Intestinal polyps are the outgrowths of tissue from the gastrointestinal epithelium that protrude into the lumen of the small and large intestine. It can be grouped into adenomatous and non-adenomatous polyps. The colorectal adenomas are the most common polyps and associated with some risk of cancer.
Adenomatous polyps originate from the epithelium and develop as a result of abnormal cell proliferation (dysplasia). These types of polyps are very common and may occur in 50% of people over the age of 60 years. It is more frequently found in the distal part of the colon and the rectum, therefore it is often referred to as colorectal adenomas.
Adenomatous polyps can be of various sizes, ranging from small pedunculated lesions to large sessile polyps. These are neoplastic polyps meaning that it has the potential to become malignant (cancerous). This malignant potential increases with severity of the dysplasia and with increasing size of the polyp.
A strong familial predisposition is seen with adenomatous polyposis syndromes. Adenomatous polyps associated with hereditary polyposis syndromes carry a significantly higher risk of colorectal cancer. Having a first-degree relative with familial polyps increases the risk for adenomatous polyps by four-fold.
Types of Adenomatous Polyps
Adenomatous polyps develop when genetic damage leads to an imbalance between mucosal cell proliferation and cell death. This means that the cells are replicating at a faster rate than those that are dying leading to an overgrowth of tissue. This eventually results in development of the abnormal mass in the form of polyps. Adenomatous polyps are of tubular, villous or tubulovillous (mixed) type based on histological examination.
Tubular adenomas are the most common type of adenomatous polyps accounting for about 80% of all adenomatous polyps. More than 50% of the tubular adenomas are located in the rectum and sigmoid colon and are associated with a 2% to 5% risk of cancer. Glands or cysts-like structures are usually found in the submucosa. Tubular adenomatous polyps which are smaller than 1 cm rarely become malignant.
Pure villous adenomas are relatively rare and account for less than 5% of adenomas. These adenomas are usually sessile cauliflower-like mass, mostly confined to the sigmoid colon and rectum. Villous adenomas which are larger than 4 cm size are often associated with severe dysplasia and has a high risk of cancer. About 40% of these lesions eventually develop cancer.
Tubulovillous type adenomas are found in 10 to 20% of the patients with adenomatous polyps. It is also known as mixed or villoglandular adenomas because it has mixed features of tubular and villous type lesions. The malignant potential of tubulovillous polyps are related to the amount of villous component of the lesions. Tubulovillous polyps have about a 20% risk for cancer development.