Last updated on 5 January 2022
What is colon cancer?
Also known as colorectal cancer, colon cancer is a serious malignant disease that affects the colon, which is the final part of the digestive tract. Cancer can occur in any part of the colon, which is generally divided into the right colon and left colon (including the rectum). The majority of cancers occur in the left colon, although some genetic cancers tend to occur more in the right colon.
Unique to cancers, colorectal cancer always starts from polyps – a noncancerous type of growth that forms along the inside of the colon. This feature provides a great opportunity to prevent colorectal cancer altogether by detecting polyps that have the potential to become cancerous and removing them early.
While some individuals with larger polyps may experience symptoms like blood in the stool or constipation, polyps seldom cause any symptoms at all. Hence, screening for asymptomatic colon polyps is very helpful in colon cancer prevention.
Colon cancer risk factors
In Singapore, colorectal cancer remains as one of the most common cancers among men. According to the Singapore Cancer Registry, it is also consistently among the leading cancers in the country, with 11,238 new cases diagnosed (about 6 cases per day) and 4,191 deaths from 2014 – 2018. In the past 50 years, colon cancer made up more than half the cases of cancer in Singapore, and this trend was also observed in other countries.
Even though colon cancer is seen more commonly in people aged 50 and above, some types, especially those that have genetic linkages, can occur at any age. Other potential factors that increase one’s risk of developing colon cancer include:
Low-fibre, high-fat diets that include red meat and processed meat may lead to a higher risk of colon cancer.
Singaporean-Chinese have a higher risk of colon cancer compared to other races ethnic groups in Singapore.
An estimated 5 – 10% of all colon cancers may be hereditary, and may develop in individuals before the age of 50.
A sedentary lifestyle, as well as excessive smoking and drinking, may all contribute to the risk of developing colon cancer.
Symptoms of colon cancer
In the early stages of colon cancer, there are usually no symptoms. However, in the later stages of cancer, some signs include:
- Blood in the stools
- Change in bowel movements and habits (such as increased frequency of diarrhoea or constipation or alternating diarrhoea and constipation)
- Unintended and unexplained weight loss
- Abdominal discomfort
- Weakness or fatigue
As these symptoms are usually more obvious in the later stages of colon cancer, it is recommended to go for regular medical check-ups for early detection.
One of the ways ensure your colon is healthy is to sign up for more frequent colonoscopy screenings once you reach the age of 50.
Frequent colonoscopy screenings are especially important if you have a family history of the disease, or if you have a family member or a first-degree relative who suffered from colon cancer. In cases like these, it is also advised to start having colonoscopy screenings early.
What is a colonoscopy?
A colonoscopy is a medical test that examines your rectum and colon to detect changes and abnormalities such as colon polyps or cancer, and investigate to discover the cause of symptoms that may originate from the colon and rectum.
During colonoscopy, a thin flexible tube called a colonoscope is inserted into the rectum. A tiny but high resolution video camera at the tip of this tube allows your doctor to view the insides of the entire colon. The tube can be advanced to the end of the colon and can even enter the terminal ileum, which is the last part of the small intestine that joins the colon.
Abnormalities of the lining of the colon and terminal ileum can be seen during colonoscopy and biopsies can be taken by means of forceps that are advanced through a working channel in the colonoscope.
Patients are usually given mild sedative medications for the procedure. As such, there is minimal, if any, discomfort.
The removal of polyps is called a polypectomy. This procedure is commonly performed during a colonoscopy. If polyps are seen, these can be excised by instruments such as a diathermy snare that is passed through the working channel. If a polyp is too large to be removed via colonoscopy, biopsies will be taken to determine its nature.
How often do I need a colonoscopy?
Based on international guidelines, if a first screening colonoscopy is negative and there is no family history or suspicion of genetically mediated colorectal cancer, screening colonoscopies can be done once in up to 10 years. However, the intervals that are appropriate for particular patients should be discussed with their doctors.
How to prepare for a colonoscopy
Some preparation is necessary prior to a colonoscopy to ensure a successful procedure, such as:
Diet before colonoscopy
For one or two days before the procedure, you may be asked to go on a low-fibre diet, which means no grains, nuts, seeds or fruits and vegetables. You will also need to fast for at least 8 hours before the procedure, although sips of water will generally be allowed. For a morning procedure, fasting from midnight is usually sufficient.
