In Singapore, colon cancer is the most common cancer in men and the second most common cancer in women. We look at the importance of colorectal cancer screening, the methods available, and who has a greater risk of developing the condition.
The colon and rectum jointly comprise the large intestine, and form the last section of the digestive system. Colon and rectal cancers originate from the inner lining or mucosa of the colon and rectum, which is continuously growing and shedding. As such, the lining undergoes cell replication all the time.
Each replication presents a chance of a mutation. Most mutations are detected as an error by the body and the mutant cell is eliminated. Sometimes, however, these mutant cells may evade the body’s defence mechanism and continue to grow. As they grow, more mutations may occur. Following a sequence of four or five mutations in the cell, they may turn cancerous.
Most instances of colon cancer follow the adenoma-carcinoma sequence ― the colon wall first mutates into an adenoma (a non-cancerous growth usually in the form of a polyp). Over time and after more mutations, the adenoma evolves into carcinoma or cancer. Once the cancer permeates the colon wall, it can then spread to distant organs either via the bloodstream or the lymphatic system.
Screen Before It’s Too Late
There are several ways to examine the colon and screen for cancer. All tests except one require proper cleansing of the colon.
This approach entails inserting a flexible tube through the anus and advancing it up along the the beginning of the colon. During the procedure, patients are usually sedated, though some may opt to be awake and watch the entire process. This technique uses direct visualisation rather than indirect imaging methods. One major advantage of colonoscopy is that, besides being a diagnostic tool, it also allows for removal of polyps and retrieval of tissue samples for testing. Contrary to many people’s fears, most patients do not feel any pain after they wake up from sedation.
A bag of white liquid (barium) is poured into the colon through a tube inserted into the anus beforehand. The patient is rotated to facilitate proper flow of barium around the colon. The bag is then dropped onto the floor and the barium allowed to flow out. Air is then pumped through the tube to distend the colon, and X-ray images are taken with the patient in various positions.
CT colonography (virtual colonoscopy)
Air is pumped through a tube inserted into the rectum via the anus to inflate and distend the colon. The patient first lies down on his back in the CT scanner and is given an intravenous injection, after which a scan will be taken. The process is repeated with the patient lying prone on his tummy. The virtual image of the colon is then created using digital software.
Stool occult blood test
Stool samples are sent to be analysed for traces of blood, which cannot be seen with the naked eye and suggest the presence of cancerous growths. With an accuracy level of 70–85%, this test is done based on the assumption that any cancerous growth would bleed as the stools pass over it. Compared to the above three which are both screening and diagnostic methods, this test is used only for screening.
Are You At Risk?
People with a history of polyps have a higher chance of getting colorectal cancer, and people with inflammatory bowel disease (IBD) are also said to be at greater risk of developing the disease. Other risk factors include:
Less than 5% of cancers are attributed to abnormalities in the genetic structure of the patient.
Most colon and rectal cancers occur in patients over the age of 50 years. In Singapore, almost 20% of colon cancer occurs in people aged 55 years and below, and 6% in people aged 45 years and below. However, this does not mean that younger people will not get colorectal cancer. Those with other risk factors should be especially watchful.
Family and personal history
The risk increases when one has close family members with colorectal cancer. When there is a first-degree relative (parents, siblings or children) with colorectal cancer, the risk increases by three times to about 6%. Screening is especially important for those who are in the high-risk category because, when detected early, colorectal cancer is highly treatable.