Sessile serrated lesion of the colon

What is a sessile serrated lesion?

A sessile serrated lesion (SSL) is a non-cancerous growth in the colon. However, it can become cancerous if left untreated or not completely removed. Sessile serrated lesions are most often found in the right side of the colon, including parts of the colon that are designated as the cecum, ascending, and transverse colon, but they may occur in any location including the rectum.

A sessile serrated lesion is a relatively new type of polyp, having only been recognized for the past two decades. Before this, most of these polyps were called hyperplastic polyps, although we know now that they are in fact different. Hyperplastic polyps still occur and are common, but they tend to be found on the left side of the colon, as opposed to the right-sided location of most sessile serrated lesions.

The colon

The colon is a part of the gastrointestinal tract which also includes the mouth, esophagus, stomach, small bowel, and anus. The colon is a long hollow tube that starts at the small bowel and ends at the anal canal. The colon is divided into sections which include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

The functions of the colon are to absorb water from the food that we eat and to move waste out of the body.

The colon is made up of six layers of tissue:

  1. Mucosa – The mucosa is the tissue that lines the inside surface of the colon. The mucosa includes epithelial cells that form structures called glands. The glands are surrounded and supported by a tissue called lamina propria.
  2. Muscularis mucosa – This is a thin layer of muscle that sits just below the mucosa.
  3. Submucosa – The submucosa contains many thick blood vessels and lymphatic channels.
  4. Muscularis propria – The muscularis propria is a thick bundle of muscle. The muscles in the muscularis propria help move digested food and waste along the colon.
  5. Subserosal adipose tissue – This is a layer of fat that sits directly below the muscularis propria. The subserosal adipose tissue is near the outside surface of the colon.
  6. Serosa – The serosa is a thin layer of tissue that covers the subserosal adipose tissue and the outside of the colon.
normal colon layers

How do pathologists make this diagnosis?

The diagnosis of sessile serrated lesion is usually made after the adenoma is removed during a medical procedure called a colonoscopy. The sessile serrated lesion may be removed in one piece or in multiple pieces. The tissue sample is then sent to a pathologist for examination.

Unfortunately, it is possible for gastroenterologists and surgeons to miss sessile serrated lesions when doing a colonoscopy as they can be very difficult to see. They can be flat and indistinct and may blend in with the surrounding lining of the colon.

sessile serrated lesion

Dysplasia

Colon cancer can start in a sessile serrated lesion. When cancer develops in a sessile serrated lesion, it usually starts in a pre-cancerous condition called dysplasia.

Dysplasia is a word pathologists use to describe an abnormal pattern of growth. In the colon, dysplasia can be either low grade or high grade, depending on how abnormal it looks to a pathologist when viewed under the microscope. High-grade dysplasia is more closely related to cancer than low-grade dysplasia.

All sessile serrated lesions require complete removal at colonoscopy but the presence of dysplasia may require a quicker repeat colonoscopy if the removal wasn’t complete on the first visit.

Cancer in the sessile serrated lesion

​Your pathologist will carefully examine the sessile serrated lesions for any evidence of cancer. If cancer is seen, further surgery may be required. The need for further surgery will depend on a variety of factors, such as the size of the cancer and whether or not it has been completely removed.

Margins

A margin in a sessile serrated lesion is the part of the colon or rectum that the gastroenterologist or surgeon cut to remove the abnormal tissue.  Pathologists do not usually state in their report whether the sessile serrated lesion has been completely removed or not, as this is thought to be best judged by the physician doing the colonoscopy.

If there is cancer within the sessile serrated lesion, the pathology report must indicate the distance of the cancer from the margin (the point at which the sessile serrated lesion was attached to the colon).

Sessile serrated lesions are often removed and sent to pathology as multiple pieces (fragments) of tissue. In some of these cases, it will not be possible for your pathologist to determine which piece is the real margin and the changes seen at the margin will not be described in your report.

by David Driman, MBChB FRCPC (updated July 23, 2021)
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