Colonoscopy Prevents Cancers in Left Side of Colon

Line drawing of colon and rectumWhile colonoscopy is effective in preventing cancers and advanced polyps in the lower part of the colon and rectum, it is less successful in stopping them in the right side or upper colon.

Looking back at almost 3,300 colonoscopies performed in community gastroenterology practices in Germany, researchers found a substantial reduction in large polyps or cancers in the left side of the colon and rectum among patients who had had a colonoscopy in the past ten years compared to those who hadn’t had one.

However, in the upper part of the colon, risk for an advanced polyp or cancer was the same whether or not the patient had a previous colonoscopy.

Overall, colonoscopy reduced the risk of a cancer or an advanced adenoma by 50 percent.

Doctors found an advanced colorectal neoplasm, defined as cancer or an advanced adenoma, in about 1 in 10 people who had not had a previous colonoscopy (11.4 percent).  If the person had a colonoscopy in the past ten years, fewer advanced neoplasms were found (6.1 percent), but location in the colon mattered.

The right side of the colon (proximal) was defined as the cecum, ascending colon, and transverse colon.  The left side (distal) was the descending colon, sigmoid colon, and rectum.

If an individual had a colonoscopy in the previous ten years, the risk of finding a large polyp or cancer in the left side was about a third of that of someone who hadn’t had an exam (relative risk 0.33).  However, there was no reduction in the risk of right sided advanced neoplasms.

Risk of cancer itself strongly impacted by previous colonoscopy. Cancer was found in 41 of 2,701 patients who hadn’t had colonoscopy (1.5 percent) compared to only 1 person in the 586 who had one. (0.2 percent).

Hermann Brenner, MD, MPH and his team concluded,

Prevalence of left-sided advanced colorectal neoplasms, but not right-sided advanced neoplasms, was strongly reduced within a 10-year period after colonoscopy, even in the community setting.

In an editorial that accompanied the Brenner study in the Journal of the National Cancer Institute, surgeon Nancy Baxter, MD, MPH and gastroenterologist Linda Rabeneck MD, MPH asked,

Is there an incremental benefit of colonoscopy over flexible sigmoidoscopy for colorectal cancer screening? If so, is the incremental benefit of sufficient magnitude to justify the additional risks and costs of colonoscopy for screening in the population?  Simply put, is the effectiveness of colonoscopy “good enough” for population-based screening? As more observational evidence accumulates, the answer to this question becomes less certain.

Drs. Baxter and Rabeneck point out that there may be biological differences between lesions on the right side of the colon that make it more difficult for colonoscopy to identify and remove them.  They tend to be flatter and may arise through different molecular pathways.

SOURCES: Brenner et al., Journal of the National Cancer Institute, Advanced Access, December 30, 2009.

Baxter and Rabeneck, Journal of the National Cancer Institute, Advanced Access, December 30, 2009 (free complete text).

What Does This Mean for Patients?

This study adds evidence that screening colonoscopy may be mostly effective in preventing cancers on the left side of the colon but may not reduce risk of proximal or right-sided cancer.

This is not a reason to avoid colonoscopy at all. Overall, in the German study it reduced advanced polyps and cancers by 50 percent — a significant improvement.

However, people need to be aware that colonoscopy is not perfect.  Even in people without family history or medical problems like ulcerative colitis or Crohn’s disease, cancer can occur in between screening colonoscopies.  It is critical to be aware of the symptoms of colorectal cancer and get an evaluation of them, including another colonoscopy, if they occur.

It may also strengthen the value of flexible sigmoidoscopy as a screening tool, particularly in situations where colonoscopy is not easily available or affordable.

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