Positron Emission Tomography (PET)

Diagnosis: Positron Emission tomography (PET)

Image courtesy of Dr. Carlos Cuevas, University of Washington

Since the early 1970s, PET imaging has been used as a research tool.  In the 1990’s, advancements in technology and materials brought PET imaging to hospitals, diagnostic clinics, and doctor’s offices.  Since then, over 600 PET and combination PET/CT scanners have been installed at institutions worldwide, and close to one million PET scans are performed annually. PET imaging holds promise in the accurate and timely diagnosis and monitoring of pancreatic cancer.

What is PET imaging?
Positron Emission Tomography, or PET imaging, is a technology that painlessly examines the body’s chemistry and gives information about the function of organs and cells.  By providing information about the chemical reactions of the body, PET scans often detect abnormalities before other imaging tests such as CT (Computed Tomography) or MRI (Magnetic Resonance Imaging) scans.  Some scientific studies suggest PET 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 does PET imaging work?
Cells in the body rely on glucose, a simple sugar, as an energy source.  When an individual develops a disease, the tissues and cells within their body undergo biochemical changes.  Cells of the body that require more fuel, such as actively growing cancer cells, break down glucose at a faster rate than other cells.  PET works by providing a visual representation of this change.

A small amount of radioactivity is attached to a naturally occurring compound, usually glucose.  This radioactive form of glucose, called fluorodeoxyglucose (FDG), is injected into the patient’s body and is allowed time to be distributed throughout the body as the patient rests (about 45 to 60 minutes).  As it is metabolized by organs and tissues during normal cell functioning, FDG emits traces of radiation that are recorded by the PET scanner.  Areas that are using more FDG than others show up as bright or highlighted areas on the resulting images.

A computer assembles all of these signals and creates whole-body images that show areas where rapid use of the glucose is present.  With a single PET procedure, physicians may detect extremely small cancerous masses that might otherwise go undetected.  Early detection allows physicians to treat these diseases earlier and more accurately.

How is a PET scan performed?
PET is usually done on an outpatient basis.  The medical care team will give the patient detailed instructions on how to prepare for the examination.  Generally, a patient will be asked not to eat or drink anything and to avoid strenuous exercise for four hours prior to the appointment.  Both of these activities can contribute to increased metabolism which will appear during PET imaging.  In addition, patients are encouraged to wear comfortable, warm clothing since scanner rooms are often cool.

The entire process of having a PET scan usually lasts 2 to 3 hours.  Upon arrival at the imaging facility, the patient will receive an injection of FDG.  The patient will then be required to sit quietly while the tracer is distributed through the body.  When the scan begins, the patient should be prepared to lie still for 15-75 minutes while the imaging is performed.

In the case of pancreatic cancer, studies have shown that it may be useful to perform a dual-phase PET.  In a dual-phase PET scan, the PET imaging procedure is performed twice.  The first PET scan is performed one hour after the FDG is injected; this is referred to as the early scan.  A second, delayed PET scan is performed two hours after the initial FDG injection.  More studies are needed to validate the effectiveness of dual-phase PET imaging in detecting pancreatic cancer.

After a PET scan is completed, the patient should drink plenty of water to help flush out the remaining FDG.  Results of a PET scan are interpreted by a radiologist and sent to the physician who ordered the imaging.  Results should be obtained within a few business days.  Follow the physician’s recommendations regarding how often imaging should be performed.

When is a PET scan usually done?
PET images are sometimes performed prior to treatment to assist the medical team in determining the appropriate methods of treatment.  In pancreatic cancer, PET scans are used to

  • differentiate a benign or chronic pancreatic mass from a malignant tumor.
  • determine whether a tumor is resectable (eligible for surgical removal).
  • pre-operatively locate a pancreatic mass.
  • identify potential metastases to the liver or elsewhere.
  • assess tumor responsiveness to chemotherapy or other treatment.

How does PET compare to other imaging techniques?
CT and MRI images provide information such as size, shape, and location of physical structures in the body while PET monitors body functioning.  For pancreatic cancer diagnosis, the helical CT is the most common, and a newer technology called the Multi-Detector CT scan is also used.  Studies show varying results in the reliability of CT or MRI versus PET in detecting tumors when each technique is used alone.

Most of the time, PET is used in addition to CT or MRI scans.  In 2000, a machine that combines PET and CT scans was developed.  By running both tests concurrently, trained radiologists are now able to join the strengths of two imaging modalities to determine the precise location of an area showing high metabolic activity indicating a tumor.  This combined imaging has rapidly proven itself a useful tool in diagnosing cancers.  In pancreatic cancer, studies have shown that the combined PET/CT is more effective than PET alone in determining if surgical resection is possible.  The combined imaging has also shown improvements in identifying malignant pancreatic tissue and detecting distant metastases in advanced pancreatic cancer.

Do all hospitals have the ability to use PET?
The number of hospitals that have PET imaging machines is increasing, however this test is not available at all treatment centers.  There are some centers that now have combined CT/PET imaging machines.

Is PET imaging widely accepted in the medical community?
There is increased acceptance of the potential uses for PET imaging.   As with any emerging technology, scientific literature reflects varied opinions on its usefulness, accuracy and practical application.  The data has shown PET scans to be more sensitive and useful in some cancers than others.  There have been some studies that indicate potential use of PET scans in both the diagnosis and the management of pancreatic cancer.  However, further studies are needed in order to scientifically validate PET scans for widespread acceptance and use.  Until it has proven to be the best imaging technique for pancreatic cancer, it is important that PET be used in conjunction with other imaging techniques to build a complete picture.

Currently, most centers use PET scanning selectively in pancreatic cancer patients, and only in situations where conventional imaging studies raise questions.  Pancreatic cancer specialists may or may not recommend the use of PET imaging based on their individual experiences, the patient’s specific case of pancreatic cancer, coverage by patient insurance, and/or access to PET or PET/CT machines.  Some cases of pancreatic cancer can be followed sufficiently by CT imaging alone.  There is currently no clearly indicated area in which PET should be used routinely.

Will the sugar feed the tumor?
Fluorodeoxyglucose (FDG), the glucose used to perform a PET image, will not “feed” the tumor or cause the tumor to grow in size.

What are the potential limitations of PET?
PET can give false results if an individual’s chemical balances are not normal.  For example, the inflammation associated with pancreatitis can lead to a larger glucose uptake.  Other diseases, infections, or surgeries can also affect the use of glucose.

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 can detect, it may show small tumors that CT 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 adversely affected 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 that is used to perform a PET scan has not been shown to be detrimental to diabetics.

Is PET covered by insurance?
The cost of a PET scan is between approximately $3,000 and $6,000.  At this time, Medicare guidelines state that PET images will be covered for some select cancers, including lung, colorectal, and melanoma.  For pancreatic cancer, Medicare currently provides “Coverage with Evidence of Development”.  This means they provide coverage in specific settings if sufficient benefit to the patient is established, including possible coverage for clinical trials.  Visit Medicare’s coverage web site at www.cms.gov/coverage for the latest details.

While many private insurance companies follow the guidelines set forth by Medicare, they also handle coverage on a case-by-case basis.  Call the patient’s insurance provider to determine complete coverage information.

Internet resources for PET imaging:

 

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