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

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