In Singapore, colorectal cancer is the second most common cancer in men and in women. Combined it would be the most common cancer.
In Singapore, the risk of a person getting colorectal cancer during his or her lifetime is 5.6% — about 1 in 20. There are about 1500 new colorectal cancers cases diagnosed every year. If diagnosed early, there is a chance for a cure.
Rectal cancers account for around 30% of the total number of colorectal cancers seen here.
It is cancer arising from the colon or rectum. Usually it arises from the epithelium (inner lining) of the gut wall.
Colorectal cancer can affect any age, any race and both sexes. It is more common after the age of 50 years, more common among the Chinese. For colon cancer, the incidence is the same between males and females, whereas rectal cancer is more common among males.
These are the warning symptoms that would alert us to look out for colorectal cancer.
Usually the doctor will be suspicious if you have some of the warning symptoms, or if she or he feels a mass in your belly or rectum during examination. Oftentimes the diagnosis is confirmed during the colonoscopy (when the tumour is visualized) and when a biopsy is taken. Sometimes the diagnosis can be made via barium enema, CT colonography or CT scan findings as well.
No one really knows what causes colorectal cancer; it is a combination of genetic causes and environmental causes. About 15% of colorectal cancers have a strong genetic basis. There are been certain well-defined genetic syndromes, namely, the Familial Adenomatous Polyposis (FAP) and Hereditary Non-Polyposis Colorectal Cancer (HNPCC).
Dietary causes account for about 90% of environmental causes. There is some link between colorectal cancer and certain factors — obesity, high caloric intake, alcohol intake and tobacco smoking, just to name a few.
The best known way to decrease the likelihood of colorectal cancer at this point in time is via colorectal screening.
There is no guaranteed formula to prevent colorectal cancer. But certain general measures can be helpful, such as exercise, taking fibre, having moderate caloric intake, eating lean meat and fish, moderate carbohydrate intake and reducing refined sugars and starches.
There is no evidence that 'antioxidants', colonic irrigation or herbal remedies help lower the risk of colorectal cancer.
The special thing about the rectum is that it is located within the pelvis, which is a limited space roughly the volume of a jam jar. For men, the rectum, the bladder and the prostate are all within that area and for women, the rectum, uterus and bladder. This plays a role because of the limited space; it is easy for the cancer to grow from the rectum into the prostate or bladder, or for women, the uterus and bladder.
The rectum is near the anus (the end of the gastrointestinal tract). The anal muscles are the structures responsible for faecal continence; the anus is closed when a person is moving around so that stool doesn't leak out and it is also able to relax and open to let stool out during a bowel movement.
For rectal cancers which are very close to the anal sphincter (anal muscles), there is a possibility that cancer cells have invaded the anal sphincter and have to be removed as well. The implication of that is after surgery, the patient would have to 'wear a bag' and have his bowel movements through an ostomy. Not all rectal tumours would need such surgery; it is only those that are very close to the anal sphincter.
The majority of colorectal cancers arise from adenomatous polyps. Malignant transformation of adenomatous polyps (adenoma-carcinoma sequence) takes 5 - 10 years via multiple gene mutations. 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:
Screening should begin at age 50 years for individuals without any risk factors. In individuals with an increased risk, screening should begin earlier, before the age of 50, depending on the risk factor(s) present.
For a screening test to be widely applicable, it must be inexpensive, reliable and acceptable. Various screening tests for colorectal cancer have been reported. Faecal occult blood testing (FOBT) is the only screening modality that has been shown in three large randomised 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 average-risk individuals age 50 and above. The other commonly employed screening test is colonoscopy.
Other screening alternatives include barium enema, sigmoidoscopy and CT colonography (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.
Immunochemical FOBTs detect human haemoglobin 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.
Further evaluation will be recommended if any of the two stool samples provided by the patient 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.
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, the 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) polyethene glycol (PEG) or low-volume (90 ml) 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:
Patients should go on a low fibre diet 3 days before colonoscopy, and avoid:
Foods allowed include:
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.
A 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 enemas.
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.
