Why Does Colonoscopy Protect Against Left-Sided Cancers?

Polyps with advanced pathology are significantly smaller in the right than left colon, and are therefore more likely to be missed during colonoscopy examinations, according to the December issue of Clinical Gastroenterology and Hepatology.

Mortality from colorectal cancer (CRC) is decreasing, in part because colonoscopy screening is increasing, leading to the detection and removal of more adenomatous polyps—the precursors of most colorectal tumors.

However, many studies have shown that colonoscopy consistently protects against left-sided, but not right-sided CRC. This could be because dangerous polyps on the right side of the colon tend to be small, and can therefore be missed and progress to cancer.

Samir Gupta et al. investigated whether polyps with advanced pathology in the right colon are smaller in size than those that develop in the left colon, analyzing a large national sample of polyps from patients undergoing routine colonoscopy.

They collected results from histologic analyses of 233,414 polyps (from 142,686 patients), performed by a diagnostic services company in 2009—samples were assessed by 1779 endoscopists in 43 US states. Gupta et al. obtained information on polyp histology, location, and size of largest fragment submitted, and compared size distribution of right vs left polyps with high-grade dysplasia (HGD) or adenocarcinoma and advanced neoplasia.

Gupta et al. found that the average size of right-sided polyps with HGD or adenocarcinoma was smaller than that of left-sided polyps (8.2 vs 12.4 mm)—the same was true for polyps with advanced neoplasia (7.6 vs 11.1 mm).

Most right-sided polyps with HGD, adenocarcinoma, or any advanced neoplasia were ≤9 mm, whereas most left-sided polyps with these findings were >9 mm. Polyps with advanced pathology were 5-fold more likely to be <6 mm in the right vs left colon (see below figure).

Distribution of polyps with high-grade dysplasia (HGD), adenocarcinomas (AdenoCa), and advanced neoplasia (AdNeo) in right and left colon, of 3 different size categories. In the left colon, most polyps with advanced histology were >9 mm in size. However, in the right colon, most of the polyps containing AdenoCa/HGD, AdNeo, and AdenoCa were ≤9 mm in size.

Although previous studies have examined the overall size distribution of advanced polyps, this study is one of the first, and the largest, to stratify size distribution of advanced polyps by colon site.

In addition to the large sample size, this study was also significant because samples were collected during routine colonoscopy examinations and assessed by a large group of pathologists who specialized in gastroenterology, with a consensus approach to diagnosis.

Gupta et al. do not offer an explanation for why dangerous polyps might be smaller in the right colon. However, they propose optimizing colonoscopy procedures to include adequate withdrawal time and careful inspection of all folds. New innovations in colonoscopy might be required to identify and remove small right-sided polyps with advanced pathology.

Read the article online.
Gupta S, Balasubramanian BA, Fu T, et al. Polyps with advanced neoplasia are smaller in the right than in the left colon: implications for colorectal cancer screening. Clin Gastroenterol Hepatol 2012;10:1395-1401.e2.

About Kristine Novak, PhD, Science Editor

Dr. Kristine Novak is the science editor for Gastroenterology and Clinical Gastroenterology and Hepatology, both published by the American Gastroenterological Association. She has worked as an editor at biomedical research journals and as a science writer for more than 12 years, covering advances in gastroenterology, hepatology, cancer, immunology, biotechnology, molecular genetics, and clinical trials. She has a PhD in cell biology and an interest in all areas of medical research.
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