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Positron Emission Tomography (PET) May Prove a Useful Adjunct to CT in Detecting Early-Stage Pancreatic Cancers

Positron Emission Tomography (PET) May Prove a Useful Adjunct to CT in Detecting Early-Stage Pancreatic Cancers

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 Medical Institutions.

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."

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