Table 1. Estimated Relative and Absolute Risk of Developing Colorectal Cancer (CRC) continued...
In the context of high-risk syndromes such as LS or FAP, surveillance implies examining patients in whom adenoma or cancer occurrence is highly probable, and the examination is done for early detection. It is not screening in the traditional sense. The meaning of the terms screening versus surveillance has evolved over time and their usage in this summary may not be consistent with other oncologic and epidemiologic contexts.
Primary prevention (eliminating the causes of CRC in people with genetically increased risk) is addressed later in this section.
State of the evidence base
Currently, there are no published randomized controlled trials of surveillance in people with a genetically increased risk of CRC and few controlled comparisons. While a randomized trial with a no-surveillance arm is not feasible, there is a need for well-designed studies comparing various surveillance methods or differing periods of time between procedures. An observational study that compared surveilled subjects with unsurveilled (by choice) controls evaluated a 15-year experience with 252 relatives at risk of LS, 119 of whom declined surveillance. Eight of 133 (6%) in the surveilled group developed CRC, compared with 19 in the unsurveilled group (16%, P = .014). In general, however, people with genetic risk have been excluded from the trials of CRC screening that have been published thus far, so it is not possible to estimate effectiveness by subgroup analyses. Therefore, prevention in these patients cannot be based on strong evidence of effectiveness, as is ordinarily relied on by expert groups when suggesting screening or surveillance guidelines.
Given these considerations, clinical decisions are based on clinical judgment. These decisions take into account the biologic and clinical behavior of each kind of genetic condition, and possible parallels with patients at average risk, for whom screening is known to be effective.
The evidence base for the effectiveness of screening in average-risk people (those without apparent genetic risk) is the benchmark for considering an approach to people at increased risk. (Refer to the PDQ summary on Screening for Colorectal Cancer for more information.)
The fact that screening of average-risk persons reduces the risk of dying from CRC forms the basis for recommending surveillance in persons at a higher genetic risk of CRC. As logical as this approach seems, it is important to note that randomized trials of surveillance have not been performed in this special population, though observational studies performed on families with LS [68,69] and FAP  support the value of surveillance. These studies demonstrate a shift towards earlier stage at diagnosis and a corresponding reduction in CRC mortality among colonoscopy-detected cancers.
(Refer to the Major Genetic Syndromes section of this summary for more information about surveillance in high-risk populations.)
Rationale for screening
Widely accepted criteria (1–3 below) for appropriate screening apply as much to diseases with a strong genetic component (more than one affected first-degree relative or one first-degree relative diagnosed at younger than 60 years) as they do to other diseases.[71,72] Additional criteria (4 and 5) were added below.
- A high burden of suffering, in terms of morbidity, mortality, and loss of function.
- A screening test that is sufficiently sensitive, specific, safe, convenient, and inexpensive.
- Evidence that treating the condition when it is detected early, by screening, results in a better prognosis than treatment after it is detected because of symptoms.
- Evidence on the extent to which the screening test and treatment do harm.
- The value judgment that the screening test does more good than harm.