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Tests That Help Evaluate Colorectal Cancer

Your doctor did a biopsy to determine that you had cancer. The doctor did this by taking a sample of cells or fluid from an area that looked suspicious in your colon. Your doctor may now ask for more tests. These tests will help the doctor learn about your specific cancer. Your doctor needs to know what stage your cancer is and whether it has spread. With this information, you and your doctor can decide on a treatment plan. The doctor may also ask for tests to assess your overall health. These tests will show if the cancer has caused other health problems. They will also show whether other health problems might interfere with your treatment for cancer.

Imaging tests

You may also need to have imaging tests, which help your doctor see what’s happening in your body. Here are some the imaging tests you may need to have:

  • Computed tomography (CT scan)

  • Magnetic resonance imaging (MRI)

  • Positron emission tomography (PET scan)

  • Ultrasound

  • Chest X-ray

CT scan

Your doctor may ask for a CT scan to examine local tumors and to locate tumors that may have spread to other parts of your body, such as your lungs or liver. The scan is actually a process for taking multiple X-ray pictures of the inside of your body from more than one angle. While you lie on a table that moves into the center of the scanner, a thin X-ray beam rotates around you. As the pictures are taken, a computer generates a detailed image of your internal organs. To be able to see a tumor, the doctor may give you an injection of a contrast dye. A complete CT scan takes several minutes. Metal objects can get in the way of the X-rays, so you will be asked to remove any jewelry and may be asked to wear a hospital gown during the test. X-rays are painless, so you won’t feel the scan. Some people, though, feel uncomfortable having to lie so still while the test is going on.


An MRI uses a magnet, radio waves, and a computer to take pictures of the inside of your body. MRIs are not a routine test for evaluating colon cancers (CT scans are more commonly done), although they may be done for rectal cancers. While CT scans usually create a clear picture of your internal organs, there are some advantages to MRIs. MRIs do not use radiation, and they may not require an injection of a dye. The dye used for MRIs is not recommended if you have kidney disease. Your doctor may ask for an MRI if you are allergic to the contrast dye, usually iodine, that is used for CT scans. Your doctor may also ask for an MRI to help evaluate lesions that showed up but were unclear during a CT scan.

MRIs are not painful. They can, though, take a long time--up to an hour--to complete. During that time you will need to lie still on a table that is moved into a long narrow tube. Some people say the test makes them feel claustrophobic. If you have experienced claustrophobia in the past, discuss it with your doctor before agreeing to the test. Newer, more open MRI machines can sometimes be used instead, but the images may not be as sharp in some cases. The equipment also makes loud banging noises during the procedure. You can ask for earplugs if you think the noise will bother you.

Because the test uses powerful magnets, you will not be allowed to have anything metal in the room. Even eyeglasses and ballpoint pens can become dangerous projectiles when the magnets are turned on. If you have any kind of metal implant, such as a heart valve or a joint pin, you may not be able to have an MRI (depending on the type of metal it is made of).  And the equipment can affect implants, such as a pacemaker. So people with a pacemaker cannot be in the room. The doctor may give you a sedative to help you stay calm during the test. If you have a sedative, you will need to allow time for its effect to wear off. Otherwise, you should be able to go about your normal routine immediately after the MRI.

PET scan

A PET scan can give the doctor a better idea of whether an abnormal area seen on a CT scan or other imaging test is a tumor or not. If you have already been diagnosed with cancer, your doctor may use this test to look for spread of the cancer to lymph nodes or other parts of the body. A PET scan can also be helpful if your doctor thinks the cancer may have spread, but doesn't know where. A PET scan can provide helpful information about your whole body. The picture is not finely detailed like a CT scan, but it can be used along with a CT scan to look for tumors. 

For this test, you are injected with a sugar that has a mildly radioactive substance or radioisotope. Cancer cells absorb more of this sugar than normal cells, and the radioactive material shows up during the image from the scan. To have the scan, you’ll need to lie still on a table that is pushed into the PET scanner, a machine that rotates around you, taking pictures that show where the sugar is in your body. The entire process may take several hours. A PET scan is painless and noninvasive. But if you’re sensitive to the sugar, you may have nausea, headache, or vomiting.


Ultrasound uses sound waves to make a picture of the inside of your body. A small instrument called a transducer emits sound waves and picks up the echoes as they bounce off body organs. Ultrasound can be used to look for tumors in your liver, gallbladder, pancreas, or elsewhere in your abdomen, but it can't look for tumors of the colon. For the exam, you lie on a table while the transducer is moved along the skin over the part of your body being examined. Usually, the skin is first lubricated with gel. The test is painless and fairly quick. 

Two other kinds of ultrasound can be useful for colorectal cancer. Endorectal ultrasound can show the depth of a tumor in your rectum. This is important for finding out how advanced your cancer is. It can also find suspicious lymph nodes. The doctor puts a thin transducer into your rectum through a proctoscope. The entire exam takes 5 to 10 minutes, and while it may be uncomfortable, it is not painful. The only preparation is an enema the morning of the procedure.

For an intraoperative ultrasound, the transducer is put against your liver during surgery. This test can show whether colorectal cancer has spread to your liver.

Chest X-ray

To see if the cancer has spread to your lungs, the doctor may ask for an X-ray. An X-ray uses an energy beam to make a picture of areas inside your body. The test is simple and routine. It is very effective in showing lung nodules. It can also show whether you have lung or heart disease. Therefore, it’s a standard examination before any major surgery. Some doctors may also ask for a chest CT scan because it is more exact for staging purposes.

Other tests

Your doctor may do other tests to evaluate your colorectal cancer. These may include blood tests or other tests described below.

Complete blood count

Your doctor may ask for a complete blood count, also known as a CBC. A CBC involves drawing a sample of blood and checking it for the following information:

  • The number of red blood cells

  • The number of white blood cells

  • The number of platelets

Information from the CBC will help your doctor evaluate the possible effects of the cancer. For instance, a low red blood cell count may indicate anemia that’s caused by the tumor bleeding.

Blood chemistry tests

Other blood tests measure chemicals in the blood to see how certain organs, such as the liver and kidneys, are working. They may also indicate possible spread of the cancer to the liver. 

CEA tests

Your doctor may ask for a test that measures a special kind of protein in your blood. The protein is called carcinoembryonic antigen or CEA. This is a special protein that can be released by colorectal tumors. To measure CEA, a technologist draws a sample of blood from your arm. The blood is then sent to a lab that tests for CEA. Your doctor is most likely to ask for this if you have already been diagnosed with colorectal cancer. A number of things besides cancer, such as smoking, can increase the CEA. So testing for it is not a good way to confirm tumors if you never had cancer before. But CEA tests are a good way to see if a cancer is coming back after treatment. Sometimes, too, the test is used to gauge how well treatment is working.

Tests of biopsy and surgery samples

Usually surgery is used to remove tumors and to see how far they’ve spread through your colon and surrounding tissues and lymph nodes. These tissues are then analyzed to see if there are any cancer cells present. A pathologist looks at the cells under a microscope.

Special lab tests may be done on any biopsy or surgery samples to see if the cancer cells have certain gene changes. These tests can help show whether certain types of cancer drugs are likely to be effective. 


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