Cancer guidelines recommend screening for African Americans beginning at age 45. The potential for harm from screening must be weighed against the likelihood of benefit, especially in elderly patients with comorbid illnesses and shorter life expectancy. Although routine screening is not recommended in adults above age 75, it may be considered on a case-by-case basis in adults age 76 through 85 years who have excellent health and functional status. Patients with first-degree relatives with colorectal neoplasms (cancer or adenomatous polyps) are at increased risk; earlier and more frequent screening is recommended (preferably with colonoscopy) for these individuals. Recommendations for screening in families with inherited cancer syndromes or inflammatory bowel disease are provided in Chapter 15. For patients at average risk for colorectal cancer, the recommendations of the Task Force are discussed below. Screening tests may be classified into two broad categories: stool-based tests and exams that visualize the structure of the colon by direct endoscopic inspection or radiographic imaging.
A. Stool-based Tests 1. Fecal occult blood test—Most colorectal cancers and some large adenomas result in increased chronic blood loss. A variety of tests for fecal occult blood are commercially available that have varying sensitivities and specificities for colorectal neoplasia. These include guaiac-based tests (Hemoccult I and II and Hemeoccult SENSA) that detect the pseudoperoxidase activity of heme or hemoglobin and fecal immunochemical tests (FITs) that detect human globin. Guaiac-based tests have undergone the most extensive testing and currently have the greatest clinical use. For optimal detection, one specimen card must be prepared from three consecutive bowel movements. To reduce the likelihood of false-positive tests, patients should abstain from aspirin (in doses > 325 mg/d), NSAIDs, red meat, poultry, fish, and vegetables with peroxide activity (turnips, horseradish) for 72 hours. Vitamin C may cause a false-negative test. Slides should be developed within 7 days after preparation. The reported sensitivities of a single guaiac-based test for detection of colorectal cancer vary widely, but are lower for Hemeoccult II (35%) than Hemeoccult SENSA (65%). When fecal occult blood test is administered to the general population as part of a screening program, 2–6% of tests are positive. Of those with positive tests, 5–18% have colorectal cancer that is more likely to be at an earlier stage (stage I or II). Several FITs are commercially available. These tests are highly specific for detecting human globin and therefore eliminate the need for pretest dietary restrictions. As with guaiac-based tests, sampling of three consecutive bowel movements is recommended. In clinical trials that compare FIT with guaiac-based tests, FIT had at least comparable sensitivity to Hemeoccult SENSA for detection of cancers (60–85%) with higher specificity. Because FITs are not affected by diet or medications and have superior accuracy, they are increasingly being substituted for guaiac-based tests despite a higher cost per test ($10–20).
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The US Multi-Society Task Force emphasizes that colon cancer prevention should be the primary goal of screening. For that reason, the lower sensitivity of fecal occult blood tests for advanced neoplasia (cancer and advanced adenomatous polyps) makes them a less attractive choice for population-based screening than endoscopic or radiographic tests. Currently, fecal occult blood tests are most suitable in settings where health care resources are limited or in patients who desire a noninvasive method of screening. The Task Force recommends that tests with higher sensitivity for colorectal cancer (Hemeoccult SENSA or FIT) be used rather than less sensitive tests (Hemeoccult II). Regardless of which stool-based test is used, patients should understand that annual testing is required to achieve the maximum screening benefit and that a positive test will require evaluation by colonoscopy accompanied by removal of any polyps identified. If colonoscopy reveals no colorectal neoplasm, further screening for colorectal cancer can be deferred for 10 years. In a meta-analysis of four large, prospective, longitudinal studies, annual or biennial screening with Hemeoccult or Hemeoccult II reduced mortality from colorectal cancer by 25% among those who were compliant with regular testing. Long-term studies with more sensitive stool tests (Heme SENSA or FIT) have not been performed. 2. Multitarget DNA assay—A fecal DNA assay (Pre-Gen Plus) is available for screening for colorectal neoplasia. The test analyzes fecal DNA for 22 gene mutations and DNA integrity. The commercially available first-generation fecal DNA panel detected only one-half of cancers; however, a second-generation assay detected almost 90%. Although the Multi-Society Task Force concluded that fecal DNA is an acceptable option for colorectal cancer screening, this test is not yet practical for populationbased screening due to its high cost and cumbersome requirements for stool collection and mailing.
B. Endoscopic Examinations of the Colon 1. Flexible sigmoidoscopy—Use of a 60-cm flexible sigmoidoscope permits visualization of the rectosigmoid and descending colon. It requires no sedation and in many centers is performed by a nurse specialist or physician’s assistant. Adenomatous polyps are identified in 10–20% and colorectal cancers in 1% of patients. Case-control studies suggest that screening sigmoidoscopy programs lead to a 60% reduction in colorectal cancer mortality. A 2010 randomized controlled trial in 170,000 participants in the United Kingdom comparing a one-time flexible sigmoidoscopy screening to usual care confirmed a 50% reduction in distal colorectal cancer incidence and 43% reduction in mortality after a median follow-up of 11 years. The risk of serious complications (perforation) associated with flexible sigmoidoscopy is < 1:10,000 patients. The chief disadvantage of screening with flexible sigmoidoscopy is that is does not examine the proximal colon. The prevalence of proximal versus distal neoplasia is higher in people over age 65 years of age, African Americans, and women. In men, approximately 50%