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What is a PET scan?

A positron emission tomography (PET) scan is used to visualize and measure changes in the blood flow, metabolism, and chemical composition of your body. It uses a special dye containing radioactive sugar (fluorodeoxyglucose, or FDG) to image the body.

Like we said earlier, PET scans detect metabolic activity. Because cancer cells consume more energy than healthy cells, they consume the radioactive tracer in higher amounts — causing them to light up on the scan! A PET scan is usually combined with a CT scan, so the lit up area can be layered on top of your body. This is so we know where the tumor is! This type of scan help determine whether a smaller spot is cancerous or benign, as cancerous lesions are much more likely to light up on a PET scan than benign spots or scar tissue. 

Here’s what a PET machine looks like:

What do I need to do before the scan?

Your doctor will provide you with specific instructions. Typically, you’ll be asked not to eat or drink anything except water on the day of the scan. You might also be asked to avoid strenuous exercise for 24 hours before your scan.

What happens on scan day?

Arrive at the test center a bit early. When you get there, you will register with the front desk, then be taken to a dressing area. You should remove all your jewelry and clothing, and put on a hospital gown. You will also be asked to sign some consent forms, outlining the minimal risks of PET scans.

Then, the technician will explain the procedure. First, you will be asked to drink a dye, followed by in intravenous (IV) injection of a radioactive form of sugar (fluorodeoxyglucose, also known as FDG). The IV will typically be placed in your arm. After the IV, you’ll be asked to sit or lie still for about 45 minutes. This allows the dye to distribute through your body.

The technician will likely ask if you’ve ever had an allergic reaction to the dye used. It’s important to notify the technician if you have any metal in your body.

What happens during the scan?

A PET scan takes between 30 and 45 minutes, and is completely painless.

During a PET scan, patients lie flat on a narrow table sticking out of a doughnut-shaped machine. The table will move, sliding you through the machine.

The technician will give you various directions from a cubicle next to the scan room, such as when to hold your breath or stay completely still. You’ll be able to hear the technician, and the technician will be able to hear you. If you need help with anything, just ask!

It’s very important that you stay very still during the scan, so the images don’t come out blurry.

If you are claustrophobic, you can request a sedative — but this is usually not needed.

What happens after the scan?

It’s a good idea to drink plenty of water following a scan with contrast, to help your body get rid of the dye. Depending on the type of dye used, you might be advised to avoid close contact with kids, pregnant women, or pets for about 8 hours after the scan. Ask your technician if this applies to you.

Your PET scan report should be available within a few days. It will be sent to your oncologist and uploaded to your patient portal, if you have one. Your oncologist will discuss the results with you at your next appointment.

You should also request a CD copy of your scan photos for your records. Place this copy in a safe space. Some hospitals have a $5 charge for them, but they are legally obligated to provide you with a copy.

How often do I need to get a PET scan?

This will depend on your individual circumstances. PET scans are usually ordered when your doctor suspects mestastases, or when a patient has rising CEA levels but clear CT scans. In these situations, a PET/CT scan can help find areas with metastatic cancer.

Oncologists will use different types of surveillance scans (CT, MRI or PET) depending on what they think is the best way to monitor your cancer. So you’ll likely get a variety of scans at different points in your treatment. 

In fact, PET scans are used much less frequently than CT and MRI scans for surveillance. Many patients will have a CT and/or MRI scan initially, and only be given a PET scan if there’s something your oncologist wants to see in greater detail.

How useful are PET scans?

PET scans are used to detect metabolically active areas in your body — so cancer shows up. PET scans can detect smaller tumors than CT or MRI scans, although they can’t reliably detect tumors smaller than a centimeter. Also, small tumors in the peritoneum can be very difficult to detect with most scanning technologies. Areas of inflammation can also show up on PET scans. 

If your doctor is unsure of the results of a PET scan, they’ll likely order another test to get a better look.

Should I be concerned about radiation exposure?

When you have a PET scan, a radioactive tracer is injected into your veins. This exposes you to radiation, which can cause DNA damage — and in rare cases, increase your chances of developing a secondary cancer.

That said, the amount of radiation exposure is relatively low, and the benefits of being able to accurately monitor your cancer far outweigh the risks. However, it’s a good idea to speak with your team about your concerns. They may be able to recommend an alternative screening test, or clarify the risks and benefits. It’s always important to follow your oncologist’s screening recommendations to make sure you’re receiving the best care possible.

COLONTOWN University has so much more to offer, from DocTalk videos with CRC experts to easy-to-understand biomarker test breakdowns. We’re here for you! See our list of Learning Centers here.

Last updated: May 18, 2022