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Diagnosis: Positron Emission tomography (PET)
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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 the function of organs and cells.
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 cancer strikes, cells 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.
A small amount of radioactivity is attached to glucose. This radioactive form of glucose is called fluorodeoxyglucose (FDG). FDG is injected into the patient’s body. As it is broken down, or metabolized, by organs and tissues, FDG emits traces of radiation that are recorded by the PET scanner. A computer assembles all of the signals from the radiation and creates whole-body images that show areas where rapid metabolism is present. Areas that metabolize more FDG than others show up as bright or highlighted areas on the PET 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 procedure may help the doctor see extremely small cancerous masses that might otherwise go undetected. 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 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 for 45-60 minutes while the tracer travels 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 occurs twice, once early and once delayed. The early PET scan is performed one hour after the FDG injection. The second, delayed PET scan is performed two hours after the initial FDG injection. More studies are needed to determine the effectiveness of dual-phase PET imaging in detecting pancreatic cancer.
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 a benign (non-cancerours) or chronic pancreatic mass from a malignant (cancerous) tumor.
- determine whether a tumor is resectable (eligible for surgical removal).
- 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 methods?
PET 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 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 is more effective than PET alone in determining if surgical removal is possible. The combined imaging can also more accurately identify distant metastatic pancreatic masses.
PET scans are more sensitive and useful in some cancers than others. Further studies are needed to determine if PET scans should be widely used in detecting 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, 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 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?
In April 2009, Medicare expanded its guidelines to cover PET scans that are part of initial treatment for some solid tumors, including pancreatic cancer. This means that one PET scan used to diagnose and stage the cancer is covered. Visit Medicare’s coverage web site at www.cms.gov/coverage for the latest details.
Conversely, Medicare has determined that there is not enough scientific evidence to show the value of follow-up PET scans for many cancers, including pancreatic cancer. This means that Medicare generally does not pay for PET scans ordered to evaluate the effectiveness of treatment or to determine if the cancer has returned or spread. However, there is an exception. Medicare Part B may pay for follow-up PET scans if the doctor ordering the test participates in the “Coverage with Evidence of Development” program. In this program, results of the scan are reported to the National Oncologic PET Registry (NOPR). Data from this registry may later help Medicare decide to cover the follow-up scans as it does the initial scans. Ask the doctor if he or she participates in the “Coverage with Evidence of Development” program. Visit www.cancerpetregistry.org/patients.htm for complete details.
While many private insurance companies follow the guidelines set forth by Medicare, they also handle coverage based on the patient’s specific health insurance plan. Call the patient’s insurance provider to determine complete coverage information. Make sure the patient’s insurer is aware of the changes that Medicare has made in paying for initial PET scans for pancreatic cancer.
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