Before you can have a colonoscopy, your bowels have to be empty and clean. Bowel preparation by laxatives will be necessary to ensure a clean colon. Your doctor will prescribe specific laxatives and a schedule for taking these, as is appropriate to you.
If you are taking any regular medications, you need to inform your doctor as it may be necessary to modify or stop some medications across your procedure. Typically, some diabetic medicines may need to be deferred in view of fasting, and blood thinners may need to be stopped for a number of days, depending on the precise medicine, before undergoing colonoscopy.
There are several options available:
Faecal Immunochemical Test (FIT)
The Faecal Immunochemical Test (FIT) is one of the more convenient screening options, and you can get a kit to carry out the procedure at home on a recommended schedule. This preliminary test helps doctors ascertain the health of your colon by checking for traces of blood in your faeces that are invisible to the naked eye. This is known as occult blood. As a rule of thumb, it is recommended to take this test once a year.
The advantage of this test is that it is convenient and non-invasive. The disadvantage is that not all polyps shed blood and may thus be missed, hence by the time occult blood is evident, the polyp may be larger, or it may have progressed to cancer. It is also possible to have false positive tests caused by bleeding from other sites in the digestive tract.
Virtual colonoscopies – also known as computed tomography or CT colonography – employ X-rays to detect polyps. Virtual colonoscopies are non-invasive, takes only 10 – 15 minutes to complete, does not require anaesthesia, and may be preferred for those who are on blood-thinning medication which would be unsafe to stop or who are not suitable candidates for a conventional colonoscopy due to existing medical conditions.
Do bear in mind that a virtual colonoscopy may be less effective in detecting polyps that are smaller than 10mm and there may be difficulty distinguishing polyps from adherent lumps of stool. Also, polyps and other growths that are discovered during a virtual colonoscopy can only be biopsied or removed through conventional colonoscopy, so if there is a positive finding (or an ambiguous finding that needs to be confirmed), then colonoscopy will need to be done as well. This is the reason why colonoscopy is the best first line test, unless there are very good reasons to try to avoid it.
A sigmoidoscopy is performed using a sigmoidoscope, which is a thin, flexible tube with a small light and camera attached, just like a colonoscope, but the procedure is intended to examine only the left side of the colon, or about one third of it. This allows the doctor to check for several things, such as polyps, tumours, ulcers and other changes in the lower colon but, obviously, if there are polyps or tumours further up, they will be missed.
The preparation for a sigmoidoscopy is less complicated than a colonoscopy and often all that is needed is one or more enemas to be applied in the hour or so prior to the procedure. Fasting is still required and sedation is also often needed as well. Sigmoidoscopy is not as comprehensive as colonoscopy and while left sided polyps and tumours will be seen, those higher up will be missed. In practice, sigmoidoscopy is not recommended for colon screening because there would be very few reasons to deliberately not examine the whole colon if an endoscopy is going to be done at all.
Double-contrast barium enema
A double-contrast barium enema, also known as a colon X-ray, injects a liquid into the rectum that contains barium. This provides improved contrast and a clearer image of the features on the lining of the colon. To further improve the quality of X-ray, air is also pumped into the colon. In addition to detecting polyps and other growths, it can also help to diagnose other conditions such as inflammatory bowel disease.
After the procedure. the liquid containing barium will be removed via the enema tube. Any residual liquid and air will be expressed through normal bowel movement, and your stools may appear whitish in colour. Some people may experience constipation so it’s important to consume more fluids, or speak to your doctor for a laxative if it persists for more than two days.
As with other non-endoscopic methods of examination, should there be a positive finding, or an ambiguous finding, conventional colonoscopy would be required as well to confirm a diagnosis, to biopsy a feature or remove a polyp. Hence, unless there are very good reasons to avoid colonoscopy in the first instance, colonoscopy is still the best option for colon screening.
To learn more about colon cancer and whether you should be screened for this, make an appointment with a specialist for a full discussion.
Article reviewed by Dr Tan Chi Chiu, gastroenterologist at Gleneagles Hospital, Singapore
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