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Neoplasms of the Large Bowel: Colon Polyps and Colon Cancer
Colon Polyps
A colon polyp is any elevation of the colon mucosal surface ( Fig. 99.1 ). It may be of any size, sessile or pedunculated, and benign or malignant. Benign polyps are categorized as neoplastic, nonneoplastic, and submucosal. Neoplastic polyps are usually considered premalignant, and most are adenomas. Adenomas may be classified histologically as tubular, tubulovillous, or villous. The polyp may demonstrate low-grade or high-grade dysplasia. High-grade dysplasia in a polyp is often referred to as “intermucosal carcinoma (malignant),” or carcinoma in situ.
Nonneoplastic polyps include mucosal, hyperplastic, and inflammatory (pseudopolyps) types, hamartomas, and other rare types. Submucosal polyps are lipomas, lymphoid collections, leiomyomas, hemangiomas, fibromas, and rare presentations of endometriosis, pneumatosis cystoides intestinalis, colitis cystica profunda, or metastatic lesions.
The prevalence of colorectal neoplasia varies worldwide: 30 to 40 per 100,000 population in the United States; 15 to 30 per 100,000 in Europe; and less than 5 to 10 in South America and Asia. The prevalence varies with the population being studied.
Hereditary polyposis syndromes show that colorectal neoplasia has a genetic component (see Chapter 100 ). The progression of normal mucosa to neoplasia is associated with a loss of the APC gene in the cell, and the progression to carcinoma is associated with K- ras, DCC, and p 53 activity. The genetic component is complex; the interested reader is referred to the Additional Resources. It is estimated that up to 20% of neoplastic polyps result from genetic effects. Strong epidemiologic evidence shows that dietary factors play a major role in polyp formation through the microflora and intestinal microbiologic relationships. Diets with high levels of fat and red meat and low levels of fiber are associated with a higher incidence of neoplastic polyp formation. Australian patients who combined a low-fat diet with increased bran intake had a lower incidence of recurrent polyps; increased bran or low-fat diet alone did not result in a decrease. Not all studies are in agreement, but the very low incidence of neoplastic polyps in societies whose diets are high in fiber and low in saturated fats is incriminating for these nutrient factors. Dietary carcinogens and micronutrient deficiencies are also thought to play a role, but no proof of a cause-and-effect sequence is yet available.
Polyps usually develop in the rectum and the sigmoid and descending colon. Incidence seems to be greater on the right side of the colon. Most polyps can progress from an adenoma to a carcinoma, flat adenomas have a potential for malignancy, and the amount of severely dysplastic tissue in a polyp is related to its size. Flat polyps are of great interest to endoscopists because of their malignant potential and because they are more challenging to identify and remove completely. Polyps shown on histologic examination to contain villous elements are associated with a higher incidence of malignancy. Therefore, patients with flat adenomas and polyps with significant villous elements are at higher risk for malignancy.
Patients with hyperplastic or inflammatory polyps are also at risk for carcinoma. Serrated polyps, a combination of hyperplastic and adenomatous elements, are being found with increasing frequency and have malignant tendencies. Most submucosal polyps are benign. Many patients with carcinoids, metastatic lesions, melanomas, lymphomas, and Kaposi sarcoma have malignant polypoid formation in the colon. Except for its association with some malabsorption syndromes, lymphoid hyperplasia has no malignant significance.
Clinical Picture
Polypoid lesions are often accompanied by occult or gross bleeding. Depending on their position, they may cause intussusception or obstruction of the bowel; therefore, they rarely cause pain. Usually, polyps are detected during colonoscopic or barium enema screening for other symptoms. If the polyp is large, which is now unusual, the patient experiences a change in bowel habits and obstruction. Large lesions are rarely benign. Unlike benign polyps, malignant formation is life threatening.
Three findings with a polyp are important in risk for malignancy: size (>2 cm), histologic type (villous formation), and degree of dysplasia (severe).
Screening
The diagnosis of colon polyps can be made through sigmoidoscopy, colonoscopy, barium enema, or virtual colonoscopy ( Fig. 99.2 ). The diagnosis is made when symptoms indicate the need for a polyp search or a screening procedure to prevent colon cancer. Screening allows small lesions to be detected and removed before they can advance to carcinoma.
Screening procedures include fecal occult blood testing, digital rectal examination, sigmoidoscopy, colonoscopy, and virtual colonoscopy. All may detect a lesion, but colonoscopy results in the greatest yield and enables biopsy and possible removal and identification of the polyp. Therefore, colonoscopy is the screening procedure of choice for most gastroenterologists. Statistical evidence shows that a combination of fecal occult blood testing and sigmoidoscopy can be as effective as colonoscopy or double-contrast barium enema in preventing mortality from colon cancer, if cost is an issue and colonoscopy is unavailable. Endoscopy and stool screening procedures are described in Chapters 79 and 81 . However, there is now a 10-year study involving the Veterans Administration to evaluate effectiveness of colonoscopy as compared to fecal immunochemical tests in reducing the mortality from colon cancer. The study just began and will be finished in 10 years (see references). We hope that this study will answer the question which is better and how often it is done.
Treatment and Management
Once identified, a polypoid lesion should be removed. Most lesions smaller than 3 cm can be removed during endoscopy. Biopsy, snaring, elevating the polyp with water injection and removing it piecemeal, and cauterization are all used effectively to ablate polyps. It is hoped that the entire polyp has been removed; the removed tissue is evaluated histopathologically for possible carcinoma in situ. The histopathology is important to ensure that all neoplastic tissue and highly dysplastic or serrated lesions are removed with no possibility of spread. The histology and correlation with genetic and growth factors (e.g., microsatellite instability, hereditary nonpolyposis colorectal cancer [HNPCC]) are important (see “ Additional Resources ”). Complication rates of screening procedures are low.
After a polyp is removed, the question is how often colonoscopy should be repeated. If there is any question about total removal, or if the colonoscopy was difficult, a repeat procedure is performed within 1 year. However, if there is only a single polyp, the question is whether repeat colonoscopy should be performed in 3 to 5 years. Recent studies reveal that small tumors can grow rapidly. A physician or patient may want a repeat examination in 3 years, but if examination findings are excellent, screening can be delayed until 5 years, with fecal occult blood testing in the “off” years. The guidelines are clear, but variations occur depending on each patient's health and circumstances.
The patient identified with polyps should be treated for risk prevention and possible polyp chemoprevention. The patient should also be advised to maintain adequate weight, decrease the intake of foods high in polyunsaturated fat, and increase the intake of foods high in dietary fiber. An 81-mg dose of aspirin daily seems effective in decreasing the risk for cancer. Other chemopreventive agents, such as nonsteroidal antiinflammatory drugs (NSAIDs [e.g., sulindac, COX-2 inhibitors]), and increased calcium intake show some statistical effectiveness, but low-dose aspirin is the most widely accepted chemopreventive regimen.
Course and Prognosis
In the United States, vigorous screening programs in conjunction with colonoscopic removal of polyps and small lesions, combined with chemopreventive agents, can decrease the incidence of colon cancer and related deaths. In other countries, incidence and mortality appear to be stabilizing, but vary with environmental and economic conditions.
The chemopreventive agents include aspirin, celecoxib, and now statins. These drugs can be associated with cardiovascular complications and gastrointestinal bleeding, and their use and selection depend on the physician's recommendations for the individual patient.
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