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

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20 Screening for Colorectal Cancer KCE reports vol.45 1.4 SCREENING VERSUS SURVEILLANCE The New Zealand Guidelines Group 27 defines screening and surveillance as follows: 1. Screening is the examination of asymptomatic individuals in order to classify them as unlikely or likely to have a disease. A national screening program is an example of a population preventive strategy, where everyone in a particular age-group is invited to participate. Such strategy has the potential to identify a high proportion of individuals with early disease in a given population. This proportion depends on the uptake of screening and the sensitivity of the test chosen. Even in cancer screening programs where uptake is high and the screening test is very sensitive, the vast majority of individuals who take part will not have cancer, so that the potential benefits of screening are available to a relatively small group. 2. Surveillance, as opposed to screening, refers to monitoring individuals known to have a disease or to be at increased risk for a disease. For this population the potential benefit of surveillance is higher than that of screening in the population at large, because the prevalence of the disease is higher in this population. Thus, the benefit-to-risk ratio of surveillance is more favourable than the benefit-to-risk ratio of screening. Therefore, for example, individuals who believe themselves to be at increased risk of developing colorectal cancer (CRC) may be more willing to accept the risks associated with surveillance. Others 28, 29 more restrictively define surveillance as monitoring of patients known to have a specific disease or a genetic predisposition to it and screening as the examination of individuals not (yet) known to have it, irrespective of the risk (average or increased). In this definition screening can either refer to a population based screening program (average risk screening, population or mass screening), or to targeted screening for individuals with established risk factors or personal concerns about it (also called sensitive screening). Both methods imply different objectives and thus different requirements: if the aim is to reduce the burden of disease on a community the approach needed is population screening; this requires the use of a test associated with a high uptake and low cost. If, on the other hand, the aim is to respond to an individual s request for information regarding his disease status, the emphasis must be on a test of high sensitivity and specificity 30. 1.5 IMPLICATIONS FOR COLORECTAL CANCER SCREENING In dealing with guidelines for CRC screening, one must recognize that in many countries sensitive case finding on an individual basis forms the foundation of so-called screening 31. Implementation of CRC mass screening programs is indeed tedious, expensive and requires several barriers to be overcome. It might therefore seem logical in some countries to concentrate public health resources on a selected sub-population, in which screening appears to be most meaningful and probably more cost-effective. Returning to definitions, we have to recognize that the demarcation line between surveillance and targeted screening remains fuzzy: should we consider a proven genetic predisposition as a pre-clinical phase of the disease and thus needing surveillance (follow-up)? And what should be done for patients with a
KCE reports vol.45 Screening for Colorectal Cancer 21 high risk category family history and with an inconclusive genetic predisposition test? Should we label them for targeted screening or for surveillance? However, from the clinical point of view, such questions remain chiefly academic. Based on the above considerations we framed a general classification scheme for CRC screening and surveillance (Table 1). Table 1: Classification of CRC screening & surveillance Classification (Sub)population involved CRC risk category Mass / population Asymptomatic or unreported symptoms Average risk screening Personal and family history negative (Low risk) Targeted / sensitive Positive or suspected family history Possible increased or screening Strong personal concerns high risk Surveillance Positive personal history = presence of related disease or proven genetic predisposition Increased risk or high risk (hereditary) Key messages In population screening programs the target disease should be a common problem that has a better outcome when treated at an earlier stage. The test used should be acceptable and sufficiently sensitive, specific, and inexpensive as to be cost-effective. To prove that screening really is beneficial, 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 (intention-to-screen principle) and those who develop cancers that are not detected by screening. Typically, lead-time bias, length-time bias and selection bias can skew the results of screening trials. The use of surrogate outcome measures rather than hard outcome measures always bears a risk of biased conclusions. Overdiagnosis can cause stress and discomfort through unnecessary diagnostic procedures or even unnecessary treatment and its complications. It can also make a study look well performing at picking up abnormalities, even though they might be harmless. Potential harms of screening are mainly unnecessary investigation and treatment of false positive results and a false sense of security caused by false negative results, which may even delay final diagnosis. Mass or population screening programs imply different objectives and thus different requirements compared to targeted screening. For mass screening programs to be successful, utmost care should be devoted to a proper organisation and a suitable test in order to maximise participation rates and minimise potential harms. In many countries sensitive case finding on an individual basis historically forms the foundation of colorectal cancer screening.
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KCE reports 1. Effectiviteit en kos
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