By Jason Wasserman MD PhD FRCPC
March 29, 2026
A sessile serrated adenoma is an older name for a 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 adenomas are found and removed during a routine colonoscopy, and removing them is the most effective way to prevent progression.
Current pathology guidelines no longer recommend this term. The same growth is now called a sessile serrated lesion or, less commonly, a sessile serrated polyp. The name changed because pathologists determined that these growths are not true adenomas — they arise through a different biological process — and the older term was misleading. If your report uses the term “sessile serrated adenoma,” you are reading about the same condition that newer reports call a sessile serrated lesion. The change in name does not change what the finding means or how it is treated.
What do the words “sessile” and “serrated” mean?
Sessile means the growth 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 growths lie flat against the lining, which makes them harder to see — one reason a careful, thorough colonoscopy matters.
Serrated refers to the saw-tooth pattern seen in the glands of the growth when examined under a microscope. This distinctive appearance helps pathologists tell these growths apart from other colon polyps, such as conventional adenomas or simple hyperplastic polyps.
What causes a sessile serrated adenoma?
Sessile serrated adenomas develop when cells in the colon lining acquire mutations — small errors in their DNA — that cause them to grow in an abnormal pattern. These changes follow what pathologists call the serrated pathway, which is estimated 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 drives the serrated growth pattern. Over time, a chemical process called methylation can switch off other important genes. When methylation affects a gene called MLH1, it leads to mismatch repair deficiency — a state in which DNA copying errors go uncorrected and further harmful changes can accumulate. This entire sequence typically takes many years, which is why finding and removing the growth early is so effective at preventing cancer.
What are the symptoms?
Most sessile serrated adenomas cause no symptoms. They are flat, often small, and typically found by chance during a colonoscopy performed for routine screening or an unrelated reason.
Occasionally, a larger growth may cause rectal bleeding or a change in bowel habits. These symptoms are not specific to this type of growth — many other conditions cause them — and investigation is the only way to identify the cause.
How is the diagnosis made?
The diagnosis is made by a pathologist who examines the removed tissue under a microscope. The growth is removed during a colonoscopy using a procedure called a polypectomy — removal with a small wire loop passed around the base of the growth. Larger or flatter growths may require a more extensive technique called endoscopic mucosal resection (EMR), which lifts and removes a wider area of tissue in one piece.
Under the microscope, the pathologist looks for specific changes in the glands (called crypts) that line the colon wall. Features that support this diagnosis include:
- Horizontal crypt growth at the base. The glands at the bottom of the growth extend 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 reaches further down 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 distinguish this growth from 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 progressing toward cancer. Not all sessile serrated adenomas show dysplasia. When dysplasia is present, the growth 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 these growths because the abnormal changes tend to be uneven and mixed across the tissue, making a reliable grade difficult to assign. Whether dysplasia is present or absent is more important than any grade applied to it.
Margin
The margin is the edge of the tissue that was removed. The pathologist examines it to determine whether the entire growth was taken out.
- Negative margin (clear margin). No growth cells are found at the edge of the removed tissue. This means the growth appears to have been completely removed.
- Positive margin. Growth 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 examine the area.
- Cannot be assessed. If the growth was removed in multiple pieces or if the tissue edges were affected by the removal technique, the pathologist may not be able to evaluate the margin reliably. Your doctor will advise on what follow-up is appropriate.
What is the risk of developing cancer?
The great majority of sessile serrated adenomas will never develop into cancer, particularly when they are removed completely and show no dysplasia. The risk is higher when dysplasia is present or when the growth exceeds 1 centimetre (roughly the width of a fingernail).
When cancer does arise through the serrated pathway, it is typically a type called colorectal adenocarcinoma. These cancers usually develop over many years, which is why removing precancerous growths during a colonoscopy is such an effective cancer prevention strategy.
What happens next?
In most cases, removing the growth during colonoscopy is the treatment. No additional surgery or medication is needed. Your doctor will recommend a follow-up colonoscopy at an interval based on what was found:
- Sessile serrated adenoma without dysplasia, completely removed. Follow-up colonoscopy is typically recommended in 3 to 5 years, depending on the size of the growth and any other findings from the same procedure.
- Sessile serrated adenoma with dysplasia, or incompletely removed. Follow-up is usually recommended sooner — often within 1 year — to check the area and look for any remaining tissue.
- Multiple sessile serrated adenomas. More frequent surveillance may be recommended. Your doctor will discuss a schedule tailored to your situation.
If this feels like a lot to take in, that is completely understandable. The most important thing is that the growth was found and removed. Your doctor can walk you through what the findings mean and what the right follow-up plan looks like for you.
Questions to ask your doctor
- Was the growth completely removed, or was the margin positive or unable to be assessed?
- Was dysplasia found?
- 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 growths?
Related articles
We are proud to partner with:
