Polyps 101

When gastroenterologists perform a colonoscopy, they may tell the patient that they found a “polyp”.  But what exactly does that mean?  Before we discuss the many forms of intestinal polyps, a very brief anatomy lesson is in order.

The intestines are made of five layers.  Starting from the inside of the intestine or lumen (lumen refers to the empty space or cavity), the first layer is the mucosa where nutrients,
fats and proteins are absorbed from the foods we eat (this is also the layer that is biopsied during a colonoscopy).  Next is the muscularis mucosa that forms a barrier between the mucosa and the third layer called the submucosa.  The submucosa is rich in vessels that provide the intestines with their blood supply while carrying the digested nutrients away to other vital organs.  The fourth layer is the muscularis propria, which acts to propel food through the intestines, and the outermost layer is the serosa.

Cut away view of rectum and magnification of the various layers colonic tissue.

Examples of benign polyps include hyperplastic polyps (also called metaplastic polyps), lipomas (benign tumor of fat cells), neuromas (benign tumors of nerve cells), inflammatory polyps (localized inflammation) or lymphoid aggregates (a cluster of immune cells).  These polyps are not generally thought to increase the risk for colorectal cancer.  Other benign polyps are sometimes referred to as “pre-cancerous.”  These polyps are not malignant, themselves, but have a chance of becoming cancerous if not removed.  Examples include adenomatous and hamartomatous polyps.  Adenomatous polyps that have tubular or villous characteristics have a higher chance of becoming cancerous.  Adenomatous and hamartomatous polyps are also the types of polyps that are usually associated with hereditary colorectal cancer syndromes.  The only truly “malignant polyp” is one that has been shown to contain invasive carcinoma.  Sometimes the carcinoma is confined to the polyp, and other times it has invaded one or more layers of the intestine.

Progression from Polyp to Cancer

When a gastroenterologist sees a polyp, he/she usually remove it by polypectomy.  This is necessary because it is not possible to distinguish a benign polyp from a malignant one just by colonoscopy.  Therefore, the gastroenterologist sends the polyp to a pathologist who determines the type and characteristics.  The gastroenterologist then uses the pathology report to determine if more tissue should be removed from the area that contained the polyp.  The information is also used to determine how often the patient should have a colonoscopy, and it can influence screening recommendations for the patient’s family members.  For these reasons, it is always a good idea to know what type of polyps you and/or your family members have had.