eMedicine Specialties > Radiology > Gastrointestinal

Colon, Polyps

Author: Justin D Pearlman, MD, PhD, ME, MA, Director of Dartmouth Advanced Imaging Center, Professor of Medicine, Professor of Radiology, Adjunct Professor, Thayer Bioengineering and Computer Science, Dartmouth-Hitchcock Medical Center
Coauthor(s): Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, LRCP, Chairman of Medical Imaging, Professor of Radiology, NGHA, King Fahad National Guard Hospital, King Abdulaziz Medical City, Riyadh, Saudi Arabia; Michael J Tsapakos, MD, PhD, Assistant Professor of Radiology, Dartmouth Medical School; Consulting Staff, Director of Body MRI, Department of Radiology, Dartmouth-Hitchcock Medical Center
Contributor Information and Disclosures

Updated: Feb 27, 2008

Introduction

Background

A polyp is a small growth of tissue shaped like the head or stalk of a mushroom. Two types of polyps develop in the wall of the colon: hyperplastic (harmless) and adenomatous (precursor to cancer). An adenomatous polyp 10 mm in diameter takes 5-10 years to become dangerous, leaving a large window of opportunity in which to find and remove it.

In optical colonoscopy, one of the techniques used to locate polyps, a long, flexible fiberoptic tube is employed to examine the walls of the colon. However, this study requires sedation to suppress the pain of the procedure, and it incurs a small risk of bowel perforation. For individuals at average risk, the American Cancer Society recommends that an optical colonoscopy be performed every 5 years, starting at age 50 years, to remove polyps 5-10 mm in size.^1,2 This practice reduces the incidence of colon cancer more than 90%.³ However, compliance with this recommended procedure is only 40%; therefore, efforts to develop reliable alternative tests are continuing.

For excellent patient education resources, visit eMedicine's Digestive System Center and Procedures Center. Also, see eMedicine's patient education articles Colitis, Colonoscopy, Colon Cancer, and Anatomy of the Digestive System.

See also the following related eMedicine topic:
Colonoscopy

See also the following related Medscape topics:
CME ACG 2007: Select Topics in GI Oncology -- Issues in CRC and Applications of EUS
CME A Cure for Metastatic Colorectal Cancer? Advances in the Treatment of Hepatic Metastases
Resource Center Colorectal Cancer

Pathophysiology

Colonic polyps are protruding, space-occupying lesions that occur in the lumen of the colon. When used in a pathologic context, the term polyp is usually restricted to a mucosal overgrowth that can be either benign or malignant. Polyps are of epithelial origin and arise from the epithelial cell lining of the colon. Colonic polyps can be subdivided into 3 groups: hyperplastic polyps, adenomas, and polyps associated with polyposis syndromes.

Hyperplastic polyps

Usually less than 0.5 cm in diameter, hyperplastic polyps represent 90% of all epithelial polyps, as shown in an autopsy series. However, hyperplastic polyps account for only 20% of surgically removed polyps. Hyperplastic polyps appear most commonly in the rectosigmoid colon, but they can occur anywhere. Although they can affect individuals of any age group, these lesions are diagnosed primarily in persons aged 50-70 years.

On endoscopy, hyperplastic polyps are typically seen as small, round, smooth, sessile (stalkless) lesions lying astride a mucosal fold. Lesions that are larger than 0.5 cm in diameter (10%) may have a short, broad-based stalk that can resemble that of a pedunculated (stalked)tubular adenoma. Histologically, hyperplastic polyps are composed of well-formed glands and crypts that are lined with non-neoplastic epithelial cells. Most of these epithelial cells show differentiation into mature goblets or absorptive cells. Larger hyperplastic colonic polyps rarely develop foci of adenomatous change.

The major clinical significance of these polyps is their differentiation from the premalignant adenomatous polyps. In general, hyperplastic polyps have no malignant potential, but in very unusual cases, polyps with adenomatous foci may undergo malignant transformation.

