Sessile Serrated Lesion of the Colon or Rectum: Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC
August 16, 2026

A sessile serrated lesion is a type of precancerous polyp that develops on the inner lining of the large intestine, which includes the colon and the rectum. It is not cancer, but it can slowly change into a cancer called adenocarcinoma if it is left in place. Most sessile serrated lesions are found and removed during a colonoscopy.

You may also see this diagnosis called a sessile serrated polyp, or, in older reports, a sessile serrated adenoma or SSA/P. All of these names refer to the same growth. Current pathology guidelines prefer the term sessile serrated lesion, which is why newer reports use it. This article explains what you may see in a pathology report after a sessile serrated lesion has been removed, what each term means, and how this diagnosis differs from the more familiar colon adenomas.

What do the words sessile and serrated mean?

The two words in the name sessile serrated lesion describe how this polyp looks, one to the eye and one under the microscope.

Sessile means the polyp is flat or only slightly raised and has no stalk. Polyps on a stalk stand up from the bowel wall like mushrooms and are easy to spot during a colonoscopy. Sessile lesions lie flat against the lining, often with a rim of mucus over them, which makes them harder to see and easier to miss. This is one reason a careful, unhurried colonoscopy with good bowel preparation matters.

Serrated refers to a saw-tooth pattern along the inner edge of the glands when the tissue is examined under the microscope. That jagged outline separates serrated polyps as a group from conventional adenomas such as tubular adenomas.

What causes a sessile serrated lesion?

Sessile serrated lesions develop when cells in the lining of the colon or rectum acquire mutations, which are errors in the DNA that allow the cells to grow abnormally. These lesions follow a different route to cancer than conventional adenomas do. Pathologists call it the serrated pathway, and it accounts for roughly 20 to 30% of all colorectal cancers.

One of the earliest steps is a mutation in a gene called BRAF, which drives the serrated growth pattern. Over many years, a chemical process called methylation can then switch off other genes by attaching molecular tags to them without changing the DNA sequence itself. When methylation silences a gene called MLH1, the cell loses part of its DNA proofreading system, a state called mismatch repair deficiency. Errors then accumulate quickly. This whole process takes years, which is why finding and removing the lesion is so effective.

Factors that increase the chance of developing a sessile serrated lesion include increasing age, smoking, heavy alcohol use, excess body weight, and a family history of colorectal cancer or polyps. Smoking is more strongly associated with serrated polyps than with conventional adenomas.

What are the symptoms of a sessile serrated lesion?

Most sessile serrated lesions cause no symptoms. They are usually flat, often small, and are often found incidentally during a colonoscopy performed for routine screening or for an unrelated reason. They occur most often in the right side of the colon, farthest from the rectum, where a polyp can grow for some time without being noticed.

A larger lesion occasionally causes rectal bleeding or a change in bowel habits. These symptoms are not specific to this diagnosis, since many conditions cause them, and only examination of the bowel and microscopic examination of any tissue removed can identify what is responsible.

How is the diagnosis made?

A pathologist makes the diagnosis of a sessile serrated lesion only after the polyp is removed and examined under a microscope. During a colonoscopy, a procedure called polypectomy removes the lesion using a wire loop passed through the scope. Because these lesions are flat and often have indistinct edges, larger ones frequently require endoscopic mucosal resection, a technique that lifts and removes a wider area of the lining.

Under the microscope, the pathologist examines the glands, called crypts, that make up the colon lining. In a normal colon, these run straight down like test tubes in a rack. In a sessile serrated lesion, the bottom of the crypt is distorted: it becomes widened and turns sideways along the base of the tissue, giving it an L or boot shape, and the saw-tooth pattern extends all the way down into the base rather than staying near the surface. These base changes are what distinguish a sessile serrated lesion from a hyperplastic polyp, which can look similar closer to the surface but is not precancerous.

Distinguishing the two depends on seeing the base of the crypts, so a fragmented or superficial sample can make the distinction uncertain. When that happens, the report will say so, and your doctor generally manages the lesion as the more serious of the two possibilities. Even with a well-oriented sample, pathologists do not always agree on this distinction, which is a recognized limitation rather than an error.

Because a sessile serrated lesion is not cancer, no imaging scans are needed after the diagnosis.

Dysplasia

Dysplasia means the cells in a sessile serrated lesion have developed abnormal features under the microscope, a sign that they have moved further along the path toward cancer. Unlike conventional adenomas, which show dysplasia by definition, most sessile serrated lesions show none. Your report will state whether dysplasia is present or absent.

Sessile serrated lesion without dysplasia. The crypts show the characteristic serrated shape, but the cells lining them still look close to normal. This is the usual finding.
Sessile serrated lesion with dysplasia. The cells lining part of the lesion now look clearly abnormal. This is a more advanced stage and carries a higher risk of progressing to cancer, so it leads to closer follow-up. This is often when MLH1 has been silenced, which can accelerate the change compared with a conventional adenoma.

Some reports describe the dysplasia as low grade or high grade. Current guidelines do not recommend grading dysplasia in sessile serrated lesions, because the abnormal changes are typically uneven across the tissue and a reliable grade is difficult to assign. What matters for your care is whether dysplasia is present at all, not the grade assigned.

Size and number of lesions

Two other findings recorded after a sessile serrated lesion is removed are its size and the total number of polyps found. Both influence how soon the next colonoscopy is recommended.

