By Jason Wasserman MD PhD FRCPC
March 29, 2026
A sessile serrated lesion is a type of polyp — a small precancerous growth that develops on the inner lining of the colon or rectum. It is not cancer, but it can develop into cancer over time if left in place. Most sessile serrated lesions are detected and removed during a routine colonoscopy, and removal is the most effective way to prevent progression.
You may also see this lesion referred to as a sessile serrated polyp or, in older reports, a sessile serrated adenoma. These names all refer to the same type of growth. “Sessile serrated lesion” is the current preferred term recommended by pathology guidelines, which is why newer reports use it — but if your report uses a different name, you are reading about the same thing.
What do the words “sessile” and “serrated” mean?
Sessile means the lesion is flat or only slightly raised and has no stalk. Stalked polyps protrude from the colon wall like mushrooms, making them easy to spot during a colonoscopy. Sessile lesions lie flat against the lining, making them harder to see, which is why a careful colonoscopy is important for finding them.
Serrated refers to the saw-tooth pattern seen in the glands of the lesion when examined under a microscope. This jagged appearance distinguishes sessile serrated lesions from other colon polyps, such as conventional adenomas or simple hyperplastic polyps.
What causes a sessile serrated lesion?
Sessile serrated lesions develop when cells in the colon lining acquire mutations — small errors in their DNA — that cause them to grow abnormally. These changes follow what pathologists call the serrated pathway, which is thought to account for roughly 20 to 30% of all colorectal cancers.
One of the earliest changes is a mutation in a gene called BRAF, which triggers the serrated growth pattern. Over time, a chemical process called methylation can silence other important genes. When methylation affects a gene called MLH1, it leads to mismatch repair deficiency — a state in which DNA errors go uncorrected and further dangerous changes can accumulate. This entire sequence takes many years, which is why finding and removing the lesion early is so effective at preventing cancer.
What are the symptoms?
Most sessile serrated lesions cause no symptoms. They are flat, usually small, and found by chance during a colonoscopy performed for routine screening or an unrelated reason.
Occasionally, a larger lesion may cause rectal bleeding or a change in bowel habits. These symptoms are not specific to this type of lesion — many conditions can cause them — and the only way to know what is responsible is through investigation.
How is the diagnosis made?
The diagnosis is made by a pathologist who examines the removed tissue under a microscope. The lesion is taken out during a colonoscopy using a procedure called a polypectomy — removal of the polyp using a small wire loop. Larger or flatter lesions may require a more involved technique called endoscopic mucosal resection (EMR), which lifts and removes a wider area of tissue.
Under the microscope, the pathologist looks for specific changes in the glands (called crypts) that line the colon wall. Features that point to this diagnosis include:
- Horizontal crypt growth at the base. The glands at the bottom of the lesion grow sideways along the base of the tissue rather than straight down, giving them an L-shaped or boot-shaped appearance.
- Distortion and widening of the crypt base. The lower portion of the glands becomes irregular and enlarged.
- Serrated pattern deep in the glands. The saw-tooth appearance extends further into the gland than it would in a simple hyperplastic polyp.
- Asymmetry between the two sides of the gland. The left and right halves of a crypt do not mirror each other.
These features allow the pathologist to tell this lesion apart from other types, including hyperplastic polyps and tubular adenomas.
What does the pathology report say?
Dysplasia
Dysplasia means the cells have developed abnormal features under the microscope — a sign that they are moving toward cancer. Not all sessile serrated lesions show dysplasia. When dysplasia is present, the lesion is at a more advanced precancerous stage and carries a higher risk of progressing to cancer if not removed.
Some reports mention low-grade or high-grade dysplasia. Current guidelines do not recommend grading dysplasia in sessile serrated lesions because the abnormal changes tend to be uneven and mixed across the tissue, making a reliable grade difficult to assign. The most important question is whether dysplasia is present or absent,ot what grade it is.
Margin
The margin is the edge of the tissue that was removed. The pathologist examines it to determine whether the entire lesion was taken out.
- Negative margin (clear margin). No lesion cells are present at the edge of the removed tissue. This means the lesion appears to have been completely removed.
- Positive margin. Lesion cells are present at the very edge of the tissue. Some tissue may have been left behind, and your doctor may recommend a follow-up colonoscopy to check the area.
- Cannot be assessed. If the lesion was removed in multiple pieces or if the tissue edges were damaged during removal, the pathologist may not be able to evaluate the margins reliably. Your doctor will advise on follow-up.
What is the risk of developing cancer?
The great majority of sessile serrated lesions will never develop into cancer, particularly if they are removed completely and show no dysplasia. The risk is higher when dysplasia is present or when the lesion measures more than one centimetre (roughly the width of a fingernail).
When cancer does develop along the serrated pathway, it is typically a type called colorectal adenocarcinoma. These cancers tend to develop over many years, which is exactly why regular colonoscopy screening works so well: removing precancerous lesions early interrupts the process before cancer has a chance to form.
What happens next?
In most cases, removing the lesion during colonoscopy is the complete treatment. No additional surgery or medication is needed. Your doctor will then recommend a follow-up colonoscopy at an interval based on what was found:
- Sessile serrated lesion without dysplasia, completely removed. Follow-up colonoscopy is typically recommended in 3 to 5 years, depending on the size of the lesion and any other findings.
- Sessile serrated lesion with dysplasia, or incompletely removed. Follow-up is usually recommended sooner — often within 1 year — to examine the area and look for any remaining tissue.
- Multiple sessile serrated lesions. More frequent surveillance may be recommended. Your doctor will discuss a schedule based on your individual situation.
If this feels overwhelming, that is completely understandable. The key point is that the lesion was found and removed. Your doctor can help you understand what the specific findings mean and what the right follow-up plan looks like for you.
Questions to ask your doctor
- Was the lesion completely removed, or was the margin positive or unable to be assessed?
- Was dysplasia found in the lesion?
- When should I have my next colonoscopy?
- Does this finding change my overall colorectal cancer risk?
- Should any of my close family members be screened earlier or more often?
- Are there lifestyle changes — such as diet, physical activity, or quitting smoking — that could reduce my risk of future lesions?
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