How to Prep for a PET Scan

If your doctor thinks you might have cancer or that your cancer has spread, they’ll likely order imaging tests to look for signs of cancer. Different types of imaging tests give different clues about what is happening inside the body. One of the most common imaging tests used for cancer is called a positron emission tomography (PET) scan.

“A PET scan is used for cancer diagnosis, through disease monitoring, and after treatment to see how well the treatment worked,” said Nancy Lee, MD, an American Society of Clinical Oncology (ASCO) expert and radiation oncologist at Memorial Sloan Kettering Cancer Center.

How is a PET scan different from other imaging tests?

Other imaging tests, like an x-ray, CT or CAT scan, or MRI, mainly show what organs and possible tumors look like in a specific area of the body. A PET scan is different because it looks at most or all of the body in a single exam. It shows how the body’s cells, tissues, and organs are working.

Before a PET scan takes place, a radioactive sugar called a tracer is injected into a vein (intravenous or IV). As the tracer travels through the bloodstream, the sugar is absorbed by the body’s cells. Cells that are very active absorb or “take up” more sugar than cells that are less active. As a result, the tracer collects in areas where the cells take up the most energy.

Because cancer uses energy more actively than healthy cells, cancer cells absorb more of the sugar than normal cells. Areas that absorb more tracer “light up” on the scan, indicating that cancer might be present and the area may need a closer look.

Often a PET scan is done along with a CT or CAT scan. A CT scan uses radiation to create more detailed images of a specific part of the body. When these two tests are used together, called a PET-CT scan, they can help show exactly where a tumor is located.

How to prepare for a PET scan

You’ll need to fast for at least 6 hours before a PET scan. This helps keep your blood sugar at a stable level so the sugar in the tracer can be absorbed better. “That means no coffee, no candy, no gum, nothing except water, because all of that can affect your results,” said Dr. Lee.

When getting ready for a PET scan, you should:

• Ask whether you should take your usual medicines or supplements on the day of the scan.
• Be sure your doctor knows if you have diabetes, allergies, or kidney problems.
• Avoid wearing anything metal to your appointment because it can interfere with the scan. Leave all jewelry, such as watches, rings, or earrings, at home. Metal implants,  including pacemakers and joint replacements, are usually not a problem.
• Contact your health insurance provider before the scan to ask if it will be covered and if you will have any out-of-pocket costs. Sometimes health insurers require pre-authorization for PET scans.

What happens during a PET scan?

When you arrive for a PET scan, you will get an IV line to inject the radioactive tracer. To give the sugar time to travel through the body and get absorbed, you’ll have to wait 30 to 60 minutes before the scan will start.

If you’re having a PET-CT scan, you’ll also be given a contrast medium to help make the CT images clearer. You’ll either drink the CT contrast liquid or it will be put into your IV.

The PET scanner is a large machine with a hole in the center that’s big enough for an exam table to slide slowly back and forth through it. You’ll lie on your back on the exam table for the duration of the scan, which typically lasts about 30 minutes.

You’ll need to lie as still as possible and may need to hold your breath at certain points during the scan. The table might also move up, move down, or tilt during parts of the scan to get certain images. You’ll hear some clicks and noises, too. If you’re uncomfortable at any point, tell the technologist. They may have ways to help make the process easier, like playing your choice of music to help you relax.

How are PET scans used for cancer diagnosis and treatment?

A PET scan cannot always detect cancer by itself. And areas that “light up” on a PET scan may or may not be cancer. “Just because something lights up doesn’t mean it’s cancer,” said Dr. Lee. “Patients will usually need to have a biopsy to find out more.”

A PET scan can be used in a number of ways, including:

• To help make an initial cancer diagnosis based on where the tumor is located and whether the cancer has spread elsewhere in the body.
• During treatment for certain cancers to help doctors learn how well a treatment is working.
• After cancer treatment is complete to help doctors look for signs the cancer has come back (known as a recurrence).

“Frequently, we order a PET scan to look at whether the tumor is active or not, if the tumor is gone or not. And we couple that with additional imaging depending on the disease status,” said Dr. Lee.

You can usually expect your scan results within 24 hours, said Dr. Lee, though it can vary from provider to provider. Ask your healthcare team when to expect results and how you’ll receive them. You can also ask what the next steps will look like if the scan comes back with abnormal results.

Dr. Lee is an ASCO member.