How Colon Cancer Starts

By Richard Asa @RickAsa
May 09, 2017

Colon cancer starts in polyps. You may be more likely to develop polyps than other people, especially if you have a family history of polyps or colon cancer.

Colon (or colorectal) cancer begins with small polyps that line wall of your large intestine.

"Colon cancer is a growth in the colon that usually arises from a polyp. Sometimes the polyps look like stalks of cauliflower, sometimes they're flat," says Richard Goldberg, MD, the physician-in-chief and a professor of medicine at The Ohio State University Comprehensive Cancer Center. "When they spread that's when [people] have life-threatening issues."

Colon cancer usually develops in the cells lining the inside of the large intestine.


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The intestines are made of five layers. Starting from the empty space of the colon, the first layer is the mucosa, where nutrients, fats and proteins are absorbed from the food you eat, according to the John Hopkins University Colon Cancer Center.

Next is the muscularis mucosa that forms a barrier between the mucosa and a third layer called the submucosa. The fourth layer is the muscularis propria, which acts to push food through your intestines. The outermost layer is the serosa.

Colon cancers usually begin as benign polyps that grow from the mucosa. You may be more likely to develop polyps than other people, especially if you have a family history of polyps or colon cancer.

Most polyps remain benign and are sometimes called hyperplastic. The likelihood that hyperplastic polyps will become cancer is very low. Other benign polyps are sometimes referred to as pre-cancerous. These polyps are not malignant, but could become cancerous if they’re not removed.

A common polyp is referred to as an adenoma, which usually grows on a stalk. These polyps tend to grow slowly over a decade or more. The risk of an adenoma developing into cancer increases as the size of the adenoma increases and with the amount of time the adenoma has been growing in the colon, according to Johns Hopkins.

In early stages, abnormal cells are contained inside the polyp and can be removed by colonoscopy before they develop into invasive cancer. As cancer cells grow and divide within the polyp, though, they can grow into nearby colon tissue and become invasive cancer. Recent research has found that some polyps long considered benign can become cancerous, so all polyps are removed during the colonoscopy with the use of a snare that’s attached to the scope used to view the large intestine. This tissue can then be examined in the lab for signs of cancer.

While fewer than 10 percent of all adenomas become cancerous, more than 95 percent of colorectal cancers develop from adenomas, according to Johns Hopkins.

According to one study, the lifetime risk of developing an adenoma is nearly 19 percent in the U.S population. Nearly 95 percent of sporadic colorectal cancers develop from those adenomas. If you have a history of adenomas, you have an increased risk of developing colon cancer.

But a long latency period, from 5 to 10 years, is usually the time span it takes for an adenoma to turn malignant.

The American Cancer Society says it's a myth that colon cancer can’t be prevented. There is colonoscopy, flexible sigmoidoscopy, double-contrast barium enema, or CT colonography (virtual colonoscopy), all of which can detect polyps in their early stages.

Other ways to help prevent developing dangerous polyps include getting to and staying at a healthy weight, being physically active, eating at least 2.5 cups of fruits and vegetables every day, choosing whole grains over refined grain products, limiting the amount of red and processed meat you eat, limiting alcohol consumption, and not using tobacco in any form.

If you’ve had one or more polyps, you’re more likely to develop more, so make sure you’re getting colonoscopies at regular intervals to keep cancerous polyp form forming or cells from spreading into your blood stream and other parts of your body.


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March 18, 2020

Reviewed By:  

Christopher Nystuen, MD, MBA