Chapter 18 COLORECTAL CANCER SCREENING
BACKGROUND
The third millennium is witnessing the emergence of preventive medicine as a cornerstone in our concept of health. Colorectal cancer (CRC) is a major health concern (Chapter 17), and one that fits the criteria of a preventable disease. Approximately two-thirds of the cases involve average-risk men and women, with a sharp increase of its incidence starting from the fifth decade of life.
Detection and removal of colonic adenomas, the non-invasive neoplasm that are the precursors of CRC, could prevent up to 90% of CRC cases. The transition process between adenomas to carcinoma is estimated as 5–10 years and that of normal mucosa thorough adenoma to adenocarcinoma as 15–20 years. This interval provides a unique window of opportunity for screening and effective interventions to reduce CRC-associated mortality. The respective curative benefits that can be gained by detecting CRC at early stages, e.g. node-negative stages I and II, are as high as 90% and 75%, respectively, with surgery alone, while the 5-year survival rate is close to zero for metastatic CRC.
FAECAL OCCULT BLOOD TESTING
Support for the ability of annual FOBT to reduce the mortality from CRC emerged from several large prospective randomised trials. Mortality was reduced by 15%–33% if the test was done yearly, and when positive results were followed by colonoscopy. A meta-analysis that pooled the results of these studies estimated a 16%–23% reduction in CRC mortality. The estimated FOBT sensitivity for cancer ranges between 30% and 90% depending upon the test used. The major limitation of FOBT is its low sensitivity as a screening test, since some carcinomas and most adenomas do not bleed. Only 24% of advanced neoplasia cases had a positive FOBT result on the three consecutive days’ samples obtained prior to bowel preparation. Proper FOBT testing requires examining three different bowel movements. Prerequisite compliance is difficult to achieve from both patients and physicians. In addition, the test gives an indirect result; hence, individuals who test positive have to undergo colonoscopy to confirm the presence of polyps, cancer or other pathology. Survival benefit may therefore reflect the benefit of colonoscopy, for detecting incidental lesions including those of subjects with false positive FOBT results. Lastly, upper gastrointestinal (GI) tract sources of occult bleeding, NSAID use or false positive results due to dietary ingredients may lead to unnecessary colonoscopies. There is often a low referral rate of patients with positive FOBT screening findings.
FOBT testing has no merit as a single test. It should definitely not be done when a patient has overt rectal bleeding (see Chapter 20) or any alarm symptoms. In such cases colonoscopy must be performed.
SIGMOIDOSCOPY
The main drawback of sigmoidoscopy is the limit of its extension, which is up to the splenic flexure at best. Unfortunately, the distance is often significantly shorter. Sensitivity actually depends on the varied experience of the examiners, and on patient discomfort, two factors that have a major impact on the depth of insertion and adequacy of mucosal inspection. Even in the hands of expert endoscopists, the sigmoidoscope was found to traverse the sigmoid colon in only 66% of cases. For more than 50% of proximal advanced lesions (i.e. advanced adenoma or carcinoma) there were no lesions in the distal colon, so those would have been missed by sigmoidoscopy. Sigmoidoscopy is even less rewarding in subjects aged 65–75 years, as a proximal shift of neoplasia in this age group is suggested.
COLONOSCOPY
Colonoscopy is the gold standard procedure to identify colorectal neoplasia. Skilled gastroenterologists perform the examination after a cathartic bowel preparation. Using back-to-back colonoscopies, it was shown that the sensitivity of a single colonoscopy is about 90%–95% for cancers and large adenomas and 75% for polyps <1 cm. The detection rates for adenomas ≥10 mm, 5–10 mm and 1–5 mm were found to be 98%, 87% and 74%, respectively. Colonoscopy miss rates are related to the skills of the endoscopist, withdrawal technique and, in particular, withdrawal time that reflects the time spent to inspect the colon. Although there are no published prospective studies on direct reduction of CRC mortality by primary screening colonoscopy, there is a large body of evidence to support it. The National Polyp Study has demonstrated a 76%–90% decrease in the incidence of CRC at 6 years after the index colonoscopy and polypectomy, compared with several appropriately selected control groups. A prospective 13-year follow-up demonstrated a relative risk of 0.2 for CRC in subjects who underwent colonoscopy with polyp removal compared with the control group. The prevalence of CRC in asymptomatic patients being screened aged 50–75 years in the USA is approximately 1%. Overall, the findings support the use of colonoscopy rather than sigmoidoscopy for screening in this age group.