Positron Emission Tomography (PET)

Image courtesy of Dr. Carlos Cuevas, University of Washington

What is Positron Emission Tomography (PET)?
Positron Emission Tomography (PET) is a noninvasive way for the doctor to examine the inside of a patient’s body.  PET imaging quickly and painlessly examines the body’s chemistry and gives information about how organs and tissues are functioing.

How does PET imaging work?
Cells in the body rely on glucose (sugar) for energy.  Some cells normally require more of this fuel than others.  When cells become cancerous they often change the way they use glucose.  For example, actively growing cells, such as pancreatic cancer cells, break down glucose at a faster rate than other cells.  PET provides a visual representation of this change.

The most common PET scan is a FDG-PET.  For this procedure, a small amount of radioactive material is attached to glucose.  This radioactive form of glucose, called fluorodeoxyglucose (FDG), is injected into the patient’s body.  As it is broken down (metabolized) by organs and tissues, FDG emits traces of radiation that are recorded by the PET scanner.  Since cancer cells are more metabolically active than normal cells they use more FDG than most normal cells. A computer turns the signals recorded by the PET scanner into whole-body images that show areas where cancer cells may be present.  Areas that metabolize more FDG than others show up as bright or highlighted areas on the PET images. Therefore, because cancer cells use more FDG, they appear brighter on computer images.

Will the sugar feed the tumor?
Fluorodeoxyglucose (FDG), the glucose used to produce a PET image, will not “feed” the tumor or cause the tumor to grow in size.  In a person with cancer, metabolic changes can cause the body to break down body fat and lean body mass to make energy for both cancer cells and healthy cells.  This is the case regardless of sugar intake.

Why is PET imaging used?
By providing information about metabolic activity in the body, PET scans may detect abnormalities before other imaging tests such as computed tomography (CT) or magnetic resonance imaging (MRI) scans.  A PET scan may help the doctor see extremely small cancerous masses that might otherwise go undetected.  Some scientific studies suggest PET imaging has enhanced sensitivity so it can reveal the presence of an abnormal mass and also indicate whether the mass is benign or malignant, based on metabolic function.

How should I prepare for a PET scan?
PET is usually done on an outpatient basis.  The healthcare team will give the patient detailed instructions on how to prepare for the procedure.  Generally, a patient should avoid strenuous exercise and not to eat or drink anything for 4 hours prior to the appointment.  Both of these activities can contribute to increased metabolism which will appear during PET imaging.

Patients are encouraged to wear comfortable, warm clothing since scanner rooms are often cool.

What happens during a PET scan?
A PET scan usually lasts 2-3 hours.  First, the patient will receive an injection of FDG and be required to sit quietly and rest for 45-60 minutes while the injected FDG travels through the blood and collects in organs and tissues.  When the scan begins, the patient should be prepared to lie still for 15-75 minutes while the imaging is performed.

What can I expect after a PET scan?
After a PET scan is complete, the patient should drink plenty of water to help flush out the remaining FDG.

A radiologist interprets the results of a PET scan and sends the results to the doctor who ordered the procedure.  Results should be available within a few business days.  Follow the doctor’s recommendations regarding how often PET imaging should be performed.

When is a PET scan usually done?
PET scans are sometimes performed prior to treatment to help the healthcare team determine the appropriate methods of treatment.  In pancreatic cancer, PET scans are used to

  • differentiate between benign (non-cancerours) and malignant (cancerous) pancreatic tumors.
  • determine whether a tumor is resectable (eligible for surgical removal).
  • locate a pancreatic mass.
  • detect the spread of pancreatic cancer to the liver or other parts of the body.
  • assess tumor responsiveness to chemotherapy or other treatment.

How does PET compare to other imaging methods?
PET imaging monitors body functioning while CT and MRI images provide information such as size, shape and location of physical structures in the body.  For pancreatic cancer diagnosis, CT scans are more common than PET scans.  Most of the time, PET is used in addition to CT or MRI scans.  Studies show varying results in the reliability of CT or MRI versus PET in detecting tumors when each technique is used alone.

Some hospitals and institutions have a machine that combines PET and CT scans.  By running both tests at the same time, trained radiologists are able to use the strengths of both imaging procedures.  The combined PET-CT scan helps determine the precise location of an area showing high metabolic activity indicating a cancerous tumor.  In pancreatic cancer, studies have shown that the combined PET-CT scan is more effective than PET scan alone in determining if surgical removal is possible.  The combined imaging can also more accurately identify distant metastatic pancreatic masses. However, PET-CT is not a substitute for high-quality, contrast-enhanced CT or MRI scans, but may be used in addition to these tests.

PET scans are more sensitive and useful in some cancers than others.  Studies are still underway to determine the usefulness of PET scans in pancreatic cancer.  Until it has proven to be the best imaging technique for pancreatic cancer, it is important to use PET in conjunction with other imaging techniques to build a complete picture.

There is currently no clearly indicated situation in which PET should be used routinely.

What are the potential limitations of PET?
PET can give false results if an individual’s chemical balances are not normal.  For example, pancreatitis or infections can give false positives on PET scans.  The inflammation associated with these conditions can lead to a larger glucose uptake.  Other diseases or surgeries can also affect the use of glucose. Therefore, a positive PET scan does not always mean that a person has pancreatic cancer.

The opposite is also true; a negative PET scan does not always mean that a person does not have pancreatic cancer.

Both PET and CT imaging have limitations at detecting tumors that are 1 centimeter in diameter or smaller.  While PET does not always show smaller tumors than CT imaging can detect, it may show small tumors that CT imaging misses.  The reverse may also be true.

Test results of diabetic patients or patients who have eaten within a few hours prior to the examination can be inaccurate because of blood sugar or blood insulin levels.  Diabetic patients should ask for specific diet guidelines to control glucose levels during the day of the test.  The glucose used to perform a PET scan is not harmful to diabetics.

Is PET covered by insurance? 
Insurance providers, including Medicare and private insurance companies, cover the cost of most PET scans. Check with the patient’s insurance provider to determine complete coverage information.

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