PET
(Positron Emission Tomography) Imaging
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 quickly and painlessly examines
the body's chemistry and gives information about the function of
organs and cells. By providing information about the metabolic activity
(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 disease strikes,
the biochemistry of tissues and cells within the body 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 metabolizing more FDG than others show up as bright
or highlighted areas on the resultant images.
A computer assembles all
of the radiated signals and creates whole-body images that show
areas where rapid metabolism 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, once early and once delayed. 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. 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 $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.
For the latest report
on Medicare's coverage of PET scans, click on the following link:
http://www.cms.hhs.gov/DeterminationProcess/
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:
References:
Heinrich S, Goerres GW,
Schafer M, Sagmeister M, Bauerfiend P, Pertalozzi BC, Hany TF, vonSchulthess
GK, & Clavien PA. (2005). Positron emission tomography/computed
tomography influences on the management of respectable pancreatic
cancer and its cost-effectiveness. Annals of Surgery, 242(2), 235-243.
Nishiyama Y, Yamamoto
Y, Yokoe K, Monden T, Sasakawa Y, Tsutsui K, Satoh K, & Ohkawa
M. (2005). Contribution of whole body FDG-PET to the detection of
distant metastasis in pancreatic cancer. Annals of Nuclear Medicine,
19(6), 491-497.
Nishiyama Y, Yamamoto
Y, Yokoe K, Monden T, Sasakawa Y, Tsutsui K, Satoh K, & Ohkawa
M. (2005). Evaluation of delayed additional FDG PET imaging in patients
with pancreatic tumour. Nuclear Medicine Communications, 26(10),
895-901.
Pasquali C, Sperti C,
Lunardi C, Scappin S, Chierichetti F, Liesse G, & Pedrazzoli
S. (2005). [Positron emission tomography with fluorodeoxyglucose
in gastro-entero-pancreatic tumors: diagnostic role and prognostic
implications]. Suppl Tumori, 4(3), S68-71.
Sperti C, Pasquali C,
Decet G, Chierichetti F, Liessi G, & Pedrazzoli S. (2005). F-18-fluorodeoxyglucose
positron emission tomography in differentiating malignant from benign
pancreatic cysts: a prospective study. Journal of Gastrointestinal
Surgery, 9(1), 28-29.
If
you have questions about PET Imaging, please contact PanCAN's PALS
program via our toll-free phone number at 877-272-6226 or by email
at pals@pancan.org.
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