associated with a 67% reduced risk foradvanced neoplasia in the left side of the colon and the rectum, but norisk reduction was seen in the right side of the colon.
"However, because most neoplasms in the colon and rectum are located onthe left side, substantial overall risk reduction for colorectalcancers and advanced adenomas was observed," they add.
"Remarkably Consistent" With Previous Studies
These results are "remarkably consistent with a number of recentlypublished studies, all of which demonstrate the overall effectivenessof colonoscopy for reduction of colorectal cancer incidence andmortality but with a marked variance in effectiveness for proximal anddistal cancers," comment editorialists Nancy Baxter, MD, PhD, from St.Michael's Hospital, Toronto, Ontario, Canada, and Linda Rabeneck, MD,MPH, from the Odette Cancer Center, Sunnybrook Health Sciences Centre,Toronto, Ontario, Canada.
They add that the study is an important contribution to theobservational evidence that "must inform our understanding of theeffectiveness of colonoscopy."
The editorialists also point out that observational evidence is allthat is available at present to address the question of how effectivecolonoscopy is at reducing the burden of colorectal cancer in thepopulation. A definitive answer can come only from a randomized trial,and although one is underway (the Northern European Initiative onColorectal Cancer), outcome data are not expected until 2026.
In the meantime, as the observational evidence continues to mount, theanswer to this question becomes less certain, they suggest.
Is Right-Sided Colon Cancer Different?
One of the limitations of this study was the small number of advancedneoplasms found at any location, the researchers comment."Nevertheless, consistent patterns of risk reduction in all parts ofthe left colon and rectum were observed, with absence of risk reductionin all parts of the right colon," they add. These site-specificfindings are also "remarkably consistent" with several previousstudies, they note.
"Possibly, the lack of effect in the right colon could be overcome tosome extent by enhanced training of endoscopists, by enhanced measuresof quality assurance, and by development of technology that enhancesinspection of the right colon," they suggest.
"Nevertheless, the possibility that cancer in the right colon simplydoes not lend itself equally well to early detection on biologicalgrounds has to be considered," they continue. "If this possibilitycould be demonstrated in other investigations, then the relative meritsof sigmoidoscopy and colonoscopy in the early detection and preventionof colorectal cancer would need to be re-evaluated."
Compelling evidence that colonoscopy is a less effective tool in the proximal colon than [the] distal colon
In their editorial, Drs. Baxter and Rabeneck add that there is"compelling evidence that colonoscopy is a less effective tool in theproximal colon than [the] distal colon," but they also add that the"underlying reasons for the differential performance are unclear."
"There may be biological differences that limit the potentialeffectiveness of colonoscopy in the proximal colon," the editorialistscontinue. "Right-sided colonic adenomas tend to be flatter thanleft-sided lesions and are, therefore, harder to identify and remove."
The potential limitations of colonoscopy for preventing colorectalcancer in the proximal part of the colon — as identified in this andother studies — raises very important questions, the editorialistsnote. Is there an incremental benefit of colonoscopy vs flexiblesigmoidoscopy for colorectal cancer screening? Also, if there is anincremental benefit, is it large enough to justify the additional risksand cost of colonoscopy for screening in the population, they ask.
The study was supported in part by a grant from the Central ResearchInstitute of Ambulatory Health Care in Germany. The study authors andeditorialists have disclosed no relevant financial relationships.
