Approximately 1,200 new cases of colorectal cancer are diagnosed each year, making it the most common cancer in Singapore. The age-standardized incidences of colon and rectal cancer in Singapore are among the highest in the world. In particular, male Chinese Singaporeans have the dubious honor of having the highest age-standardized incidence of rectal cancer in the world!
Colorectal cancer is the most common cancer in Singapore. Male Chinese Singaporeans are particularly at risk. Surgery is the mainstay of treatment for colorectal cancer. Early diagnosis improves the likelihood of a cure.
The majority of colorectal cancers arise from adenomatous polyps. Malignant transformation of adenomatous polyps (adenoma-carcinoma sequence) takes 5 to 10 years via multiple gene mutations (Figure 11). Adenomatous polyps are relatively asymptomatic. They are present in up to 25% of individuals at age 50 and the prevalence increases with age. Most polyps (90%) can be removed at colonoscopy, thereby precluding the need for surgery.
Thus, colorectal cancer has a detectable premalignant phase (adenoma) and a relatively long duration of malignant transformation. Mortality from colorectal cancer can be reduced by screening asymptomatic individuals for the presence of adenomas and early cancers.
Adenomatous polyps are largely asymptomatic.
The process of malignant transformation takes a relatively long time.
Screening for colorectal cancer:
a. prevents cancer by removing polyps at colonoscopy
b. detects early cancers with a good chance of a cure
sigmoid polyp in an asymptomatic patient
snare polypectomy at colonoscopy
Who to Screen
Screening should begin at age 50 years for individuals without any risk factors. In high-risk individuals, screening should begin earlier, before the age of 50, depending on the risk factor(s) present. Risk factors include a strong family history of colorectal cancer (particularly in first-degree relatives younger than 45 years) and any of the colorectal polyposis syndromes.
How to Screen
For a screening test to be widely applicable, it must be inexpensive, reliable and acceptable. Various screening tests for colorectal cancer have been reported. Fecal occult blood testing (FOBT) is the only screening modality that has been shown in 3 large randomized trials to show a 33% reduction in colorectal cancer mortality. In light of this, it would be almost medically negligent not to offer FOBT screening for averagerisk individuals age 50 and above. The other commonly employed screening test is colonoscopy.
Other screening alternatives include barium enema, sigmoidoscopy and CT colography (virtual colonoscopy). However, current evidence suggests that these alternatives may not be as effective and reliable as FOBT or colonoscopy in large-scale population screening.
Fecal occult blood tests (FOBT)
Immunochemical FOBTs detect human hemoglobin from partially digested blood in the stool. They are more sensitive and more specific than guaiac-based tests that were used in the past. Another advantage is that dietary restriction is not required in immunochemical testing.
Two samples from each of 3 consecutive stools are tested. Colonoscopy is needed if at least 1 of the 6 samples is positive. In a large UK study, 12% and 23% of FOBT-positive individuals had cancer and adenomatous polyps respectively on colonoscopy. Cancers detected at screening were of an earlier stage than symptomatic ones (Duke's A: 26% screened vs 11% in controls).
The main disadvantage of FOBT screening is its low sensitivity. An estimated 50% of cancers will be missed on each screening round. To enhance the pick-up rate, FOBT must be done annually.
How to collect a stool sample for FOBT
Lay toilet paper in toilet bowl as shown on the right.
Reverse sitting position as shown below need be adopted to allow for stool to collect on the toilet paper to simplify collection of the stool sample for the FOBT test
Immunochemical FOBTs do not need dietary restriction. Individuals with positive FOBT require colonoscopy. Individuals with negative FOBT are tested annually.
Colonoscopy is the gold standard for complete large bowel evaluation. The main disadvantages are its higher cost, need for full bowel preparation and sedation. There is also a small risk of bowel preparation. For high-risk patients e.g., individuals at risk of hereditary non-polyposis colorectal cancer, colonoscopy is the screening investigation of choice.
The main advantages are its high sensitivity and specificity and the long recommended screening interval of 10 years. The protective effect of colonoscopy is attributed to the ability to remove asymptomatic polyps before malignant transformation occurs.
Usually, bowel preparation takes 1 of 2 forms: high-volume (3-4 litres) polyethylene glycol (PEG) or low-volume (90 mls) oral fleet. Oral fleet is contraindicated in patients with renal impairment due to its high phosphate content. For suitable patients, it is a more palatable option as it can be mixed with sweetened fluids. Patients taking oral fleet must be encouraged to drink plenty of water to decrease the likelihood of phosphate toxicity.
General advice to patients on bowel preparation for patients undergoing colonoscopy
Oral medications which need to be stopped before colonoscopy:
- iron supplements (1 week before appointment)
- anticoagulation medications e.g. aspirin, ticlid, wafarin (5 days before appointment)
Low fibre diet 3 days before colonscopy, patients need to avoid:
- fruits and vegetables including fresh fruit and vegetable juices
- vegetable soup
- red meat
- milk products
- Cereals and grains e.g. oats, bran, wheat, muesli, barley nuts and beans
Foods allowed include:
- Simple carbohydrates (white rice, white bread, mee swa, bee hoon, kway teow, potatoes)
- Plain coffee, tea, gluocose, honey or clear soup
Colonoscopy is the gold standard for large bowel evaluation. The screening interval for colonoscopy is 10 years. Bowel preparation with low-volume oral fleet is feasible in the absence of contraindications.
Barium enema is an alternative to colonoscopy for large bowel evaluation. However, bowel preparation is still needed and in some studies, the false negative rate is as high as 50%. Furthermore, colonoscopy may still be needed to rule out suspicious lesions on the enema. There are currently no population screening studies using barium enema.
CT Colography/Virtual Colonoscopy
Virtual colonoscopy is a new radiologic technique used to generate images of the colon and rectal wall. Bowel preparation is still needed and like barium enema, colonoscopy may be needed for ruling out suspicious lesions and for therapeutic polypectomy.
A recent meta-analysis suggests that overall polyp detection rate is woefully inadequate, making this new technique unsuitable for population screening.
Colorectal Cancer at a glance
- Colorectal cancer screening reduces cancer-related
- Risk stratification determines age of onset of
screening, screening test and screening interval.
- For average-risk individuals, screening to start at
- FOBT annually or
- Colonoscopy every 10 years or
- Barium enema every 5 years
- For high-risk individuals, colonoscopy with age of
onset and frequency according to recommendations
in Table (shown above).