Colon cancer is the second leading cause of cancer deaths in the United States, although effective screening methods have been available for decades. Walsh and Terdiman searched the literature to review the status of colon cancer screening.
The authors consulted with experts in colon cancer screening and reviewed meta-analyses, systematic reviews, and proceedings of national professional organization meetings. Randomized controlled trials (RCTs) and controlled trials were available for fecal occult blood testing (FOBT), sigmoidoscopy, and FOBT plus sigmoidoscopy; observational and diagnostic accuracy studies were available for colonoscopy.
Colon cancer can be prevented by the detection and removal of colorectal adenomas. Screening with FOBT reduces colon cancer mortality risk by 15 to 33 percent, with absolute risk reduction ranging from 0.8 to 4.6 per 1,000 persons. Three stool samples should be provided; current recommendations do not support rehydration. Although the positive predictive value of the test increases with the number of positive samples, even one positive sample is an indication for colonoscopy.
Flexible sigmoidoscopy is associated with a reduction of up to 70 percent in mortality from cancers within reach of the sigmoidoscope. Colonoscopy is recommended if an adenomatous polyp larger than 1 cm or advanced histologic findings are detected on sigmoidoscopy, although there is controversy over whether to investigate the whole colon if one or two small, tubular adenomas are found. Sigmoidoscopy should be repeated every five years.
Colonoscopy is considered highly sensitive and specific for detection of advanced polyps and cancers, but lesions can be missed. The procedure involves preparation on the part of the patient, as well as sedation, with a two- to three-hour recovery. Colonoscopy appears to reduce mortality, but this benefit has not been confirmed by RCTs. Indirect evidence suggests that if no adenomas are detected, the protective effect of colonoscopy lasts about 10 years.
Combined sigmoidoscopy and FOBT may reduce mortality more than either method alone, but the results are inconsistent. A recent large study found that more than 600 patients would need to be screened with FOBT to detect one advanced adenoma or cancer that otherwise would have been missed.
Double-contrast barium enema detects a majority of advanced adenomas and cancers. The examination is not as sensitive as colonoscopy, but that deficit may not be clinically important. A positive finding requires follow-up colonoscopic examination.
Two new screening methods are on the horizon: (1) virtual colonoscopy uses computed tomographic or magnetic resonance imaging to generate images, and (2) stoolbased molecular screening detects DNA mutations indicative of colorectal neoplasia. Cost, sensitivity, specificity, and acceptability to patients should be compared with current screening methods.
The authors conclude that multiple screening modalities for detecting potential colon cancers and reducing mortality from this disease are available, and that current evidence does not support the use of one method over another.