Colorectal Polyps

1

What are polyps?

A gastrointestinal (GI) polyp is an abnormal growth of tissue projecting from the mucosal layer anywhere along the GI tract. Almost half of patients with GI polyps do not complain of bowel symptoms. Bowel habit alteration is more common than abdominal pain. The majority of colorectal polyps found at screening colonoscopy are ≤5 mm. Two-thirds of polyps occur in the rectosigmoid and descending colon.

2

What are the major types of polyps?

Until recently, colonic polyps were traditionally classified as either hyperplastic or adenomatous, and only the latter were believed to have the potential to progress to carcinoma.

Polyps can be classified by morphology (the Paris classification)—pedunculated, sessile, flat, or depressed.

• Pedunculated polyps have a head attached by a stalk to the mucosa of the colon or rectum. The stalk is usually covered with normal mucosa and <1.5 cm in length.

• Sessile polyps are relatively flat where the base is attached to the colon wall.

• Flat polyps have a height less than one-half of the diameter of the lesion. These account for 27%–36% of polyps.

• Depressed polyps have an increased likelihood of showing high-grade dysplasia and are more often seen in Asian populations. In general, the muscularis mucosa is an important histologic landmark for differentiating invasive from noninvasive lesions because lymphatics and veins do not extend across the muscularis mucosa. Submucosal lesions such as carcinoids and lipomas may resemble colorectal polyp.

3

At what age do polyps develop?

Adenomatous colorectal polyps infrequently develop under the age of 30. The incidence increases with age. Colonic screening studies in asymptomatic individuals suggest the prevalence of adenomas is 25%–30% at age 50. Autopsy studies are consistent with this clinical prevalence.

4

Which polyps do not have malignant potential?

• Hyperplastic (metaplastic) polyps are composed of normal cellular components, do not exhibit dysplasia, and have a characteristic serrated (“saw tooth”) pattern, typically <5 mm, and located in the rectosigmoid.

• Hamartomas are made up of tissue elements normally found at that site, but which are growing in a disorganized mass.

• Inflammatory pseudopolyps are residual, intact but inflamed colonic mucosa subject to ulcerations. They represent healing/healed islands of mucosal epithelium. Seen in ulcerative colitis, Crohn’s disease, and schistosomiasis.

5

Which polyps have malignant potential?

• a.

  Adenomatous polyps are known to have malignant potential and are classified pathologically:

  • i.

    Tubular adenomas are characterized by a network of branching adenomatous epithelium; the tubular component must be >75%. These account for more than 80% of colonic adenomas.

  • ii.

    Villous adenomas have long glands extending down from the surface to the center of the polyp; the villous component must be >75% to be villous. These account for 5%–15% of adenomas.

  • iii.

    Tubulovillous adenomas have 36%–75% villous component and account for 5%–15% of adenomas.

• b.

  Serrated polyps are a heterogeneous group with a variable malignant potential. Histologically distinct from adenomas, the characteristic feature of all serrated polyps is the “saw-toothed” infolding of the crypt epithelium. Generally, these are broken into:

  • i.

    Traditional serrated adenoma histologically display overall protuberant growth pattern with viliform projections and contain cytologically dysplastic cells with elongated nuclei and eosinophilic cytoplasm with more prevalence in the rectosigmoid colon.

  • ii.

    Sessile serrated adenomas have extensions of the serrations to the crypt base and dilated L- or inverted T-shaped crypts and are more prevalent in the proximal colon.

• c.

  There is recent evidence implicating large hyperplastic polyps, which are mostly benign, as a potential precursor to serrated polyps.

6

What are the risk factors for high-grade dysplasia and cancer?

Villous histology, increasing polyp size, and high-grade dysplasia are risk factors for focal cancer within an individual adenoma.