Colorectal cancer is one of the commonest cancers in Singapore. Over 1000 new cases are diagnosed annually here. As is true with most cancers, early detection of the disease results in improved chances of cure. Colorectal cancer frequently arises from non-cancerous tumours in the colon known as polyps. Over a period of years, these polyps transform into cancer. Detection and removal of polyps can avoid cancer developing.

WHY SCREEN?

Screening refers to the process of investigating for a given disease in individuals who have no symptoms of that disease. This is especially important in diseases that only demonstrate symptoms when they are more advanced and less amenable to treatment. Colorectal cancer and breast cancer are 2 good examples of such diseases. Colorectal cancer has the further advantage of having a pre-cancerous stage that can easily and safely be detected, and treated without the need for major surgery. As it is not practical to screen everybody for these diseases, risk profiles have been developed to identify persons at higher risk.

Screening has been shown to reduce mortality from colon cancer by:

1. Detecting cancer in its early stages, before it has spread to other organs

2. Reducing the risk of cancer developing in the first place, by identifying and removing polyps.

WHO TO SCREEN?

In general, individuals can be divided into different risk profiles. A simplified schema is provided below:-

Average RiskAge:

≥50yrsNegative family history for colon cancer

, Screening Test:

Colonoscopy, orFaecal Occult Blood test annually plus 5-yearly flexible sigmoidoscopy, orDouble-contrast barium enema 5-yearly

Increased Risk:

Personal history of polyp/resected colon cancer

, First degree relative with colorectal cancer

, Two related second degree relatives with colorectal cancer.

Screening Test: Colonoscopy

High Risk:

Family history of Familial Adenomatous

Polyposis

, Family history of Hereditary Non-polyposis

Colorectal Cancer

, Inflammatory Bowel Disease

. Screening Test: Colonoscopy

SCREENING TESTS:

Faecal Occult Blood Test (FOBT)

FOBT detects human haemoglobin from digested blood in the faeces. As the tests are not very sensitive, several stool samples are required, and the tests should be done annually. Positive tests have been shown in studies to aid in detecting cancer at an early stage, as compared to relying on symptomatology.

Colonoscopy

This is the gold standard in large bowel evaluation. In this procedure, a tube with a camera is inserted through the anus and is navigated through the entire large bowel. This procedure is usually done as a day case under light sedation. Its main disadvantages are its higher cost, need for bowel cleansing prior to the procedure, and a very small risk of bowel perforation (about 0.02% risk). Its main advantages are that its very sensitive, and abnormalities encountered can be biopsied or removed straight away.

Bowel cleansing is performed the day before the colonoscopy, and involves drinking a solution that clears the bowel. This is necessary as stool residue in the large bowel can obscure small lesions.

Sigmoidoscopy

This procedure is similar to the colonoscopy except that only the left side of the large bowel is examined. Its benefit is that it is a shorter procedure, and easier to perform, with very low risk. It is therefore useful in the population with average risk of colon cancer, and is used in adjunct with the FOBT.

Double-contrast Barium Enema

This is a radiographic procedure where a dye is inserted via the anus into the large bowel. This dye outlines the large bowel. Serial X-rays are taken of the abdomen, and large lesions or tumours in the wall of the bowel can be detected. Its benefits are that its not as invasive a procedure as colonoscopy, so carries an even lower risk of bowel perforation. Unfortunately, it is a less sensitive test, and takes longer to complete. Patients who have a positive test will still need to undergo a colonoscopy to obtain tissue for examination.