Adenomas

Adenomatous polyps account for approximately 10% of all colonic polyps. More than 90% of these are smaller than 1.5 cm in diameter and have a small potential for malignancy. Larger adenomatous polyps have a greater potential for malignant change (10%).

Adenomas are traditionally divided into 3 types: tubular, tubulovillous, and villous. Tubular adenomas are the most common of the 3 types (75%) and can be found anywhere in the colon. They may pedunculated or sessile, that is (as previously mentioned), with or without a stalk, respectively.

The risk of malignant potential is related to the size of the adenoma. Tubulovillous adenomas are most commonly found in the distal colon and rectum (75%); the cecum and ascending colon are the next most common sites. The lesions can range from 1-10 cm in diameter. The degree of villous component of these polyps is correlated with the risk of progression to carcinoma.

Villous adenomatous polyps occur more frequently within the rectum than they do elsewhere. Generally larger than the other 2 types, they tend to be nonpedunculated and to have a velvety surface or cauliflower-like (or frondlike) appearance. Compared with other polyps, villous adenomas are associated with the highest morbidity and mortality rates. Villous adenomas are symptomatic more often than are the other types of polyps; most commonly, villous adenomas cause rectal bleeding, but they can also lead to hypersecretory syndromes characterized by hypokalemia and profuse mucous discharge. Villous adenomas often harbor a carcinoma, with those that do accounting for 10% of in situ villous adenomas and 30% of the more invasive forms.

Polyposis syndromes

A variety of polyposis syndromes can affect the GI tract. These can be classified as familial inherited (autosomal dominant) or nonfamilial.

The inherited polyposis syndromes can be further divided into 2 groups, depending on whether the polyps are adenomas or hamartomatous polyps (which contain a mixture of normal tissues). They include the following:

• Adenomatous polyposis syndromes
  • Familial adenomatous polyposis (FAP)
  • Gardner syndrome
  • Turcot syndrome
• Hamartomatous familial polyposis syndromes
  • Peutz-Jeghers syndrome
  • Juvenile polyposis syndrome
  • Cowden disease
  • Ruvalcaba-Myhre-Smith syndrome

The noninherited polyposis syndromes include Cronkhite-Canada syndrome and a variety of other nonfamilial conditions featuring polyposis.

From a prognostic viewpoint, the polyposis syndromes must be recognized, because the adenomatous polyps are premalignant. These syndromes should be considered when an intestinal polyp is recognized in young patients, when 2 or more polyps are seen in any patient, when colonic carcinoma is discovered in patients younger than 40 years, and when extra-intestinal manifestations associated with these syndromes are discovered.^4,5 Progress has been made in understanding some of the genetic factors contributing to the development of these syndromes.⁶ FAP is best understood in terms of its genetic basis and of the subsequent pathologic and genetic events leading to carcinoma.

Adenomatous polyps, whether they are sporadic or are secondary to a familial polyposis syndrome, are the only premalignant type of polyp.

Another polyp group, the inflammatory polyps, consists of benign growths that contain an inflammatory epithelial reaction.

See also the following related eMedicine topics:
Colon Cancer, Adenocarcinoma
Colonic Polyps
Colorectal Tumors
Cowden Disease (Multiple Hamartoma Syndrome)
Familial Adenomatous Polyposis
Gardner Syndrome [Dermatology]
Gardner Syndrome [General Surgery]
Intestinal Polyposis Syndromes
Peutz-Jeghers Syndrome

Frequency

United States

Colon cancer is deadly, accounting for 12% of cancer deaths. When male and female data are combined, its frequency is second only to that of lung cancer. The American Cancer Society estimated that in the United States, almost 150,000 new colorectal cancer diagnoses (108,070 for colon cancer, 40,740 for rectal cancer) would be made in 2008, with 49,960 colorectal cancer deaths occurring that year.⁷

Alaskan natives and African Americans have an increased risk of developing colorectal cancer, and roughly 6% of Jewish Americans have a single misspelled gene that raises their risk for the disease from 6% to 12%.⁸ The risk has been found to be lowest among American Indians in New Mexico. In all groups, the risk is higher for men than for women.

