Colorectal cancer or colon cancer is an abnormal growth in the colon or rectum (bowel). The colon is the longest part of the large intestine and the rectum is the passageway connecting the colon to the anus. Most colorectal cancers begin as a polyp (a benign or non-cancerous growth on the lining of the colon or rectum) that develops into an abnormal (cancerous) growth. Screening for polyps can detect colorectal cancer early.

The exact cause of colorectal cancer is not known, but certain risk factors are strongly linked to the disease.

Risk factors of colorectal cancer include:

• Older age. People who are older than 50 years old are at a higher risk, as the majority of cases are diagnosed in people above 50.
• Personal history of polyps. People who have had colorectal polyps before, which has the potential to develop into colorectal cancer.
• Personal history of cancer. People who have had colorectal cancer before and women who have had cancer of the ovary, uterus or breast also face higher risk.
• Radiation therapy. People who have received radiation therapy directed at the abdomen for previous cancers.
• Family history of colorectal cancer. You are more likely to develop colorectal cancer if you have family members with colorectal cancer or, rarely, gene mutations passed through generations that increase the risk of colon cancer.
• Inflammatory intestinal conditions. Chronic inflammatory intestinal diseases such as ulcerative colitis or Crohn’s disease (inflammation of the colon) can increase your risk of colorectal cancer.
• Smoking and alcohol consumption. People who smoke or consume alcohol, may have an increased risk of various forms of cancer, including colorectal cancer.
• High-fat and low-fibre diet. Some studies have found an increased risk of colorectal cancer in people with a diet high in fat and low in fruits and vegetables.
• Sedentary lifestyle. Getting regular physical activity may reduce your risk of colorectal cancer.
• Obesity. People who have co-morbid conditions such as obesity and diabetes or insulin resistance have an increased risk of colorectal cancer.

Preventing colorectal cancer

In addition to screening, lifestyle changes can help to reduce the risk of colorectal cancer. These include:

• Eat more fruits, vegetables and whole grains
• Eat less red meat and processed meats
• Exercise regularly
• Maintain a healthy body weight
• Avoid smoking
• Limit alcohol intake

The symptoms of colorectal cancer depend on the location of the cancer, how advanced it is and how it affects the nearby organs and tissue. Because colorectal or colon cancer symptoms often do not appear until the cancer has advanced, it is important to have regular colorectal screening.

Signs and symptoms of colorectal cancer include:

• Change in bowel habits (diarrhoea or constipation)
• Feeling that your bowel does not empty completely
• Feeling very tired all the time
• Finding blood (either bright red or very dark) in your stools
• Finding your stools are narrower than usual
• Frequently having gas pains or cramps, or feeling full or bloated
• Losing weight with no known reason

Screening tests, conducted prior to the onset of symptoms, are recommended for individuals at the age of 50 years and above to detect precancerous polyps. This allows the polyps to be removed before they develop into cancer or detect colorectal cancer at its early stages, when treatment is more likely to be effective.

Once symptoms appear, diagnostic tests are recommended to allow doctors to confirm if it is colorectal cancer, assess the stage and determine the best course of treatment.

These tests are commonly used to diagnose colorectal cancer:

Faecal occult blood test or blood stool test

Faecal occult blood test or blood stool test checks for the presence of blood in stools (faeces). However, as several conditions could cause the presence of blood in stools, a positive result may not be sufficient to confirmation a diagnosis of colorectal cancer.

Sigmoidoscopy

Sigmoidoscopy uses a flexible, slender tube with a light inserted into the rectum and sigmoid (the part of the colon just before the rectum) to detect polyps or colon cancer in the end-third of the colon and rectum. If either are detected, a colonoscopy may be recommended.

Colonoscopy

Colonoscopy is similar to a sigmoidoscopy but with longer tube and with an attached video camera and monitor, allowing the doctor to view the entire length of the colon and rectum. This procedure also allows the doctor to remove any polyps, or take tissue samples for further testing.

Barium enema

Barium enema is an imaging test that uses barium (contrast dye) and x-ray to create a clear image of the colon and rectum, allowing the doctor to detect signs of colorectal cancer. In a double-contrast barium enema, air is added to expand the colon and provide a more detailed view of the inner lining of the colon.

CT Colonography

CT colonography or virtual colonoscopy, uses low dose radiation CT scanning or magnetic resonance imaging to obtain an interior view of the colon (the large intestine) and rectum (back passage) that are otherwise only seen with a more invasive procedure where an endoscope is inserted into the rectum and passed through the entire colon.

In 40% of cases, diagnosis of colorectal cancer occurs at an advanced stage, when surgery is likely the best option.

The type of surgery will depend on the stage and extent of the cancer. Laparoscopic surgery can be performed in early to mid stage cancers.

For more advanced cancers, open surgery will be required to ensure complete removal of the tumour and associated lymph nodes. Open surgery is also performed for urgent situations such as tumour obstruction.

Where possible, the healthy sections of the colon are reconnected. If this is not possible, a separate procedure called an ostomy is performed to create an alternate way for stools to be safely eliminated from the body.

Non-surgical colorectal cancer treatments may be recommended as well to improve the prognosis and prevent the cancer from coming back. These include:

Chemotherapy

Chemotherapy is the use of specific drugs to help destroy cancer cells, especially if it has spread to the lymph nodes. It may also be recommended pre-surgery to help shrink the tumour and make its removal easier.

Radiation therapy

Radiation therapy uses high-energy x-rays and protons in designated area to destroy cancer cells or shrink tumours prior to surgery. Sometimes radiation therapy and chemotherapy may be used in combination.

Targeted therapy

Targeted therapy focuses on specific abnormalities found within cancer cells. It is usually reserved for advanced colorectal cancer.

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Complications and related conditions of colorectal cancer include:

Bowel obstruction

Bowel obstruction or blockage of the colon which can happen when something prevents the contents of the intestines from passing normally through the digestive tract. If your digestive system comes to a halt, you will not be able to have a bowel movement or pass gas.

Metastatic cancer

Cancer cells in the colon or rectum can sometimes spread to the liver, lungs, brain, peritoneum (the lining of the abdominal cavity), or to distant lymph nodes. Once it spreads to another organ, that's called metastatic, or stage IV colorectal cancer. Treatment of stage IV colorectal cancer is mostly palliative and its median survival rate is reported to be approximately 9 months with the best supportive care.