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Medical Notes 


By Dr Desmond Wai

Mr P, a 56 years old Chinese, consulted me for colorectal cancer screening as one of his distant relatives was recently diagnosed with colon cancer. Mr P was totally asymptomatic, and he was not directly related to the distant relative who had colon cancer.

The Ministry of Health has recently announced a screening program for colorectal cancer. Citizens are encouraged to undergo screening for colorectal cancer. So…

1.       Why should we be concerned about colorectal cancer?

2.       Who should go for screening?

3.       What types of screening method are there? and

4.       Which screening test should Mr P undergo?


1.   Why should we be concerned about colorectal cancer?

Based on the Singapore Cancer Registry Report 2004-2008, colorectal cancer is the commonest cancer in Singapore, affecting about 1,500 Singaporeans every year. It is the number 1 cancer in man (surpassing lung cancer), and the number 2 cancer in woman (trailing after breast cancer).

The cancer process starts when a polyp is formed on the inner lining of the colon. The polyp will gradually increase in size. When it reaches one centimetre, the lining on the polyp surface may start to develop early cancer. Later on, the cancer will invade into the base of polyp, and then into the layer of the colon. If left untreated, it will continue to grow to invade nearby lymph nodes, and even other organs like the liver and the lungs.

Fortunately, the process of colorectal cancer development is slow. Research has shown that it takes approximately 7-10 years for a polyp to turn into cancer. This gives us a window of opportunity for screening. If a polyp can be detected and removed, it will not become cancer and hence, cancer can be prevented. And even if a polyp has transformed into cancer, if it is found in the early stage before it invades lining of the colon, cure is achievable by surgical resection. 

Screening of colorectal cancer has been shown to save lives and hence, most doctors and health organisation like the American Cancer Society and the Singapore Ministry of Health recommend at-risk individuals to undergo screening of colorectal cancer.


2. Who should be screened?

Screening refers to the use of simple tests across a healthy population to identify individuals who have the disease. Screening programmes should only be undertaken if the screening methods are acceptable and cost-effective.  Colorectal cancer fits well into the criteria, as it is common, deadly if left untreated, and has cost effective screening methods.

At-risk individuals refer to those 50 years or older, as incidence of colorectal cancer increases after that age. Exceptions are for those with family history of colorectal cancer or longstanding inflammatory bowel disease where screening is recommended at younger age.

Mr P belongs to the at-risk individuals as he is more than 50 years, so he should undergo colorectal cancer screening.


3. What type of screening method are there?

Screening can be done by several methods:

  1. Fecal occult blood test

Colorectal cancer may cause bleeding and hence, detecting occult blood (or hidden blood) in the stools will help identify colon cancer. Many studies have shown that yearly fecal occult blood testing helps diagnose early colorectal cancer, and reduce cancer related deaths.

Fecal occult blood test is inexpensive and easy to do. It is also available in all government polyclinics and most GP clinics.

As the cancer may not bleed all the time, participants have to collect 3 specimens on 3 consecutive days to improve accuracy of screening. And the tests should be repeated yearly. A positive fecal occult blood test would require further evaluation by a colonoscopy.


  1. Colonoscopy

This is the gold standard for colorectal cancer screening.

A flexible tube of diameter 1.3cm is inserted through patient’s anus to reach the end of colon. A camera is attached to the tip of the scope and the doctor can visualise the inner lining of the whole colon directly. Any polyp or tumor detected during the procedure will be removed or biopsied through the scope. Patients are given sleeping medications during the procedure so they do not feel any pain or discomfort during the procedure.

Ironically, the most uncomfortable part of the procedure was the bowel preparation. Patients have to consume orally 1-3 litre of special solution to wash out all their fecal materials one day before the procedure. This is uncomfortable as they have to pass motion 5-10 times over a few hours duration.

If the findings are normal, it should be repeated 10 years later. But high risk individuals (those with family history of colorectal cancer or past history of colon polyp) should undergo repeat colonoscopy 3 to 5 years later.

There is a small risk of about 0.5% associated with colonoscopy, as complications like perforation or bleeding of the colon lining can occur during the procedure. Colonoscopy has to be performed by specialist doctors like a gastroenterologist or general surgeon, and is only available in the hospital setting.


  1. CT colonoscopy, or virtual colonoscopy

This is a special CT scan whereby the colon can be visualised through a CT scan. If an abnormality is detected on virtual colonoscopy, the patient would need a formal colonoscopy for proper examination. Similar to colonoscopy, bowel preparation is also required.

While most research studies showed virtual colonoscopy is almost as accurate as formal colonoscopy, some studies showed it being less accurate in detecting smaller polyps. If the scan is normal, it should be repeated 5 years later.


  1. Barium enema

A special dye is injected through the anus to coat the whole colon. X-Ray pictures are then taken to see shadow of the whole colon. Bowel preparation is not required. If the results are normal, it should be repeated in 5 years’ time. If abnormality is detected, a formal colonoscopy would be needed for full examination of the colon.

There is relatively less research on barium enema on colorectal cancer screening so this test is not recommended as a first line test.


4. Which screening test should Mr P undergo?

There is no absolute answer to this question. I went through all the screening modalities with Mr P and discussed the pros and cons of each modality. Mr P chose a formal colonoscopy, as that was the most accurate screening  method and it only has to be performed once in 10 years.

Fortunately, for Mr P, his colon was entirely normal on colonoscopy, and his next screening colonoscopy will be in 10 year’s time.



Colorectal cancer is the number one cancer in Singapore. Fortunately, there are effective screening methods to diagnose precancerous lesions and early cancer. Screening is recommended for at-risk individuals, ie, those aged 50 years or above. For high risk individuals like those with an immediate family members having colorectal cancer, screening should be started even earlier.

But as everything in medicine, there is risk and benefits associated with each procedure. It is recommended that individuals discuss with their family doctors to decide on the best method for that individual. Further information on screening of colorectal cancer can be obtained in the official websites: and .