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

BALTIMORE-Although standard imaging methods are not sensitiveenough to catch early-stage pancreatic tumors, positron emissiontomography (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 HopkinsMedical Institutions.

Patients with pancreatic cancer face a grim prognosis. "Bythe 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 ofthe pancreas, and may be curable if discovered while less than2 cm in diameter, Dr. Pasricha said. However, the 20% of tumorsfound in the body of the pancreas and the 10% in the tail arenot treatable, because the disease has already spread too far.

Symptoms appear only late in the development of the tumor. Thesemay 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 pancreaticcancer from other causes of obstructive jaundice.

"Less than 3% of patients with pancreatic cancer will havea 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-endoscopicultrasound-will help in making the diagnosis when standard imagingis negative in patients with suspected pancreatic cancer.

Dr. Hubner said that PET offers an alternative to structural imagingof the pancreas. Radiolabeled amino acids and F-18-2-fluoro-2-D-deoxyglucose(FDG) have been studied as radiotracers that might discriminatebetween 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 demonstratedthe feasibility of using carbon-11-labeled natural and unnaturalamino acids for imaging the pancreas.

Dr. Hubner pointed to current studies being conducted at the Universityof Tennessee's Biomedical Imaging Center evaluating the potentialof aminocyclo-butanecarboxylic acid (C-11 ACBC) and FDG in patientswith suspected pancreatic cancer.

In 58 patients to date with a diagnosis confirmed histologicallyor by clinical follow-up, PET with C-11 ACBC showed a sensitivityof 83%, specificity of 87%, and accuracy of 85%, compared with75%, 50%, and 67%, respectively, for FDG PET.

For both radiotracers, the time activity curve (TAC) profilesshowed a continuously rising slope for pancreatic cancer, comparedto 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 PETmethods could be complementary to CT in further characterizingpancreatic masses, he said.

Dr. Hubner noted that these preliminary data suggest better resultswith ACBC than with FDG.

"The most important observation with ACBC is the absenceof ACBC uptake in chronic pancreatitis and intense uptake in pancreaticcancer," he commented. "This indicates the potentialof this unnatural amino acid for the differential diagnosis ofa pancreatic mass, and it deserves further clinical investigationon a larger scale."