Hyperplastic polyps

 • Very common polyps, occurring most frequently in the distal colon.

 • Usually small lesions, <1 cm in size, found on the crest of a mucosal fold.

 • Microscopically, they are composed of crypts which are dilated and serrated in the superficial portion and narrow at the base.

 • Benign lesions with no risk of progression into carcinoma, unless they show dysplasia which is very uncommon. the hyperplastic polyposis syndrome, in which there are very large numbers of hyperplastic polyps, is also associated with an increased risk.

Sessile serrated lesions (or polyps)

 • relatively recently characterized polyps which tend to be >1 cm and more likely to be found in the right colon.

• Genetically, they tend to harbour mutations in mismatch repair genes.

 • Microscopically, they show markedly dilated serrated crypts which are widened at their base: ‘boot- shaped crypts’.

• there may be coexisting dysplasia.

* Associated with an increased risk of subsequent colorectal carcinoma.

Adenomatous polyps

 • Very common polyps which may occur anywhere in the large bowel.

• Most occur sporadically, but they are also associated with familial adenomatous polyposis (FAP). FAP is an inherited condition in which the colon becomes carpeted with thousands of adenomas at a young age, with the inevitable development of colorectal carcinoma without prophylactic colectomy. there may be polyps in the duodenum as well. Gardener’s syndrome, which is a variant of FAP, may be associated with extraintestinal manifestations such as desmoid tumours.

• Neoplastic polyps which harbour frequent mutations of APC, KRAS, and p53.

 • Microscopically, the polyps contain dysplastic glands lined by epithelial cells with stratified hyperchromatic nuclei growing in complex tubules or finger- like villous projections. Depending on the relative proportion of tubules and villi, they are classified as tubular, tubulo- villous, and villous adenomas.

• the dysplasia is graded into low or high grade, according to the degree of cytological and architectural abnormality.

• ~10% of adenomas develop carcinoma.

 • the likelihood of malignant transformation is higher with larger polyps, high- grade dysplasia, and a villous architecture. When carcinomas arise in pedunculated polyps, they are staged using the Haggitt system to assess the risk of the presence of lymph node metastasis.

Inflammatory polyps

 • thin, filiform lesions which occur following any mucosal injury, but are often seen in patients with inflammatory bowel disease.

• Microscopically, they are covered by mucosa on all sides, with only a tiny amount of submucosal tissue.

Mucosal prolapse (‘solitary rectal ulcer syndrome’)

 • Prolapsed pieces of mucosa which appear as polypoid projections.

 • Can occur at any point in the large bowel, but characteristically seen on the anterior rectal wall or in association with diverticular disease.

 • Can ulcerate and mimic colorectal carcinoma.

 • Microscopically, they show distorted angulated crypts set in the lamina propria containing bundles of smooth muscle running up from the muscularis mucosae.

Benign fibro- epithelial polyps

 • Almost always incidental polyps picked up in adults undergoing screening colonoscopy.

 • Microscopically, they show a bland spindle cell proliferation in the lamina propria. the spindle cells show no specific line of differentiation immunohistochemically.

Leiomyomas

 • Benign smooth muscle tumours arising from the muscularis mucosae.

 • Usually small polyps, located mostly in the distal large bowel.

 • Microscopically, they show bundles of bland smooth muscle cells.

 Juvenile (hamartomatous) polyps

 • Most common colonic polyp found in children.

 • thought to be hamartomatous in nature.

 • Microscopically, they show irregular, markedly dilated, disorganized colonic glands set in an oedematous stroma.

 • Presence of multiple juvenile polyps may be a marker for juvenile polyposis, an autosomal dominant condition caused by germline mutations in either SMAD4 or BMPR1A.