Background
Colonic polyps are slow-growing overgrowths of the colonic mucosa that carry a small risk (< 1%) of becoming malignant. However, because colonic polyps are highly prevalent in the general population (especially with increasing age), they confer an important predisposition to colon cancer and are therefore removed when detected.
Patients with isolated colonic polyps are usually asymptomatic but can experience overt or occult colonic bleeding. Colonic polyps can occur as part of inherited polyposis syndromes in which their number is greater and the risk for malignant progression is much greater compared to the risk with isolated colonic polyps.
In the context of clinical studies of chemoprevention, efforts are being directed at suppressing colonic polyp formation (eg, by use of sulindac) and/or at preventing their progression to colon cancer (eg, by use of aspirin).
Pathophysiology
Colonic polyps, or adenomas, are benign epithelial neoplasms that arise from the epithelial cells lining the colon. Colonic polyps are traditionally divided into 3 groups, as follows: hyperplastic polyps, adenomas, and polyposis syndromes.
Hyperplastic polyps
Hyperplastic polyps comprise about 90% of all polyps and are benign protrusions. They are usually less than 0.5 cm in diameter. Hyperplastic polyps most commonly occur in the rectosigmoid region during adulthood.
Thought previously to be entirely clinically insignificant, hyperplastic polyps are now recognized to possess some malignant potential in the setting of hyperplastic polyposis syndrome. Patients who are affected have an occurrence of hyperplastic polyps proximal to the sigmoid colon, with (1) 2 or more that are greater than 10 mm in diameter, (2) a total of more than 30 polyps, or (3) a first-degree relative with the syndrome. The polyps in this syndrome may have adenomatous components; display a serrated, saw-tooth surface epithelium; and harbor methylation of specific target genes, including mismatch repair genes.
Adenomas
Adenomas comprise approximately 10% of polyps. Most polyps (approximately 90%) are small, usually less than 1 cm in diameter, and have a small potential for malignancy. The remaining 10% of adenomas are larger than 1 cm and approach a 10% chance of containing invasive cancer.
Adenomas are traditionally divided by histology into 3 types, as follows: tubular, tubulovillous, and villous. Tubular adenomas are the most common of the 3 types and can be found anywhere in the colon. Villous adenomas most commonly occur in the rectal area; tend to be larger than the other two types; and tend to be nonpedunculated, velvety, or cauliflowerlike in appearance. Villous adenomas are associated with the highest morbidity and mortality rates of all polyps. They can cause hypersecretory syndromes characterized by hypokalemia and profuse mucous discharge and can harbor carcinoma in situ or invasive carcinoma more frequently than other adenomas.
Thus, the risk of progression to carcinoma is related to both the size and the histology of the adenoma. Adenomas that are greater than 1 cm, contain a substantial (>25%) villous component, or have high-grade dysplasia are commonly referred to as advanced neoplasms and carry an increased cancer risk.
The shape or gross structure of the polyp is also clinically significant. Those polyps with a stalk are called pedunculated. Those polyps without a stalk are called sessile. Sessile polyps are more concerning than large pedunculated polyps for two reasons. First, the pathway for migration of invasive cells from the tumor into submucosal and more distant structures is shorter. Second, the complete endoscopic removal is more challenging and more difficult to ascertain.
Some premalignant neoplasia is now recognized to be flat, rather than protuberant. Such nonpolypoid neoplasia is more common in the setting of chronic colitis and may be detected more readily by nontraditional endoscopic imaging methods, such as narrow-band width imaging or mucosal staining.
Polyposis syndromes
Polyposis syndromes are hereditary conditions that include familial adenomatous polyposis (FAP), hereditary nonpolyposis (a misnomer) colorectal cancer (HNPCC)/Lynch syndrome, Gardner syndrome, Turcot syndrome, Peutz-Jeghers syndrome, Cowden disease, familial juvenile polyposis, and hyperplastic polyposis.
Progress has been made in understanding some of the genetic factors contributing to the development of these syndromes. Some of the syndromes have extraintestinal features that help differentiate one syndrome from the other. For example, FAP is best understood in terms of the genetic basis and subsequent pathological and genetic events leading to carcinoma.
Two other types of benign polyps are hamartomatous polyps, which contain a mixture of normal tissues, and inflammatory polyps, which contain an inflammatory epithelial reaction and are typically found in the context of colitis.
Epidemiology
Frequency
United States
Population and autopsy studies suggest that about 30% of middle-aged or elderly individuals have colonic polyps. In comparison, the incidence of FAP in the United States is 1 case for every 6580-8300 persons.
International
Accurate comparison of colonic polyp incidence and prevalence among countries is difficult because of differences in the methods used for detection. Colonic polyp prevalence in patients older than 60 years appears to vary substantially within and among countries, but it appears to be greater than 10% in most areas.
Mortality/Morbidity
Untreated, colonic polyps can and do progress to carcinoma over several years. Morbidity from colonic polyps is related to complications, such as bleeding, diarrhea, intestinal obstruction, and progression to cancer. Bleeding can be frank hematochezia but is often chronic and goes unnoticed by the patient. If uncompensated, intestinal blood loss can cause anemia, typically due to iron deficiency.
A study by Stryker et al suggests that the risk of cancer development from sporadic 1-cm colonic polyps is 8% at 10 years and 24% at 20 years.[1] The risk for cancer development depends on the size of the polyp, villous histology, and its association with polyposis syndromes. In FAP, cancer inevitably develops 10-20 years after the initial appearance of colonic polyps.
Race
Race per se is not a major risk factor for colonic polyps. However, studies indicate that blacks have a somewhat higher incidence and an earlier onset of colorectal carcinoma. An American Gastroenterological Association task force recommended beginning colorectal cancer screening in blacks at age 45 years, rather than the standard age of 50 years.
Sex
Males appear to have a moderately higher colonic polyp incidence than females, with earlier onset observed in some studies.[2]
Age
Colonic polyps are strongly associated with increasing age (typically after age 40 y), but they can occur early in patients with polyposis syndromes. For example, colonic polyps can be detected in adolescents with familial adenomatous polyposis or in patients aged 20-40 years with hereditary nonpolyposis colorectal cancer (HNPCC).
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