Positron emission tomography (PET) scans can detect cancer earlier than other imaging tests. But some types of cancer are harder to detect on a PET scan. In particular, they may miss cancers that don’t use a lot of glucose.
Positron emission tomography (PET) is a nuclear medicine imaging test. Using a special dye that contains radioactive tracers, PET scans help doctors diagnose a variety of diseases, including cancer, particularly in the early stages.
PET imaging can also help determine how well cancer treatment is working.
It’s an effective way to find cancer, but a negative PET scan doesn’t always mean there’s no cancer.
There are some conditions that can lead to a false-negative or a false-positive result. That’s why it’s often performed in combination with computed tomography (CT) scans, magnetic resonance imaging (MRIs), and other diagnostic tests.
Let’s take a closer look at PET scans for cancer and what a negative result may mean.
PET scans are effective imaging tests. They can detect abnormal activity in the body and often find cancerous tumors earlier than other imaging tests can.
But the degree of accuracy varies with the type of cancer and how it’s combined with other tests.
But F-FDG isn’t cancer-specific. So, PET scans may reveal hot spots that aren’t necessarily related to cancer.
False positives can happen due to conditions such as:
- bacterial and fungal infections
- radiation pneumonitis
- post-operative surgical conditions
Blood sugar and insulin levels can also affect results. So, you can get a false-positive result if you have diabetes or if you ate something within a few hours of the test.
For these reasons, PET scans are often performed along with other tests, such as CT scans or MRIs. It takes a specially trained radiologist or nuclear medicine specialist to evaluate and interpret the results.
Other tracers may be used to look for specific types of cancer. For example, prostate-specific membrane antigen (PSMA) can help detect prostate cancer.
A negative PET scan means that the test did not detect cancer. But certain types of cancer don’t use a lot of glucose.
A negative PET scan may miss certain cancerous tumors, such as:
- bronchioalveolar carcinomas (a type of lung cancer)
- carcinoid tumors (tumors that grow from neuroendocrine cells)
- low-grade lymphomas
Low-activity tumors are a
Some cancers can. A PET scan may not detect tumors with low activity or those that are very small and slow growing.
If a PET scan is negative, your doctor may recommend other tests to help diagnose or rule out cancer. Depending on the type of cancer they suspect, this may include:
- other imaging tests, such as CT, MRI, or bone scans
- blood tests
- urine tests
Most of the time, you’ll need a biopsy to confirm a cancer diagnosis.
Your doctor will provide instructions on how to prepare, which includes not eating for several hours before you have a PET scan.
You may get more detailed instructions if you:
- are pregnant
- have diabetes
- are breastfeeding
- have claustrophobia
A nurse or technician will insert an intravenous (IV) catheter into your arm or hand to inject the radiotracer solution. It takes about 30 to 60 minutes for your body to absorb the tracer. During this time, you’ll need to move as little as possible.
You’ll lie flat on a narrow bed that will slide into a cylindrical scanner. It’s important to stay perfectly still and not talk during the scan.
It can take 30 minutes to an hour, depending on the area being scanned. You can go home soon after the scan is complete, and the results will go to your doctor.
Possible side effects
A PET scan is painless, with the possible exception of inserting the IV. You might feel a cold sensation in your arm when you get the radiotracer injection. Some people may have temporary discomfort, swelling, or redness at the injection site.
You may experience nervousness or anxiety if you have:
- trouble staying still for a long time
- a fear of needles
- a fear of tight or closed-in spaces
PET scans, especially when combined with CT scans or MRIs, can help diagnose, stage, and monitor treatment for cancer.
Potential benefits include:
- more detailed pictures of body structures and how they’re functioning than other imaging tests can provide
- finding cancer earlier than other diagnostic tests would
- possibly being able to determine whether cancer is spreading and if treatment is working
- pinpointing the precise location and size of tumors
- offering a possible alternative to exploratory surgery
Of course, medical tests come with some potential risks as well. PET scan risks include:
- false-positive or false-negative results
- an allergic reaction to the radiotracers
- exposure to radiation
Radiation exposure from PET scans is low, as the radiation usually passes out of your body in a matter of hours. Speak with your doctor if you have any concerns about radiation exposure from a PET scan.
A PET scan is a type of nuclear imaging test that can help detect cancer. It’s an effective tool in diagnosing, staging, and monitoring cancer treatment.
But you can still have cancer if a PET scan is negative. That’s because a few types of tumors are harder for PET scans to detect.
For this reason, many doctors will also request PET scans in combination with CT scans, MRIs, and other diagnostic tests. This helps radiologists or nuclear medicine specialists evaluate the results more accurately.
Your doctor can give you more information about the need for a PET scan and provide details on what you need to do before you have this procedure.