Colonic Polyps and Colorectal Cancer
Colorectal cancer (CRC) is the second leading cause of cancer mortality in the United States. In men, CRC is second in prevalence only to lung cancer, and in women, it is third behind breast and lung cancer. More than 95% of the cancers are thought to have their origin in adenomatous polyps. In the United States, the prevalence of CRC is 30 to 40 per 100,000 and it increases with age.
I. COLONIC POLYPS
A. Pathogenesis.
The term polyp refers to any protrusion into the lumen of the gastrointestinal (GI) tract. A sessile polyp is a raised protuberance with a broad base. A pedunculated polyp is attached to the bowel wall by a stalk that is narrower than the body of the polyp. Submucosal polyps are lipomas, leiyomyomas, hemangiomas, fibromas, lymphoid tissue, endometriomas, melanomas, or metastatic lesions. Most submucosal polyps are benign; however, many patients with carcinoid metastatic lesions, melanomas, lymphomas, and Kaposi’s sarcomas have malignant polyp formation in the colon.
Polyps may be benign or malignant. In the colon, polyps can be described as adenomatous, hamartomatous, hyperplastic, or inflammatory, according to their histopathologic appearance. Hyperplastic and inflammatory polyps are usually benign, but are also at risk for carcinoma. Adenomatous polyps can be classified as tubular, villous, or tubulovillous, depending on whether their histologic appearance is primarily glandular, villous, or mixed, respectively. Although most adenomatous polyps are benign, some may contain carcinoma and others may degenerate later to carcinoma. The risk of a polyp containing carcinoma increases directly with the size of the polyp (Table 39-1). Cancer is more likely to occur in villous adenomas than the other types, and benign adenomatous polyps may coexist with adenocarcinoma elsewhere in the bowel. In short, a colon that has a tendency to produce polyps also is at higher risk of the development of cancer.
B. Clinical presentation.
Because colonic polyps are so common—some estimates of prevalence are as high as 50% in people over age 50—they are an important risk factor in the development of CRC in the general population.
Most patients with polyps are asymptomatic. In those instances, the polyps remain undiscovered or are diagnosed during surveillance examination. Sometimes polyps cause occult or gross bleeding. Occasionally patients complain of abdominal discomfort, which may be caused by tugging on the polyp by peristaltic contrac-tions. If the polyp is large, frank obstruction can occur. Rarely, polyps may cause intussusception of the small or large intestine.
TABLE 39-1 Relation of Polyp Size to Risk of Cancer in the Polyp | ||||||||
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 Figure 39-1. Air-contrast barium enema showing a sigmoid polyp on a long stalk (arrows). |
C. Diagnostic and screening.
Studies for colonic polyps and CRC include colonoscopy, virtual colonoscopy, CT colography, sigmoidoscopy, and air contrast barium enema x-ray examination (Fig. 39-1). Often the barium enema has been ordered for symptoms that have no relation to the polyp. If the possibility of a polyp is raised by barium enema findings, complete colonoscopy should be performed. Similarly, identification of a polyp by sigmoidoscopy is justification for colonoscopy because the chance that there is a synchronous polyp in the colon above the reach of the sigmoidoscope exceeds 20%. For discussion of cancer surveillance, see section V.
D. Treatment
1. Polypectomy.
Nearly all polyps can be removed during colonoscopy. Polypectomy is performed by encircling the polyp with a wire snare through which an electrocauterizing current is passed. Sessile polyps may be removed in a piecemeal fashion. Injection of saline to the base of sessile lesion before snaring may give better visualization of the polyp and help its complete removal. If a polyp is too large to be removed, it should be adequately biopsied.
2. Careful histologic examination of resected polyps
is essential for formulating appropriate recommendations for the patient. Nonadenomatous polyps are thought to have little or no malignant potential, and removal of those polyps is sufficient treatment. On the other hand, adenomatous polyps not only predispose to the subsequent development of cancer but also may contain cancer at the time of removal. Thus, it is important that all polyps, particularly those larger than 1 cm in diameter, be examined carefully for adenocarcinoma.
