Colorectal cancer occurs when abnormal cells grow uncontrollably in the large bowel or rectum, forming a cancerous tumour.

Most colorectal cancers develop on the inner wall of the colon and rectum and usually start with growths called polyps.

These polyps are benign growths and usually harmless, however they can become cancerous if left undetected.

Colorectal cancer includes colon cancer or rectal cancer, depending on where the cancer is located and the area it originates from. Colorectal cancer is the most common cancer in Hong Kong and the second leading cause of death.^1,2

One third of colorectal cancer cases are caused by genetics and family history.³

The three common inherited disorders which have been linked to colorectal cancer are:

• Hereditary Non-Polyposis Colorectal Cancer (HNPCC) (known as Lynch Syndrome) – results in mutations to the genes that protect cells from growing abnormally and turning into cancerous cells.²
• MYH – Associated Polyposis (MAP) – caused by a genetic mutation in the gene MYH gene which results in numerous (10-100) polyps (abnormal tissue growth) which can become cancerous.²
• Familial adenomatous polyposis (FAP) – causes significant numbers (100 – 1000) of adenomatous polyps (gland-like tissue growths) in the lining of the large intestine.²

The TNM system is used to stage colorectal cancer, which helps doctors understand how your cancer spreads. The TNM stands for:

• Tumour – the degree to which the tumour has affected the inner wall of the colon and other tissue, for example has it grown outside the wall of the rectum or colon? ⁶
• Node – is a measure of whether lymph nodes have been affected.
• Metastasis – the degree to which the cancer has spread to other organs of the body.

Not everyone experiences symptoms of colorectal cancer, however some common signs include: ⁵

Cancer screening plays an important role in identifying colorectal cancer in people who do have any signs or symptoms. The Hong Kong Colorectal Cancer Screening Program is open to eligible people between 50 and 75 years of age – who will have the opportunity to perform a stool occult blood test to check for any blood particles within the stool (a potential sign of colorectal cancer).³

For people who have symptoms of colorectal cancer or the diagnosis is uncertain, a full examination of the colon is required. The two common tests used for this are:

• Colonoscopy – A colonoscopy involves inserting a camera on a flexible tube into the rectum to examine the entire large bowel. Most commonly patients with severe iron deficiency anaemia and minimal bowel symptoms are diagnosed through colonoscopy. It can be used for both diagnostic and therapeutic purposes, with the ability to obtain tumour biopsies, stop tumour bleeding and place stenting to relieve obstruction.⁴
• CT colonography – CT colonography (or virtual colonoscopy) uses a CT scanner to produce 3D images of the large bowel and rectum, and provides a less invasive option for people who are unable to have a colonoscopy. However, a colonoscopy may still be required if a biopsy needs to be taken.

Further tests

If bowel cancer is diagnosed, further tests are usually carried out to see if the cancer has spread beyond the colon or rectum.

These tests can include:

• Computed tomography scan (CT scan)
• Magnetic resonance imaging (MRI)
• Positron emission tomography (PET scan)