Screening for Colon Cancer
Screening of symptomatic patients for colon cancer includes routine digital rectal examination and stool guaiac testing (Hemoccult), proctosigmoidoscopy and/or pancolonoscopy, and barium enema with air-contrast studies. The individual choice of test and order of test priority depend on the precise symptoms of the patient.
Rules for screening asymptomatic patients for colon cancer are empiric or dependent on data derived from statistical modeling. Major studies that attempted to determine the benefit of screening large population groups for occult blood in the stool have led to mixed results, although most studies show a reduction in colorectal cancer mortality rates.More than likely, the increased accuracy of modifications of the Hemoccult or Hemoquant tests, the new designs for testing occult blood in the stool that make application of the test more palatable to the general population, and, perhaps most important, increased communication of the algorithms for correctly interpreting the tests and correctly studying patients who have truly positive tests will all increase the cost-effectiveness of mass screening approaches in the future. Nevertheless, at the present, screening is effective only among high-risk populations (∼25% of all colorectal cancers) and is generally thought to include mandatory testing of stool for occult blood at the time of routine history taking and physical examination.
Data from the Strang Clinic study by Winawer and coworkers have shown what most people would have predicted. Patients whose colorectal tumors (polyps as well as carcinomas) are diagnosed in the asymptomatic state have more superficial lesions than do patients who present with signs or symptoms that lead to the diagnosis of colorectal cancer. The colorectal cancer mortality rate was lower in patients who underwent screening with both fecal occult blood tests and rigid sigmoidoscopy than in those who were screened with annual sigmoidoscopy alone. Although lead time bias is an issue, most thoughtful persons, given the choice, would rather have colon or rectal cancer diagnosed earlier!
The American Cancer Society now recommends that “average”-risk individuals undergo annual fecal occult blood testing and either flexible sigmoidoscopy or total colon examination for detect colon cancer (colonoscopy or air-contrast barium enema studies) every 5 years beginning at age 50.Which endoscope should be used (colonoscope, 60-cm flexible colonoscope, flexible sigmoidoscope, rigid proctosigmoidoscope), whether double-contrast enema or endoscopy is more cost effective, and precisely when screening should be applied to patients who are asymptomatic but are related to individuals with sporadic nonfamilial colon or rectal cancer would ideally be defined in clinical trials. These trials will never be done because they are extraordinarily difficult to design and prohibitively expensive. Therefore, we will likely rely on statistical modeling and commonsense empiricism for future screening rules. Asymptomatic individuals in the standard-risk group who ask for screening recommendations should be told that 60-cm flexible sigmoidoscopy should be initiated between the ages of 40 and 50 years. At the present, carcinoembryonic antigen (CEA), as well as other more experimental tumor markers, is ineffective for colon cancer screening purposes, because no marker, including CEA, is tumor specific (large numbers of false-positive and false-negative results).
