PET/CT scan for cancer

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on October 29, 2021.

What is a PET scan?

A positron emission tomography (PET) scan is an advanced nuclear imaging technique used to look for cancer and its spread. PET scanners trace a form of radioactive sugar as it’s absorbed by the body's cells. Because cancer cells grow quickly, they take up larger amounts of sugar (glucose) than normal cells. The patient will drink or be injected with the sugary tracer before the test.

During the nuclear medicine scan, the patient rests on a table and slides into a large, tunnel-shaped scanner. The outpatient procedure is painless and varies in length, depending on the part of the body being evaluated.


What is a PET/CT scan?

This advanced nuclear imaging technique combines a PET scan and a computed tomography scan (CT) into one machine. A CT scan is similar to traditional X-rays. However, it takes pictures in thin slices from different angles. A computer is used to compile these thin slices and create a 3D picture of the X-rayed area. The combination of CT and PET imaging reveals information about both the structure (from the CT scan) and the function (from the PET scan) of cells and tissues in the body during a single session.

Compared with an X-ray or magnetic resonance imaging (MRI) scan, a PET/CT enables doctors to examine medical conditions and abnormalities at a cellular level.

PET scan vs. CT scan

Whereas CT scans take pictures from a variety of angles to show images of the patient's body organs, tissues and bones, the PET scan shows how the patient's cells react to a radiotracer, which may indicate cancerous areas. The combined PET/CT scan joins these two technologies together.

What is PET/MRI?

A PET/MRI scan is used to obtain anatomic and quantitative information from an MRI at the same time as physiologic information from PET imaging. The advantage of a PET/MRI is that it shows soft tissue more clearly. Soft tissue cancers occur most frequently in connective tissue and are often found in the legs and pelvis. They may also develop in the arms and upper body.

What does a PET/CT scan show?

The PET/CT scanner is able to "see'' damaged or cancerous cells where the radiotracer mixture is being taken up. The rate at which the tumor is using the sugary substance may help determine the tumor grade in some tumors. This helps stage your tumor by determining which parts of the body have abnormal activity.

By combining information about the body's anatomy and metabolic function, a PET/CT scan provides a more detailed picture of cancerous tissues than either test does alone. The PET and CT images appear in a single scan, allowing for a high level of accuracy. The use of PET/CT scan helps the oncology team develop the optimal cancer treatment plan. Follow-up may involve additional testing and biopsy or treatment protocols, such as radiation therapy and chemotherapy.

Many oncologists perform a CT scan and/or a bone scan prior to ordering a PET/CT scan.

A PET/CT scan may also:

  • Provide information on how well a treatment is working
  • Help plan future radiation therapy 
  • Determine the right place in the body to perform a biopsy, if needed
  • Check for new cancer growth, after treatment ends, during follow-up care
  • Be used during the diagnostic testing phase of theranostics (cancer care that combines diagnosis and treatment)

A PET/CT scan can be more sensitive than other imaging tests and may find cancer sooner than other tests do. Not all tumors take up the radiotracer, but PET/CT is highly accurate in differentiating from the benign and malignant tumors it finds, particularly in some cancers such as lung and musculoskeletal tumors.

How to prepare for a PET/CT scan

Before the scan

  • Find the right facility for you: PET/CT scans are typically performed at a hospital’s radiology or nuclear medicine department, but you may also find appointments at outpatient imaging facilities.
  • Dress accordingly: You may be asked to disrobe for the procedure, so it may be more convenient to wear loose clothing. The facility may provide a wrap or gown to wear. You’ll also likely be asked to remove jewelry, piercings and metal objects such as dentures or hearing aids.
  • Bring medical records: The technologist may ask for your personal and medical history, including past scans and surgeries and a list of current medications you’re taking.
  • Review instructions for food and drink: You may be asked to avoid eating for about six hours before the PET/CT scan and to drink only water. This may be case by case, so ask your doctor ahead of the procedure. While there’s no specific PET scan diet, eating could interfere with the distribution of the radioactive tracer, resulting in suboptimal images. Avoid drinks with sugar and coffee. The caffeine in coffee can affect sugar (glucose) metabolism.
  • Ask about exercise: Your doctor may require that you avoid exercise 24 to 48 hours before the PET/CT scan.
  • Allot enough time: To help the scan produce its images, you’ll be given a small amount of a radioactive sugar substance, which takes 30 to 90 minutes to travel throughout the body. The scanning procedure takes about 30 minutes. The total time it will take depends on which area of the body requires scanning. Plan to be at the imaging facility for one to three hours.
  • Lean on a friend or family member: If you have anxiety, it may help to bring a loved one for support. Ask the facility whether there are any restrictions on bringing support.

During the scan

  • Be prepared for the placement: You may be asked to lie down on a table that slides into a large scanning machine with a doughnut-shaped hole in the middle. To get clear images, the technologist may ask you to hold your breath at certain times during the scan.
  • Expect an injection: A small amount of a radioactive sugar substance is injected into your body through an intravenous (IV) line. Cancer is attracted to the energy of this substance and absorbs it, which allows the scanner to detect the substance more easily and produce images of the inside of your body.
  • Don’t be afraid to speak up: It’s not uncommon to feel claustrophobic or anxious during a PET/CT scan, so let the technologist know if you’re too uncomfortable.

After the scan

  • Resume activities: Typically, you may resume normal activities right after the scan.
  • Drink water: It’s a good idea to drink plenty of water afterward to flush out the dye or radioactive sugar.

What are possible side effects and risks of a PET/CT scan?

While the benefits of a PET/CT generally outweigh the risks, there are some risks.

  • The radiation risk is low, but not zero.
  • You could have an allergic reaction to the radiotracer. Most allergic reactions are mild. Tell your care team about any allergies beforehand.
  • You may experience slight pain if you’re injected with the radiotracer. It should go away in a short time.
  • You may feel warm or flushed as the contrast is injected.
  • If you’re given a radiotracer to drink, you may have a metallic taste in your mouth.
  • Some procedures require a catheter to be placed in your bladder. The catheter placement may cause temporary discomfort.
  • Be sure to tell your doctor if you could be pregnant or are breastfeeding because the radiation could be harmful.

What do the results mean?

A radiologist who specializes in reading PET/CT scans will interpret the images and report back to the doctor who ordered the procedure.

The radiologist’s report will include:

  • Whether your scans show signs of cancer (and if there is cancer, what stage it is)
  • Whether the cancer has spread and where

How long it takes for your doctor (the ordering physician) to get your results depends on a number of factors:

  • Did your doctor request immediate results? Is there an urgency?
  • How complex is your particular test?
  • Has your doctor given the radiologist all the information needed to interpret the images?
  • Is this your first PET? If you had prior scans, the reader may need to see them for comparison. Does the reader have your previous scans or need to get them?
  • How is the report from the reader being sent: phone, email, fax or regular mail?

Don’t be afraid to ask the facility when your doctor is likely to receive the report.

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