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How is bowel cancer diagnosed?

Some people have tests for bowel cancer because they have symptoms. Others may not have any symptoms, but have a strong family history of bowel cancer or have received a positive result from a screening test.

The tests you have to diagnose bowel cancer depend on your specific situation. They may include general tests to check your overall health and body function, tests to find cancer, and tests to see if the cancer has spread (metastasised).

Some tests may be repeated during or after treatment to check how well the treatment is working. It may take up to a week to receive your test results. If you feel anxious while waiting for test results, it may help to talk to a friend or family member, or call Cancer Council 13 11 20 for support.

Physical examination

Your doctor will examine your body, feeling your abdomen for any swelling. To check for problems in the rectum and anus, your doctor will insert a gloved, lubricated finger into your anus and feel for any lumps or swelling. This is called a digital rectal examination (DRE).

The DRE may be uncomfortable, but it shouldn’t be painful. Because the anus is a muscle, it can help to try to relax during the examination. The pressure on the rectum might make you feel like you are going to have a bowel movement, but it is very unlikely that this will happen.

Blood test

You may have a blood test to assess your general health and to look for signs that suggest you are losing blood in your stools.

The blood test may measure chemicals that are found or made in your liver, and check your red blood cell count. Low red blood cell levels (anaemia) are common in people with bowel cancer, but may also be caused by other conditions.

Immunochemical faecal occult blood test (iFOBT)

Depending on your symptoms, you may have an iFOBT. This test is generally not recommended for people who are bleeding from the rectum, but may be used for people with unexplained weight loss, abdominal pain, changes to their bowel habits or anaemia.

The iFOBT involves taking a sample of your stools at home. The stool sample is examined for microscopic traces of blood, which may be a sign of polyps, cancer or another bowel condition. An iFOBT does not diagnose cancer, but if it finds blood, your doctor will recommend you have a colonoscopy as soon as possible, but no later than 120 days after getting the result.

The main test used to look for bowel cancer is a colonoscopy. Other tests that are sometimes used to diagnose bowel cancer include CT colonography and sigmoidoscopy.

Colonoscopy and biopsy

A colonoscopy examines the whole length of the large bowel. It is still possible, however, that small polyps may be missed, especially if they are behind one of the many folds in the bowel or the bowel is not completely empty.

Before a colonoscopy, you will have a bowel preparation to clean your bowel. On the day of the procedure, you will usually be given a sedative or light anaesthetic so you don’t feel any discomfort or pain. This will also make you drowsy and may put you to sleep.

During the procedure, the doctor will insert a colonoscope (a flexible tube with a camera on the end) into your anus and up into your rectum and colon. Carbon dioxide or air will be passed into the colon.

If the doctor sees any suspicious-looking areas (such as polyps), they will remove the tissue for examination. This is known as a biopsy. During the colonoscopy, most polyps can be completely removed (a polypectomy). A pathologist will examine the tissue under a microscope to check for signs of disease and may look for specific genetic changes.

A colonoscopy usually lasts about 20–30 minutes. You will need to have someone take you home afterwards, as you may feel drowsy or weak. An occasional side effect of a colonoscopy is temporary flatulence and wind pain, especially if air rather than carbon dioxide is passed into the bowel during the test. More serious but rare complications include damage to the bowel or bleeding. Your doctor will talk to you about the risks.

Less commonly used tests

CT colonography – This uses a CT scanner to create images of the colon and rectum and display them on a screen. It is also called virtual colonoscopy. It may be used if the colonoscopy was unable to show all of the colon or when a colonoscopy is not safe.

A CT colonography is not often used because it is not as accurate as a colonoscopy and exposes you to radiation. Your doctor also may not be able to see small abnormalities and cannot take tissue samples. This test is covered by Medicare only in some limited circumstances.

Flexible sigmoidoscopy – This test allows the doctor to see the rectum and lower part of the colon (sigmoid colon) only. To have a flexible sigmoidoscopy, you will need to have a light bowel clean-out, usually with an enema. Before the test, you may be given a light anaesthetic.

You will lie on your side while a thin, flexible tube called a sigmoidoscope is inserted gently into your anus and guided up through the bowel. The sigmoidoscope blows carbon dioxide or air into the bowel to inflate it slightly and allow the doctor to see the bowel wall more clearly. A light and camera at the end of the sigmoidoscope show up any unusual areas, and your doctor can take tissue samples (biopsy).

Bowel preparation

Before some diagnostic tests, you will have to clean out your bowel completely. This is called bowel preparation and it helps the doctor see inside the bowel clearly. The process can vary, so ask if there are specific instructions for you. It’s important to follow the instructions so you don’t have to repeat the test. Talk to your doctor if you have any questions about the bowel preparation process or side effects.

