Bowel cancers usually develop through multiple stages, from early polyps to advanced adenomas to invasive malignancy. Cure of malignancy is also more likely for small lesions that do not invade through the muscularis propria (T1-2) versus advanced lesions (T3) or those with lymph node metastases (N1). Therefore if the tumour can be detected in its early stages, the patient's quality of life and life expectancy could be improved.
Australia is implementing a immunochemical faecal occult blood test for people aged 50, 55 and 60. The NHMRC recommended screening interval is 2 years but the government is implementing the scheme slowly to ensure its effectiveness and allow for expansion in diagnostic tests (colonoscopy). Screening is recommended yearly in the USA
Potential screening tests are divided into three categories:
- Faecal tests (faecal occult blood guanic/immunochemical/DNA)
- Radiological tests (barium enema/CT colonography)
- Direct visualisation (colonoscopy/sigmoidoscopy)
- Faecal tests are the safest but least accurate; immunohistochemical tests which specifically target human haemoglobin are more sensitive and specific than guaiac tests which detect all forms of blood, including dietary meats. Guaiac tests have been studied in randomised controlled trials (e.g. Minnesota trial) and were found to reduce colorectal cancer mortality by 20-30%. The immunohistochemical tests, which are more acceptable, more sensitive and more specific, should have better results than the guaiac tests; although they cost more per test, the overall cost should be less due to lower rates of colonoscopy for false positive tests.
- Radiological tests vary in effectiveness and involve low doses of radiation
- Double contrast barium enema is more sensitive than faecal occult blood tests, but is significantly more invasive and requires specific skills which are waning in the era of CT scanning and colonoscopy
- CT Colonography is an evolving discipline that may reduce the need for actual colonoscopy. It still requires bowel preparation and insertion of carbon dioxide into the bowel, making it significantly more invasive. It is less cost effective than other strategies and has not been evaluated in a clinical trial as a screening test.
- Direct visualisation is most time intensive and intrusive for patients but also has the best detection rates
- Colonoscopy is a diagnostic as well as potential screening test; patients with positive results from the other screening tests above will generally be referred for colonoscopy.
- Colonoscopy is an expensive, invasive test with complications occurring in about 1/1000 patients.
- There are no randomised studies of colonoscopy for screening purposes; an observational study demonstrated halving of colorectal cancer rates in a population who underwent colonoscopy versus the general population. It is recommended in the US as a potential screening test every 10 years.