In individuals who smoke more than 20 cigarettes per day, the number of polyps that occur is 250% higher than it is for nonsmokers, and in persons who smoke and drink, the number is 400% greater than it is for nonsmokers.

International

As much as 20% of the world's population is genetically predisposed to FAP. For example, people of Jewish ancestry who are of Eastern European descent may have a genetic predisposition that doubles their risk of developing FAP.

Mortality/Morbidity

• If adenomatous polyps are not removed and progress to cancer, the patient's prognosis depends on the extent of disease, which is reflected in the following disease characteristics:
  • Limited to the mucosa
  • Entering the muscularis layer lining the colon
  • Penetrating through the muscularis layer
  • Spreading to nearby lymph nodes
  • Reaching distant lymph nodes and/or organs (liver, lung, and/or bone)
• Five-year survival rates are predicted by using the Duke classification, as follows:
  • Duke A - The disease is limited to the mucosa.
  • Duke B1 - The disease has penetrated into the muscular layer (the muscularis propria), but there is no lymph node involvement.
  • Duke B2 - The disease has penetrated through the muscularis propria, but there is no lymph node involvement
  • Duke C1 - The disease has penetrated into the muscularis propria, and there is lymph node involvement
  • Duke C2 - The disease has penetrated through the muscularis propria, and there is lymph node involvement
  • Duke D - The disease has spread to other organs (eg, liver, lung, and/or bone).
• Other poor prognostic indicators are deoxyribonucleic acid (DNA) aneuploidy, an undifferentiated cell type, a 1p53 mutation, a pre-operative carcinoembryonic antigen level of greater than 5 ng/mL, venous invasion, penetration of the bowel wall, perforation of the colon, and adherence to adjacent organs.

Race

• Alaskan natives have the highest incidence (75 cases per 100,000) followed by (1) Japanese, black, and non-Hispanic white populations; (2) Chinese, Hawaiian, and white Hispanic populations; and (3) Filipino, Korean, and Vietnamese populations.
• The incidence is substantially lower among American Indians in New Mexico (18.6 cases per 100,000 men and 15.3 cases per 100,000 women) than in other populations.⁹
• See also Frequency.

Sex

In all groups, a male preponderance is found.

• The greatest difference in incidence between males and females (60%) is found in Filipino and Japanese populations.
• In other groups, the incidence in males and females differs by 13-22%.

Age

The incidence of polyps increases with age. In the general population, the incidence rises from 10 cases per 100,000 population (in persons aged 40-45 years) to 300 cases per 100,000 population(in persons aged 75-80 years).

For individuals with no risk factors, screening colonoscopy is recommended starting at the age of 50 years. For individuals with 1 or more risk factors (eg, a first-degree relative who had colon cancer), screening should start 5-10 years earlier.

The lifetime risk is 1 in 20.

Presentation

Lynch syndrome (autosomal dominant inheritance) accounts for 5% of colon cancers. This syndrome is diagnosed if colon cancer occurs before the age of 40 years and if 3 or more relatives over more than 1 generation have had colon cancer. Patients with Lynch syndrome have a hereditary predisposition to colon cancer without polyposis.

People who are exposed to cigarette smoke have more colonic polyps than do those who are not. Chronic alcohol intake also may increase the incidence of colonic polyps. Ulcerative colitis increases the risk of colonic polyps and cancer, and first-degree relatives of patients with ulcerative colitis also have an increased risk; however, the risk is not greater in patients with diverticular or inflammatory bowel disease.

Individuals with acromegaly have a 3-fold higher risk of colon cancer than do those who do not. Individuals with Peutz-Jeghers disease or Gardner syndrome also have an increased risk. Among individuals with skin tags, the incidence of colonic polyps is reported to be 10-77%. Individuals who have undergone irradiation of the pelvis have a 2- to 4-fold increase in the incidence of colonic polyps.

Elevated methane levels in the breath are another reported risk factor for colon cancer. Among individuals with septicemia resulting from Streptococcus bovis infection, 55% have colon cancer. High fat intake also is described as a risk factor. Although high fiber intake has long been thought to help prevent colon cancer, research data have negated that theory. However, phytochemicals in vegetables apparently do protect against the disease.

