Bowel (colorectal) 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.
This disorder is most common in areas such as New Zealand, Australia, Europe and North America where lifestyles and diets are similar. The incidence in New Zealand is one of the highest in the world. Approximately 7 % of New Zealanders develop cancer of the rectum or the colon. Throughout the world it is the third most common cancer in males and the fourth most common female cancer. There is now a lot of evidence suggesting a link between diet and the risk of developing colorectal cancer.

Risk factors

  • Genes
  • Family history of colorectal cancer or polyps
  • Previous colon or rectal cancer or adenoma (polyp)
  • Inflammatory bowel disease (ulcerative colitis or Crohn’s disease)
  • Obesity
  • Age > 40 years
  • Diet


In all cancers there are abnormalities of the genes in the cells of the tumour. These abnormalities can be acquired during a person’s lifetime, or in some cases inherited from a parent.

Family history — inherited colorectal cancer

A person with a gene that causes colorectal 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”.

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. Other genes associated with colorectal cancer include the BRCa gene which also confers a risk of breast & prostate cancer.

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.


A polyp is a mass or lump on the wall of the colon. It is an overgrowth of glandular cells in the lining of the colon. Adenomatous polyps have the potential to turn cancerous, though it takes years for a polyp to turn into a cancer.

The cause of polyps is unknown. If your family members have had polyps or colorectal cancer your risk of developing the disease is higher. Also, if you have had a polyp or cancer in the past, you are more at risk of developing future polyps or cancer. When a polyp is found it should be removed and examined by a pathologist. When one polyp is found there is a 40 % chance there will be others present in the bowel so it is important to examine the entire colon.

Adenomas have a risk of becoming cancerous especially if large, multiple or dysplastic (abnormal cells). 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 require surgery for removal.

Previous cancer

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.

Inflammatory bowel disease

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.

Dietary factors

Dietary factors potentially related to the incidence of colorectal carcinoma:

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
Beer and ale intake
Low dietary selenium

Possibly protective:

Yellow/green vegetables
Carotene (vitamin A) rich foods
Vitamins C and E
Polyunsaturated fat (olive oil, fish oil etc)
Aspirin and non-steroidal anti-inflammatory medications

* Dietary fat and fibre vary and not all types may be important in causing or preventing colorectal

Prevention of colorectal 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.

Techniques for screening for colorectal carcinoma include:

  • Testing the bowel motion for traces of blood (faecal occult blood)
  • Examination of the rectum and the lower end of the large bowel (colon) using a flexible instrument (sigmoidoscope)
  • Complete colon and rectum examination (colonoscopy)
  • Genetic testing
  • CT colonography

Screening can be offered to the general population or focused only on those with an increased risk of the disease. Colorectal screening of the general population is currently free in a number of European countries but it is not Government funded in New Zealand.

The American Cancer Society currently recommends testing for faecal occult blood (see above) on an annual basis along with sigmoidoscopy every three to five years and many medical organisations recommend a screening colonoscopy for everybody between ages 50 and 60 years.

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.

Because polyps take some years to develop into cancer, intervals of up to five years between screening are possible in most cases. This interval should be discussed on an individual basis. CT colonography or “Virtual colonoscopy” is now being developed that will allow examination of the colon by scanning and reconstruction of the images. Any detected polyps would require removal by colonoscopy. Neither colonoscopy nor CT colonography is 100 % accurate. Up to 10 % of polyps over 1cm in diameter may be missed on a single examination and CT colonography may also be interpreted as showing polyps that are not actually present.

Click here for more information about our colorectal cancer screening service.


Most bowel cancers do not cause symptoms until they have been present for many months or years and have become reasonably large. Alarm 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 and a feeling of incomplete rectal emptying after passing a bowel motion. Medical investigation is advised for any of these symptoms.

The diagnosis of colorectal 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.

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.

Radiation therapy

Radiation treatment is not normally used for colon cancer but may be needed either before or after surgery for advanced cancer of the rectum.

Laparoscopic surgery

Certain colorectal cancers can be removed by laparoscopic (“keyhole”) surgery. The potential advantages include a quicker recovery and less pain than after conventional surgery. You should discuss this with your surgeon.


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 colorectal 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 where high risk for colorectal cancer. Surgical techniques have been refined and minimally invasive techniques adopted to improve outcomes for this disease.

Common Questions

What is bowel cancer?

Bowel cancer is an uncontrolled growth in a cluster of cells from the lining of the colon (large bowel) or rectum. These cells spread into surrounding tissues and can travel to other parts of the body through lymphatic channels and blood vessels.

Why does bowel cancer occur?

Bowel cancer occurs when damage to the genes of cells lining the colon results in a loss of control of cell growth. This uncontrolled growth produces a lump (or “polyp”) in the lining of the bowel which can eventually progress to a cancer.

Who gets colon cancer?

New Zealand has one of the highest rates of colon cancer in the world. One in fifteen New Zealanders get colon cancer. Some of the genetic abnormalities that result in colon cancer can be inherited. If you have one first degree relative with colon cancer, your risk is nearly doubled. If you have two first-degree relatives with colon cancer your risk is nearly trebled. Despite this, most people who get colon cancer do not have any relatives with the disease.

Can I prevent colon cancer by eating carefully?

While there is some evidence that a diet high in fresh fruit and vegetables and low in animal fats may reduce your risk of colon cancer and a number of other diseases, no diet can completely eliminate your risk of colon cancer.

How do I know if I have colon cancer?

Colon cancer usually does not cause any symptoms until it has been present for some time. Once the tumour is of sufficient size, it can cause symptoms such as bleeding or anaemia, change in bowel habit or blockage of the bowel. Cancer rarely causes pain unless it has grown through the wall of the bowel. By this time, the cancer is usually at an advanced stage.

Can colon cancer be prevented?

Yes. More than 95 % of colon cancers begin as a benign (non-cancerous) growth in the lining of the colon called an adenomatous polyp. Except in people with a large inherited risk or with ulcerative colitis, these polyps usually take several years to progress into cancer. If they are found and removed before this occurs, cancer will not develop.

How do I know if I have colonic polyps?

The vast majority of colonic polyps cause no symptoms at all. Occasionally they will cause a small amount of bleeding or mucus. The only way to know if you have polyps is to look for them.

How are polyps found and removed?

Polyps are found with an examination called colonoscopy. A flexible tube with a tiny video camera on the end is passed around the colon under mild sedation. Polyps are seen on a TV monitor and can be removed at the same time through the colonoscope.

Should I have a colonoscopy?

You should have regular colonoscopy if you have more than one first degree relative with colon cancer or polyps, or if you have ulcerative colitis. You should also have a regular colonoscopy if you have had polyps or colon cancer in the past,or symptoms.

Will I die if I get colon cancer?

The majority of people with colon cancer can be cured. The earlier the cancer is detected the greater the chances of cure. Once the cancer has spread beyond the colon, cure becomes more difficult but modern treatment can result in significant prolongation of good quality life, or occasionally cure, even in this situation.

How is colon cancer treated?

The primary cancer in the colon is treated by surgical removal of the section of colon involved by the cancer. Usually it is possible to join the ends of the colon together. Very occasionally it may be necessary to create a colostomy where the colon comes onto the abdominal wall into a bag. Generally this is only necessary for cancers very close to the anus. In some circumstances, surgical excision is proceeded or followed by a course of chemotherapy or radiotherapy.