BALTIMORE-Although standard imaging methods are not sensitive
enough to catch early-stage pancreatic tumors, positron emission
tomography (PET), currently under investigation for this purpose,
may prove to be a useful addition to CT scans, said Karl F. Hubner,
MD, professor of radiology, University of Tennessee Medical Center,
Knoxville. Dr. Hubner spoke, along with Pankaj Jay Pasricha, MD,
at a meeting on nuclear medicine sponsored by the Johns Hopkins
Patients with pancreatic cancer face a grim prognosis. "By
the time a patient is symptomatic, it's already too late,"
said Dr. Pasricha, assistant professor of medicine at Johns Hopkins.
Seventy percent of pancreatic cancers are found in the head of
the pancreas, and may be curable if discovered while less than
2 cm in diameter, Dr. Pasricha said. However, the 20% of tumors
found in the body of the pancreas and the 10% in the tail are
not treatable, because the disease has already spread too far.
Symptoms appear only late in the development of the tumor. These
may include jaundice, epigastric pain, weight loss, or acute pancreatitis.
Dr. Pasricha said that endoscopic retrograde cholan-giopancreatography
(ERCP) is the method of choice for differentiating pancreatic
cancer from other causes of obstructive jaundice.
"Less than 3% of patients with pancreatic cancer will have
a normal ERCP," he said. But of the tumors detected by ERCP,
only 40% are in stage I/II, leaving room for improvement. Dr.
Pasricha is hopeful that the development of a new technique-endoscopic
ultrasound-will help in making the diagnosis when standard imaging
is negative in patients with suspected pancreatic cancer.
Dr. Hubner said that PET offers an alternative to structural imaging
of the pancreas. Radiolabeled amino acids and F-18-2-fluoro-2-D-deoxyglucose
(FDG) have been studied as radiotracers that might discriminate
between inflammatory and malignant pancreatic masses.
Bares et al reported a sensitivity of 92% and specificity of 85%
for FDG PET in detecting pancreatic cancer (Radiology 192:79-86,
1994), Dr. Hubner said. In the late 1970s, researchers demonstrated
the feasibility of using carbon-11-labeled natural and unnatural
amino acids for imaging the pancreas.
Dr. Hubner pointed to current studies being conducted at the University
of Tennessee's Biomedical Imaging Center evaluating the potential
of aminocyclo-butanecarboxylic acid (C-11 ACBC) and FDG in patients
with suspected pancreatic cancer.
In 58 patients to date with a diagnosis confirmed histologically
or by clinical follow-up, PET with C-11 ACBC showed a sensitivity
of 83%, specificity of 87%, and accuracy of 85%, compared with
75%, 50%, and 67%, respectively, for FDG PET.
For both radiotracers, the time activity curve (TAC) profiles
showed a continuously rising slope for pancreatic cancer, compared
to an initial rapid rise and slow decline in normal pancreas,
and no or low uptake in pancreatitis.
Better Results With ACBC
This small University of Tennessee series suggests that both PET
methods could be complementary to CT in further characterizing
pancreatic masses, he said.
Dr. Hubner noted that these preliminary data suggest better results
with ACBC than with FDG.
"The most important observation with ACBC is the absence
of ACBC uptake in chronic pancreatitis and intense uptake in pancreatic
cancer," he commented. "This indicates the potential
of this unnatural amino acid for the differential diagnosis of
a pancreatic mass, and it deserves further clinical investigation
on a larger scale."