The role of colonscopy and polypectomy in the prevention of colorectal cancer Free

CN14-03

Adenomas are the most common neoplastic outcome in all screening modalities. they are far more common than cancer. Removal of these adenomas has been shown to reduce the incidence and mortality of colorectal cancer. In the National Polyp Study, patients were enrolled after one or more adenomas were removed and randomized into a more aggressive and less aggressive followup surveillance. The cohort was pooled and the number of cancers detected were 5 as compared to more than 40 in 2 reference groups that were adenoma bearing and more than 20 in the SEER database, showing a reducion in incidence of between 76 and 90%\ in the 1418 patients followed for 8401 person years of followup. These results supported the long standing belief in the adenoma -carcinoma sequence and the practice of removing adenomas to prevent colorectal cancer. Subsequent studies confirmed this finding. An Italian study of post polypectomy patients showed an incidence reduction of 68% and a U.K sigmoidoscopy study also showed a reduction in incidence of rectal cancer following rectal polypectomy. A Norwegian randomized study demonstrated an 80% reduction in incidence in people having polyps detected on sigmoidoscopy and removed by colonoscopy. Guidelines have been reported that recommend appropriate followup intervals following polypectomy based on risk stratification at baseline. However , colonoscopy is not a perfect examination. Interval cancers have been reported that range from less than 1% to more than 5%. The reasons for these interval cancers include; technological limitations of the instrument , the manner in which the proceedure is performed, and biological factors such as cancers with rapid appearance. In one study, the proportion of MSI positivity was 30% in interval cancers as compared to 10% in non-interval cancers. The 'fast track' cancers seen in HNPCC are considered now to occur in sporadic cancers without this strong family history. Aspects of the technique that impact on the finding of interval cancers include not reaching the cecum, poor preparation and a fast withdrawal time. A recent paper demostrated that endoscopists who have a mean withdrawal time of less tha 6 minutes tend to find fewer adenomas. Technological aspects of the colonoscope are being improved to increase its accuracy including; greater magnification, NBI, wider angle of vision. 3rd eye retroscope and other modifications. The demonstration of prevention of colon cancer by colonoscopic polypectomy has been based on the use of mathematical analyses and modelling. This has shown not only a reduction in incidence of cancer , but also a reduction of mortality over a period of 20 years. These reductions appear to be the effect of the baseline colonoscopy with little additional effect of the followup surveillance until after 10 years following polypectomy. Unfortunately there have been no prospective randomized trials of screening colonoscopy. There are several screening tests that if positive lead to colonoscopy. However most of the tests are much more sensitive for cancer than for polyps. Direct screening colonoscopy is the test most likely to find adenomas and thus have the greatest impact on reducing colorectal cancer incidence and thus prevent the cancer entirely. Virtual colonoscopy has been shown to be equivalent to optical colonoscopy for the detection of polyps 6mm or larger. The miss rate for smaller polyps is substantial but the significance of this miss rate needs to be determined. Colonoscopy also has a miss rate for adenomas as well as for cancers, the former estimated as being 88-98%. The major problem today is the low screening rates for colorectal cancer, about 40-50% as compared to the higher rates for mammography which is in the range of 70-80%. It is not clear whether direct screening of the population with colonoscopy is feasible in this country. A two stage approach using other tests first is another option. Each country and community must decide on the best option based on resources, patien t population, and other priorities. However, it is clear that any test will reduce the risk of dying from colon cancer. Any test is better than none. The best test is the one that gets done.