What is bowel cancer?

What is bowel cancer?

Bowel cancer is cancer in any part of the large bowel (colon or rectum). Depending on where it starts, it may be called cancer of the colon (large bowel) or cancer of the rectum(lower end of the large bowel). Sometimes bowel cancer is also called colorectal cancer. Large bowel cancer occurs when cells grow abnormally and form a lump or tumour. Most bowel cancers are found in the colon.

Types of bowel cancer

Most bowel cancer is adenocarcinoma. This means that the cancer started in the gland cells of the lining of the bowel. Rarer types of bowel cancer include:

  • mucinous tumours
  • signet ring
  • squamous cell tumours
  • carcinoid tumours (neuroendocrine tumours)
  • sarcomas (mostly leiomyosarcomas)
  • lymphoma.


Causes of bowel cancer

Bowel cancer is common in New Zealand. In most cases it is unclear why people have been affected by it. Bowel cancer usually starts as a benign polyp that grows to become cancerous. A polyp is a mushroom-like growth that occurs inside the bowel. Only about five per cent of polyps develop into cancer.

While most people do not have a cause for their cancer, there are some things that might contribute to the cancer to a small extent. These include the following.

Family/whānau history of bowel cancer
Having a parent, brother, sister or child who has had bowel cancer may contribute to a small extent.
Most bowel cancer (as much as 90 per cent) is not inherited. You cannot catch bowel cancer or pass it on through personal

Rare genetic conditions
Some people have a family/whānau history of bowel cancer, or other cancers such as endometrial cancer, stomach cancer,
brain cancer or pancreatic cancer. If you have a family/whānau history of cancer, please discuss this with your doctor. If you have a strong family history of bowel or other cancers, your doctor may recommend additional genetic tests throughthe Familial GI Cancer Service. These can help work out if your immediate family members or relatives may also be at increased risk of cancers.

Rare genetic conditions include:

  • Hereditary non-polyposis colorectal cancer (HNPCC) is a codition in some families/whānau where the tendency to develop cancer is inherited.
  •  Familial adenomatous polyposis (FAP) is a condition that causes hundreds of small growths (known as polyps) in the bowel of the person affected. If left untreated, FAP always turns into bowel cancer.

Other possible factors
Some factors may increase your risk of bowel cancer, including getting older, smoking, being overweight, consuming alcohol
and having an inactive lifestyle. Having Crohn’s disease or ulcerative colitis for more than 10 years may also be a contributing factor.


The digestive system

Diagram of the digestive system


Should people at increased risk be checked?

In some cases, people who have a higher-than-average risk of developing bowel cancer are advised to be checked. Each person has a right to make an individual decision on whether or not to be checked.

How common is bowel cancer?

Bowel cancer is the second most common cancer in New Zealand. 93 per cent of cases are found in people over the age of 50.
(Source: Ministry of Health, Cancer: New Registrations and Deaths 2013).


In New Zealand, a bowel cancer screening programme is now being rolled out region by region for those without
symptoms. For more information, talk to your doctor or visit the Ministry of Health’s website: www.health.govt.nz/