Colorectal Cancer ____________________________________________________________________________
Flexible Sigmoidoscopy Flexible sigmoidoscopy involves anal insertion of a sigmoido- scope (similar to the colonoscope) to visualize the rectum and sigmoid colon—the lower one-third of the colon. The scope inflates the large bowel with air to improve imaging, and polyp removal or biopsy may be performed during the proce- dure [159]. A 60-cm flexible sigmoidoscope was introduced decades ago that is more tolerable to patients than the older, rigid sigmoidoscope. It allows a more complete distal colon examination and can discover up to 65% of polyps, compared with 25% using the older instrument [160]. Potential Complications and Harms Sigmoidoscopy can be an uncomfortable or painful procedure. Women may have more pain during the procedure, which may discourage them from returning for future screening sigmoidos- copies. Sigmoidoscopy can also cause perforation of the colon, bleeding, severe abdominal pain, and death, although this is rare [85; 159]. Bleeding and perforation are the most common complications. Most cases of bleeding occur in patients who have polyps removed [159]. Double-Contrast Barium Enema Double-contrast barium enema (DCBE) consists of the patient receiving an enema with a barium solution. Air is then pumped into the colon, and a series of x-rays are performed to image the entire colon and rectum [161]. Potential Complications and Harms DCBE is no longer recommended as an alternative test for colorectal cancer screening, and its use has declined dramati- cally. DCBE effectiveness for polyp detection is substantially lower than that of colonoscopy and CT colonography [116]. Fecal Occult Blood Tests In FOBT testing, the patient collects stool samples that are analyzed for presence of blood. Different FOBT tests involve different collection approaches but commonly require collec- tion of consecutive stool specimens for up to three days. The first FOBTs to enter clinical use were guaiac-based (gFOBT); more recent versions employ immunochemical tests (iFOBT) or markers of DNA mutation (stool DNA tests or sDNA) [1]. Colorectal lesions and adenomatous polyps tend to bleed, and the resulting presence of hemoglobin in stool that is detectable even with intermittent or minimal bleeding formed the basis for gFOBT use in colorectal cancer screening. Hemoglobin is used as a biomarker for detecting blood in stool with guaiac, which identifies peroxidase-like activity that characterizes hemoglobin. However, gFOBT cannot discriminate human from nonhuman or intact from partially digested hemoglo- bin and is being phased out of clinical use. This results in detection of blood from ingested meat and upper airway and gastrointestinal bleeding as well as colorectal lesions. The low specificity of gFOBT requires confirmatory colonoscopy to validate positive findings [162].
iFOBT was developed to detect intact human hemoglobin originating from colorectal tissue. Unlike gFOBT, it does not detect hemoglobin from nonhuman dietary sources or partly digested human hemoglobin originating from the upper respiratory or gastrointestinal tract [163]. The sDNA variation of FOBT incorporates markers of DNA mutation that detect molecular genetic changes associated with colorectal cancer
gene mutations shed into the stool [164]. Potential Complications and Harms
The very low sensitivity of gFOBT leads to a high proportion of false-positive results when confirmed by colonoscopy or DCBE plus flexible sigmoidoscopy, which a systematic review of published clinical trials estimated at greater than 80% [165]. iFOBT is increasingly recognized as superior to gFOBT for sensitivity, accuracy, and compliance, and it shows greater ability in detecting advanced neoplasia. While iFOBT requires colonoscopy confirmation of positive results and cannot detect many precancerous polyps, higher participation in iFOBT than in colonoscopy screening may offset some of its comparative limitations [121]. DNA fecal testing is emerging as a potentially important addi- tion to the stool-based tests for colorectal cancer screening. More research is needed to understand the role of sDNA test- ing in organized colorectal cancer screening and unaddressed factors, such as screening interval, patient adherence, and costs [121]. PRACTICE GUIDELINE RECOMMENDATIONS FOR COLORECTAL CANCER SCREENING American College of Physicians The American College of Physicians (ACP) published their practice recommendations for colorectal cancer screening based on the review and synthesis of guidelines for screen- ing colorectal cancer produced by several other professional organizations. Several tests to detect adenomatous polyps and cancer were evaluated for colorectal cancer screening efficacy, including flexible sigmoidoscopy, colonoscopy, DCBE, and CT colonography. Tests to primarily detect cancer (e.g., gFOBT, iFOBT, and sDNA) were also assessed [85]. Screening Initiation The ACP recommends that individualized assessment of colorectal cancer risk should be performed in all adults [85]. Screening of asymptomatic, average-risk patients should begin at 50 years of age with a stool-based test, flexible sigmoidoscopy, or optical colonoscopy [85]. The ACS supports a qualified recommendation for colorectal cancer screening in average-risk adults 45 to 49 years of age. This recommendation is based on the increasing incidence of colorectal cancer in this age group, the availability of accurate screening tests, and modeling results from other organizations [85]. . Screening is not recommended in adults older than 75 years of age or with a life expectancy of less than 10 years [85].
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