Robert H. Fletcher
Screening for colorectal cancer
Harvard Medical School, Boston, MA, USA
C olorectal cancer is the second leading cause of cancer death in the U .S . and com m on throughout the w orld. T he bio-logic behavior of this cancer suggests that it can be prevented: m ost cancer arise from adenom as, w hich progress over m any 'years from sm all «5m m ) lesions to cancer and can be identi-fied during a precancerous phase. N ow there is strong evi-dence that screening prevents incidence and m ortality form colorectal cancer but disagreem ent about the best strategy.
F our screening m ethods are available. T hey differ is the strengthy: of evidence supporting them , cost and acceptabil-ity. I) T he strongest evidence is for testing for fecal occult blood (F O B T ): a guaiac-based test on2 specim ens from 3 consecutive stools yearly, w ith colonoscopy for patients w ith positive tests. A random ized controlled trial of this strategy show ed a reduct on in m ortality of one third and others now in progress are show ing com parable results. T his m ethod aim s m ainly at early detection of cancer, not polyps, because pol-yps infrequently bleed. 2) S igm oidoscopy has been show n to reduce m ortality from colorectal cancer by tw o thirds in the segm ent of the colon exam ined but the evidence, from tw o case control studies, is not as strong as w ith random ized trials and only about half of the colon is exam ined. 3) F inding rom oving polyps, by colonoscopy, seem s to prevent colorectal cancers from arising. W inaw er and colleagues have show n that patients w ho have had their colon cleared of polyps and rem ain under surveillance w ith colonoscopy every 1-3 years have an incidence of cancer of only about 20 percent of sim i-lar patients, in other cohO lts. 4) F inally, double contrast barium enem as detect large (high-risk) polyps and early cancers about as w ell as colonoscopy and are less expensive and m ore ac-ceptable. H ow ever, there is no direct evidence that screening
w ith barium enem as prevents cancer incidence or m ortality and evidence that barium enem a and colonoscopy have com -parable sensitivity and specificity is not strong.
S creening usually begins at age 50 years: about 90% of cancers occur after that age. P atients are reluctant to subm it stools or undergo endoscopies but their com pliance can be im proved. T he m onetary cost of screening for colorectal can-cer appears com parable to screening for other com m on can-cers such as breast. T he false positive rate is high; as m any as 40 positive F O B T tests m ust be w orked up for every cancer found.
S om e patients are know n, from history and phisical exa-m ination, to be at increased risk, for their age, for colorectal cancer. T he m ain risk factors are specific syndrom es (fam i-lial adenom atous polyposis, hereditary non-polyposis colon cancer, inflam m atory bow el disease) and a fam ily history of colon cancer, especially first degree relatives w ith onset be-fore age 45 years. T here is consensus that patients w ith these conditions should be screened differently (earlier onset oftest-ing, at shorter intervals and w ith m ore pow erful testing such as colonoscopy rather than F O B T ) but no strong evidence favoring any of these strategies. G enes for the inherited syn-drom es have been identified and can be used to predict risk w ithin affected kindreds but are not presently useful for gen-eral screening.
M ost current recom m endations include yearly F O B T w ith or w ithout sigm oidoscopy every 5 years, both after age 50 years, w ith colonoscopy for patients w ith positive tests. H ow ever, new form s of testing and evidence of effectiveness for existing m ethods are accum ulating rapidly and these rec-om m endations are likely to change.
