Bowel (colorectal) cancer

Bowel cancer is the most common registerable cancer in New Zealand, and New Zealand has the highest incidence of colorectal cancer in the world.

What is bowel cancer?

Cancer of the colon (large bowel) or rectum usually begins as a small non-cancerous growth called a polyp. This polyp may then undergo changes in its gene make-up which result in uncontrolled growth and spread – at this stage it is called cancer. 

Approximately 7% of New Zealanders develop cancer of the rectum or the colon and about 1200 New Zealanders die from colorectal cancer every year. This is four times the number who die from road traffic accidents and twice the number who die from breast cancer.

What are the causes of bowel cancer?

The exact causes of bowel cancer aren’t fully understood, however research has shown that several factors may make you more likely to develop it.

The main risks of bowel cancer include:

About 20% of people with bowel cancer have a parent or sibling who had the disease. A person with a gene that causes bowel cancer can pass the gene to either a son or a daughter even though that person never actually developed cancer. This is because some genes are not always “expressed”. 

Overall, the risk of developing colorectal carcinoma increases two to three times in siblings or children of people with colorectal cancer when compared to the general population. If families have several affected members and, particularly if these cancers develop at a young age, this means an even higher level of risk. It is advisable that these relatives are screened by colonoscopy to detect any cancers or polyps at an early stage.

The strongest identified inherited risk for colorectal cancer is known as Familial Adenomatous Polyposis (FAP) where multiple polyps develop in the colon and rectum. Cancer develops at an early stage in this condition so family members must be tested in their early teens.

The most common type of inherited colorectal cancer is HNPCC or ‘Herediary Non-Polypous Colorectal Cancer’. This accounts for 3-5% of all colorectal cancers. Individuals who carry the gene have about an 80% chance of developing colorectal cancer. They also have an increased risk of some other cancers including endometrium (womb), small bowel, stomach and ovary.

In New Zealand, 90% of people who develop bowel cancer are over the age of 50 years.

Polyps are protrusions arising from the lining of the colon or rectum due to an overgrowth of the gland cells lining the colon. A simple polyp is called a hypoplastic polyp. If the cells in the polyp are abnormal it is called an adenoma. 

Adenomas have a risk of becoming cancerous especially if large, multiple or dysplastic (abnormal cell structure). If these adenomas are identified before they become too large they can be removed by means of colonoscopy which prevents a cancer from forming. Larger polyps may require surgery for removal.

Hyperplastic polyps behave differently from adenomas, and are believed not to become cancerous but large or multiple hyperplastic polyps indicate an increased risk of colorectal cancer.

Find out more about polyps.

About 5% of people with colorectal cancer will have another colorectal cancer either at the same time or later in life, making it important that the entire bowel is completely examined when a cancer is diagnosed, and follow up colonoscopies are performed to check for new polyps/cancer.

The risk of developing colorectal cancer associated with either ulcerative colitis or Crohn’s disease depends on the duration of the disease and the extent of the inflammation. Regular surveillance colonoscopy is recommended for people with extensive disease of long standing to detect any pre-cancerous (dysplastic) changes.

Find out more about ulcerative colitis and Crohn’s disease.

  • Definitely related.
    • Excessive calorie intake
  • Probably related.
    • High unsaturated fat
    • Low fibre consumption
  • Possibly related.
    • Environmental cancer forming substances, i.e. from bowel bacteria and from burnt, charbroiled meat and fish
  • Possibly protective.
    • Yellow/green vegetables
    • Calcium
    • Carotene (Vitamin A) rich foods
    • Vitamins C and E
    • Polyunsaturated fat (olive oil, fish oil etc)
    • Aspirin and non-steroidal anti-inflammatory medications

Preventing bowel cancer

Colorectal carcinoma is preventable. Most cancers develop from non-cancerous growths (polyps) present for some years before becoming cancerous.  If these growths are removed cancer can be prevented. Regular colonoscopy and polyp removal has been shown to reduce the number of bowel cancers in scientific studies.


Screening means testing people with symptoms or signs of the disorder, aiming to diagnose it at an early stage which is curable, or requires less aggressive treatment.

Regular screening by colonoscopy is recommended for people who have close relatives (siblings or parent) with colorectal cancer. If a relative developed colorectal carcinoma before the age of 50 years, these people should start screening when they are 10 years younger than the age of the youngest affected relative.

Find out more about bowel cancer screening.

What are the symptoms of bowel cancer?

Most bowel cancers do not cause symptoms until they have been present for many months or years and have become reasonably large. Alarming symptoms that can be associated with colorectal carcinoma include:

  • Bleeding from the rectum
  • A change in bowel function such as diarrhoea or constipation
  • Abdominal pain or distension
  • Vomiting
  • Weight loss
  • A feeling of incomplete rectal emptying after passing a bowel motion. 

Medical investigation is advised for any of these symptoms.

How is bowel cancer diagnosed?

The diagnosis of bowel cancer includes a physical examination, testing the bowel motion for blood and blood tests to check for anaemia or disturbed liver function. The most accurate method of confirming the diagnosis is colonoscopy where a flexible video camera is guided around the colon and samples are taken for laboratory testing. Other techniques include a CT colonogram or barium enema, which involve X-rays, taken after air and/or liquid barium are passed around the colon.

If the diagnosis is confirmed it is important to determine how advanced the cancer is. This is known as “staging” and usually requires tests such as liver scans and chest X-rays.

What are the treatment options?

Untreated colorectal cancer usually continues to grow, spreading to the liver and other organs, which is eventually fatal.  If detected at an early stage the disease can be cured by removal either during colonoscopy (if small) or surgery.

Surgical excision involves removal of the affected segment of bowel and associated lymph nodes. Complete surgical clearance of disease is the best way of ensuring long-term survival.

Sometimes, if limited spread has occurred to the liver or lungs, cure can be achieved but this is less likely.  In the majority of colorectal cases treated by surgical excision, the bowel ends can be rejoined, avoiding a permanent colostomy (stoma bag).

A temporary stoma is occasionally needed to allow healing of the join (anastomosis) and this is then closed at a later date.  If a rectal cancer is very close to the anus this has to be removed during surgery.  In this situation a permanent colostomy is needed.

For early cancers which have not yet spread beyond the bowel, chemotherapy is not required.  For those cancers which have spread to the glands (lymph nodes), chemotherapy can significantly improve the outlook from approximately a 58% five-year survival rate to a 70% five years survival rate.

Most colorectal cancers can be removed by laparoscopic (“keyhole”) surgery. The advantages include a lower complication rate, less likelihood of needing a blood transfusion or developing adhesions afterwards, a quicker recovery and less pain than after conventional surgery. You should discuss this with your surgeon.

Radiation treatment is not normally used for colon cancer but it is often recommended before surgery for cancer of the rectum.


The duration of survival after developing colon or rectal cancer is determined predominantly by two factors, the stage (how advanced it is) of the disease and the treatment given.

Early tumours confined to the bowel are cured by surgery in about 90% of cases. Until recently, tumours that had spread to the liver were incurable. There are now some treatments that can cure, or delay progression in a number of tumors that are isolated to the liver.

The future

The future will see greater emphasis placed on the prevention of bowel cancer. Identification of those at increased risk of the disease will allow for more accurate screening techniques and genetic testing will become more available. It is important that changes in diet and lifestyle occur for patients who have a high risk for colorectal cancer. Surgical techniques have been refined and minimally invasive techniques adopted to improve outcomes for this disease.