Decorative image

PET scan

You might have a PET scan to find out more about the stage of your cancer. 

PET stands for positron emission tomography. This type of scan can show how body tissues are working, as well as what they look like.  

You’ll usually have a PET scan in the x-ray (radiology) department as an outpatient. These scanners tend to be only in the major cancer hospitals. So you might have to travel to another hospital to have one.

A radiographer operates the scanner. It usually takes between 30 and 60 minutes.

Why you might have a PET scan

A PET scan can help to:

  • diagnose cancer
  • stage the cancer, so doctors know whether it has spread
  • decide the best treatment for your cancer
  • show how well a cancer treatment is working

After you have had treatment for cancer, a CT scan may show an area that looks like there might still be some cancer left. This might not be cancer but scar tissue left over from the cancer killed off by your treatment. A PET scan can sometimes show whether this tissue is active cancer or not.

Preparing for your PET scan

For most PET scans, you need to stop eating for about 4 to 6 hours beforehand. You can usually drink water during this time.

You might have instructions not to do any strenuous exercise for 24 hours before the scan.

Call the number on your appointment letter if not eating is a problem for you, for example if you’re diabetic. You might need to adapt your diet and sugar control and your appointment time could change.

Some people feel claustrophobic when they‘re having a scan. Contact the department staff before your test if you’re likely to feel like this. They can take extra care to make sure you’re comfortable and that you understand what’s going on. Your doctor can arrange to give you medicine to help you relax, if needed.

What happens

Your radiographer might ask you to change into a hospital gown. You have to remove any jewellery and other metal objects such as hair clips. Metal interferes with the images produced by the scanner.

You have an injection of a dye called a radiotracer about an hour before the scan. You’ll have this injection through a small plastic tube called a cannula in your arm.

You need to rest and avoid moving too much during this hour. This allows the drug to spread through your body and travel to places where glucose is used for energy, like your brain.

When you arrive at the scan department

Your radiographer takes you into the scanning room. The PET machine is large and shaped like a doughnut.

You have most scans lying down on the machine couch on your back.

Once you’re in the right position, your radiographer leaves the room. They can see you on a TV screen or through a window from the control room.  You can talk to each other through an intercom.

Having the PET scan

The couch slowly slides backwards and forwards through the scanner. The machine takes pictures as you move through it. 

The scan is painless but can be uncomfortable because you have to stay still. Tell your radiographer if you’re getting stiff and need to move.

It’s not particularly noisy but you’ll hear a constant background noise.

When it’s over, your radiographer will come back into the room and lower the couch so you can get up.


After your PET scan

Your radiographer removes the tube from your arm before you go home.

You can then eat and drink normally.

Someone will need to take you home if you’ve had medicine to help you relax. You won’t be able to drive for the rest of the day as you might be drowsy.

The radiotracer gives off very small levels of radiation that go away very quickly. But for the rest of the day you shouldn’t have close contact with pregnant women, babies or young children.

Possible risks

A PET scan is a safe test for most people. But like all medical tests it has some risks. Your doctor and radiographer make sure the benefits of having the test outweigh these risks.


Pregnant women should only have the scan in an emergency. There’s a risk that the radiation could harm the developing baby. Contact the department beforehand if you're or think you might be pregnant.


If you're breastfeeding, let the department know a few days before your appointment. They will let you know if you need to stop breastfeeding for a length of time after having the radioactive drug. You might need to store enough expressed milk for at least one feed.


Exposure to radiation from the radiotracer during a PET scan slightly increases your risk of developing cancer in the future. Talk to your doctor if this worries you. 

Bruising and swelling

You might get a small bruise around the area where they put the needle in. 

There's is a risk that the radioactive tracer will leak outside the vein. This can cause swelling and pain in your arm but it's rare.

Allergic reaction

Rarely, people have an allergic reaction to the radioactive tracer. This most often starts with weakness, sweating and difficulty breathing. Tell your radiographer immediately if you feel unwell.

Getting your results

Your scan will be looked at by a specialist doctor and you should get your results within 1 or 2 weeks. You won't get any results at the time of the scan. 

Waiting for test results can make you anxious. Ask your doctor or nurse how long it will take to get them. Contact them if you haven’t heard anything after a couple of weeks.

You might have the contact details for a specialist nurse. You can contact them for information and support if you need to. It may help to talk to a close friend or relative about how you feel. 

For support and information, you can call the Cancer Research UK nurses on freephone 0808 800 4040. The lines are open from 9am to 5pm, Monday to Friday.

Contact the doctor that arranged the test if you haven't heard anything after a couple of weeks.

Last reviewed: 
01 Dec 2020
Next review due: 
01 Dec 2023
  • Guidelines for the diagnosis and treatment of cholangiocarcinoma: an update.
    S Khan and others 
    Gut. 2012 December, Volume 61, Issue 12, Pages 1657-69

  • Guidelines for the diagnosis and management of intrahepatic cholangiocarcinoma
    J Bridgewater and others
    Journal of Hepatology, 2014. Volume 60, Pages 1268-1289

  • Biliary cancer: ESMO clinical practice guidelines for diagnosis, treatment and follow up
    JW Valle and others
    Annals of Oncology, 2016. Volume 27, Pages 28-37