3. Adenocarcinoma
may be present in adenomatous polyps in one of four ways (Fig. 39-2, Table 39-2).
 Figure 39-2. Stages of involvement of adenomatous polyps with cancer. |
TABLE 39-2 Diagnosis and Treatment of Cancerous Adenomatous Polyps | |||||||||||||||
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a. The cancerous change involves only the mucosa and does not penetrate the muscularis mucosa into the stalk of the polyp. This condition is sometimes called carcinoma in situ or high-grade dysplasia. Colonoscopic resection of the polyp in this instance is regarded as curative. No surgical treatment is indicated. The patient should be scheduled to return in 1 year for follow-up colonoscopy.
b. The cancer penetrates the muscularis mucosae of the polyp into the stalk but does not involve blood vessels or lymphatics within the resected portion of the stalk, and the cancer is moderately to well differentiated.
Although a small number of patients with this finding has cancerous involvement of the bowel or local lymph nodes, the mortality (<2%) and morbidity of surgery to resect the portion of the colon that contained the polyp exceeds the risk of residual cancer. Thus, additional surgery for these patients is not recommended. Repeated colonoscopy should be scheduled for 1 year later.
c. The cancer not only penetrates the muscularis mucosae of the polyp but also has invaded blood vessels or lymphatics within the stalk. In these patients, cancer also is likely to be present in the bowel or local lymph nodes. Also in this category are patients with poorly differentiated cancers that do not involve blood vessels or lymphatics. If operative risk is not prohibitive, these patients should undergo resection of the segment of colon that contained the polyp. Repeated colonoscopy to examine the anastomosis for recurrent tumor should be scheduled for 3 to 6 months if residual cancer is found at surgery. If no residual cancer is found, colonoscopy should be scheduled for 6 to 12 months.
d. The cancer involves the resectional margin of the polyp, indicating that residual cancer remains in the patient. If the patient is an operative candidate, segmental resection of the colon is indicated. These patients should undergo follow-up colonoscopy in 3 to 6 months to check for residual tumor at the anastomosis and for other polyps.
II. Polyposis syndromes differ in their clinical manifestations,
pathology, patterns of inheritance, and predisposition to carcinoma (Table 39-3).
A. Familial adenomatous polyposis (FAP) and Gardner’s syndrome.
These conditions probably are related and expressions of the same genetic syndrome: one that is inherited in an autosomal dominant pattern. The prevalence in the United States is 3 per 100,000 persons. Affected individuals with FAP have hundreds of colorectal polyps in the first three decades of life. Polyps may also occur in the stomach and small intestine. The risk of polyp occurrence in the duodenum is 3% to 5%. Two percent of patients may have pancreatic, thyroid cancer, and hepatoblastoma. If the colon is not resected, virtually 100% of the patients eventually develop cancer. Gardner’s syndrome is distinguished from FAP by the presence of osteomas, fibromas, and other features (Table 39-3) in addition to the intestinal polyps. Yearly colonoscopy should begin at age 10 in asymptomatic individuals carrying the gene for FAP. Total proctocolectomy with an ileostomy or an anal sphincter-saving procedure is indicated if the diagnosis of FAP is made. Genetic testing is available for family members.
B. Turcot’s syndrome.
In this rare disease, colonic polyposis is associated with brain tumors. Both recessive and dominant patterns of genetic transmission have been described. Screening and treatment of affected individuals are the same as for FAP.
C.
In Peutz-Jeghers syndrome, intussusception, obstruction, or infarction of the polyps may develop with consequent abdominal pain and bleeding. For these reasons, surgery may be indicated. Because the risk of cancer is less than 3%, prophylactic surgery is not indicated.
D.
The other polyposis syndromes are not associated with an increased risk of cancer, although relatives of patients with juvenile polyps may have cancers of the stomach, small intestine, colon, or pancreas. However, patients with these syndromes may have complications of the polyps, such as bleeding and obstruction; if conservative treatment fails, they require surgery.
III. COLON CANCER
A. Pathogenesis.
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