PET Scans and PET/CT for Cancer

A PET scan, short for positron emission tomography, is a type of nuclear scan. It makes detailed 3-D pictures of areas inside your body where cells take up glucose, a sugar. Cancer cells often take up more glucose than healthy cells, so those areas can show up on the scan, helping doctors find cancer and see how it behaves.

Your doctor will usually ask you not to eat for several hours before the scan. Drink only water in that window. Eating beforehand can change how the tracer spreads through your body. That can force a reschedule. If you have diabetes, tell your team well ahead of time. The test depends on how your cells take up sugar, so your blood sugar level matters. You may get special instructions for your diabetes medicines that day.

Tell your team about all medicines, vitamins, and supplements you take, and mention if you are pregnant or breastfeeding. On the day, wear loose, comfortable clothing and be ready to remove jewelry, eyeglasses, dentures, or other metal items before the scan.

Before the scan, you get an injection of a tracer. Most often, it is FDG, a radioactive form of glucose. You then wait about an hour while it spreads through your body. During the scan, you lie still on a table that moves through the scanner. A PET/CT combines two scans in one visit. The CT part takes less than two minutes. The PET part takes about 20 to 30 minutes. Depending on which part of your body is being studied, the full visit can run from about an hour up to roughly three hours.

A PET or PET/CT scan uses a small, carefully controlled amount of radioactive tracer. A combined PET/CT also adds the CT's own low-dose X-ray radiation. Doctors have used nuclear medicine scans like this for more than six decades. There are no known long-term harms from a dose this low. The tracer naturally breaks down. It leaves your body through urine or stool within hours to days. Drinking plenty of water afterward helps your body clear it faster.

PET is frequently paired with CT in a single procedure. The CT creates detailed "anatomic" pictures of your organs and structures. The PET adds "functional" information. It shows where cells are more active, since cancer cells often use energy differently from normal ones. Combining the two gives a fuller picture of a tumor's location, and whether it is growing or spreading, than either test alone.

According to the National Cancer Institute, PET/CT may improve the ability to diagnose cancer. It can show how far a tumor has spread, help plan treatment, and track how well treatment is working. It may also reduce how many extra imaging tests you need.

A PET scan is not perfect. Inflammation and infection can also take up extra glucose. So an active area on a PET scan is not automatically cancer. That is why doctors read PET results alongside your other tests. They are often confirmed with a biopsy, rather than treated as a stand-alone diagnosis.

Like all imaging, a PET or PET/CT scan is one part of the overall picture. Your doctor reads its results alongside lab tests and, in most cases, a biopsy, then explains what the findings mean specifically for your care and what, if anything, happens next.

You can usually go back to your normal activities right away. A small amount of the radioactive tracer stays in your body for a while as it clears through your urine and stool. Ask your team whether they recommend any precautions for that window, such as extra fluids or limiting close contact with certain people, and for how long.

Ask what your team hopes to learn from this scan, and how you should prepare, including fasting and diabetes medicine timing. Ask what tracer will be used, how long the appointment will take, and how this scan fits with your other tests and treatment plan. Ask when results will be ready and who will walk you through them.

• National Cancer Institute — Diagnosis of Cancer.
• RadiologyInfo.org (Radiological Society of North America / American College of Radiology) — Positron Emission Tomography (PET) / Computed Tomography (CT).