Sessile serrated lesion of the colon and rectum

By Jason Wasserman MD PhD FRCPC
July 19, 2025

A sessile serrated lesion is a type of polyp that develops on the inner lining of the colon or rectum. Although these polyps are not cancerous, they are considered precancerous, which means they can turn into cancer over time if not removed. Most sessile serrated lesions are detected during routine colonoscopies and are removed at that time.

What does sessile mean?

The word sessile means that the polyp is flat or only slightly raised, without a stalk. This makes sessile serrated lesions harder to see during a colonoscopy than other types of polyps that grow on a stalk.

What does serrated mean?

The word serrated refers to the saw-tooth shape seen in the lining of the polyp when examined under a microscope. This serrated pattern helps pathologists distinguish these polyps from other types, such as conventional adenomas.

What causes a sessile serrated lesion?

Sessile serrated lesions form when cells in the lining of the colon or rectum begin to grow abnormally due to mutations (changes) in their DNA. These changes are part of a process called the serrated pathway, which is thought to be responsible for up to 30% of colon and rectal cancers.

A key early step is a change in a gene called BRAF, which causes the cells to grow in an unusual pattern, forming a serrated polyp. Over time, these cells can undergo methylation, a chemical change that affects how genes are turned on or off. When methylation affects certain genes like MLH1, it can lead to mismatch repair deficiency, a condition that allows DNA damage to go unrepaired. This can eventually result in the development of dysplasia, a precancerous condition, and in some cases, invasive cancer.

Other genetic changes, such as mutations in the WNT signaling pathway, also contribute to the progression from a sessile serrated lesion to cancer. These changes typically occur over many years.

What are the symptoms of a sessile serrated lesion?

Most sessile serrated lesions do not cause any symptoms. Because they are flat and usually small, they are often found by chance during a colonoscopy performed for another reason, such as routine screening.

In some cases, especially if the lesion is larger or located in a certain part of the colon, it may cause rectal bleeding, changes in bowel habits, or abdominal discomfort. However, these symptoms are not specific to sessile serrated lesions and may be caused by many other conditions.

What does a sessile serrated lesion look like under the microscope?

Under the microscope, a sessile serrated lesion shows a unique growth pattern in the tissue lining the colon. Pathologists look for specific changes in the shape and structure of the glands (called crypts), including:

  • Crypts that grow sideways along the bottom layer of tissue (horizontal growth).

  • Enlargement and distortion of the base of the glands.

  • Serrated (saw-tooth) pattern that extends deeper into the glands.

  • Asymmetry between the left and right sides of the glands.

These changes help pathologists tell the difference between sessile serrated lesions and other non-serrated polyps like hyperplastic polyps or tubular adenomas.

What does dysplasia mean in a sessile serrated lesion?

Dysplasia is an abnormal pattern of cell growth that increases the risk of developing cancer. Not all sessile serrated lesions show dysplasia. When dysplasia is present, it means that the cells are starting to behave abnormally and may progress to cancer if not removed.

Some pathology reports may use the terms low grade dysplasia and high grade dysplasia. However, for sessile serrated lesions, this division is not recommended. This is because the dysplasia in these lesions can be very mixed or variable, making it difficult for pathologists to separate the changes clearly into low or high grade. The presence of dysplasia, regardless of the grade, is important because it means the lesion has a higher risk of becoming cancer.

Sessile serrated lesions that show dysplasia are often removed completely, and your doctor may recommend more frequent follow-up colonoscopies.

What is the risk of developing cancer?

Most sessile serrated lesions will not turn into cancer. However, some can develop dysplasia, which increases the risk of progression to colorectal adenocarcinoma, the most common type of colon cancer. The risk is highest when dysplasia is present or when the lesion is larger than one centimeter.

This type of cancer tends to develop slowly over many years. That is why it is important to remove sessile serrated lesions when they are found and to follow your doctor’s advice about future screening.

What is the treatment for a sessile serrated lesion?

Sessile serrated lesions are usually completely removed during colonoscopy. Because these lesions are precancerous, removing them reduces your risk of developing colon cancer in the future. If a sessile serrated lesion shows dysplasia or is very large, your doctor may recommend additional treatment or closer follow-up.

Questions to ask your doctor

  • Was dysplasia found in the sessile serrated lesion?

  • Was the polyp removed completely?

  • Will I need a follow-up colonoscopy, and if so, when?

  • Does this finding increase my risk for colorectal cancer?

  • Is there a chance that similar polyps could develop in the future?

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