Change diet – For a few days before the diagnostic test, you may be told to avoid high-fibre foods, such as vegetables, fruit, wholegrain pasta, brown rice, bran, cereals, nuts and seeds. Instead, choose low-fibre options, such as white bread, white rice, meat, fish, chicken, cheese, yoghurt, pumpkin and potato.

Drink clear fluids –  Your doctor might advise you to consume nothing but clear fluids (e.g. broth, water, black tea and coffee, clear fruit juice without pulp) for 12–24 hours before the test. This will help to prevent dehydration.

Take prescribed laxatives – You will be prescribed a strong laxative to take 12–18 hours before the test. This is taken by mouth in tablet or liquid form over several hours, and will cause you to have several episodes of watery diarrhoea.

Have an enema, if required – One common way to clear the bowel is using an enema. This involves inserting liquid directly into the rectum. The enema solution washes out the lower part of the bowel, and is passed into the toilet along with any faeces. You may be given an enema before a colonoscopy if the laxative hasn’t completely cleaned out the bowel or on its own before a sigmoidoscopy.

If the tests show you have bowel cancer, you will have additional tests to see if the cancer has spread to other parts of your body.

CEA blood test

Your blood may be tested for a protein called carcinoembryonic antigen (CEA). This protein is produced by some cancer cells. If the results of the blood test show that you have a high CEA level, your doctor may organise more tests. This is because other factors, such as smoking or pregnancy, may also increase the level of CEA. Your CEA level may be retested after treatment to see if the cancer has come back.

CT scan

A CT (computerised tomography) scan uses x-ray beams to create detailed, cross-sectional pictures of the inside of your body. A scan is usually done as an outpatient. Most people are able to go home as soon as the test is over.

Before the scan, dye is injected into a vein to make the pictures clearer. This dye may make you feel hot all over and leave a strange taste in your mouth for a few minutes. You might also feel that you need to urinate, but this sensation won’t last long.

During the scan, you will lie on a table that moves in and out of the CT scanner, which is large and round like a doughnut. Your chest, abdomen and pelvis will be scanned to check if the cancer has spread to these areas. The scan takes 5–10 minutes and is painless.

MRI scan

An MRI (magnetic resonance imaging) scan uses a powerful magnet and radio waves to create detailed, cross-sectional pictures of the inside of your body. An MRI is recommended to more accurately determine the position and extent of rectal cancer. An MRI may also be used to scan the liver if your doctor suspects the cancer has spread to the liver. Usually only people with cancer in the rectum have an MRI; it is not commonly used for cancers higher in the bowel.

A dye might be injected into a vein before the scan to help make the pictures clearer. During the scan, you will lie on a treatment table that slides into a large metal tube that is open at both ends. The noisy, narrow machine makes some people feel anxious or claustrophobic. If you think you could become distressed, mention it beforehand to your medical team. You may be given a medicine to help you relax and you will usually be offered headphones or earplugs. The MRI scan may take between 30 and 90 minutes, depending on the size of the area being scanned and how many images are taken.

PET-CT scan

A positron emission tomography (PET) scan combined with a CT scan is a specialised imaging test. The two scans provide more detailed and accurate information about the cancer. A PET-CT scan is most commonly used after surgery to help find out where the cancer has spread to in the body or if the cancer has come back after treatment.

Before the scan, you will be injected with a glucose solution containing a small amount of radioactive material. Cancer cells show up brighter on the scan because they take up more glucose solution than the normal cells do. You will be asked to sit quietly for 30–90 minutes as the glucose spreads through your body, then you will be scanned. The scan itself will take around 30 minutes. Let your doctor know if you are claustrophobic as the scanner is a confined space.

Medicare only covers the cost of PET-CT scans for bowel cancer in limited circumstances. If this test is recommended, check with your doctor what you will have to pay.

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This information is reviewed by

This information was last reviewed January 2021 by the following expert content reviewers: A/Prof David A Clark, Colorectal Surgeon, Royal Brisbane and Women’s Hospital, and The University of Queensland, QLD, and The University of Sydney, NSW; A/Prof Siddhartha Baxi, Radiation Oncologist and Medical Director, GenesisCare Gold Coast, QLD; Dr Hooi Ee, Specialist Gastroenterologist and Head, Department of Gastroenterology, Sir Charles Gairdner Hospital, WA; Annie Harvey, Consumer; A/Prof Louise Nott, Medical Oncologist, Icon Cancer Centre, Hobart, TAS; Caley Schnaid, Accredited Practising Dietitian, GenesisCare, St Leonards and Frenchs Forest, NSW; Chris Sibthorpe, 13 11 20 Consultant, Cancer Council Queensland; Dr Alina Stoita, Gastroenterologist and Hepatologist, St Vincent’s Hospital Sydney, NSW; Catherine Trevaskis, Gastrointestinal Cancer Specialist Nurse, Canberra Hospital, ACT; Richard Vallance, Consumer.