Size is usually reported in millimeters, and the threshold that matters most is 10 mm, roughly the width of a fingernail. A sessile serrated lesion 10 mm or larger is treated as a higher-risk finding, regardless of whether dysplasia is present.

The number of serrated polyps also matters. Finding several places puts you in a higher risk group. Finding many raises the possibility of serrated polyposis syndrome, a condition defined by either at least five serrated polyps above the rectum with two or more of them 10 mm or larger, or more than twenty serrated polyps of any size spread throughout the colon. It is uncommon; its cause is not fully understood, and in most people, no inherited gene change is identified. It does carry an increased risk of colorectal cancer, and it leads to a much shorter surveillance interval, so if your colonoscopy showed multiple serrated polyps, you can ask your doctor whether you meet the criteria.

Margins

The margin is the cut edge of the tissue removed during the procedure. The pathologist examines it to determine whether the whole sessile serrated lesion was removed. Your report will describe the margin in one of three ways:

Negative (clear) margin. No abnormal cells are seen at the cut edge. The lesion appears to have been removed completely.
Positive margin. Abnormal cells reach the cut edge, so some lesion tissue may remain in the bowel. Your doctor will usually arrange a repeat examination of that area.
Cannot be assessed. Larger lesions are often removed in several pieces, and the edges of the tissue are frequently sealed with heat during removal, leaving a cautery artifact. Either situation can make the margin impossible to evaluate reliably.

Margin status is harder to interpret for these lesions than for a rounded polyp on a stalk, because a sessile serrated lesion is flat and its edges blend into the surrounding normal lining. Incomplete removal is more common than with conventional adenomas for the same reason, and it is the usual explanation when a lesion appears at the same site on a later colonoscopy.

How is a sessile serrated lesion different from an adenoma?

A sessile serrated lesion and a conventional adenoma are both precancerous polyps of the colon and rectum, but they arise through different biological routes and differ in several ways that affect what your report says and what happens next.

Appearance. A sessile serrated lesion is flat and covered in mucus. Conventional adenomas such as the tubular adenoma, tubulovillous adenoma, and villous adenoma are more often raised or on a stalk.
Location. Sessile serrated lesions favor the right side of the colon. Conventional adenomas are more evenly distributed.
Dysplasia. Every conventional adenoma shows dysplasia. Most sessile serrated lesions do not.
Genetic route. Conventional adenomas usually begin with a change in the APC gene. Sessile serrated lesions usually begin with a BRAF mutation and progress through methylation.
Speed. Once dysplasia appears in a sessile serrated lesion, progression can be faster than the decade-or-more typical of conventional adenomas.

Neither type is more serious than the other in every case. The features on your own report matter more than which category the polyp falls into.

What is the risk that a sessile serrated lesion will become cancer?

Most sessile serrated lesions never become cancer, particularly when they are removed completely and show no dysplasia. The risk rises when dysplasia is present, when the lesion is 10 mm or larger, and when many serrated polyps are found.

When cancer does develop along the serrated pathway, it is usually a colorectal adenocarcinoma, and it is often mismatch repair deficient because of the MLH1 silencing described above. Mismatch repair deficient colorectal cancers tend to respond well to immunotherapy, so this feature affects treatment if cancer is ever found.

In a sessile serrated lesion, mismatch repair deficiency arises from MLH1 promoter methylation, which is acquired during life and is not inherited. It does not indicate Lynch syndrome and is not passed to your children.

What happens after this diagnosis?

For a sessile serrated lesion, removing it during the colonoscopy is the whole treatment. No surgery, chemotherapy, or radiation is involved. Next, your doctor sets a follow-up interval based on your report findings, your age, overall health, and family history.

Current guidelines from the US Multi-Society Task Force on Colorectal Cancer group findings roughly as follows:

One or two sessile serrated lesions, each smaller than 10 mm, without dysplasia. The next colonoscopy is generally considered at 5 to 10 years.
Three or four sessile serrated lesions, each smaller than 10 mm. Generally 3 to 5 years.
Five to ten sessile serrated lesions, a lesion 10 mm or larger, or any lesion with dysplasia. Generally 3 years.
A lesion 20 mm or larger removed in pieces. A repeat examination of that specific area is usually arranged within 6 months, separately from the routine schedule above, to confirm the site is clear.
Findings that meet the criteria for serrated polyposis syndrome. Surveillance every 1 to 2 years, and family members are usually offered screening as well.

These intervals are a starting point, not a rule, and the quality of the examination matters a great deal for flat lesions. If the bowel preparation was poor or the colonoscopy could not be completed, your doctor may recommend repeating it sooner regardless of what the polyp showed. Your pathology report is one of several things your doctor weighs when setting the plan, and what happens after your pathology report describes the wider process.

Questions to ask your doctor

Where in the colon was the lesion found?
How large was it, and how many polyps were removed in total?
Did my report say whether dysplasia was present?
Was the lesion removed in one piece or in fragments?
What did the margin show, and does it mean any tissue was left behind?
Do I need a separate follow-up look at the site where the lesion was removed, and when?
Was there any uncertainty about whether this was a sessile serrated lesion or a hyperplastic polyp?
Was the bowel preparation good enough to find flat lesions reliably?
Do my findings meet the criteria for serrated polyposis syndrome?
When should I have my next colonoscopy?
Does this finding change my overall risk of colorectal cancer?
Should my close family members be screened earlier or more often?
Would quitting smoking or other changes lower my risk of developing more serrated polyps?