What’s the best way to test for colon cancer?

Colorectal cancer, also known as colon cancer or bowel cancer, can be a worrying prospect, especially as we get older. Regular screening is important, especially for those over the age of 60. But how do we test for colorectal cancer? Expert colorectal surgeon Mr David McArthur explains.

Symptoms

The most appropriate investigation depends on the symptoms the patient presents with, whilst taking into account other patient-related factors such as age and other co-morbidities.

Symptoms of colorectal cancer include:

• Rectal bleeding
• A change in bowel habit
• Abdominal pain or bloating
• Weight loss
• Anaemia

However, colorectal cancer may be asymptomatic and is only detected as part of a screening process.

Colonoscopy

The gold standard investigation is colonoscopy. Patients take an oral bowel prep to cleanse the colon prior to the test, which is usually performed using light sedation or entonox (gas and air). The procedure involves passing a flexible telescope through the anal canal and the entire colon to where it transitions into the small bowel, allowing direct visualisation of the bowel lining.

It is possible to take biopsy specimens of abnormalities seen, in addition to allowing therapeutic options, such as the removal of polyps (berry-like growths in the bowel lining). If cancer is detected, it is also possible to tattoo the site in the colon to facilitate laparoscopic (keyhole) surgery for the treatment of the condition. 

The test usually takes 15-20 minutes to complete and is performed as a day-case. It is generally regarded as a safe test, with small risks of bleeding if samples are taken, pain, and a very small risk of perforation of the bowel. It has the highest sensitivity for detecting lesions within the bowel.

Flexible sigmoidoscopy

An alternative to colonoscopy, if patients present with symptoms suggestive of a problem closer to the anal canal (e.g. bright red rectal bleeding), is a limited endoscopic examination of the lower bowel called flexible sigmoidoscopy. For this test, an enema is usually administered to the patient to clear the left side of the colon. Then, the procedure is performed in the same way as colonoscopy, although the entire colon is not examined.

Occasionally, if flexible sigmoidoscopy reveals a polyp, they will need to return on another occasion to have a full colonoscopy to exclude polyps in the rest of the colon. The risks of the procedure are similar to colonoscopy.

CT scan

Another modality to investigate the bowel involves the use of CT scanning, in the form of a CT virtual colonoscopy. For this test, patients are given bowel preparation, similar to that for colonoscopy.

The patient lies in the CT scanner in two different positions, whilst gas (carbon dioxide) is inserted into the lower bowel through a narrow tube in the anal canal to distend the colon. Computer reformatting of the scan allows a radiologist to effectively fly though the colon and visualise any growths or abnormalities. The sensitivity is almost as high as that of visual colonoscopy, although small or flat polyps can be missed.

There is also the disadvantage that if there are abnormalities detected, patients often have to subsequently undergo a visual colonoscopy (e.g. to remove a polyp or biopsy a potentially cancerous growth).

CT virtual colonoscopy is generally less onerous for patients to go through. It is often used for older patients, and in those whose symptoms might have an alternative abdominal cause, whereby the CT scan examines not only the bowel, but also the rest of the abdominal structures.

Blood tests

Some GPs will perform blood tests on patients who present with bowel-related symptoms. CEA (carcinoembryonic antigen) is sometimes elevated in patients who have bowel cancer, and occasionally, patients will be referred who have had a CEA test that showed above-normal levels.

Unfortunately, it is not a very sensitive screening test and can be raised in other circumstances, such as in patients who smoke. It is generally reserved for monitoring patients after they have undergone treatment for bowel cancer.

Other blood tests that are non-specific, but can point to a potential problem in the bowel, include a low haemoglobin level (anaemia), associated with reduced iron levels, which can result from bowel cancer-related bleeding very slowly over a period of time into the bowel, which has gone unnoticed by the patient.

Screening with faecal tests

Screening for colorectal cancer aims to detect the condition, as well as pre-cancerous bowel polyps, before symptoms develop – if the disease is caught earlier, the outcome is likely to be better. Currently, in the UK, screening applies to patients between the ages of 60-74, although the age range is due to be reduced to include younger patients.

At present, patients are sent a home test kit every two years called a faecal occult blood test (FOBT). The investigation involves testing a faecal sample for the detection of blood. A positive test results in patients being invited for a colonoscopy. FOBT, although useful in this context, is unfortunately not very sensitive or specific, meaning it will not detect everyone with cancer or polyps, and it will result in a positive test for a proportion of patients who have no underlying condition.

A newer screening test called FIT (faecal immunochemical test) is currently being trialled and rolled out across the UK. It has advantages in terms of its acceptability to patients, and improved sensitivity and specificity over FOBT.

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