See also Frequency.

Preferred Examination

Efforts are underway to find all polyps 5-10 mm in diameter using computed tomography (CT) scanning or magnetic resonance imaging (MRI). These modalities are employed in the evolving techniques for so-called computed colonography (also called virtual colonography or virtual colonoscopy) examinations.

The impact of computed colonography on health-care costs depends on the component costs and the percentage of patients that go on to intervention. For example, if computed colonography provides high confidence that no lesions are present, it saves the difference in cost between it and colonoscopy. However, if a computed colonography examination finds a lesion that merits treatment by colonoscopic polypectomy, then computed colonography represents an added cost. Thus, computed colonography offers the most financial benefit when the ratio of its cost to colonoscopy is lower than the incidence of negative cases. In practice, computed colonography can lower costs, but not at all centers.¹⁰

Other tests, including stool heme testing, sigmoidoscopy, and double-contrast barium enema (DCBE) study, are less accurate; with these tests, between one third and one half of all treatable lesions can be missed. (As many as 55% of precancerous lesions can be missed with sigmoidoscopy.)

Of patients tested for heme in the stool, 1 in 5 has a positive result, and of those who test positive, one third have colonic polyps and 1 in 20 have colon cancer. Among individuals with negative heme results, approximately 1 in 30 has colon cancer. Conversely, of patients with colon cancer, one half have negative results on stool heme tests. (In fact, more than 90% of known colonic lesions test negative for occult blood, because polyps do not cause bleeding unless they are torn as a result of a shear force. Moreover, cancerous lesions do not commonly bleed.) Careful analysis of statistical data indicates that the primary benefit of these tests is to motivate patients to comply with recommended optical colonoscopy examinations.

See also the following related Medscape topic:
CME Virtual Colonoscopy May Be Used First in Screening for Colorectal Cancer

Limitations of Techniques

The false-positive rate for stool heme testing is high, as is the false-negative rate. The stool heme test in patients with known colon cancer commonly provides negative results. Studies show a benefit in one third of those patients who undergo a stool heme test who are subsequently referred for colonoscopy, and the net reduction in mortality rates is one third when everyone is referred; therefore, stool heme testing is not effective.^11,12,13,14

Sigmoidoscopy and barium enema study do not permit examination off the right side of the colon, where an increasing percentage of lesions occur.^{8,12,15,16,17,18,19,20,21} A colonoscopy, performed every 5 years starting at the age of 50 years (or 40 years in patients with risk factors, such as a family history), is the best screening method.

Failure to diagnose cancer is a leading cause of malpractice litigation against gastroenterologists.^22,23

A study in which different examiners performed 2 consecutive colonoscopy procedures on the same day revealed a miss rate of 24% for adenomas overall and a miss rate of 6% for adenomas of 1 cm or more.^22,23,24 Of the misses on colonoscopy, one half are ascribed to perception, and one half are ascribed to incomplete viewing.

Optical colonoscopy cannot be used to visualize areas behind tissue folds or to look through retained feces, and the tip of the scope may not reach the end of the colon.²⁵

Retrospective data support the conclusion of a >5% miss rate. Of 941 patients with colon cancer who underwent colonoscopy within the preceding 3 years, 5% of the results were interpreted as negative even though dangerous cancer precursors must have been present. Because most lesions do not progress to cancer, the miss rate for lesions is considerably higher. Nevertheless, colonoscopy compares favorably with DCBE study, which had a miss rate of 17% in 719 of patients who underwent barium enema study within the previous 3 years. Most of the difference between colonoscopy and barium enema study is in the ability to detect early, curable colon cancers and precursor lesions.

Differential Diagnoses

Other Problems to Be Considered

Intestinal neurofibromatosis
Benign nodular lymphoid hyperplasia
Postinflammatory polyposis (pseudopolyposis)
Lymphoid hyperplasia
Lymphosarcoma

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