Most colon cancers start as polyps. Those are small growths on the lining of the colon. However, most polyps do not become cancerous. Polyps are usually harmless. Some people have these growths scattered throughout their colon. Over time, though, cancer may start in certain polyps.
Doctors find polyps during routine screenings, like a colonoscopy. It's hard to tell which polyps will progress to colon cancer. They must be removed and examined under a microscope.
There are certain clues that help doctors figure out whether a polyp is likely to become cancer. These include the type of polyp, its growth pattern, and the location and size of the polyp. How abnormal the cells are is another clue.
Determining Cancer Risk
There is more than one type of colon polyp. The two most common types are:
Hyperplastic polyps: These are usually harmless and typically small. They are often found in the rectum, which is at the end of the colon.
Adenomas: These polyps could become cancer. Adenomas account for more than 66% of colon polyps. Still, most adenomas are harmless—only about 10% of adenomas turn into cancer.
Not all adenomas are alike. Polyps with a sawtooth appearance are called serrated. Most serrated polyps are hyperplastic, but some are adenomas. People with a serrated adenoma are at greater risk for developing colon cancer. Serrated polyps account for up to 30% of all colon cancers.
Adenomas also have various growth patterns:
About 80% are tubular. Tubular polyps have less than a 5% chance of becoming cancerous.
About 5 to 10% have finger-like projections. These are called villous polyps. They are the most serious type—about half of these growths are cancerous.
Up to 15% of adenomas are both tubular and villous, hence their name: tubulovillous. About 22% of tubulovillous adenomas become cancer.
Adenomas may be flat or look like a small bump on the colon wall. Others have a stalk and resemble a mushroom. Flat polyps more often become cancer.
Other Things to Consider
Whether a polyp will turn into cancer also depends on:
Location: Hyperplastic polyps are usually found in the rectum. They pose little risk. But, sometimes they grow in what's called the ascending colon. That's the first main part of the large intestine. Polyps that grow there are more likely to become cancer.
Size: Polyps also vary in size. Some are just a few millimeters. Others may grow to several centimeters (cm). Larger polyps are more likely to become cancer. In fact, polyps larger than 2.5 cm (equivalent to 1 inch) are five times more likely to be cancerous than those smaller than 1.5 cm. Larger adenomas are also more likely to have a more risky villous growth pattern.
Cells: Doctors also look at polyps (removed during colonoscopy) under a microscope to look for abnormal cells. The medical term for abnormal cells is dysplasia. Some polyps are only slightly abnormal. Others look much more like cancer cells. High-grade dysplasia greatly increases the likelihood of the polyp becoming colon cancer. People with polyps that have highly abnormal cells need more aggressive monitoring for the disease.
