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Understanding Cancer Tests

Doctors use tests like mammography, MRI, and CT scans to help them screen for, diagnose, treat, and monitor cancer. If you have been diagnosed with cancer, you may have had one or more of these tests. You may have wondered why you had certain tests and not others, and what exactly the doctor was looking for.

Why do I need to be tested?

There are several reasons why a doctor may order a test for you. Tests may be done to:

  • Screen for cancer. Screening tests are done in people without any symptoms as a way to find a cancer early. An example of this is the Pap smear to screen for cervical cancer and a mammogram to screen for breast cancer.

  • Help diagnose and stage the cancer. If your doctor thinks that you may have cancer, he or she may recommend a test to help diagnose and stage the cancer (find out how far it has spread). An example of this might be a chest X-ray and CT scan for someone with symptoms suggesting lung cancer.

  • Guide treatment for the cancer. These tests may be used during treatment to help guide or direct the surgery or radiation that is being done. An example of this might be MRI-guided brain surgery. In this surgery, an MRI is used to create virtual images that help the surgeon remove the tumor with less damage to the nearby tissues.

  • Monitor the cancer and its treatment. If you have been diagnosed with cancer or if you have had treatment for cancer, your doctor will use tests to monitor the cancer, see if it has come back, or to check for side effects of the treatment. An example of this might be checking the prostatic-specific antigen (PSA) levels in someone who has been treated for prostate cancer.

What types of tests exist?

There are many different types of tests. Some tests require you to give a sample of your blood, urine, or tissue. Others are done on you (for example, an X-ray or CT scan). Often, more than one test is done to diagnose and monitor cancer. Many of these tests are not cancer specific, but some are.

Tumor markers

The cancer or the body's response to the cancer may make substances that can be measured. They may be specific to one or more types of cancer. Or they may be caused by noncancer (benign) problems. Some of these markers are used more in monitoring during and after treatment. Here are some common molecular markers:

Selected tumor markers

Cancers linked with increased levels

Alpha-fetoprotein (AFP)

Liver and germ cell cancers of the ovary or testicle

CA 15-3

Breast cancer

CA 19-9

Pancreatic, and biliary tract cancers

CA 27-29

Breast cancer

CA 125

Ovarian cancer

Carcinoembryonic antigen (CEA)

Colorectal cancer

Human chorionic gonadotropin (HCG)

Choriocarcinoma or gestational trophoblastic disease (a rare type of uterine cancer involving placental tissue) and germ cell cancers of the ovary or testicle

Prostate-specific antigen (PSA)

Prostate cancer

Molecular tumor markers may be measured in blood, urine, or tissue tests. They are used along with other tests to diagnose and monitor treatment. Following these tumor markers may or may not be necessary based on the stage and specific subtype of cancer. Other tumor marker tests may be done as well.

Imaging tests

Imaging tests are used to locate the cancer within the body. This information is helpful in determining the stage of the cancer and can help target the treatment to the area where the cancer is located. Here are some common imaging tests:


Imaging test

How it works

Computed tomography (CT scan)

A combination of X-rays and computer technology that produces horizontal, or axial, images (often called slices) of the body.

Positron emission tomography (PET)

A radioactively-linked substance is injected into the body. Images are then taken to see where the body is using these substances.


Radio waves in a strong magnetic field are used to create a computer-generated, detailed image of inside the body.

Radionuclide imaging

The person is given radioactively-linked substances through injection or by mouth (orally). Images are then taken to see where the body is using these substances (for example, bone, liver, or thyroid scans).


Sound waves are used to create a picture of what is inside the body.


A radiograph is used to create a two-dimensional image of the body.

The following questions may be helpful when tests are being recommended:

  • What tests are being recommended?

  • What is the purpose of the test?

  • What is it like to have the test?

  • Are there any special preparations that are needed for the test?

  • Are there any risks from the test?

  • How long does it take to get the results?

  • How will the information you get from the test help?

  • What other tests might be needed?

  • Are there any clinical trials regarding the tests that should be considered?

Unfortunately, no test is 100% perfect. Sometimes a cancer is missed when it is present (false negative). Or it may look like a person has cancer when he or she does not (false positive). Researchers are working on developing better, more sensitive tests to help diagnose, treat, and monitor cancer. This development process is similar to the process for developing new cancer treatments. Tests must be developed, tested, and approved for use through the U.S. Food and Drug Administration (FDA). That process can take a lot of time and money. And, once approved, it may take some time before the tests are available on a widespread basis.

Online Medical Reviewer: Dozier, Tennille, RN, BSN, RDMS
Online Medical Reviewer: Gersten, Todd, MD
Online Medical Reviewer: LoCicero, Richard, MD
Online Medical Reviewer: Sather, Rita, RN
Date Last Reviewed: 6/1/2018
© 2021 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare provider's instructions.
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