Report in English with a Dutch summary (KCE reports 45A)

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18 Screening for Colorectal Cancer KCE reports vol.45 who are not. If people with a higher risk of a disease are more eager to be screened, for instance women with a family history of breast cancer joining a mammography program, then a screening test could cause the disease related mortality to look worse than in non-participants. This is because more people with the disease will participate, resulting in a higher incidence, in the screened population, of people dying from it. Selection bias may also make a test look better than it really is. If a test is more available to young and healthy people (for instance if people have to travel a long distance to get checked) then less people in the target population would be detected with disease, and the test will seem to make a positive difference. 4. Volunteer bias is a special kind of selection bias, created by the fact that screening invitations tend to be more readily accepted by health-conscious individuals who are likely to have a better outcome for reasons other than early detection of the disease. In other words, individuals with a strong interest in health issues are more likely to participate, whereas the risk of these individuals may be lower, due to a more healthy lifestyle. 5. Overdiagnosis bias: screening may identify abnormalities that would never cause a problem in a person's lifetime. An example of this is prostate cancer screening 8, 14. It has been said that "more men die with prostate cancer than from it". Autopsy studies have shown that a high proportion of men who have died in other ways, have prostate cancer when the prostate is examined under a microscope. Aside from issues arising from unnecessary treatment (prostate cancer treatment is by no means without risk), overdiagnosis makes a study look good at picking up abnormalities, even though they are sometimes harmless. 6. Observation bias or observer bias is error introduced into measurement when observers overemphasize what they expect to find and fail to notice what they do not expect. This is why medical trials are normally double-blind rather than single-blind. The overall effect of these biases tends to exaggerate the beneficial effect of screening. To prove that screening is producing a real benefit, it is essential to carry out population-based randomized controlled trials in which the group randomized to screening is analyzed as a whole and includes those who refuse the invitation to be screened and those who develop cancers that are not detected by screening (intention-to-screen principle). 1.3.2 End points in RCTs on cancer screening The most widely accepted end point in randomized cancer screening trials is disease-specific mortality. Only if the disease-specific mortality in the whole of this group is significantly lower than in the randomly selected group, that is not offered screening, can we be sure that the screening process is producing a truly beneficial effect on disease outcome. The validity of this end point, however, rests on the assumption that cause of death can be determined accurately. An alternative end point is all-cause mortality, which depends only on the accurate ascertainment of deaths 15. Major inconsistencies were identified in disease-specific and all-cause mortality endpoints in randomized cancer screening trials 15. Because all-cause mortality is
KCE reports vol.45 Screening for Colorectal Cancer 19 not affected by bias in classifying the cause of death, it should be examined when interpreting the results of randomized cancer-screening trials. The use of surrogate outcome measures in screening trials always bears a risk of biasing conclusions. So, the real question to be answered before spending considerable public health resources on the implementation of a mass screening program, is whether one should consider overall mortality 16-18 and quality of survival 19 as the only hard outcome measure of interest, especially in case of unsatisfactory screening uptake. 1.3.3 Adverse effects Although screening may lead to an earlier diagnosis, not all screening tests have been shown to benefit the person being screened 20-24. Like any medical test, the tests used in screening are not perfect. The test may miss people who have the disease (false negative) or may appear positive for those without disease (false positive). Besides misdiagnosis and overdiagnosis, other potential adverse effects of screening are: 1. Stress and anxiety caused by a false positive screening result 25. 2. Unnecessary further investigation and treatment of false positive results. 3. Prolonging knowledge of an illness if nothing can be done about it. 4. A false sense of security caused by false negative results, which may even delay final diagnosis. 5. Overuse/waste of medical resources. 6. Unnecessary and uncomfortable procedures looking for a disease that is unlikely. 1.3.4 Target population and appropriateness of screening It is often assumed that screening applicants are asymptomatic but this is not necessarily so: a screening offer may be more readily accepted by a patient with unreported symptoms. Indeed, a study 26, published in 2005, on 563 consecutive individuals with a positive fecal occult blood test (FOBT) in the Scottish arm of the national colorectal cancer screening pilot has shown that 439 (78,0 %) had one or more lower gastrointestinal symptoms and 124 (22,0 %) were symptom free. Taking adenoma and carcinoma together, 322 (57,2 %) of the subjects were found to have colorectal neoplasia, and 128 (22,7 %) had a completely normal colon, the remaining 113 having minor colorectal lesions. Rectal bleeding was the most common symptom, followed by recent change in bowel habits, abdominal pain, tenesmus, rectal pain besides unexplained weight loss or anaemia. This undoubtly raises the question whether the FOBT was being ordered appropriately, but this issue will be dealt with in the chapter on clinical effectiveness and potential harms of CRC screening.
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KCE reports 1. Effectiviteit en kos
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