PET Scan for Cancer

A PET scan (positron emission tomography scan) can be used to find cancer or plan further cancer treatment. This test is sometimes combined with a CT (computed tomography) scan and referred to as a PET/CT scan.

What is a PET scan?

A PET scan is a type of nuclear medicine scan. It uses radiotracers to show abnormal activity in your organs and tissues. These radiotracers travel to tissues in your body. Special cameras can then pick up signals and take pictures of the places the tracer traveled to.

Other types of imaging tests, like MRI and CT scan, show anatomy and structure. A PET scan shows how active the tissues and cancers in your body are.

What does a PET scan show?

A PET scan shows areas of abnormal activity in your body. This abnormal activity could be because of cancer, inflammation, infection, or other changes.

PET scans can help doctors:

• Find cancer
• Determine the amount of cancer in the body and where it is located (staging)
• See if a current treatment is working
• Find out if treatment worked after it is finished
• Check for new cancer growth or recurrence after treatment ends

PET scans combined with other imaging may also help:

• Plan cancer treatment, such as radiation therapy
• Find the right place for a biopsy

How does it work?

PET scans usually use a form of radioactive sugar (radiotracer) called fluorodeoxyglucose-18 or FDG-18. It is injected into a vein.

Body cells take in different amounts of this sugar, depending on how fast they are growing or how much energy they use. Cancer cells grow quickly, so they are more likely to take up larger amounts of the sugar compared to normal cells.

The sugar shows up as a bright spot on the PET scan.

Certain tissues in your body, such as your heart, kidneys, and brain, will take up more of the sugar than other organs. This is normal. It does not always mean there is cancer in these areas.

Other places in your body might also take up larger amounts of the sugar. This includes areas of inflammation or infection, or areas that have changed because of recent surgery or radiation.

To make sure the test results are accurate, you’ll be asked not to drink any sugary liquids or eat for several hours before the test.

PET/CT scan

Many clinics and hospitals have machines that can combine a PET scan with a CT scan. With this type of scanner, the PET shows any areas of increased cell activity while the CT shows more detail in these areas. This helps doctors pinpoint tumors, but it also exposes a person to more radiation.

How do I get ready for the scan?

PET and PET/CT scans are generally painless. They are usually done in the nuclear medicine department of a hospital or clinic.

Your healthcare team will tell you exactly what you need to do to get ready for your scan. You may be asked to:

• Stop eating or drinking for 4 to 12 hours before the scan.
• Limit heavy exercise for 24 hours before the test. Exercise can affect your test results.
• Avoid some prescription and over-the-counter medicines before the test.
• Drink lots of fluid after the scan to help clear the tracer from your body.

Tell your care team everything you take. This includes over-the-counter medicines, vitamins, and herbs. Certain medicines may affect your test results.

What is it like having a PET scan?

A nuclear medicine technologist (tech) usually does the scan. For your safety and to get the best images, the tech might ask you to:

• Undress and change into a gown.
• Remove metal objects that might interfere with the test.
• Empty your bladder.
• Follow other instructions for a CT scan.

Getting the tracer and contrast dye

Before your scan, the radioactive tracer will be put into your vein through an IV. You will sit in a chair and limit your activity so the tracer doesn’t go into places it’s not supposed to go.

It may take 30 to 90 minutes for the tracer to reach the parts of your body being studied.

If you are having a PET/CT scan and need contrast dye too, you may be asked to drink liquid contrast. Or it may be put into your IV.

The dye could make you feel hot, itchy, or leave a metallic taste in your mouth. This usually goes away quickly, but tell your tech if you have any of these symptoms in case they are signs of an allergic reaction.

During the scan

During the scan, you will lie on a table while the tech positions you. The machine looks like a large donut with a table that slides through the center. The table will slowly slide back and forth as the images are taken.

The tech will be in a nearby room, but they can always see, hear, and talk to you.

The scan is usually painless, but you may find it uncomfortable to be in certain positions for long periods of time.

How long does a PET scan take?

A PET scan takes 20 to 30 minutes, but you must wait about 30 to 90 minutes while the tracer collects in the organ or tissues being studied.

After the test, the tech will review the images to make sure they are clear and show everything. If not, you may need more images.

What are the possible risks and side effects?

Tracer reactions: Reactions to the tracer are usually rare. If they do happen, they are mild. You might have some pain or swelling where the tracer was put into your vein.

Contrast dye reactions (PET/CT scan): Some people react to the contrast dye used for the CT portion of a PET/CT scan.

Possible contrast dye reactions include:

• Rash
• Nausea or vomiting 
• Flushing
• Feeling of warmth 
• Itchy or scratchy throat 
• Taste changes 

Most symptoms are usually mild and often go away on their own, but they can be a sign of a more serious reaction that needs treated. Tell your healthcare team or tech if you have any of these problems.

Severe reactions: In rare cases, people can have a severe reaction that causes blood pressure issues, swelling or rashes, or breathing problems like shortness of breath or wheezing. This must be treated right away. Tell your healthcare team or tech if you notice any changes after getting the contrast dye.

Kidney problems: The IV contrast dye can also cause kidney problems. This is rare, and it’s more common in people whose kidneys already do not work well. If you need a scan with contrast dye, your healthcare team might first do a blood test to check your kidney function. They might also give you extra fluids in an IV or medicines to help your kidneys get rid of the dye safely.

What else should I know about a PET scan?

• If you are pregnant or breastfeeding and need a PET scan, talk to your healthcare team about the best plan for you.
• These tests are expensive. Check to make sure they are covered by your insurance.
• PET scans may be able to detect cancer earlier than other imaging tests.
• A combined PET/CT scan is often more convenient because you do not have to do the tests separately.

Learn more

• Imaging Tests for Cancer