Approximately, 65–75% of the population are considered to be low or average risk, i.e., there are no identifiable risk factors including, a lack of first-degree relatives with CRC or advanced adenomata (Table 10.2). Another 20–30% are at an increased risk of CRC, based on having one first-degree relative with an age of less than 60 years or two or more first-degree relatives of any age with CRC or advanced polyps, or a respective personal history; there are also a number of ethnicities who have been associated with an increased risk of CRC, including African-Americans and Ashkenazi Jews [4]. Additionally, 6–8% of the population are linked to a high-risk constellation for developing CRC based on the presence of genetic mutations/syndromes such as familial adenomatous polyposis (FAP) or its attenuated form (AFAP), Lynch syndrome (HNPCC), or MUTYH-associated polyposis (MAP) , or based on the presence of chronic inflammatory bowel disease (IBD) [12].

In the asymptomatic average-risk individual , it is recommended to start screening colonoscopy at age 50, and if negative to repeat it every 10 years (Table 10.3). Data from larger cohort studies suggest that the first colonoscopy was associated with the overall greatest benefit in risk reduction, and that an earlier start of general screening (e.g., at age 40) was of only limited value.

However, it has been postulated by some of the professional societies to start routine screening earlier in some subgroups and ethnicities that were associated with an overall increased risk or have shown no or an insufficient decrease of CRC incidence rates over the past decades. Among these groups are African-Americans who are recommended to start screening at the age of 45 years [4]. Moreover, if there is a positive family history involving, in particular, first-degree relatives, and no known genetic mutation is identifiable, the first screening colonoscopy should be recommended to start at age of 40 or 10–15 years before the age at diagnosis of the youngest family member with CRC or advanced adenoma (whichever comes first).

Recommendations for high-risk categories include not only a much earlier start, but due to the accelerated adenoma-carcinoma sequence, much shorter intervals, more frequent repeat exams, and potentially screening for extra-colonic pathology. The specifics are also outlined in Table 10.3 and depend on the nature of the risk (e.g., genetic syndrome versus IBD) [12]. Preferably, the gene carrier status in families with known genetic syndromes (FAP , Lynch, MAP) should be established by genetic testing rather than “screening” for the presence of polyps. Patients with established clinical or genetic diagnosis of FAP have traditionally been recommended to start screening at age of 10–12 years with annual flexible sigmoidoscopy. In reality however, there is no nonsurgical, pharmacological, or endoscopic intervention that could obviate the necessity for a prophylactic surgical resection (typically proctocolectomy ) which should be planned for an appropriate time between ages 16 and 25. Particularly with wide availability of genetic testing, it is therefore our recommendation that these patients wait with “screening” until they reach the age of 14, as those 2–4 additional years allow these young patient to mature and get an opportunity to understand and participate in the process of screening rather than being traumatized. The risk of this delay is negligible as a proctocolectomy is almost never needed before the age of 14. The purpose of flexible sigmoidoscopy or colonoscopy in FAP is less to prevent CRC but to get a relative growth profile and establish the right timing for the inevitable surgery.

In contrast, Lynch syndrome (HNPCC) has a more variable phenotype . Patients with a clinical or genetic confirmation of a carrier status are recommended to begin colonoscopy screening at age 20–25 years or 10 years before the youngest family member with CRC or advanced polyps and to subsequently continue every 1 or 2 years.

The efficacy of colonoscopy as a screening tool has been linked to a number of quality parameters that involve (a) the endoscopist, (b) the patient and the bowel preparation, and (c) potentially some technological aspects (see Table 10.4) [13, 14]. The clinically most relevant though unpractical parameter would be the detection rate of interval cancers. Hence, the most important surrogate parameter appears to be the overall adenoma detection rate. Other similar parameters such as polyp detection rate (which includes hyperplastic polyps), the overall cecal intubation rate with photo documentation, and the average withdrawal time (typically greater than 6 min) have been used as quality benchmarks even though strong supportive evidence is lacking. Unquestionably, visibility is highly dependent on the completeness of the bowel cleansing. An adequate bowel preparation is critical for the accuracy and cost-effectiveness of colorectal cancer screening while inadequate cleansing should trigger an earlier reexamination [15].

After a previous polypectomy or colon resection for CRC, the aim of repeat colonoscopies is to detect and remove adenomata that were potentially missed on the initial exam as well as metachronous new adenomata with advanced pathologic features [16]. Defining the exact length of recommended interval depends on the number of factors to not only include the previously mentioned overall risk categories but also the individual findings (Table 10.5). In particular, the number of detected and removed adenomatous or serrated polyps, the completeness of the previous removal, the size of lesions, and the presence or absences of unfavorable features (e.g., high-grade dysplasia) have to be taken into account. Furthermore, the time interval may need to be shortened depending on the quality of the previous examination, e.g., if it was complete or the bowel cleansing and visibility were inadequate.

If there were only a limited number of small adenomata (tubular adenoma ), a 5- to even 10-year interval is sufficient. A shorter interval of 3 years would be recommended if there were more advanced or multiple polyps (≥ 3), including sessile serrated adenomata proximal to sigmoid colon. In patients who were found to have numerous adenomata (including serrated adenoma), a malignant adenomatous polyp with high-grade dysplasia or focal adenocarcinoma (cancerous polyp), large sessile polyps including sessile serrated adenomata, incomplete removal of polyps, or whose colonoscopy was incomplete or otherwise unsatisfactory, an interval of a few months may have to be recommended (unless a surgical resection is carried out). No adjustment to the screening schedule of 10 years is needed if there were only hyperplastic polyps with typically distal distribution in the rectum and sigmoid colon. Patients with proximal serrated adenomata/polyps or with hyperplastic polyposis syndrome are exceptions from that.