Diagnosis and Screening for Colorectal Cancer

Colorectal cancer is common and readily treated in many cases, making it an ideal cancer to try to prevent using screening strategies. Because almost all colorectal cancers arise from adenomatous polyps, one might conclude that the vast majority of deaths from CRC would be preventable if screening tests were used to their maximum benefit. Yet, in general, less than 30% of persons for whom screening is recommended have been screened. In 1997, a large population-based study of 52,754 persons age 50 or older assessed whether people had had fecal occult blood testing (FOBT) and/or sigmoidoscopy within the recommended 1 or 5 year intervals, respectively. Only 19.8% said they had FOBT within the prior year and 30.4% had sigmoidoscopy. In that light, it is incumbent on the medical profession and those concerned with the public's health to better identify the barriers to screening so that its maximum benefit can be achieved.

There are a variety of methods available for the diagnosis of CRC, each having its own associated controversy. In addition, there are several currently accepted screening guidelines being employed for the purpose of detecting CRC in otherwise asymptomatic patients with no additional risk factors. Although similarities exist, new information regarding the utility of endoscopic and radiographic techniques may change significantly those established recommendations.

Beginning at age 50, the U. S. Preventive Services Task Force recommends annual FOBT and flexible sigmoidoscopy (periodicity unspecified).

The American Cancer Society recommends one of five screening options:

• Yearly FOBT
• Flexible sigmoidoscopy every 5 years
• Yearly FOBT plus flexible sigmoidoscopy every 5 years
• Double contrast barium enema every 5 years
• Colonoscopy every 10 years

Digital Rectal Examination

Digital rectal examination has traditionally aided in the diagnosis of many rectal cancers. Today, however, the proximal shift in the distribution of CRCs makes a greater number of lesions undetectable with digital rectal examination alone. As digital rectal examination at best can reach only the most distal 5 to 7 cm of the rectum, its diagnostic virtue lies only in its simplicity and low cost but in no way should be considered a substitute for a more thorough examination.

Fecal Occult Blood Testing

Fecal occult blood testing continues to be the subject of significant study. Its allure remains its relatively low cost, the potential for mass screening, and most importantly the detection of asymptomatic cancers. Most currently available guaiac tests rely on a peroxidase-like reaction to the heme component of blood. Despite the flaws associated with FOBT, including failure to detect 75% of adenomas and 30% to 50% of colorectal carcinomas in association with an overall false-negative rate of 40%, most reports on their use emphasize that a relatively high percentage of the cancers detected are early-stage lesions. The 33% reduction in mortality observed in individuals who underwent annual FOBT screening in the Minnesota Colon Cancer Control Study strongly supports FOBT as a useful diagnostic approach. Dietary precautions prior to collection of the FOBT can decrease the likelihood of false positives. Patients should be instructed to avoid aspirin and nonsteroidal antiinflammatory drugs (NSAIDs) for 1 week prior to collection of specimens, and red meat and vitamin C for 3 days prior. Current recommendations are for obtaining a total of six samples (two from each of three specimens).