By Jason Wasserman MD PhD FRCPC
March 29, 2026
A sessile serrated polyp is a type of polyp — a small growth that develops on the inner lining of the colon or rectum. These polyps are not cancerous, but they are considered precancerous, which means they can develop into cancer over time if they are not removed. Most sessile serrated polyps are found during a routine colonoscopy and are removed during that same procedure. If this finding appears in your report, it is a good sign: the polyp was detected and removed before it had a chance to cause harm.
What do the words “sessile” and “serrated” mean?
Sessile means the polyp is flat or only slightly raised and has no stalk. Stalked polyps stand up like a mushroom, making them easier to spot during a colonoscopy. Sessile polyps lie flat against the colon wall, making them harder to see, which is why colonoscopists must look carefully to find them.
Serrated refers to the saw-tooth pattern seen in the lining of the polyp when examined under a microscope. This jagged pattern is what distinguishes sessile serrated polyps from other types, such as conventional adenomas or simple hyperplastic polyps.
What causes a sessile serrated polyp?
Sessile serrated polyps form when cells in the colon lining acquire mutations — small errors in their DNA — that cause them to grow in an abnormal pattern. These changes are part of 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 causes the cells to divide in an uneven, serrated way. Over time, a chemical process called methylation can silence other important genes. When methylation affects a gene called MLH1, it can lead to mismatch repair deficiency — a condition in which DNA errors are no longer corrected. If that happens, the risk of further changes leading to cancer increases. These events unfold over many years, which is why removing the polyp early is so effective at preventing cancer.
What are the symptoms?
Most sessile serrated polyps cause no symptoms. Because they are flat and often small, they are typically found by chance during a colonoscopy performed for routine cancer screening or for an unrelated reason.
Occasionally, a large sessile serrated polyp may cause rectal bleeding or a change in bowel habits. These symptoms are not specific to this type of polyp — many conditions can cause them — so the only way to know what is going on is through examination and testing.
How is the diagnosis made?
The diagnosis is made by a pathologist who examines the removed polyp under a microscope. The polyp is typically taken out during a colonoscopy using a technique called a polypectomy — the removal of a polyp using a small wire loop. Larger or flatter polyps may require a more involved technique called endoscopic mucosal resection (EMR), which removes a wider area of tissue.
Once the tissue reaches the pathology laboratory, the pathologist looks for specific changes in the shape and structure of the glands (called crypts) that line the colon. Features that support this diagnosis include:
- Horizontal crypt growth at the base. The glands at the bottom of the polyp grow sideways along the base of the tissue rather than straight down, creating 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 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, giving an uneven appearance.
These features help the pathologist confirm the diagnosis and distinguish this polyp 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 in a precancerous direction. Not all sessile serrated polyps show dysplasia. When dysplasia is present, it means the polyp was at a more advanced precancerous stage and carries a higher risk of cancer if left untreated.
Some pathology reports mention low-grade or high-grade dysplasia. For sessile serrated polyps, current guidelines do not recommend separating dysplasia into these two grades. The reason is that the abnormal changes in these polyps are often mixed and uneven throughout the tissue, making a reliable grade difficult to assign. What matters most is whether dysplasia is present or absent — not which grade it is.
Margins
The margin is the edge of the tissue that was removed. The pathologist examines the margin to determine whether the entire polyp was taken out.
- Negative margin (clear margin). No polyp cells are found at the edge of the removed tissue. This means the polyp appears to have been completely removed.
- Positive margin. Polyp cells are present at the very edge of the tissue. This suggests 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 polyp was removed in multiple pieces or if the tissue edges were damaged during removal, the pathologist may not be able to evaluate the margin reliably. Your doctor will advise on what follow-up is needed.
What is the risk of developing cancer?
The great majority of sessile serrated polyps will never turn into cancer — especially if they are removed completely and do not show dysplasia. The risk is higher when dysplasia is present or when the polyp is larger than one centimetre (about the width of a fingernail).
When cancer does develop along the serrated pathway, it tends to be a type called colorectal adenocarcinoma. These cancers often develop over many years, which is precisely why regular colonoscopy screening is so effective: finding and removing these polyps early interrupts the process long before cancer has a chance to form.
What happens next?
Sessile serrated polyps are usually completely removed during the colonoscopy. Because they are precancerous, removing them is the treatment — no additional surgery or medication is needed in most cases.
Your doctor will recommend a follow-up colonoscopy at an interval that depends on the findings. In general:
- Sessile serrated polyp without dysplasia, completely removed. Follow-up colonoscopy is usually recommended in 3 to 5 years, depending on the size of the polyp and other findings.
- Sessile serrated polyp with dysplasia, or incompletely removed. Follow-up colonoscopy is usually recommended sooner — often within 1 year — to check the area and look for any remaining tissue.
- Multiple sessile serrated polyps. More frequent surveillance may be recommended. Your doctor will discuss a schedule tailored to your situation.
If this feels like a lot of information to process, that is completely understandable. The most important takeaway is that the polyp was found and removed. Your doctor can walk you through the specific follow-up plan that makes sense for you.
Questions to ask your doctor
- Was the polyp completely removed, or was the margin positive or unable to be assessed?
- Was dysplasia found in the polyp?
- 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 polyps?
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