A recent study suggests that some types of colon polyps previously thought as harmless may actually develop into cancers.

Since the 1970s, doctors have known that some adenomatous polyps, benign mushroom-shaped growths inside the colon, can become cancerous. This discovery was important in that doctors can find these adenomatous polyps by a variety of screening tests and usually can remove them through a colonoscope, thereby preventing the cancer from forming.

On the other hand, hyperplastic polyps, which tend to be smaller than their adenomatous relatives and have cells that appear less abnormal when viewed under a microscope, were thought to be unlikely to ever become cancerous.

But, according to two Australian researchers, hyperplastic polyps and serrated polyps (a type of polyp that shares some characteristics of hyperplastic and adenomatous polyps) may be responsible for some colon cancers, especially those found in the right side of the colon. Nicholas John Hawkins, MBBS, and Robyn Lynne Ward, MBBS, PhD, of the University of New South Wales, published their results in the Journal of the National Cancer Institute (Vol. 93, No. 17: ).

Hawkins and Ward examined the noncancerous colon tissue removed along with colon cancers from 58 patients. Twenty-nine of the cancers had a type of DNA abnormality known as microsatellite instability (MSI) that often occurs in cancers of the right side of the colon. Adenomatous polyps were often found near cancers, whether or not they had MSI. But hyperplastic polyps and serrated polyps were four times as likely to be found near cancers with MSI, especially those found in the right side of the colon, than near cancers lacking MSI.

The researchers conclude that this association suggests a link between MSI-related colon cancers and right-sided hyperplastic and serrated polyps. They note that their data does not provide an estimate of how likely these polyps are to turn cancerous, and they suggest that more research is needed to address this important point. And if that risk turns out to be substantial, more attention to finding and removing these previously ignored polyps will be needed.

In an editorial in the same journal, Stanley Hamilton, MD, of the MD Anderson Cancer Center in Houston, agrees with the Hawkins and Ward study.

Hamilton writes that for now, “The implications of the study for patient management are uncertain… It is therefore unclear how aggressively removal of these lesions should be pursued…”

He also notes that early studies of drugs intended to prevent colon cancer have started by asking whether the drugs prevent formation of adenomatous polyps, the precursors of most colon cancers. But, Hamilton suggests, testing these drugs’ ability to prevent formation of hyperplastic and serrated polyps may also be important.

According to Robert Smith, PhD, director of cancer screening at the American Cancer Society, “These findings are likely to stimulate new interest in hyperplastic polyps, and interest in the clinical implications of whether risk varies depending on whether they are right-sided or left-sided lesions. The emphasis now should be on identifying when these lesions need to be regarded seriously.”