Doctors use tests such as blood tests, mammograms, MRIs, PET scans, CT scans, and biopsies to help them screen for, diagnose, treat, and monitor cancer. If you've 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.

There are many reasons why you may need certain tests done. Tests may be done to:

• Screen for cancer. Screening tests are done in people who don't have any symptoms. They help find a cancer early, when it's small and it hasn't spread beyond where it started. Examples are Pap smears to screen for cervical cancer and mammograms to screen for breast cancer.
• Help diagnose and stage the cancer. If your doctor thinks that you may have cancer, tests will be needed to help diagnose and stage the cancer (find out how big it is and how far it has spread). For example, a chest X-ray, CT scan, and biopsy might be done for someone who has symptoms that could be caused by lung cancer.
• Guide cancer treatment. Sometimes tests are used during treatment to help guide or direct the surgery or radiation that's being done. One example is MRI-guided brain surgery. In this surgery, an MRI is used to create images that help the surgeon remove the tumor and limit damage to the nearby tissues.
• Keep track of cancer and its treatment. If you've been diagnosed with cancer or had treatment for cancer, your doctor will use tests to keep track of the cancer, see if it has come back, or check for side effects of the treatment. An example is checking prostate-specific antigen levels in someone who has been treated for prostate cancer.

There are many different kinds of tests that can be used. Some tests are done on a sample of your blood, urine, or tissue. Others are done on your whole body (such as an X-ray or CT scan). Most of the time, more than one test is done to diagnose or keep track of cancer. Many of the tests used are not cancer-specific, but some are.

A biopsy is a procedure to remove a tissue or cell sample and check it for cancer. A biopsy is often the only way to tell for sure if you have cancer. There are many types of biopsies. The type you have will depend on the type of cancer your doctor is checking for. Common types of biopsies include:

• Fine needle biopsy. A thin, hollow needle is used to remove a small cell or tissue sample.
• Core needle biopsy. A slightly larger needle is used to remove a larger sample of tissue.
• Excisional biopsy. A surgeon removes the entire tumor.
• Incisional biopsy. A surgeon removes part of the tumor.
• Endoscopic biopsy. A tissue sample is removed with the help of an endoscope (a thin, flexible tube with a light and camera at the end). For example, during a colonoscopy, a biopsy may be taken to check for colon cancer.

The cancer or the body's response to the cancer may make substances that can be measured. These substances are called tumor markers, or molecular tumor markers. Markers may only be made for one or more types of cancer. Or they may be caused by noncancer (benign) problems. Some of the markers are used to keep track of changes during and after treatment. In this case, your doctor is looking for changes in the marker levels over time. Here are some common tumor markers:

Molecular tumor markers may be measured in blood, urine, or tissue tests. They're used along with other tests to diagnose and keep track of treatment. The usefulness of tumor markers depends on the stage and specific subtype of cancer. There are many other tumor marker tests that might be used.

Imaging tests are used to find cancer inside the body. They are also used to find the stage of the cancer. And they can help focus treatment (such as radiation or surgery) to the part of the body with cancer. Here are some common imaging tests:

These questions may be helpful when you talk with your doctor about tests:

• What tests do you think are needed?
• What will the test tell us?
• How is the test done?
• Are there any special preparations needed for the test?
• Are there any risks from the test?
• How long does it take to get the results?
• How will the information from the test help?
• What other tests might be needed?

No test is perfect. Sometimes a cancer is missed when it's really there (false negative). Or it may look like a person has cancer when they don't (false positive). Researchers are working on better, more sensitive tests to help diagnose, treat, and keep track of cancer.