Screening for colorectal cancer
Evidence to suggest that sigmoidoscopy may be effective for colorectal cancer screening, with benefits lasting for up to ten years, has come from two case-controlled studies (Selby et al., 1992; Newcomb et al., 1992). As such studies cannot eliminate the effect of selection bias, however, this benefit may have been overestimated. Trials are now under way to evaluate flexible sigmoidoscopy and colonoscopy for screening.
Several trials have evaluated the effect of the faecal occult blood test (FOBT). A trial in Minnesota, United States of America, used the FOBT annually in one group and biennially in another. This initially indicated that annual, but not biennial, FOBTs reduce mortality from colorectal cancer after about a ten year period (Mandel et al., 1993). A more recent report, with follow-up for up to 18 years, showed mortality reduction at a lower level from biennial screening (Mandel et al., 1999). Trials in Europe also showed mortality reduction from biennial screening (Hardcastle et al., 1996; Kronborg et al., 1996).
It is clear that a major difficulty with screening using the FOBT is lack of specificity, especially if the test is rehydrated, which substantially increases the costs of programmes. Further, there seems to be a lack in sensitivity for detecting adenomas.
Taken together, the FOBT trials suggest that, after an interval of about 10 years, there could be a reduction of up to 20% in colorectal cancer mortality from biennial screening, and a greater reduction as a result of annual screening. Unless high compliance with the test can be achieved, however, the benefit that could be obtained in the general population would be much less, and not commensurate with the expense of the screening programme.