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 the community setting has shown that it does reduce the risk for colorectal cancer, but this reduction is seen only for left-sided, not right-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%) of 586 participants who had undergone a previous colonoscopy in the preceding 10 years vs 308 (11.4%) of 2701 participants with no previous colonoscopy.
This is a "substantial risk reduction," comment the study authors, headed by Hermann Brenner, MD, MPH, from the German Cancer Research Center in Heidelberg, Germany.
However, the risk reduction was seen only in left-sided advanced neoplasms and not right-sided ones, the researchers point out. A previous colonoscopy was associated with a 67% reduced risk for advanced neoplasia in the left side of the colon and the rectum, but no risk reduction was seen in the right side of the colon.
"However, because most neoplasms in the colon and rectum are located on the left side, substantial overall risk reduction for colorectal cancers and advanced adenomas was observed," they add.
"Remarkably Consistent" With Previous Studies
These results are "remarkably consistent with a number of recently published studies, all of which demonstrate the overall effectiveness of colonoscopy for reduction of colorectal cancer incidence and mortality but with a marked variance in effectiveness for proximal and distal 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 the observational evidence that "must inform our understanding of the effectiveness of colonoscopy."
The editorialists also point out that observational evidence is all that is available at present to address the question of how effective colonoscopy is at reducing the burden of colorectal cancer in the population. A definitive answer can come only from a randomized trial, and although one is underway (the Northern European Initiative on Colorectal Cancer), outcome data are not expected until 2026.
In the meantime, as the observational evidence continues to mount, the answer 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 advanced neoplasms found at any location, the researchers comment. "Nevertheless, consistent patterns of risk reduction in all parts of the left colon and rectum were observed, with absence of risk reduction in all parts of the right colon," they add. These site-specific findings are also "remarkably consistent" with several previous studies, they note.
"Possibly, the lack of effect in the right colon could be overcome to some extent by enhanced training of endoscopists, by enhanced measures of quality assurance, and by development of technology that enhances inspection of the right colon," they suggest.
"Nevertheless, the possibility that cancer in the right colon simply does not lend itself equally well to early detection on biological grounds has to be considered," they continue. "If this possibility could be demonstrated in other investigations, then the relative merits of sigmoidoscopy and colonoscopy in the early detection and prevention of 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 the proximal 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 potential effectiveness of colonoscopy in the proximal colon," the editorialists continue. "Right-sided colonic adenomas tend to be flatter than left-sided lesions and are, therefore, harder to identify and remove."
The potential limitations of colonoscopy for preventing colorectal cancer in the proximal part of the colon — as identified in this and other studies — raises very important questions, the editorialists note. Is there an incremental benefit of colonoscopy vs flexible sigmoidoscopy for colorectal cancer screening? Also, if there is an incremental benefit, is it large enough to justify the additional risks and cost of colonoscopy for screening in the population, they ask.
The study was supported in part by a grant from the Central Research Institute of Ambulatory Health Care in Germany. The study authors and editorialists have disclosed no relevant financial relationships.
J Natl Cancer Inst. Published online December 30, 2009.
