Screening to find growths before they become cancerous or before symptoms appear is highly recommended to prevent colorectal cancer, advises Dr N. Vikneswaran, a Senior Consultant with Singapore General Hospital (SGH).
Colorectal cancer arises from benign growths, known as polyps. These grow in size and develop into cancers over several years. When they bleed or block the passage of faeces, they result in colorectal cancer symptoms such as rectal bleeding, low blood count or change in the bowel habit patterns. These symptoms, however, occur after the cancer is well established. Polyps, in contrast, do not give rise to symptoms.
Colorectal cancer can be largely prevented by screening
As precancerous polyps typically take several years to develop into cancer, the detection and removal of polyps in the colon prevent their progression to cancer.
Not all polyps develop into cancer. Some are completely benign and others are precancerous. Precancerous polyps (called adenomas) are found in approximately 25 per cent of the general population above the age of 50.
The nature of polyps is determined by the pathologist after their removal during colonoscopy. Patients who have precancerous polyps will typically require close follow-up with a repeat colonoscopy every few years depending on the number and type of polyps found.
Who should be screened for colorectal cancer?
Screening is currently recommended to begin at age 50 or earlier in those with a family history of colorectal cancer.
What screening tests are available?
Screening involves two main methods –
faecal occult blood testing (FOBT) and screening colonoscopy.
FOBT detects blood in the stool and is based on the principle that colorectal cancers may bleed.
Those with a positive FOBT will have to undergo a colonoscopy to rule out colorectal cancer. Those with a negative FOBT should undergo repeat FOBT annually. The current FOBT that is in use is also known as FIT (faecal immunochemical test).
FOBT enables detection of cancers but is not meant to detect adenomas as adenomas do not normally bleed.
Screening colonoscopy examines the inner lining of the entire large intestine. It is the only test that detects and removes polyps should these be found during the procedure. For the majority of individuals with normal results on colonoscopy, a repeat colonoscopy is advised within 10 years.
In individuals with polyps or patients deemed to be high-risk, colonoscopy is repeated at an earlier interval –typically 3-5 years- depending on the number and type of polyps.
Which colorectal cancer screening test is suitable for me?
The choice of screening test is influenced partly by your preference. A FOBT test is easier to administer as it involves the collection of stool specimen and may be ordered by a family practitioner. However, the test has to be repeated every year as it is meant to detect cancers and not polyps.
A positive FOBT must be followed up with a colonoscopy to look for cancer.
A colonoscopy is a more thorough test that provides a detailed assessment of the entire colon, detects polyps and removes them thus preventing cancer. However, this involves undergoing an outpatient procedure.
While the choice of test may vary from individual to individual, getting tested by either test is better than not being screened at all.
What should I do if I already have symptoms such as rectal bleeding or change in bowel habits?
Screening recommendations do not apply to individuals with symptoms. If you already have symptoms, such as altered bowel habits or rectal bleeding, prompt evaluation is advised, usually with a colonoscopy. Please consult your doctor for advice.
FOBT is available through general practitioners (GPs) and polyclinics while colonoscopy is available at the
Department of Gastroenterology and Hepatology and
Department of Colorectal Surgery,
Singapore General Hospital.
Free FIT kits (a type of FOBT) are available from
Singapore Cancer Society for eligible Singaporeans and Permanent Residents.